Gas Turbine

TRAINING PROGRAM - PHASE IIA FOR

LNG PLANT at DAMIETTA

Gas Turbine All Rights Reserved

As per the provision of the CONTRACT, SEGAS are permitted to the use of this Product for reproduction and future reference only for SEGAS internal use. No part of this Presentation may be reproduced in any form or resold for other purposes without written permission from The Consortium.

UNION FENOSA gas SEGAS Services

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Lesson 1 : Engine Design

Lesson 2 : Operating Principles

Lesson 3 : Engine Case – Air Inlet

Lesson 4 : Compressor Section

Lesson 5 : Diffuser & Combustion

Lesson 6 : Turbine & Exhaust

Lesson 7 : Ignition System

Lesson 8 : Bearings & Seals

Lesson 9 : Lubrication & Lube Oil

Lesson 10 : Lube Oil Pumps

Lesson 11 : Lube Oil Filters & Coolers

Lesson 12 : Lube Oil Instrumentation

Lesson 13 : Hydraulic Oil System

Lesson 14 : Trip Oil System

Lesson 15 : Fuel Systems

Lesson 16 : Fuel Gas Supply System

Lesson 17 : Fuel Gas Control System

Lesson 18 : Liquid Fuel System


Lesson 20 : Pneumatic Starting

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Lesson 21 : Hydraulic Starting

Lesson 22 : Diesel Starting

Lesson 23 : Enclosures

Lesson 24 : Fire Detection

Lesson 25 : Gas Detection l

Lesson 26 : Extinguisher System

Lesson 27 : Principles of Power Gen

Lesson 28 : Generator Components

Lesson 29 : Lube Oil System

Lesson 30 : Generator Components

Lesson 31 : Generator Control

Lesson 32 : Principle of Components

Lesson 33 : Compressor

Lesson 34 : Compressor Lube Oil

Lesson 35 : Compressor Cooling l

Lesson 36 : Compressor Control

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IntroductionGas turbines are designed for many different purposes. In the petroleum industry they are commonly used to drive:

· compressors for transporting gas through pipelines

· generators that produce electrical power

In this lesson, you will learn about the basic design of gas turbine engines, their sections, and how they operate.

Lesson 1: Gas Turbine Engine Design


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Engine Design : SectionsThe purpose of a gas turbine engine is to create energy to turn a shaft that drives other rotating equipment such as compressors and generators.

The figure shows a two-shaft turbine engine. However, this lesson discusses a single-shaft turbine engine. The operation of the two types is similar.

A gas turbine engine is divided into five sections:· air inlet section· compressor section · combustion section· turbine section· exhaust section

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The sections of a gas turbine are contained in the engine case. The engine case is a horizontal tube-like container that is open at both ends.

Each section of the gas turbine has a specific function. Engine Design: Air Inlet & CompressorsHuge quantities of air enter the case through an opening at the front end called the engine air inlet.

After passing through the engine air inlet, the air flows to the compressor section. The compressor section contains the first moving part, the compressor.

Compressor wheels increase the pressure of the incoming air.

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Engine Design: CombustionThe compressed air flows to the combustion section of the gas turbine engine.

As the compressed air enters the combustion chambers, fuel is added through nozzles. The result is a mixture of fuel and air.

The fuel and air mixture is ignited and burns, creating hot gases. The hot, expanding gases flow into the turbine section of the engine.

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Engine Design: Turbine & ExhaustThe turbine rotors get power from the flow of gases. This flow of gases into the turbine section causes the turbine rotors to turn. A single shaft gas turbine has only one major moving part - the rotor shaft.

The turbine rotors and the compressor are mounted on opposite ends of the same rotor shaft. This allows the turbine rotors and compressor to work as a unit.

When the gases leave the turbine section, they return to the atmosphere through the exhaust section.

Information about how a basic gas turbine works is presented next.

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Air Entry and Compression :

The engine case of the gas turbine is a tube-like container that is open at both ends and narrow in the center.

The front end of the case contains the compressor. The compressor draws air into the case and increases the air pressure by compressing (or reducing) its volume.

In the figure, the electric fan acts as a compressor.

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Combustion

In the narrow section of the case, the air is compressed.

Fuel gas is injected into the compressed air through fuel nozzles. The fuel and air are mixed together and ignited.

Burning the fuel and air mixture creates hot combustion gases. The heat increases the temperature and available energy of the gases.

A turbine captures this energy and changes it into mechanical energy.

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Turbine Rotation

The turbine in the rear of the engine case extracts mechanical energy from the flow of the gases acting on the blades.

The turbine rotates because of the pressure and velocity of the hot expanding gases acting on the blades of the turbine.

As the hot gases pass through the turbine, the energy in the gas is reduced.

The gases exit the rear of the case and into the atmosphere.

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Air Entry and Compression

The engine case of the gas turbine is a tube-like container that is open at both ends and narrow in the center.

The front end of the case contains the compressor. The compressor draws air into the case and increases the air pressure by compressing (or reducing) its volume.

In the figure, the electric fan acts as a compressor.

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CombustionIn the narrow section of the case, the air is compressed.

Fuel gas is injected into the compressed air through fuel nozzles. The fuel and air are mixed together and ignited.

Burning the fuel and air mixture creates hot combustion gases. The heat increases the temperature and available energy of the gases.

A turbine captures this energy and changes it into mechanical energy.

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Turbine Rotation

The turbine in the rear of the engine case extracts mechanical energy from the flow of the gases acting on the blades.

The turbine rotates because of the pressure and velocity of the hot expanding gases acting on the blades of the turbine.

As the hot gases pass through the turbine, the energy in the gas is reduced.

The gases exit the rear of the case and into the atmosphere.

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Turbine Rotor

The turbine uses energy from the thrust force created by the expanding gases. This energy is changed into shaft horsepower to drive the turbine compressor, the engine accessories, and the load. Most of this energy is expended to drive the turbine compressor.

(Contd.)

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The main parts of a turbine are :

· rotor shaft · rotor disc · rotor blades

The rotor blades (sometimes called buckets) are attached to the rotor disc. The rotor disc is mounted on the rotor shaft. The entire assembly of blades and disc is often called a rotor.


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Basic OperationThe figure shows how all the components of the gas turbine engine work together. The fan has been replaced by a single set of compressor blades.

The compressor creates the compressed air that is needed for combustion.

Fuel gas is mixed with the compressed air and is ignited. The burning mixture creates a force in the rear of the engine case. The force is changed into rotating mechanical energy that turns the turbine.

(Contd. )

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The turbine turns the rotor shaft that is shared by the compressor and the turbine. The compressor and turbine are connected to the same rotor shaft. The rotating force of the turbine is used to drive the compressor.


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Front and Rear Drive Gas TurbinesSome of the rotating energy created by the gas turbine can be used to drive a gear box or generator connected to either end of the rotor shaft.

Depending upon where the load is connected to the rotor shaft, the gas turbine is referred to as a

· front end drive · rear end drive

(Contd.)

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If the shaft is lengthened on the compressor end of the engine, it is a front end drive or a cold end drive.

If the shaft is lengthened on the turbine end of the engine, it is a rear end drive or a hot end drive.


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Multiple Stage Compressors

Gas turbine manufacturers may place more than one set of compressor and turbine stages in an engine as shown in the figure.

The additional stages in the compressor section provide more compression of the air before combustion.

More than one stage is used in the turbine section to extract as much power as possible from the hot, expanding gases.

The gas turbine in the figure is a two-stage turbine driving a three-stage compressor.

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Newton's First Law of MotionNewton's first law explains why a force is needed to make the gas turbine work.

In the figure, a ball on a level table will not move until it is made to move by some force such as the wind or pushing it by hand.

Similarly, until the fuel and air mixture is burned in the gas turbine, there is no force for the turbine to use to turn the rotor shaft.

Lesson 2 : Operating Principles

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Newton's Second Law of MotionNewton's Second Law explains why the air must be compressed and accelerated to create a force.

In the figure, a hammer is used to drive a nail. The force of hitting the nail is proportional to the mass (weight) of the hammer multiplied by the velocity of the hammer when it hits the nail.

If you also use a heavier hammer, it is even easier to drive the nail into the wood.

Mass and acceleration directly affect the amount of force created. The more compressed air (mass) that enters the gas turbine, the more force created from the combustion process.


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Newton's Third Law of MotionNewton's Third Law explains how the action of creating a thrust force results in the reaction of the turbine rotating.

In the figure, a boat is near the bank of a river. The person steps from the boat toward the land. As his body pushes forward, the boat is pushed backward with the same amount of force.

In the gas turbine, the thrust force is the action. This force is directed into the rear of the case and on to the turbine blades. The blades of the turbine react to the force and turn the rotor.

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Laws of ThermodynamicsThe laws of thermodynamics explain the effects of heat in an engine.

The first law states that energy can be changed but it cannot be destroyed.

In a gas turbine engine, heat energy is changed to mechanical energy.

The second law of thermodynamics states that heat cannot be transferred from a cooler body to a hotter body.

In a gas turbine engine, heat is transferred from the hotter engine to the cooler lube oil.

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Boyle's LawCompressibility is an important factor in gas turbine performance. Gas turbines use compressed air for combustion.

Boyle's Law can be explained by placing a quantity of gas in a cylinder that has a tightly fitted piston.

When a force is applied to the piston, the gas is compressed to a smaller volume.

When the force is doubled, the gas is compressed to half its original volume.

The force exerted on the turbine blades increases as the pressure of the combustion air increases.

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Charles' LawCharles' Law explains the expansion of gases when heat is added by burning fuel in an engine.

Charles' Law is explained using the figure. The first container holds a certain volume of air. If the air in the container is heated, it expands and its pressure increases.

The expanding air pushes against the container. The higher the temperature, the greater the force applied by the expanding volume of air.

In the gas turbine, the forces created by the hot, expanding gases push against the blades of the turbine and turn the rotor.

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Atmospheric Factors Affect PerformanceAtmospheric factors affect the performance of gas turbines. Some of these factors are:

· air density ·contaminants

Temperature and water content affect the density of the air. Because cold air is more dense than hot air, it has more mass. The more air in the gas turbine, the more force created.

(Contd.)

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On a very cold day, a gas turbine can exceed its peak load. On a very hot day, a gas turbine will produce much less power.

The atmosphere also contains foreign matter that is harmful to gas turbines, such as pollen and dust. The contaminants reduce the efficiency of the turbine and damage internal parts.


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Brayton Cycle. Expansion . Combustion . Compression

The Brayton Cycle defines what takes place in the gas turbine engine. These events are controlled by the physical laws described on the previous pages.

The events in the Brayton Cycle take place in specific sections of the gas turbine. These events are:

· Compression · Combustion · Expansion · Exhaust

The Brayton Cycle is unique among engine cycles because all the events in the cycle take place at the same time without interruption.The exhaust event is the only part of the cycle that does not take place in the engine.

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Axial Flow Design

The axial flow gas turbine is one of the most common gas turbine engine designs.

The figure shows the simplified airflow through an axial flow gas turbine.

As you can see from the red flow lines, the air flows in a straight or axial path from one end of the gas turbine to the other.

Lesson 3: Engine Case & Air Inlet

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The purpose of the engine case in an axial flow gas turbine engine is to house and protect internal engine parts.

The engine case usually consists of two parts, a top half and a bottom half.

This type of construction allows easy access to the internal engine parts.

Note the similarity of the engine case in this figure to the engine case displayed in a previous lesson.

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Air Inlet System: PurposeAnother main component of a gas turbine engine is the air inlet system.

Air is delivered to the gas turbine engine air inlet through the air inlet system.

The purpose of the air inlet system is to carry clean, dry air to the compressor with minimum turbulence and energy loss.

The outside covering of the air inlet system is called ducting and is usually made of galvanized or stainless steel.

The exterior of the ducting and the inlet support structure should be painted to protect them from damage caused by corrosion.

Salt air will quickly corrode any unprotected exterior surfaces.

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Air Inlet System: Main Assemblies

The air inlet system controls the air quality and directs a steady flow of air to the compressor air inlet.

The amount and quality of air affects engine performance and reliability.

The air inlet system has two main assemblies:

· filter assembly· inlet ducting assembly

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Filter Assembly

The first component of the air inlet system is the filter assembly.

The filter assembly consists of the following parts:

· weather louvers

· inlet screens

· filters( Contd. )


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The filter assembly separates moisture and particles such as dirt, dust, and insects from the inlet air. This prevents contamination and foreign object damage to the compressor. Weather Louvers, Screens, & FiltersThe purpose of the weather louvers is to prevent direct rainfall from entering the air inlet system.

Inlet screens, usually constructed of wire mesh, are designed to prevent large items and other contaminants from entering the compressor inlet.

The inlet screens are located behind the weather louvers.Filters trap small particles of dust, dirt, and other contaminants to prevent them from entering the compressor section.

Filters are constructed of materials which meet the specific operating and climatic conditions of the gas turbine location.

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Air Inlet System: Main AssembliesThe air inlet system controls the air quality and directs a steady flow of air to the compressor air inlet.

The amount and quality of air affects engine performance and reliability.

The air inlet system has two main assemblies:

· filter assembly· inlet ducting assembly


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Inlet Ducting Assembly

The second main assembly in the air inlet system is the inlet ducting assembly which consists of the following parts:

· bypass door(s)

· inlet silencer

· trash screens

· air plenum


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Bypass Doors

Some air inlet systems have bypass doors located just downstream of the air filters.

The purpose of the bypass doors is to protect the air inlet from excessive differential pressure.

High differential pressure is usually caused by excessively dirty air inlet filters or some other abnormal blockage of the air inlet system restricting airflow through the filter. ( Contd. )

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Anytime the bypass doors are open during gas turbine operation, the engine is unprotected and is operating with unfiltered air.

Operating under these conditions reduces engine reliability and operating life.

Operators should take immediate action to determine why the bypass doors are open and take corrective action.

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Trash Screens

The inlet silencer is a baffle arrangement intended to quiet the noise vibration of the compressor blading.

The trash screens are the next component in the inlet ducting assembly. Trash screens prevent foreign objects from entering the compressor inlet.

Trash screens are installed downstream in the ducting before the compressor inlet.

( Contd. )


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Access doors are provided to allow cleaning and servicing of the trash screens.

Trash screens are constructed of stainless steel and should not require maintenance other than periodic cleaning and inspection.

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Air Plenum

The final component in the duct assembly is the air plenum.

The purpose of the air plenum is to provide equal distribution of ducted air to the compressor inlet.

The air plenum is located just forward of the compressor inlet.

One of the most common types of compressor inlets is the bellmouth inlet.

Air Plenum


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Bellmouth Compressor Inlet

The bellmouth inlet is a bell-shaped funnel with rounded shoulders that reduce air resistance. A bellmouth inlet is designed to direct the outside air to the inlet guide vanes of the compressor.

In some gas turbines, the inlets are fitted with protective screens to prevent foreign objects from entering the compressor.

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Silencers

Silencers are usually installed in both the air inlet system and the exhaust section of the gas turbine to reduce operating noise.

Silencing is accomplished by baffles covered with sound-absorbing material.

In some air inlet ducts, the interior walls of the ducting and air plenum chambers are also lined with this sound-absorbing material. ( Contd. )

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Gas turbine operating noise is harmful to the human ear. Silencers help but do not totally eliminate gas turbine noise. If you work near a gas turbine that is operating, you should wear ear protection to avoid hearing loss.

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Lesson : 4 Compressor Section


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Introduction

The preceding lesson discussed the general construction of an axial flow gas turbine engine. It also described the engine case and the components of the air inlet.

In this lesson you will learn about the compressor section of an axial flow gas turbine.

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Axial Flow Compressor: Purpose

The purpose of the compressor section is to compress air for cooling and combustion. The compressor draws in atmospheric air through the air inlet and increases its pressure while reducing its volume.

In an axial flow compressor the air flows axially. This means that the air flows in a relatively straight path in line with the axis of the gas turbine.

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The main components of an axial flow compressor are the:

· case

· rotor

· stator

The first component discussed is the compressor case.


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Compressor Components: Case

The compressor case contains the rotor and the stator. The case is divided into halves. The upper half may be removed for inspection or maintenance of the rotor and stator blades while the bottom half remains in place.

The case of an axial flow compressor has the following functions:

· support the stator vanes( Contd. )

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· provide the outside wall for the axial path of airflow

· provide a means for extracting compressed air

The next compressor section component is the rotor.

· support the stator vanes

· provide the outside wall for the axial path of airflow

· provide a means for extracting compressed air

The next compressor section component is the rotor.

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Compressor Components: RotorThe rotor is the rotating element of the compressor. The rotor contains blades fixed on a spindle, drum, or wheel.

These blades push air to the rear in the same way a propeller does. The movement of air is caused by the angle and the shape of the blades.When turning at high speed, the rotor takes in air at the compressor inlet, increases the air pressure, and accelerates the air toward the rear of the engine through a series of stages.

Energy is transferred from the compressor to the air as velocity energy


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Compressor Components:Rotor Blades

Rotor blades are usually made of stainless steel. They are usually fitted into the rotor disks by either bulb-type, fir-tree type, or dove-tail type roots.The blades are then locked by means of screws, spacers, pins, keys, lock wires, or peening.The clearance between rotating blades and the outer case is critical.Rotor blades are thinner at the tips than at the base. This design helps prevent damage to the blade, stator vanes, or compressor housing if the blade contacts the compressor housing.


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Compressor Components: Rotor BladesCompressor rotor blades are shorter at the discharge than at the inlet.

This narrower working space is caused by the decrease in casing diameter, by the increase in rotor wheel diameter, or both.

Some compressor blades have knife-edge tips. At ambient temperature, the compressor rotor fits easily into the compressor case. However, as the blades expand from compression heat, they lengthen and reduce clearance between the case and rotors.


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Compressor Components: Rotor Blades

Tighter clearances increase the efficiency of the axial flow compressor.

The compressor section component discussed next is the stator.


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Compressor Components: StatorStator vanes are the non-moving elements of the compressor. They are located between each rotor stage. Stator blades are attached to the inner wall of the case.

Stator vanes receive high velocity air from each preceding rotor stage of the compressor.Stator vanes direct airflow to the next stage of compression at the desired angle. This controlled direction provides increased blade efficiency. ( Contd. )


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Stator vanes also act as diffusers, changing velocity to pressure.

Stator vanes on the discharge end of the compressor are aligned to straighten the airflow and reduce turbulence. These vanes are called straightening vanes, outlet vane assemblies, or exit guide vanes.

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Compressor Components: Stator Vanes

Stator vanes are usually made of corrosion-resistant and erosion- resistant steel.

They may be mounted to the engine case in several ways. For example:

· They are frequently shrouded or enclosed by a suitable band for fastening purposes.

(Contd. )


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· The vanes may be welded into the shrouds. The shroud is secured to the inner wall of the compressor case.

· In some cases, individual blades are inserted into slots cut in the case.

Each component plays an important role in compressor operation, which is our next topic.

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Axial Flow Compressor: Operation

When air enters the compressor through the air inlet, incoming air passes through the first row of vanes, called inlet guide vanes.

As the air enters the first set of rotating blades, it is deflected in the direction of blade rotation.The air is then caught and turned as it passes through a set of stator vanes. From there, the air is picked up by another set of rotating blades.This process continues through the compressor. The pressure of the air increases each time it passes through a rotor/stator blade set (called a stage).


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Axial Flow Compressor: OperationAs pressure is increased by successive rotor/stator blade sets, air volume is decreased.

At the compressor exit, the diffusion section finishes the compression process by decreasing air velocity and increasing pressure just before the air enters the combustion section.

A major effect of an unstable compression process is surging, which is discussed next.


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Compressor Surge

Compressor surge is a characteristic common to all types of gas turbines.

In general, surge is the result of unstable airflow in the compressor.

This unstable condition is often caused by air building up in the rear stages of the compressor.

When a compressor is not operating at its optimum speed, the forward compressor blades may provide more air than the downstream stages can compress. The air then tends to reverse flow. The compressor surges.

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Compressor Surge

Surging causes the machine to vibrate excessively.

Several methods are used to control surging. For example, the two-shaft gas turbine design reduces the possibility of surging.

Compressors with higher compression ratios have a greater tendency to surge. Compression ratios are discussed next.

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Compression RatioLarge, high-powered gas turbines require greater efficiency and higher compression than can be obtained with a single axial flow compressor.

Single axial compressors usually have a compression ratio of approximately 8:1.Compression ratio is determined by the discharge pressure (psia) divided by the suction pressure (psia).

For example, a gas turbine with a compression ratio of 8:1 discharges 117.6 psia of discharge pressure for every 14.7 psia of suction pressure.In two-shaft gas turbines, one or more turbine stages drive the compressor.


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Two-Shaft Axial Flow Gas Turbine

A separate turbine section drives the compressor.

Except for the airflow, the two rotor systems (compressor and turbine) operate independently.

Each compressor is driven at its own speed by its own set of turbine wheels, as shown in the figure.


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Coaxial Rotor Shaft

Two-shaft gas turbines use a coaxial rotor shaft. A coaxial shaft consists of a hollow outer shaft containing a solid inner shaft.

The inner shaft is mounted on bearings, which allows each shaft to independently rotate at different speeds.

The front compressor is the low pressure compressor. The rear compressor is the high pressure compressor.


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Two-Shaft Compressor Two-shaft compressors can operate with lower compression ratios. The lower compression ratio helps reduce the possibility of surging.

For example, if a gas turbine had a compression requirement of 20:1, a two-shaft (dual compressor) would share the load.

Each compressor, operating in series, may have only a 4:1 or 5:1 compression ratio. The net compression ratio of the dual compressors is higher than that of a single compressor.


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Two-Shaft Compressor Ratio

For the dual-compressor engine, compressor pressure ratio is usually given for each compressor or as:

LP compressor 4:1 x HP compressor 5:1

= Total compression of 20:1

The ratio of one compressor is multiplied by the other to give the total compressor pressure ratio.

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Lesson : 5 Diffuser & Combustion

Introduction

The previous lesson presented information about the compressor section of an axial flow gas turbine. This lesson contains information about the diffuser and the combustion section.

We will begin by discussing the purpose of the diffuser.


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Diffuser: Purpose

The diffuser is located between the compressor section and the main components of the combustion section.

The purpose of the diffuser is to prepare the air for entry into the combustion section.

The front end of the diffuser is bolted to the compressor case, and the back end is attached to the combustion section. ( Contd.)

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The diffuser has an expanding internal diameter that looks like an upside down funnel. This provides additional space, like the air plenum, for the compressed air to expand. In an operating gas turbine, the point of highest pressure is in the diffuser exit.

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Diffuser Bleed Air PortsBleed air ports are usually built into the diffuser case.

Some ports are opened and closed automatically to aid in start-up and shutdown.

The primary purpose of the ports is to provide bleed air for:

· cooling internal engine parts· operating engine sensors and controls· preventing compressor surge


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Diffuser: Airflow

The diffuser straightens the flow of air into the combustion section and provides equal distribution to each chamber.

When the air leaves the diffuser, it enters the next main section of the gas turbine, the combustion section.


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Combustion Section: Purpose

The combustion section is located between the compressor and turbine sections.The purpose of the combustion section is to add heat energy to the flowing gases.

This addition of heat causes the gases to expand and accelerate into the turbine section.The hot gases that are generated by burning fuel in the combustion chambers are used to power the turbine and the load.


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Components: Combustion ChamberThe main component of the combustion section is the combustion chamber (burner).

A basic combustion chamber consists of the following:

· outer case· perforated inner liner· fuel injectors· source of ignition ( Contd.)


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The outer case forms the shell or chamber in which the combustion process takes place.

The perforated inner liner, also called a combustion liner, is designed to provide paths for compressed air and gases to flow through the chamber for efficient combustion and expansion.

The fuel injectors are located at the inlet of the combustion chamber.

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Components: Combustion Chamber

The fuel system supplies clean, pressure-regulated fuel to the combustion chamber where it is mixed with the incoming compressed air from the diffuser.

During start-up, the fuel and air mixture is ignited by a spark plug (source of ignition).

After combustion occurs, the spark plug stops firing.

The burning gases supply the heat energy required to operate the turbine and load.


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Components: Coaxial ArrangementCombustion chambers are arranged coaxially (common axis) with the compressor and turbine to allow efficient flow-through operation.

The figure shows a typical combustion chamber arrangement.

Note the location of the spark plugs. Not all combustion chambers have spark plugs. Some are equipped with only crossfire tubes. ( Contd.)


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To supply the needed source of ignition, the flame from the combustion chamber is carried through a crossfire tube to the next chamber. It is then used as the source of ignition for the next chamber. This process continues until all chambers are ignited.

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Combustion Section: Operation

We turn now to the overall operation of the combustion section.

Compressed air enters the combustion chamber, fuel is injected, and the fuel/air mixture is ignited and burned.

The burning or combustion gases expand and travel toward a point of lower pressure at the rear of the chambers. Because high pressure compressed air surrounds the burner on all sides except the rear, the hot, expanding gases are directed toward the turbine section.


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Operation: Airflow

To operate efficiently, a combustion chamber must provide:

· a means for proper mixing of air and fuel· a way to cool the hot combustion products to a temperature the turbine section components can tolerate.

Contd.


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Operation: AirflowTo accomplish these actions, airflow through the combustor is divided into two air paths:

· primary

· secondary

The primary air is approximately 25% of the total air that enters the chamber. Primary air is sent to the fuel nozzle area for combustion.


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Operation: Primary & Secondary Air

About half of the primary air flows axially through the front of the combustion liner in the area of the fuel nozzles.

The rest of the primary air enters radiallythrough small holes in the front third of the combustion liner.

All primary airflow supports combustion.( Contd.)


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The remaining air that enters the chamber, approximately 75%, issecondary air. Half of this air provides a cooling air blanket over the inside and the outside surfaces of the combustion liner.

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Operation: Primary & Secondary AirSome of this airflow also centers the flame and keeps it from contacting the combustion liner.

The other half of the secondary air enters the combustion liner toward the rear.

This part of the airflow dilutes the combustion gases to an acceptable temperature to improve the turbine components service life. ( Contd.)


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The combustion process is accomplished in the first third of the combustion liner.

In the remaining two thirds of the combustion liner, combusted gas and air mix to provide even heat distribution to the turbine nozzle.

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Lesson 6 : Turbine & Exhaust


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Introduction

The previous lesson presented information about the diffuser and the combustion section of a gas turbine engine.

The information in this lesson focuses on the last two sections of the gas turbine engine, the turbine and the exhaust sections.

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Turbine Section: Purpose

The turbine section is located between the combustion and the exhaust sections of the engine.

The purpose of the turbine section is to convert the energy of the expanding gases into mechanical energy to drive the compressor, the accessories, and the load.

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Turbine Section: Components

Like the compressor section, the turbine section consists of two major components:

· the stator or turbine nozzles

· the rotor or turbine wheel


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Components: Stators & Rotors

The stators and the rotors of the turbine and compressor sections are similar in their construction.

The primary difference is the angle at which the vanes, nozzles, and blades are positioned.

Their positioning is critical to efficient engine operation.

Contd.


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Note the similarity of the elements of the turbine nozzle to the compressor stator vanes discussed in an earlier lesson.

We will now look at the purpose and operation of the turbine nozzle vanes.

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Components: Turbine NozzleThe turbine nozzle vanes have two purposes:

· prepare the combustion gases for driving the turbine rotor

· deflect the combustion gases in the direction of the turbine rotation

To accomplish this, the shape and position of the turbine nozzle vanes form passages for expanding gas flow.

Contd.

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These passages:

· change the direction of gas flow

· increase gas velocity

· reduce pressure and temperature of the gas

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Components: Turbine RotorThe energy of the gases leaving the first row of turbine nozzle vanes encounters the next major component of the turbine section, the rotor or turbine wheel.

The purpose of the turbine rotor is to extract mechanical energy to operate the compressor, accessories, and load.

The turbine rotor consists of the following:

· shaft · blades or buckets · disk


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Components: Turbine Rotor BladesThe blades of the turbine wheel are mounted to the disk in the same manner as compressor blades are mounted to the rotor, by either fir-tree, bulb, or dove-tail type roots.

This rotor and disk assembly is attached to the shaft.As the turbine wheel rotates it transfers energy to the shaft, which is connected to the compressor, the accessories, and/or the load.The turbine section may be either single stage or multistage.When the turbine has more than one stage, nozzle vanes are installed between each stage.


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Components: Nozzle Vane AssemblyEach set of stator vanes forms a nozzle vane assembly for the following turbine wheel.

Exit guide vanes straighten the gas flow as it enters the exhaust section.We have looked at the purpose, function, and design features of the two main components of the turbine section, the stator and the rotor.Our next focus is on the operation or flow of gases through the turbine section.


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Turbine Section: Operation ( Contd.)

Hot, expanding gases from each burner (or combustion chamber) flow through a transition duct to turbine nozzle vanes.

The nozzle vanes direct the expanding gases into the turbine section.

As you recall, the components of the compressor section convert energy by increasing the airflow pressure.


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In contrast, the components of the turbine section convert energy by reducing the pressure of the flowing gases.

Pressure is changed to velocity by the shape and position of both the turbine stator vanes and the rotor blades.

Turbine Section: Operation


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Operation: Gas FlowThe shape of the vanes causes an increase in velocity as the gases pass between the vanes.

As the gases flow into the first set of vanes, the gases accelerate because the space between the vanes is converging (a funnel effect similar to the first half of the venturitube).

As Bernoulli's principle states:· an increase in velocity causes a decrease in pressure


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Operation: Gas Flow

The gases leaving the nozzle vanes reach their maximum velocity just before they hit the first-stage turbine, causing it to rotate.

The shape of the rotor blades also accelerates the gases.

At this point, the gases still have enough energy to do work.

The turbine blades redirect the hot gases into the second row of nozzle vanes.


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The gases are again accelerated between the nozzles.

The second set of nozzles generates additional gas velocity just before the hot gases impinge on (hit) the second set of rotor blades.

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Operation: Gas Flow

The process continues through each stage of the turbine section.

Exit vanes reduce turbulence before the gases enter the exhaust section. This reduces backpressure on the turbine section.

Approximately two-thirds of the total energy available for work in a gas turbine is used to turn the compressor.


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The remaining energy available for work is used to drive the load.

The purpose and operation of the exhaust section are described next.

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Exhaust Section: Purpose

When the gases exit the turbine section they enter the last section of the gas turbine, the exhaust section.

The exhaust section is located directly behind the turbine section of the engine.

The purpose of the exhaust section is to discharge the spent gases to the atmosphere.


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Exhaust Section: Components (Contd.)

The exhaust section usually consists of the following components:

· outer housing · inner housing · struts · plenum

These components act as a diffuser, to reduce the turbulence and velocity of exhaust gases.


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The outer housing is fastened to the rear flange of the turbine section.

The inner housing is connected to the outer housing by struts and may be fitted with a cone to help in the diffusion process.

Struts also straighten the exhaust flow.


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Exhaust Section: Components

The inner housing may contain the gas turbine rear bearing assembly and overspeed trip device.

The outer housing flange is used to connect the exhaust collector or plenum to the gas turbine exhaust section.

The diffusion process occurs in the exhaust section as the volume is increased.


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Exhaust Section: Operation

As the exhaust gases pass through the exhaust section components:

· the velocity is decreased· the pressure remains relatively constant· the turbulence is reduced

The exhaust gases enter the atmosphere from the exhaust plenum.

Because exhaust gas temperatures normally exceed 700°F, external areas are covered with insulation or guards to protect personnel and to prevent fire.


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Introduction

Subsystems and accessories are separate from the main gas turbine engine assembly.

This lesson presents information on three gas turbine engine subsystems and accessories:· ignition system· accessory drives (gearbox)· vibration monitoring system

Subsystems and accessories are essential for gas turbine engine operation.The first subsystem discussed is the ignition system.


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Ignition System: Purpose & Components

The purpose of the ignition system is to supply a spark to ignite the fuel/air mixture in the combustion chambers.

A typical ignition system consists of the following components:

· igniter plugs or spark plugs· transformers or ignition exciter· ignition leads


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G.E. Ignition System Components (Contd. )

Ignition system components and operation differ among manufacturers. However, the purpose of the system is the same.

We will look at the components and operation of typical ignition systems used by General Electric (G.E.) and Solar.

We begin with the G.E. ignition system.


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The components used in a typical G.E. ignition system are:

· igniter plugs

· transformers

· ignition leads


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Components: Igniter Plug & TransformerAn igniter plug is usually installed in two of the combustion chambers.

When ignition occurs, a high-intensity spark jumps across the air gap of the ignitor plug. This spark initiates combustion in the combustion chamber.

The G.E. ignition system has two transformer units and two leads to the igniter plugs. ( Contd.)


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Components: Igniter Plug & Transformer

The two transformer units may be separate or housed in one unit.

After a normal start-up, ignition is no longer needed and the ignition system is deactivated.

After lightoff, the flame in the combustor is the ignition source for continuous combustion.


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Components: Igniter Plugs

Gas turbine igniter plugs differ from the spark plugs of reciprocating engines.

The air gap at the igniter plug tip is much wider, and the electrode is designed for a much higher intensity spark.

(Contd.)


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An igniter plug is less likely to foul because the high energy spark removes carbon or other deposits when the plug fires.

Igniter plug shells are made of high quality alloy, and the center electrode may be tungsten or iridium. These are all highly wear-resistant materials.

Igniter plugs are usually much more expensive than spark plugs.


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The hot end of the igniter plug is usually air cooled to keep it 500°F to 600°F cooler than the gas temperature.

This cooler temperature helps to prevent corrosion.

Next we focus on the ignition system components used by Solar.


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Solar Ignition System Components (Contd.)Solar gas turbines use a torch ignition system consisting of:

· ignition exciter· shielded cable spark plug lead· spark plug

The ignition exciter is mounted in a box on the gas turbine base.

The exciter is connected to the spark plug by a cable or lead.


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Solar Ignition System Components

The exciter is operated by 24 volt DC power. This power is changed to AC and then is stepped up, which charges a storage capacitor.

When the capacitor charge reaches its discharge value, the capacitor discharges through the spark plug cable to the spark plug.

Exciter output is approximately 18,000 volts. Up to fifteen sparks per second are produced as long as the exciter is energized.


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Components: Igniter TorchThe spark plug is installed in the igniter torch.

The igniter torch extends through the combustor outer liner.A small, controlled amount of gas is sent to the tip of the spark plug electrode.The spark jumps across the spark plug electrode's air gap and ignites the gas, creating a torch flame.This torch flame flares into the combustion liner to provide positive lightoff of the fuel/air mixture in the combustor.


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Ignition System Maintenance Procedures (Contd.)

In ignition systems, the term high intensity means that the electrical charge can be lethal.

Because the electrical charge can be lethal, ignition systems require special maintenance and handling according to the manufacturer's instructions.


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Typical maintenance procedures include the following:

· Ensure that system power is locked out before performing any maintenance on the system.

· To remove the igniter plug, disconnect the transformer input lead, wait the time specified by the manufacturer (usually 1 to 5 minutes), then disconnect the igniter lead and ground the center electrode to the engine to discharge the capacitor.


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Ignition System Maintenance Procedures (Contd.)

· Use caution when handling damaged transformer units that are hermetically sealed. Some transformers contain radioactive material.

· Before performing a firing test of igniters, ensure that the combustor is free of fuel. A fire or explosion could result.


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Ignition System Maintenance Procedures

· Do not energize the system for troubleshooting when the igniter plugs are removed. Transformer damage may occur.

· Discard all igniter plugs that have been dropped. Internal damage can occur that is not detectable by inspection or testing.

· Use a new gasket when the plug is reinstalled. The gasket provides a good current path to ground.


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Accessory Drives: Purpose

The next topic discussed is accessory drives. We will look at the typical accessory drives for both G.E. and Solar.

Gas turbine accessory drive systems provide gear reduction and mounting pads for accessories needed for engine operation.

( Contd.)


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Accessory Drives: Purpose

Some of these accessories are the oil pumps, hydraulic pump, fuel pump, and starting means interface.

The primary purpose of the accessory drive is to provide a means to drive each accessory at the proper speed and to connect and disconnect the engine from its starting device.

The figure shows a typical G.E. accessory drive assembly.


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G.E. Accessory DriveThe accessory drive gear is driven by a shaft that meshes with a helical gear driven by the main rotor shaft.

The gearbox is usually located at the front (forward) or the rear (aft) of the gas turbine engine, depending on the engine inlet or exhaust arrangements.

G.E. describes its typical accessory drive system as the main link between the gas turbine and the drive components of the starting system.

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G.E. Accessory Drive: Function

The gear drives several accessory devices that support gas turbine operation.

Each drive pad is a point of potential oil leakage because of the shaft seal arrangement.

Engine oil from the lube oil pump or the hydraulic pump may leak into, or from, the accessory drive assembly through the drive shaft seal.


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G.E. Accessory Drive: FunctionThe G.E. accessory gear also provides a mount for the turbine overspeed trip bolt mechanism.

The trip bolt mechanism is mounted on the exterior case of the accessory gear.

The actual overspeed trip bolt is mounted in the main or number one gear shaft. This is covered in more detail in a later lesson.

The Solar accessory drive is discussed next.


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Solar Accessory Drive: Function (Contd.)On Solar gas turbines, the accessory drive is attached to the air inlet assembly.

The accessory housing contains the accessory drive gears, pinion gears, and the necessary shafts and bearings.

Mounting pads and gear drives are provided for the starter, lube oil pump, hydraulic oil pump, speed governor, seal oil pump, and other accessories.


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Solar Accessory Drive: Function

If a particular accessory is not used, a cover plate is installed on the mounting pad.

During the starting cycle, the Solar accessory gear is driven by the starter assembly.


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Solar Accessory Drive: Function

A starter disengaging jaw clutch and accessory drive adapter connect the starter to a spur gear and shaft.

During the start cycle, the gas turbine compressor is driven by the gear.

After the starter jaw clutch disconnects, the compressor shaft drives the gear.


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Vibration Monitoring System: Purpose (Contd.)

The last topic discussed in this lesson is the vibration monitoring system. The purpose of the vibration monitoring system is to help in preventing abnormal operating conditions.

The rotating shafts of any machine or gearbox have a tendency to move axially or radially as a result of speed, loads, worn internal parts, unbalance, or other reasons.


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Vibration Monitoring System: Purpose

Axial and radial shaft movement is called vibration. Vibration is a continuing periodic change in a displacement from a fixed reference.

Excessive vibration is an abnormal operating condition that can result in equipment damage. Excessive vibration is a symptom of other abnormal conditions.

A bent shaft or improper shaft alignment could be the source of vibration.


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Vibration MonitoringSeveral protective systems are used to alert operators to abnormal operating conditions that could result in damage to the turbine or other equipment.

Vibration is one of the critical operating parameters that is monitored by a protection system.

A vibration monitoring system is usually a part of the gas turbine's programmable logic control and operator terminal.

The figure shows typical vibration detector locations in relation to the rotor.


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Shaft Movement

Vibration monitoring systems are installed on gas turbines and driven equipment to monitor and sometimes record axial and radial shaft movement.

Shaft movement is monitored in either displacement (mils), velocity (length/unit-time), or acceleration (g's).

(Contd.)


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One mil equals 0.001 of an inch. A shaft movement of 5 mils could generate an electrical impulse of one volt. Either of these measurements may be used as setpoints to initiate an alarm or shutdown.

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Vibration Monitoring Probes (Contd.)In the gas turbine, vibration probes are installed in the bearing housings near the shaft.

The probe tips operate on 24-volt DC power to establish a magnetic field between the probe tip and a burnished area on the shaft.

As the distance between the probe tip and the shaft changes, the strength of the magnetic field changes.

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The probe senses fluctuations in the magnetic field, and the monitoring systems uses this information.

The figure illustrates a typical single and double radial probe installation in a bearing.


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Vibration Monitoring Probes: FunctionIn the figure, four probes monitor the radial movement of a gas turbine shaft and two probes monitor the shaft axial location.

Axial position probes 1 and 2 monitor shaft axial movement in two places at the thrust collar.

Probes 3Y and 4X measure radial movement at the low pressure end of the compressor. (Contd.)


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Probes 5Y and 6X measure radial movement at the high pressure end of the compressor.

The probes are placed 90 degrees apart to monitor relatively both horizontal and vertical radial movement.

One probe monitors the X axis, and the other monitors the Y axis.


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Vibration Monitoring Probes:Function

Axial movement is usually monitored by two probes, as shown in the figure.

One probe is mounted at the end of the shaft, and the other is mounted at the thrust collar.

During operation, any of the vibration monitors can usually be read on the PLC display.

(Contd.)


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At some preset vibration limit (3 mils, for example), the vibration monitoring system will initiate an alarm.

If vibration increases to the high limit (5 mils, for example), another alarm is initiated and the vibration monitoring system will initiate a TRIP signal to shut the unit down before damage occurs.


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Gas TurbineLesson 8 : Bearings & Seals IntroductionThis lesson presents information about bearings and oil seals commonly used in gas turbine engines.

The main bearings of a gas turbine engine are mounted in a bearing housing.Most bearing housings contain seals to prevent oil leakage into the gas path.In this lesson, bearings are discussed first, followed by oil seals.The lesson begins with the purpose of bearings.

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Bearings: Purpose

Bearings have several purposes. They:

· support engine parts

· minimize friction

· minimize wear

· allow freedom of movement

· carry loads( Contd. )


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The primary loads that act on main bearings are:

· weight of the rotating mass (compressor, turbine, etc.)

· axial forces of power or load change

· compression and tension loads between stationary parts and rotating parts caused by thermal expansion and misalignment

· vibration


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Types: Plain

There are several types of bearings used in gas turbines:

· plain or sleeve

· ball and roller

Plain bearings are the simplest type of bearing.

Plain bearings are used in minor load locations, such as engine accessories.


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Types: Plain Bearing AssemblyA plain bearing assembly consists of the:

· bearing support or bracket

· bearing housing or container

· plain or sleeve bearing

During engine operation, pressurized lube oil is injected into the bearing through oil passages.


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This oil forms a film between the bearing and the surface that is being supported to prevent metal-to-metal contact.

The rotating part moves on a film of lube oil instead of on the surface of the bearing.


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Types: Ball & Roller BearingsBall bearings and roller bearings are called antifriction bearings because the balls and rollers minimize friction. Ball and roller bearings are commonly used because they:

· offer little resistance to rotation· provide precise alignment of rotating parts· are relatively inexpensive· can withstand momentary overloads· are easy to lubricate· work with both radial and axial loads· can endure elevated temperatures


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Types: Ball Bearings

The main disadvantages of ball and roller bearings are that they:

· are easily damaged by foreign matter· fail with very little warning

A ball bearing consists of the following components:

· an inner and an outer race· a set of polished steel balls· a ball retainer


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Types: Roller Bearings

Roller bearings also have an inner and an outer race, but use rollers rather than balls.

Roller bearings are made in different shapes and sizes for both radial and thrust loads.

Straight roller bearings primarily support radial loads. Tapered roller bearings support both radial and thrust loads.

(Contd.)


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In roller bearings, the roller is located between an inner and an outer race.

When a roller is tapered, it rolls on an angled outer race.

The inner races of ball and roller bearings are closely fitted to the rotor shafts to prevent movement of the shaft. Bearings designed to resist thrust in one direction have a heavier race on the side that supports the thrust.


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Types: Hydrodynamic Bearings

Hydrodynamic bearings use an oil wedge for support and to reduce friction.

There are two types of hydrodynamic bearings:

· radial oil-wedge

· thrust oil-wedge


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Hydrodynamic Bearings: Radial Oil-Wedge

A radial oil-wedge bearing resembles a plain bearing except the bearing or bushing is divided into several sections, or pads. Each pad is able to tilt or lean.

When the shaft rotates in the bearing, the pads tilt to allow wedges of oil to form between the pad and the shaft.

( Contd.)


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Oil wedges support the shaft as it rotates and cannot be squeezed out of the bearing housing when a heavy load is imposed.

The axial movement of a gas turbine rotor shaft is controlled by thrust bearings.


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A typical thrust oil-wedge bearing consists of:

· a bearing housing· thrust shoes· a thrust collar attached to the rotating shaftThe shaft is held in position by oil pressure acting against the thrust collar.

If the shaft moves, the thrust collar loading increases to prevent further movement.Hydrodynamic Bearings: Radial Oil-Wedge


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The thrust oil-wedge bearing operates on the same principle as a radial oil-wedge bearing.

In a thrust oil-wedge bearing, the thrust shoes are positioned against leveling plates.

As the thrust shoes pivot during gas turbine operation, oil wedges form between the thrust collar and the shoes.

The oil wedges limit axial thrust of the rotor shaft.


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Bearing assemblies must be well supported and strong enough to support the loads imposed by the rotating rotor.

Lube oil is delivered to the bearings to provide support.

Information about seals is presented next.Hydrodynamic Bearings: Radial Oil-Wedge


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Seals: Purpose & Types

To prevent leakage of oil from a narrow flow path, bearing assemblies usually contain oil seals.

The purpose of oil seals is to prevent oil from leaking from the bearing housing.

There are two types of oil seals, they are labyrinth or carbon. Both types are often used in the same seal assembly.


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Types: Labyrinth SealThe labyrinth seal is a controlled-leakage device that allows minimal leakage across the seal.

It also controls compressed air from the compressor section and hot gases from the turbine section that leak along the shaft.Air from the gas path outside the bearing housing bleeds inward through the grooves in the labyrinth seal.A labyrinth seal assembly consists of grooves in the seal and, in some cases, teeth in the shaft. Also provided is a vent and a drain for removal of gas and liquids.


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Types: Labyrinth Seal in Bearing Housing

Seal dams formed by the teeth and grooves in the labyrinth seal allow a metered amount of air from the engine gas path to flow inward.

The figure shows a typical compressor rear bearing housing arrangement.

Lube oil enters at the top of the bearing housing and drains out the bottom.

( Contd.)


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Labyrinth seals are installed in both ends of the bearing housing with bleed air pressure against the outer surfaces of the seals.

The air flows between the teeth and grooves of the seals into the bearing housing. This prevents lube oil from leaking through the seals.


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Types: Labyrinth Air-oil SealThis figure shows a labyrinth air-oil seal arrangement with a dual labyrinth seal located on each side of the bearings.

The bearing housing is contained in a cavity. The space between the bearing housing and the walls of the cavity is pressurized with bleed air from the engine compression section.

This type of arrangement takes advantage of controlled bleed airacross the seals. The bearing housing is vented to the atmosphere.


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Types: Labyrinth Air-Oil Seal

The controlled leakage of air into the bearing housing prevents oil leakage.

Pressurized air that leaks outward along the shaft prevents gases from leaking into the bearing housing.

This type of air-oil seal prevents the introduction of oil into the gas path.

Oil leaking into high velocity combustion gases will damage turbine parts.


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Types: Carbon SealsCarbon seals are a blend of carbon and graphite.

Carbon seals perform the same function as labyrinth seals.

The carbon seal rides on a surface while the labyrinth seal has an air space.

Carbon seals are usually spring loaded and sometimes pressurized with air.This causes a preload pressure on the carbon segment and provides a tighter seal.


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Types: Carbon SealsCarbon seals are used for greater control of the airflow entering the bearing housing.

Carbon surfaces are usually stationary.

A highly polished mating surface, called the seal land, is attached to the main rotor shaft.

In some engines, a full contact seal is required to hold back oil that tends to puddle before it drains from the bearing housing.


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Gas TurbineLesson 9 : Lubrication & Lube Oil IntroductionThis lesson is the first in a series on the lubricating oil system of a gas turbine engine. It describes the system as a complete unit.

Lessons presented later in this series describe the major components of the system:

· reservoir and pumps· filters and coolers· instrumentation and controls

This lesson begins by describing the purposes of the lube oil system.

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Lube Oil: Purpose & Functions

The purpose of a gas turbine lubricating oil system is to provide clean and cool oil to engine parts that are subject to friction.

Lube oil:

· reduces friction· cushions· cools· cleans

· seals (Contd.)


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The primary purpose of any lubricant is to reduce friction between moving parts.

A lubricating oil system provides oil films as surface coatings on moving parts.

The oil films slide against each other to prevent metal-to-metal contact.


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Function: Reducing FrictionWhen the oil film is unbroken, friction in the engine is fluid friction instead of metal-to metal friction.For example, oil pressure will actually lift the journal of a shaft off the bearing on which it is resting.

As the shaft rotates, a layer of oil prevents the journal from physically touching the bearing.


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Lube oil acts as a cushion between moving parts. The oil:

· prevents metal-to-metal contact

· absorbs shock, for example shock imposed on gear teeth as they mesh


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Function: Cooling & CleaningOne of the laws of thermodynamics states that heat is transferred from a hot substance to a cooler substance.

Lube oil cools the internal parts of an engine by absorbing heat. The oil carries this heat away from the engine.

The heat is removed from the oil when the oil goes through the oil cooler. Oil also cleans internal engine parts. (Contd. )


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As the oil flows through the engine, it collects foreign matter and carries it away with the oil returning to the lube oil reservoir. Foreign matter is removed in the lube oil filter.


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Function: Seal Formation

Lube oil is also used to form seals.

Mechanical seals are installed in an engine between the moving and nonmoving parts.

A very small space exists between the two parts of a seal. This space is sometimes filled with lube oil.A thin film of oil between sealing surfaces makes a mechanical seal more leak resistant.To perform these functions lube oil must meet certain requirements.


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Requirements: Viscosity

To function properly, lube oil must have a certain thickness, or viscosity.Viscosity is a measurement of the resistance of a fluid to flow.

For example, water has low viscosity because it flows easily. Honey has high viscosity because it flows slowly.

If lube oil is too viscous (thick), it may not pass through the small spaces at the required flow rate.

If the oil is not viscous enough (too thin), the oil film could be broken, causing the moving parts to wear rapidly.


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Requirements: Synthetic Oil & TestingThe viscosity requirements of oil for gas turbine engines are often met by high quality synthetic lubricants.

When synthetic lubricants are used in gas turbine engines, they must meet manufacturer's specifications.

After a period of time, lube oil (synthetic or petroleum based) will begin to break down.

To maintain the quality of lube oil, it must be tested periodically for contaminants.Contaminants are a good indication of engine wear. Oil samples are taken from a sediment-free area in the lube oil reservoir. The oil sample is submitted for testing.

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The sample is tested by spectrometric oil analysis.

A spectrometer measures silicon (dirt) and wear metal levels in parts per million (ppm).

It analyzes the color and measures the intensity of brightness that result when oil is burned in a specific light spectrum.

The result of the analysis is used to monitor gas turbine internal wear. This allows the operator to take corrective actions to avoid costly repair or loss of equipment.

The gas turbine may require maintenance as indicated by oil analysis results.


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Lube Oil System: Service Procedures

When servicing the lube oil system, the following procedures should be followed:

· Maintain cleanliness.

· Do not allow foreign matter to enter the system.

· Use a 10-micron or smaller filter when servicing with bulk oil.


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Contd.

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.If a hand pump is used to service lube oil, use that pump for one specific lube oil only.

· Do not mix incompatible lubricants. This can result in improper lubrication of the engine.

· Record the amount of oil serviced.


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Lube Oil System: Service Procedures

· Wear proper protective clothing and gloves because all lubricating oils contain additives that are irritating to the skin, toxic, or both.

The last part of this lesson focuses on the operation of a lube oil system as a complete system.


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Lube Oil System: Operation

The lube oil supply is stored in the lube oil reservoir.

Main, auxiliary, and pre/postlube oil pumps draw oil from the lube oil reservoir under pressure, to the lube oil system.

The temperature control valve regulates the oil temperature.

The oil then flows from the pump to the lube oil filter.

The oil filter removes contaminant particles that are suspended in the oil.


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Lube Oil System: Operation

After passing through the filter, the oil flows to the oil feed manifold.

An oil pressure gauge, temperature indicator, alarm switch, and shutdown switch monitor oil temperature and pressure in the oil supply manifold.

From the oil supply manifold, lube oil is distributed to the turbine rotor bearings, the hydraulic pump, the reduction gear bearings, and the generator bearings.

Contd.


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After lubricating the bearings and gears, the lube oil is returned to the lube oil reservoir.

Lube oil system operation will vary from manufacturer to manufacturer, but the components are basically the same.


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Lube Oil System: ComponentsA typical gas turbine lube oil system consists of the following components:

· lube oil reservoir· oil pumps· oil filters· oil coolers· control devices· instruments and alarmsInformation about each of these components is presented in the following lessons.


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IntroductionThe previous lesson presented information about the purpose, functions, and requirements of a gas turbine lube oil system.

This lesson contains information about the lube oil reservoir and pumps.

The figure shows a typical gas turbine lube oil reservoir and basic lube oil system components. A lube oil reservoir is normally located below the gas turbine in the base frame.


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Gas TurbineLesson 10 : Lube Oil Pumps

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Reservoir: Purpose

The purpose of the lube oil reservoir is to contain an ample supply of lubricant for the gas turbine, accessory drive systems, gearbox, and driven equipment.

The lube oil reservoir also provides the oil for starting, control, positioning inlet guide vanes, and trip oil circuits.


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Reservoir: Component FunctionsLube oil temperature is usually measured in the reservoir. Proper lube oil temperature is necessary for most gas turbines.

Low oil pressure, low oil level, and high oil temperature will initiate shutdowns of most gas turbine engines.Reservoirs may have both a level sight glass and a level indicator to indicate oil level.A sealed float device operates level transmitters (LT), indicators (LI), and switches.The switches activate high (LSH) and low (LSL) level alarms and shutdowns.


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Reservoir: Component FunctionsThe lube oil reservoir is vented to the atmosphere to maintain an even pressure in the tank.

A flame arrestor is often installed in the vent to prevent a source of ignition from entering the reservoir.

The purpose of the pressure regulator (PCV) is to control the lube oil system pressure by returning excess lube oil to the reservoir.

System protection is provided by a pressure relief valve (PSV) located at the discharge of each pump.


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Lube Oil Pumps: Purpose & TypesThis section explains the most common types of pumps used in the lube oil system.

The purpose of lube oil pumps is to provide lube oil under pressure for lubrication of the engine and associated equipment.

The lube oil pumps take suction from the oil reservoir and discharge into a common header. Contd.


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Lube oil system pumps are classified as:

· main

· auxiliary

· emergency

All three types use oil from the lube oil reservoir.


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Lube Oil Pumps: TypesThe three most common types of lube oil system main pumps are the following:

· vane· gerotor· gear

These pumps are positive displacement pumps because they send a fixed quantity or constant volume of oil to the pump outlet during each revolution.


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Sliding Vane PumpPumping action in the sliding vane lube oil pump takes place as the rotor drive shaft and the eccentric rotor drive the sliding vanes.

The space between each pair of sliding vanes fills with oil as the oil passes the oil inlet port.

This oil is carried to the outlet port as the rotors turn.

When the spaces between the vanes, the eccentric rotor, and the inner walls of the pump case reduce to minimum clearance, the oil is forced out of the pump.


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Gerotor Pump Components

The figure shows the pumping element of a gerotor pump.

The gerotor pump operates on a principle similar to that of the vane pump.

The gerotor pump uses a lobe shaped drive gear inside an elliptically shaped idler gear to move oil from an intake to a discharge port.The right side of the figure shows a complete pumping element.Several elements can be mounted on a single shaft inside the pump case.


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Gerotor Pump Operation

Gerotor pump operation is shown in the lower portion of the figure.

· From 0° to 180° of pump rotation, the space between the lobes and the openings increases from a minimum to a maximum volume.

· As the space reaches maximum volume, it is closed to the intake port and is in position to open to the discharge port. (Contd.)


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At the 270° point of rotation, the space decreases in volume, forcing oil out the discharge port.

· At 360° again, the space reaches minimum volume. The space is closed to discharge and begins to open to the intake port, repeating the cycle as rotation continues.

This action takes place in each of the seven interlobal spaces between the inner six-lobe gerotor and the outer seven-lobe gerotor.


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Gerotor Pump Operation

The inner drive gear in the figure has six lobes (teeth).

The outer idler gear has seven openings.

This extra lobe allows oil to fill the one open space as it passes the intake port.

The oil moves through the pump as the pump rotates, until a minimum clearance forces the oil out through the discharge port.


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The most commonly used lube oil pump is the gear pump shown in the figure on the following page.

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Gear Pump Components & OperationA gear pump usually consists of two close-fitting gears, the drive and idler gears, that rotate in a pump case.

The pump case provides minimum space between the gear teeth and the inner walls of the case.

The gear lube oil pump is usually engine driven by the accessory gears.

( Contd.)


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When the pump is rotating, the gears take in oil.

The gears rotate in a direction that forces the oil to move between the gear teeth and the pump inner case until the oil is delivered to the pump outlet port.

The idler gear seals the pump inlet from the outlet and prevents the oil from reversing flow.


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Gear Pump Components & OperationA gear pump usually consists of two close-fitting gears, the drive and idler gears, that rotate in a pump case.

The pump case provides minimum space between the gear teeth and the inner walls of the case.

The gear lube oil pump is usually engine driven by the accessory gears.

When the pump is rotating, the gears take in oil. Contd.


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The gears rotate in a direction that forces the oil to move between the gear teeth and the pump inner case until the oil is delivered to the pump outlet port.

The idler gear seals the pump inlet from the outlet and prevents the oil from reversing flow.


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Auxiliary Pump: Purpose & Function

The purpose of an auxiliary lube oil pump is to supply lube oil:

· during gas turbine startup

· during gas turbine shutdown or cool down

· anytime the main lube oil pump cannot supply lubricating oil

Contd.


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When gas turbine engine start-up begins, oil is delivered to the lube oil system by the AC-powered auxiliary lube oil pump.

The auxiliary oil pump operates for a preset time before the starter engages.

The auxiliary lube oil pump operates until the gas turbine engine accelerates to approximately 90% of its normal operating speed.


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Auxiliary Pump: Purpose & FunctionDuring gas turbine engine shutdown, the auxiliary lube oil pump is again started by the control system when engine speed slows to approximately 80% speed.

The auxiliary oil pump continues to operate throughout the shutdown and cool-down cycles.

The figure shows a typical vertical, centrifugal pump. Contd.


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Unlike the pumps discussed up to this point, centrifugal pumps are not positive displacement pumps.

This type of pump uses an impeller to move the oil through the system.

A vertical, centrifugal pump is sometimes used as an auxiliary and an emergency lube oil pump.


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Emergency Pump: Purpose & FunctionThe purpose of an emergency lube oil pump is to supply lube oil during an emergency shutdown if the auxiliary lube oil pump is inoperative or is unable to maintain sufficient lube oil pressure.

The emergency lube oil pump is similar to the auxiliary oil pump. The main difference is that the auxiliary lube oil pump is operated by an AC motor and the emergency lube oil pump is operated by a DC motor.

The next topic discusses how these pumps function within the operation of a typical lube oil system.


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Lube Oil Pumps: OperationThe figure shows a basic gas turbine lube oil system.

The main lubricating oil pump takes suction from the lube oil tank and supplies oil under pressure to the temperature control valve, filter and supply manifold.

After lubricating engine parts, the lube oil is returned to the oil reservoir.

Contd.


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An auxiliary oil pump, or pre/postlube oil pump, supplies lube oil under pressure during the engine start and shutdown cycles.

After lube oil pressure has been established, the engine begins to rotate.

When the engine reaches sufficient speed for the main (engine driven) oil pump to provide adequate lube oil pressure, the pre/postlube oil pump is automatically shut down.


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Lube Oil Pumps: OperationThis pump draws oil from the reservoir and sends it to the lube oil filter and oil supply manifold for distribution to the lubrication points.

Oil pressure is regulated by the relief valve (PSV) during pre/postlube auxiliary oil pump operation.

At engine shutdown, the postlube oil pump is activated by the control system.

The postlube oil pump operates for a preset time after shutdown to provide postlubrication and to cool the engine.


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IntroductionThe preceding lesson discussed the lube oil system reservoir and pumps.

This lesson provides information about the lube oil system filters and coolers.

Filters and coolers are both important in maintaining the quality of lube oil.

Filters keep the lube oil clean by removing contaminating particles from the oil. But heat can also cause rapid oil breakdown. Therefore, lube oil temperature is carefully regulated by oil coolers.

Information about lube oil filters is presented first.


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Lube Oil Filters: Purpose

The purpose of lube oil filters is to remove particles that collect in the oil.

These particles can lodge in the close spaces between bearings and seals.

Contaminants in lube oil will increase the friction between moving parts, resulting in excessive wear and bearing failure. Contd.


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Lube oil is pumped through oil filters to remove contaminant particles that collect in the oil.

Gas turbine engine lube oil filters have micron ratings.

A micron represents a size or distance equal to one millionth of a meter, or approximately .000039 of an inch.


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Lube Oil System: Contaminant SourcesContaminants in lube oil systems are primarily from the following sources:

· small particles of carbon from the breakdown of oil · metallic particles from engine wear and corrosion· airborne contaminants entering through bearing seals· dirt and other foreign matter introduced into the reservoir during servicing


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Lube Oil Filters: TypesThe most common types of lube oil system oil filters are the disposable filter and the cleanable screen filter (wire mesh).

Disposable filters are smaller than cleanable screen (wire mesh) filters.

Disposable filters are capable of filtering particles as small as 5 microns.

Disposable filters are heavily pleated.

( Contd.)


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The purpose of these pleats is to provide a maximum surface area for filtration.

The figure shows a typical disposable filter assembly.

Cleanable screen filters can be removed, cleaned, and reinstalled.


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Lube Oil Filters: Types of AssembliesTwo types of lube oil filter assemblies are used in gas turbine lube oil systems:· simplex· duplex

The simplex has only one filter case. The duplex has two filter cases.

When a simplex oil filter is used, the engine must be shut down to replace the filter elements. ( Contd.)


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When a duplex oil filter is used, oil flow can be diverted to the second filter and the engine does not have to be shut down to replace a filter.


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Lube Oil Filters: Simplex Assembly

A simplex lube oil filter assembly usually consists of the following:

· filter case· plumbing· differential pressure gauge · differential pressure alarm switch

The filter case is shaped like a cylinder and contains replaceable filter elements.

Each filter element is a pleated paper cartridge designed to filter particles that are larger than 5 microns.


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Lube Oil Filters: Protection System

During gas turbine engine operation, the filter elements become clogged with contaminants in the lube oil.

When this occurs, the lube oil pressure between the oil pump and the filter begins to rise and the pressure between the filter and the lube oil header or manifold decreases.

This difference in pressure, indicated on the differential pressure gauge, helps detect a clogged filter.Many lube oil systems contain a differential pressure switch that initiates an alarm when differential pressure reaches the setpoint.

The alarm sounds at the local control panel and often at the DCS.


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Lube Oil Filters: Duplex Filter AssemblyMany gas turbine engines have a duplex lube oil filter assembly instead of a simplex oil filter.

The operation of the lube oil system is the same except that the engine does not need to be shut down to replace a dirty filter element. The oil flow can be diverted to the clean filter case.

The duplex oil filter has two filter cases. Each case contains replaceable filter elements. Contd.


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Smaller filter cases may contain only one filter element, while large filter cases may contain nine elements in sets of three each.

Only one filter case is used at a time. A manual selector (transfer) valve is positioned to direct lube oil flow to either filter case.


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Lube Oil Filters: Duplex Filter AssemblyA typical duplex lube oil filter design is shown in the figure.

Lube oil under pressure enters one of the two filter units through the transfer valve.

Turning the transfer valve from one filter to the other positions a valve at both the inlet and the outlet of the filter element.

( Contd.)


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This position ensures maximum oil flow through the selected filter. It also traps lube oil under pressure in the filter element removed from service.

All pressure must be bled from the unused filter case before it is opened.


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Lube Oil Coolers: Purpose & TypesThe purpose of the lube oil system oil coolers is to maintain a specified lube oil temperature.

The specified temperature must be maintained under differing oil heat loads that take place with differing operating conditions.

One of the laws of thermodynamics states that heat can only be transferred from a hot surface to a colder surface.Oil coolers are heat exchangers. The two most common types of oil coolers are:

· oil-to-water· oil-to-air


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Lube Oil Coolers: Oil-to-Water CoolerThe oil-to-water cooler uses water to cool the oil. The oil-to-air cooler uses air to cool the oil. The oil-to-water cooler is discussed first.

The figure shows an oil-to-water cooler.

The heat exchanger transfers heat from the lube oil to the water and keeps the oil at the proper temperature.

Lube oil coolers require minimal operating checks and maintenance.

They should be inspected for oil and water leaks during each routine and off-line maintenance inspection.


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Lube Oil Coolers: FlowHigh bearing header lube oil temperature may indicate that the oil cooler tubes are fouled and in need of cleaning.

Lube oil coolers can be damaged by thermal shock, overpressure, and hydraulic hammer.

Thermal shock is prevented by starting the flow of cooling water through the oil cooler before the hot lube oil flow is started.If the lube oil system has been down for maintenance, the system must be gradually filled with fluids as air is vented from the system. Pulsations of fluids through oil coolers can cause vibrations that may damage the cooler and shorten its operating life.


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Lube Oil Coolers: Oil-to-Air Cooler

The oil-to-air cooler and thermostatic valve arrangement is similar to the oil-to-water cooler. The primary difference is that the actual heat exchanger resembles the radiator cooler used for water cooled reciprocating engines.

An oil-to-air lube oil cooler keeps lube oil temperature within operating limits by using an oil-to-air heat exchanger with an electrically operated fan.

(Contd.)


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During normal operation the fan blows air upward through the cooler.

Lube oil enters the cooler assembly and flows through passages in the radiator core before exiting the cooler.


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Lube Oil Coolers: Oil-to-Air Cooler

Honeycomb type passages through the core allow airflow when the fan is operating.

Heat is exchanged from the hot lube oil to the cooler air through the walls of the radiator core.

Oil-to-air lube oil coolers may be either high ambient temperature or low ambient temperature. Contd.


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High ambient temperatures require greater core size for a larger surface area.

A larger surface area distributes heat over a wider area and exchanges heat faster.

Oil-to-air cooler subsystem operation is discussed next.


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Lube Oil Coolers: OperationThe figure shows a diagram of an oil-to-air cooler subsystem.

Lube oil under pressure from the oil pump either bypasses the oil coolers or enters one of the coolers through the transfer valve.

If the oil temperature is less than approximately 60°F, the temperature control valve will open port B to port A and the oil will bypass the oil coolers. Contd.


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As the oil heats up, the thermostatic control valve begins to open port C and close port B.

This closes off bypass flow and forces oil flow through the oil coolers.


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Lube Oil Coolers: Operation

The transfer valve directs oil flow into the coolers.

As shown in the figure, the transfer valve is controlled by a flow control valve and control system.

Under high oil temperature conditions, the transfer valve can direct oil flow into both oil coolers.

The control system automatically starts the fan motor on the oil cooler.

The vent valve and the drain valve must both be opened to drain oil from the oil cooler.


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Lube Oil Coolers: OperationWhen returning an oil cooler to service, all the air in the cooler must be bled through the vent valve.

Lube oil from the coolers flows through the temperature control valve to the oil filter.

Local temperature gauges are located upstream and downstream from the temperature control valve.

The first indication that the lube oil cooler is malfunctioning is an increase in oil temperature.The second indication of oil cooler trouble is the high oil temperature alarm.The last indication of oil cooler problems is the lube oil high temperature trip.


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IntroductionThe preceding lessons in this series discussed the lube oil system, its operation, and individual components of the system.

This lesson provides information about lube oil system pressure and temperature controls, instruments, and alarms.

Proper lubrication of gas turbine engines is critical to engine performance and operating life.

Proper lubrication depends on oil pressure and oil temperature.

High oil pressure can damage lube oil system components. Low oil pressure can prevent oil from reaching internal engine parts.

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IntroductionProper lubrication also depends on oil viscosity, which is affected by oil temperature.

Cold oil is more viscous than hot oil. Cold oil also produces higher oil pressure at the pump discharge.

Hot oil loses film strength because of reduced viscosity.

Information about the devices used to control lube oil pressure and temperature begins the lesson.


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Pressure Control Devices: PurposeThe purpose of pressure control devices is to regulate lube oil pressure. Excessive pressure in the lube oil system is prevented by relief valves.

The following lube oil system pressure control devices are discussed:

· pump relief valve

· pressure relief valve

· diaphragm operated control valve The figure illustrates typical lube oil system protection devices.


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Relief ValvesLube oil pumps are protected by a relief valve. If the oil pressure at the pump discharge outlet is high, the valve is lifted off its seat and some of the excess oil is returned to the pump inlet. Some systems return the oil to the reservoir.

A lube oil system pressure relief valve relieves excessive oil pressure.

Like the pump relief valve, this valve opens when oil pressure overcomes spring pressure, and excess oil is returned to the reservoir.

The pressure relief valve can be adjusted by increasing or decreasing spring tension.


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Diaphragm Valve

A diaphragm valve regulates lube oil pressure within a very narrow range. A diaphragm valve senses lube oil pressure in the bearing header and opens or closes to maintain the pressure in the correct operating range.

As mentioned at the beginning of this lesson, oil pressure and oil temperature are closely related.

Information about temperature control devices is presented next.


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Temperature Control Devices: PurposeThe purpose of temperature control devices is to regulate lube oil temperature.

Such devices include:

· oil cooler

· thermostat

· lube oil regulator assembly (thermal bypass valve and pressure regulator)

The figure illustrates a typical oil cooler arrangement. Lube oil always flows through an oil filter, but it does not always flow through an oil cooler.


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Oil Coolers & Thermostat

The figure illustrates two water-cooled oil coolers. The oil cooler at the top of the figure shows the oil temperature control valve in the cool oil mode. The oil cooler at the bottom of the figure shows the oil temperature control valve in the hot oil mode.

In the cool oil mode the valve is partially open. Most of the oil flowing through the valve completely bypasses the cooler. Contd.


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The oil that flows through the oil cooler is mixed with the bypassed oil at the control valve outlet.

The lube oil temperature control valve is an oil temperature thermostat.

The thermostat uses a heat sensitive spring to position the valve.


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Oil Coolers & ThermostatIn the hot oil mode, the thermostat spring expands to hold the valve against the valve seat and stop the flow of oil through the bypass part of the valve.

When the thermostat is cold, the spring retracts to lift the valve off its seat.

This position allows the oil to flow through the valve and bypass the oil cooler.

As the thermostat warms, it begins to close the temperature control valve to force more flow through the oil cooler.


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Lube Oil Regulator Assembly

A typical lube oil regulator assembly contains:

· two thermal bypass valves· a pressure regulating unloading valve

The figure shows these components and the flow of the oil through the lube oil regulator assembly.

(Contd.)


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The thermal bypass valves located inside the lube oil regulator assembly serve two purposes:

· control the lube oil temperature

· protect the oil cooler against high oil pressure during a cold weather start

If lube oil temperature is below 60°F, the thermal bypass valves are fully open and lube oil bypasses the oil coolers.


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Lube Oil Regulator AssemblyWhen oil temperature exceeds 60°F, the valves begin to close and are fully closed at 140°F. At that point, all oil flows through the oil cooler.

This assembly also protects the oil cooler against high oil pressure during cold weather starts.

When the differential pressure across the bypass valves exceeds 50 psig, the valves are opened wide enough to maintain 50 psig differential pressure. ( Contd.)


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Another component of the lube oil regulator assembly is the pressure regulating unloading valve.

This valve is pilot operated and spring closed. The valve also has a center passage with an oil metering opening.


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Lube Oil Regulator Assembly

During operation, pressurized pilot oil flows through the opening to the back side of the regulating valve and assists the spring in keeping the valve closed.

Oil pressure is adjusted by means of the externally adjusted pilot oil pressure relief valve. Pressurized pilot oil comes from the lube oil filter outlet.

As the engine-driven lube oil pump pressure increases with engine speed, the unloading valve opens to maintain a constant lube oil system pressure.

Lube oil system instruments and alarms are discussed next.


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Instruments & AlarmsInstruments provide information on the operation of the lube oil system. They monitor conditions such as oil level in the reservoir and oil pressure and temperature throughout the system.

Alarms alert the operator to out-of-limit conditions that must be corrected to ensure safe operations.

The lube oil system instruments and alarms used by G.E. and Solar perform the same basic functions. However, the schematics used by the two companies are different.

Schematics for both companies are included in this lesson.


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G. E.: Level Indicator

The figure shows a typical G.E. lube oil system schematic. The devices explained are highlighted. The letter "L" in a circle represents the lube oil reservoir level indicator.

As drawn in the schematic, the level indicator is a local gauge. It also signals lube oil level information to the gas turbine control panel and to the distributed control system (DCS).

Various lube oil levels are listed to the right of the level indicator. When the oil level is 12 inches below the top of the tank, the lube oil reservoir is considered full.


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G. E.: Level Indicator

When the level is 10 inches or less from the top of the reservoir, the high lube oil level alarm is initiated by the high level switch 71QH-1.

The level gauge indicates EMPTY when the oil level is 16 inches below the top of the tank.

If the oil level is 17 inches below the top of the tank, level switch 71QL-1 initiates the low lube oil level alarm and shutdown.


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G. E.: Temperature Indicator & Relief Valve

The temperature indicator gauge is identified by the letter "T" in a circle.

This gauge indicates the temperature of the lube oil in the reservoir.

The relief valve VR1 regulates the discharge pressure of the main engine-driven lube oil pump, which is discussed next.


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G. E.: Pressure IndicatorIn the figure, the main lube oil pump is a shaft-driven gear pump.

This pump is attached to the accessory gearbox of the gas turbine.

Following the discharge line from the bottom of the main lube oil pump, the first instrument encountered is a pressure indicator identified by the letters "PI" in a circle.

Follow the pump discharge line almost to the oil cooler where another pressure indicator is located.


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G. E.: Pressure Indicator & SwitchLow lube oil pressure switch 63QA-1 is located to the right of this indicator. During the shutdown sequence or whenever the main oil supply pressure decreases to approximately 75 psig, oil pressure switch 63QA-1 actuates. This action starts the auxiliary lube oil pump.

The auxiliary lube oil pump, identified as 88QA in the figure, is a motor-driven pump. A pressure indicator and another pressure switch are located between the pump and the check valve.

The pressure switch, identifed as 63QP, signals the control system when the auxiliary lube oil pump is operating.


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G. E.: Pressure Indicator & SwitchThe emergency lube oil pump is identified as 88QE.

An emergency lube oil pump is started when AC power is lost or the auxiliary lube oil pump fails.

The emergency lube oil pump is started by actuating a low lube oil pressure switch located on the gas turbine bearing header. This pump also has a pressure indicator and a "run" switch located between the pump and the check valve.

Pressure switch 63QE sends a "pump running" signal to the gas turbine control system when the emergency lube oil pump is operating.


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G. E.: Pressure Indicator & Switch

In the G.E. lube oil system, a pressure indicator is installed in the discharge of each pump.

The indicator may be local, it may have a transmitter to signal the pressure information to the DCS, or both.

G.E. uses a pressure switch at the discharge of each motor-operated pump to initiate a "pump running" signal to the DCS.


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G. E.: Differential Pressure IndicatorIn the figure, lube oil flows from the pumps to the oil coolers, oil filters, and the bearing header.

Lube oil for the generator is discharged from the main lube oil header.

The differential pressure indicator (PDI) is connected across the lube oil filter inlet and outlet lines.

The purpose of the differential pressure indicator is to monitor the pressure drop across the lube oil filter that is in operation.

The device identified as 63QQ-1 is the main lube oil filter differential pressure alarm switch.

This switch initiates an alarm when the differential pressure reaches the setpoint.


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G.E.: Bearing Header Pressure Regulator

The bearing header pressure regulator (VPR2) is located between the generator lube oil line and the gas turbine bearing header.

A diaphragm in the regulator senses the oil pressure in the bearing header and opens or closes the control valve to maintain set pressure.


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G. E.: Emergency Pump Start Switch

To the left of the bearing header is the low lube oil pressure emergency pump start switch, 63QL.

On the right side of the gas turbine bearing header are three outgoing lines labeled hydraulic supply.

Lube oil from the bearing header provides oil to the hydraulic pumps which provides hydraulic pressure for starting, inlet guide vane operation, and fuel gas control valves.


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G. E.: Temperature SwitchesHigh lube oil temperature may indicate fouled lube oil cooler tubes.

A temperature indicator and several switches are located to the right of the hydraulic supply lines.

The temperature indicator is a local instrument as drawn. When used with a transmitter, this temperature can be signaled to the DCS.

The devices identified as 26QT are related temperature switches. If lube oil temperature in the bearing header increases to approximately 165°F, a high lube oil temperature alarm is initiated. If corrective action is not taken and lube oil temperature increases to 175°F, temperature switches 26QT-1A and 26QT-1B will trip the gas turbine.


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G. E.: Thermocouples

The device labeled LT-TH-1A,B represents thermocouples located in the bearing header.

These thermocouples provide high temperature alarms and trip signals to the control system.

To trip the unit, the trip temperature must be sensed by at least two thermocouples.

This concludes the information about oil instruments and alarms in the G.E lube oil system.

The Solar lube oil system will be discussed next.


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Solar Instruments & Alarms

The figure shows a typical Solar lube oil schematic.

Information about Solar lube oil system instruments and alarms begins on the next page with the level indicator.


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Solar: Level Indicator & SwitchesThe figure shows a float type oil level indicator on the left side of the reservoir.

This is drawn as a local instrument, but if used with a transmitter, it sends lube oil level information to the local control panel and the DCS.

The right side of the lube oil tank has a low level alarm switch and a low level shutdown switch.

If the oil level in the tank decreases to the low level alarm setpoint, the switch initiates the low oil level alarm. If the oil level decreases to the low level shutdown switch setpoint, this switch will initiate a shutdown of the gas turbine.


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Solar: Pressure Gauges

During normal operation, the main engine-driven lube oil pump takes suction from the lube oil tank.

Lube oil under pressure flows through the lube oil cooler to the oil filter.

A filter inlet pressure gauge is located near the lube oil filter inlet.

The oil filter outlet oil pressure gauge is shown just below the oil supply manifold.

The difference in pressure between these two gauges is the differential pressure across the lube oil filter.


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Solar: Differential Pressure SwitchThe oil filter differential pressure switch will initiate an alarm when a pressure drop across the filter reaches the switch setpoint of approximately 30 psig.

The ultimate relief valve protects the complete lube oil system against overpressurization. The valve opens at approximately 150 psig to return excess oil to the lube oil tank.

Oil flows from the oil supply manifold to the lube oil regulator assembly where oil pressure is regulated.

Several oil pressure switches are installed on the line to the pressure regulating unloading valve.


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Solar: Low Oil Pressure Alarm Switch

The purpose of the low oil pressure alarm switch is to initiate an alarm when lube oil pressure decreases to the setpoint of the switch. This alarm does not shut down the engine.

If lube oil pressure decreases to or below the minimum setpoint, the low oil pressure switch signals the protective system circuit.

This will shut down the engine and initiate the low oil pressure malfunction indication at the DCS.


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Solar: Prelube Oil Pressure Switch

The prelube oil pressure switch has several functions. If the prelube oil pump fails to deliver pressure or if oil pressure decreases below the minimum setpointduring the start sequence, this switch terminates start-up and initiates a malfunction indicator at the DCS.

If oil pressure decreases below minimum setpoint during the postlube cycle after engine shutdown, this switch initiates an alarm.

Contd.


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The prelube oil pressure switch also shuts down the prelube oil pump.

During the start cycle, as main lube oil pump pressure exceeds the prelube oil pump pressure, the prelube oil pressure switch actuates at the increasing oil pressure setpoint. This shuts down the prelube oil pump.


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Solar: Temperature Gauge & Switches

A temperature gauge and two temperature switches are shown on the left side of the oil supply manifold in the figure.

High lube oil temperature, and sometimes low lube oil pressure, are indications of lube oil cooler problems.

The high oil temperature alarm switch actuates if engine inlet oil temperature reaches the setpoint.

The switch will actuate the alarm, but the engine will not shut down.


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Solar: Temperature Gauge & Switches

A temperature gauge and two temperature switches are shown on the left side of the oil supply manifold in the figure.

High lube oil temperature, and sometimes low lube oil pressure, are indications of lube oil cooler problems.

The high oil temperature alarm switch actuates if engine inlet oil temperature reaches the setpoint.

The switch will actuate the alarm, but the engine will not shut down.


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Introduction

The preceding lessons provided information about the lubrication oil system, its components, and operation.

This lesson presents information about the hydraulic oil system.

The Solar gas turbine engine servo oil system and a typical G.E. gas turbine hydraulic system are used as examples.

The figure shows a schematic of a typical hydraulic oil system used by Solar.


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Hydraulic Oil System: Purpose

The purpose of a hydraulic system is to distribute fluid forces to various moving parts.

This fluid is required for the operation of gas turbine electrohydraulic control system components, the fuel system, variable inlet guide vane mechanisms, and the hydraulic components of some starting systems.

The lesson begins with information about Solar hydraulic oil systems.


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Solar: Subsystem of Lube Oil SystemSolar gas turbines have two separate hydraulic oil systems:

· subsystem of the lube oil system· separate servo oil system

In the subsystem of the lube oil system, oil is taken from the oil supply manifold by lube oil pump pressure. It is then routed to the variable vane control valve and to the actuator to move the vanes to the maximum open position.

The Solar gas turbine servo oil system is an electrohydraulic system that operates as a separate, closed-circuit hydraulic oil system.

Information about this system is presented next.


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Solar Servo Oil System

The servo oil system shares the lube oil reservoir with the lube oil system.

The purpose of the servo oil system is to operate the main fuel valve electrohydraulic servo actuator.

The actuator is operated by servo oil pressure and is controlled by the engine control system. This system is closely related to the lube oil system shown in the figure.

Contd.


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The servo oil system consists of the following:

· pump element

· servo oil filter

· servo relief valve

During engine operation, servo oil pressure is provided by the servo pump element that works with the two-element, engine-driven lube oil pump.


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Solar Servo Oil SystemOil is drawn from the lube oil reservoir through the oil line common to all three pump elements.

High-pressure servo oil flows from the servo pump outlet port through the servo oil filter to the inlet port of the servo actuator. A servo relief valve is located downstream of the servo oil pump.

The servo oil filter is between the servo oil pump and the servo actuator.

(Contd. )


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The oil filter removes particles from the servo oil that are 25 microns or larger.

The purpose of the relief valve is to protect the pump and othercomponents in the servo oil system against excessive oil pressure.

Information about a G.E. hydraulic oil system is presented next.


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G. E.: Hydraulic Oil Main PumpIn a G.E. hydraulic oil system, gas turbines use bearing header oil at 25 psig. This oil has been cooled and filtered as supply oil for the hydraulic system.

The main hydraulic pump is driven by the main shaft in the accessory gear. This pump increases the 25 psig oil supply pressure to 1500 psig for hydraulic control of fuel valve and inlet guide vane actuators.

Both the main hydraulic pump and the auxiliary hydraulic pump contain pressure compensators to control discharge pressures at approximately 1500 psig.


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G. E.: Hydraulic Oil Main PumpIn a G.E. hydraulic oil system, gas turbines use bearing header oil at 25 psig. This oil has been cooled and filtered as supply oil for the hydraulic system.

The main hydraulic pump is driven by the main shaft in the accessory gear. This pump increases the 25 psig oil supply pressure to 1500 psig for hydraulic control of fuel valve and inlet guide vane actuators.

Both the main hydraulic pump and the auxiliary hydraulic pump contain pressure compensators to control discharge pressures at approximately 1500 psig.


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G. E. Components & OperationCheck valves, located between the two manifolds, keep the lines full when the turbine is down.

Each manifold also has a pressure relief valve. One relief valve aids in controlling auxiliary pump output pressure. The other protects the main hydraulic pump circuit from damage if the main pump pressure compensator fails.

Contd.


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Hydraulic oil at 1500 psig leaves the manifold through a manual filter transfer valve to enter one of two filter manifolds. Each filter is equipped with a differential pressure switch. An alarm is initiated if the differential pressure increases to 60 psig.

Hydraulic oil leaving the filter system discharges to the inlet guide vanes and the fuel control system.

Hydraulic accumulators are located upstream of these systems.


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G. E. Hydraulic Oil System Accumulators

The purpose of the accumulator is to absorb any transient shocks that may occur when the hydraulic pumps are started or equipment is actuated.

The accumulators must be precharged with nitrogen before the hydraulic system is activated.

Each accumulator contains a sliding piston with o-ring seals located in approximately the center of the cylinder. (Contd. )


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Cavities are located above and below the piston.

The top cavity is precharged with 750 lbs. of dry nitrogen. The hydraulic system pressure forces the piston up against the nitrogen pressure until equilibrium is attained at 1500 lbs.


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G. E. Hydraulic Oil System

At this point the accumulator functions as a shock absorber. A sudden surge or drop in pressure will be compensated for by the accumulator piston movement.

A fill device and a disc safety valve are located on the top end cap. Block valves and bleed valves are provided for maintenance and service.


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G. E. Variable Inlet Guide Vane System

Hydraulic oil pressure at 1500 psig flows to the variable inlet guide vane system (IGV), the fuel gas system, and to one side of the hydraulic dump valve.

The purpose of variable compressor inlet guide vanes is to provide compressor pulsation protection during start-up and shutdown. They are also used during operation under partial load conditions when waste heat recovery is installed.

The variable inlet guide vane actuator is a hydraulically actuated assembly used to control the angle of the guide vanes.


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G. E. Variable Inlet Guide Vane SystemHydraulic oil pressure at 1500 psig flows to the variable inlet guide vane system (IGV), the fuel gas system, and to one side of the hydraulic dump valve.

The purpose of variable compressor inlet guide vanes is to provide compressor pulsation protection during start-up and shutdown. They are also used during operation under partial load conditions when waste heat recovery is installed.

The variable inlet guide vane actuator is a hydraulically actuated assembly used to control the angle of the guide vanes.


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G. E. Guide Vane System OperationFor a normal shutdown, inlet guide vanes move to the closed position.

In reference to inlet guide vanes, the term fully closed means that the vanes have moved to the minimum open ("closed") position.

Inlet guide vanes do not close completely like a door or a control valve. There is some airflow through the guide vanes during all phases of gas turbine operation. Contd.


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In case of a turbine trip, the bleed valves open and the inlet guide vanes move to their "closed" position.

Inlet guide vanes are automatically positioned during start-up and shutdown sequences to avoid gas turbine compressor surge.


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Introduction

The preceding lesson presented information about the hydraulic oil system. This lesson focuses on the trip oil system, which is one of the most critical of the lube oil subsystems.

The purpose of the trip oil system is to provide protection for the gas turbine.

A brief summary of lube oil functions is presented because of the interaction between the trip oil system and other lube oil functions.

Lesson 14 : Trip Oil System


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Lube Oil System Functions

Lube oil is pressurized to 100 psig by the main shaft-driven pump. The following systems are supplied from the lube oil flow:

· pressure reducing valve· trip oil· overspeed bolt trip assembly

(Contd.)


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The pressure reducing valve decreases the 100 psig lube oil supply to 25 psig for the following systems:

· bearing lubrication

· accessory gear lubrication

· hydraulic pump supply pressure

· starting means control fluid and lubrication


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Lube Oil System FunctionsThe hydraulic pumps take the 25 psig supply pressure and increase the pressure to 1500 psig for hydraulic control oil to the following systems:

· inlet guide vanes· fuel control valves

The lube oil at various pressures lubricates the turbine and generator, protects the machine, starts the engine indirectly, and controls fuel gas and the inlet guide vanes.


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Trip Oil System: Protective DevicesElectronic protective devices located throughout the gas turbine activate the control system. The trip oil is pressurized to run, depressurized to trip.

A mechanical or manual trip protective device is mounted on the accessory gear.

A manual trip is also located on the gauge cabinet in the accessory compartment.


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Trip Oil System: OverspeedTrip DeviceAn overspeed bolt assembly is shown in the figure. The trip oil protection system is located between the control system and the actuators (fuel control and inlet guide vane actuators).

Both the fuel valves and the inlet guide vane servovalves are controlled by hydraulic pressure at 1500 psig. These are electrohydraulic-operated valves.

An electronic signal from the control system allows hydraulic oil to either enter or exit a hydraulic cylinder, which positions the control valves.


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Trip Oil System: Dump ValveBefore the 1500 psig oil can reach a control valve, a dump valve must be closed port, which is activated by 100 psig trip oil.

The dump solenoid valve and the fuel gas stop solenoid valve are solenoid-operated trip oil dump valves.

They are activated by the control system. When these valves dump, the loss of trip oil pressure causes other pressure-operated trip valves to dump or close.


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Trip Oil System: Dump Valve

Closing the fuel control valve shuts down the turbine.

If the overspeed bolt assembly is tripped, either manually or mechanically, trip oil is dumped to the lube oil reservoir.

Loss of trip oil pressure also causes control oil pressure to dump, resulting in an engine shutdown.


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Trip Oil System: Governor OperationDuring normal operation, the governor controls engine speed through varying load demands.

If the load is removed, the engine starts to accelerate, the governor closes the throttle valve, and the engine slows to setpoint speed.

If the load is increased, the engine starts to slow down, the governor opens the throttle valve, and the engine accelerates to setpoint speed.

If a large load were suddenly removed (such as a generator breaker trip), the governor might not act quickly enough. The engine could overspeed and destroy the unit.


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Trip Oil System: Emergency Trip DeviceTo prevent an uncontrolled overspeed, turbines are equipped with an emergency overspeed trip device. If engine speed reaches 112% rated speed, the trip bolt triggers the dump valve, or trip valve, which releases control oil pressure and stops the flow of fuel. The trip bolt is held in place by a spring at all speeds below trip speed.

As the shaft rotates, centrifugal force tends to push the weighted bolt head out of the shaft, but the force of the spring holds the bolt in place.

If the shaft overspeeds, centrifugal force overcomes the spring force and the bolt moves out of the shaft and strikes the trip lever.


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Trip Oil System: Trip Device Operation

During normal operation, trip oil pressure compresses spring B and holds the trip valve open.

If the shaft overspeeds, the trip bolt strikes the trip lever, releasing the trip latch.

Spring A moves the hydraulic piston to the rear, closes the oil inlet line, and opens the trip oil dump line. Contd.


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The loss of trip oil pressure allows spring B to extend, closing the valve plug in the trip valve. Hydraulic oil pressure is released, the fuel flow is shut off, and the turbine stops.

After a turbine trip, the trip valve must be manually reset and the cause of the trip corrected before normal operation resumes.


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Introduction

This lesson is the first in a series on fuel gas and liquid fuel systems.

This lesson provides introductory information on gas turbine fuel systems.

Lesson 15 : Fuel Systems


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Gas Turbine Fuel Systems: Purpose

The purpose of a fuel system is to supply an exact amount of clean fuel to the engine under all operating conditions.

The amount of fuel is based on turbine speed and load requirements.

The fuel pressure required for a gas turbine is primarily a function of the compression ratio of the compressor section.

For example, the lower the compression ratio, the lower the fuel pressure requirement; the higher the compression ratio, the higher the fuel pressure requirement.


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Gas Turbine Fuel Systems: Purpose

Almost any combustible fluid, either gaseous or liquid, can be used for turbine fuel.

Some gas turbines operate on both liquid fuel and fuel gas.

Both fuel gas and liquid fuel must be clean for efficient turbine operations. However, fuel requirement specifications differ among manufacturers.

The following information on fuel gas and liquid fuel requirements is provided to illustrate the differences in specifications.


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Fuel Gas Requirements

The figure lists typical fuel gas requirements for gas turbine engines. These requirements are:

· lower heating value· supply pressure· gas temperature· fuel quality

The following page shows specifications for liquid fuel.


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Liquid Fuel Requirements

The figure lists typical liquid fuel requirements for gas turbine engines. These requirements are:

· fuel temperature· fuel viscosity· pour point· fuel quality

NOTE: Always check the turbine manufacturer's fuel specifications to ensure that the fuel meets the specifications for the gas turbine you are operating.

The main components of a fuel gas system and liquid fuel system are given next. We will begin with the fuel gas system.


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Fuel Gas System: ComponentsThe main components of a typical gas turbine fuel gas system are as follows:

· fuel shutoff valve (SOV)· vent valve· pressure control valve (PCV)· pressure indicator controller (PIC)· pressure safety valve(s) (PSV)· instruments and alarms· filter separators· control system· fuel gas heater (optional depending on gas dew point)


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Liquid Fuel System: Components

The main components of a typical liquid fuel system are:· manifold· nozzles· pumps· filters· pressure switches· fuel control valves· solenoid-operating valves· control system Contd.


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NOTE: Nozzles are not shown in the figure.

The components for both of these fuel systems are discussed in detail along with their operation in later lessons.

The main difference between the two systems is that the liquid fuel system has a storage tank and fuel pumps, whereas a fuel gas system does not.


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Fuel Gas Supply System: PurposeThe purpose of the gas turbine fuel gas supply system is to provide the correct amount of clean, dry fuel gas to the engine under all operating conditions.

Each component of the fuel gas supply system is important to its overall operating efficiency. The information on the following page highlights the seven main components in a typical fuel gas supply system.


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Components & Operation

The main components of a typical fuel gas supply system are as follows:

· fuel shutoff valve (SOV)· vent valve· pressure control valve (PCV)· pressure indicator controller (PIC)· pressure safety valve(s) (PSV)· filter separator· control system

The purpose and operation of these components are discussed next.


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Fuel Shutoff & Vent Valve

The purpose of the fuel shutoff valve is to allow, or prevent, the flow of fuel to the gas turbine fuel system.

The purpose of the vent valve is to release the pressure of trapped gas in the fuel lines.

The position of both valves is controlled by actuators, which are operated by instrument air or control oil.

Contd.


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In the figure, a hand switch controls a four-way air valve to send instrument air pressure to either side of the shutoff valve actuator piston. In some cases actuator piston movement is controlled by hydraulic control oil ported through a servo valve.

The actuator is operated by the hand switch or by the fire detection system by means of an interlock. This operation is discussed next.


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Fuel Shutoff & Vent Valve

The hand switch opens or closes the fuel shutoff valve through the control system. The switch may be located at the local control panel, the DCS, or both locations. In some systems, the shutoff valve is electrically operated.

The fire detection system sends a signal through the interlock when an engine fire is detected. The interlock closes the fuel shutoff valve and opens the vent valve.

Contd.


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The interlock receives signals from the hand switch and the firedetection system to change the positions of the fuel shutoff valve and the vent valve.

When the fuel shutoff valve is closed, the vent valve must be open. The interlock is usually a part of the control system logic that closes the vent valve before the fuel shutoff valve opens.


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PCV & PIC ValveThe figure shows fuel gas flow when the fuel shutoff valve is open. The first device located downstream of the shutoff valve is the pressure control valve (PCV).

In the figure, the pressure control valve regulates fuel gas pressure to the turbine according to instrument air signals from the pressure indicator controller (PIC).

The pressure indicator controller measures the fuel gas pressure, compares this pressure to the setpoint, and modulates the pressure control valve to maintain setpoint pressure.


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Pressure Safety Valve

In the figure, a pressure safety valve (PSV) is located between the pressure control valve and the pressure indicator controller. The purpose of the pressure safety valve is to vent excessive fuel gas pressure to the flare should the PCV malfunction.

Two other instruments receive fuel gas pressure from the same instrument line as the pressure indicator controller: the local pressure indicator (PI) and the pressure transmitter (PT). Contd.


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The pressure transmitter signals gas pressure information to the DCS pressure indicator (PI), low pressure alarm (PAL), and high pressure alarm (PAH).

The fuel gas pressure can be read from the turbine control system CRT and from the DCS.


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Filter SeparatorsThe purpose of the filter separators in the fuel gas supply system is to provide filtration and separation of the fuel gas before it enters the fuel control system.

The fuel gas should be relatively clean and dry by the time it reaches this part of the system.

Operators should monitor the filter separator sight glass (LG) during routine operating checks.

Any liquid accumulation must be drained off. If liquid levels in the filter separator become excessively high, the high level switch (LSH) signals the DCS high liquid level alarm (LAH) and an alarm is initiated.


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Filter Separators

Like the lube oil and some hydraulic oil filters, the filter separators are equipped with differential pressure indicators and alarms. The system shown in the figure contains a high differential pressure indicator switch (PDISH) and a DCS differential pressure alarm (PDAH).

The differential pressure initiates the high differential pressure alarm when the high differential pressure setpoint is reached.

Operators should place the standby filter in operation and service the operating filter before the high differential pressure alarm is initiated.


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Control SystemFuel gas flows from the filter to the gas turbine fuel system which is controlled by the turbine control system.

Several measuring devices are located between the filter separator and the gas turbine:

· pressure transmitter (PT)· flow transmitter (FT) - optional· temperature measuring elements (TE)

Contd.


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These devices provide fuel gas supply information to the control system instruments.

This information may be used for pressure indicators, pressure recorders, pressure alarms, flow indicators, totalizers, recorders, temperature indicators, and temperature recorders.


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Control System

The information is also analyzed and computed by the control system to schedule fuel flow to the engine as needed for speed and load requirements.

The control system is discussed in more detail in the next lesson.


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IntroductionThe preceding lesson discussed the components of a gas turbine fuel gas system. It also explained the operation of those components.

The focus of the preceding lesson was the operation of a fuel gas system before the fuel supply enters the gas turbine package.

This lesson focuses on the control of a fuel gas system in relation to gas turbine operation.

The figure shows a simplified gas turbine package fuel gas system. It represents the system on the gas turbine being controlled.

Lesson:17 Fuel Gas Control System


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Fuel Gas Control System: Purpose

The purpose of a fuel gas control system is to provide required fuel flow and pressure to the engine. It uses specially designed components to accomplish this function.

A simplified view of the fuel gas control system is shown in the figure.

The diagram on the following page shows a complete fuel gas control system with the main components highlighted. Each of the highlighted components and its operation are discussed in the following pages.


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Fuel Gas Control System: Components

The main components of the fuel gas control system are:

· strainer· gas supply pressure switch· stop/speed ratio valve assembly (stop/ratio valve and gas control valve)· fuel gas pressure transducer· fuel vent solenoid valve· four linear variable differential transformers (LVDT) position sensors· two electrohydraulic servo valves· three gas pressure gauges· Speedtronic controls

The strainer is discussed first.


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Strainer

The purpose of the strainer is to remove foreign particles from the fuel gas before it enters the stop/speed ratio valve assembly.

A blowdown connection on the bottom of the strainer body is used for periodic cleaning of the strainer screen.

The next fuel gas control system component discussed is the supply pressure switch.


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Gas Supply Pressure Switch

The gas supply pressure switch is installed in the gas piping upstream of the stop/speed ratio valve.

This switch initiates an alarm when fuel gas pressure decreases below the setpoint.

Downstream of the gas supply pressure switch is the stop/speed ratio valve and gas control valve assembly. This assembly is discussed next.


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Stop/Speed Ratio & Gas Control ValveThe combination stop/speed ratio and gas control valve assembly contains the following independent valves:· stop/speed ratio valve· gas control valve

Both of these valves are actuated by hydraulic pressure through servos which receive the signal from the Speedtronic control system (discussed later in this lesson).

The stop/speed ratio valve has two functions:

· a stop valve· gas pressure regulation


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Stop/Speed Ratio Valve

As a stop valve, the stop/speed ratio valve shuts off fuel gas flow during normal and emergency shutdowns.

The hydraulic dump valve, control oil, and servo valves control the action of the stop/speed ratio valve as a stop valve.

The hydraulic dump valve is located between the electrohydraulic servo valve and the actuating cylinder.

(Contd.)


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When control oil pressure is low, a spring moves an internal spool to the "dump" position.

When hydraulic pressure is removed, a closing spring on the stop/speed ratio valve plug closes the valve.

Fuel gas flow to the gas control valve and gas turbine is stopped.


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As a pressure regulator, the stop/ speed ratio valve regulates the gas pressure to the gas control valve.

The stop/speed ratio valve is positioned by the actuating cylinder and servo valve according to FSR (Fuel Stroke Reference) signals from the stop/speed ratio valve control.

(Contd.)


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The stop/speed ratio valve has two control loops:

· position control of the valve· gas pressure control to the inlet of the gas control valve referenced to the speed of the turbine

The stop/speed ratio valve position control loop functions by comparing an FSR signal with a valve position signal generated by the LVDTs (position sensors).


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Pressure TransducerThe pressure control loop senses the fuel gas pressure exiting the stop/speed ratio valve. The pressure transducer changes the gas pressure input signal to a DC voltage output signal. The output signal is relayed to the stop/speed ratio valve Speedtroniccontrol.

The Speedtronic control compares the signals received from the intervalvepressure, position control loop, and turbine speed. The control system then adjusts the stop/speed ratio valve to maintain fuel gas pressure requirements.


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Gas Control Valve

The purpose of the gas control valve is to meter or supply fuel gas to the turbine based on turbine speed and requirements of the load or driven equipment. The valve is activated by an electronic signal from the GCV segment of Speedtronic control system.

The gas control valve position is detected by the LVDTs. The sensors signal the valve's position to the GCV Speedtronic control, which changes the signal to a DC signal.

The DC signal is compared with the FSR input signal.


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Gas Control Valve

If the feedback signal is in error (differs) with the FSR, the control system signals the hydraulic servo valve to adjust the gas control valve in a direction to decrease the error.

This adjustment maintains a relationship between valve position and FSR.

The gas control valve then meters the correct fuel flow.

Located between the stop/speed ratio valve and gas control valve is the vent solenoid valve, the component discussed next.


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Vent Solenoid Valve

The purpose of the fuel vent solenoid valve is to vent fuel gas from the line between the stop/speed ratio and gas control valve when the gas turbine is shut down and prior to firing the machine during startup.

This venting prevents gas pressure buildup between the valves and fuel gas leakage through the gas control valve.

The solenoid valve vents the gas line when the solenoid is de-energized.

When the master control/protection circuit is energized, the solenoid is energized and the vent valve is closed.

When a turbine start signal is initiated, the vent valve is closed and remains closed until the turbine is shut down.


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LVDTs (Position Sensors)Linear variable differential transformers (LVDTs) are position sensors attached to the valve stems. A fuel gas control system contains four LVDTs:

· two sense the position of the stop/speed ratio valve

· two sense the position of the gas control valve

( Contd.)


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LVDTs sense the position of the SRV and the GCV and transmit signals to the appropriate Speedtronic control system segment.

Here the signals are changed into DC signals, which are compared to the FSR.

This signal is then transmitted to the servo valves, which regulate the flow of oil to the actuator cylinder, repositioning the SRV and the GCV.


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Servo Valves

The electrohydraulic servo valves are two-stage, four-way flow control devices.

They provide directional and proportional hydraulic flow control in response to a low power DC input signal from the Speedtroniccontrol system.

The servo valves control the direction and rate of movement of the pistons in the stop/speed ratio valve and gas control valve actuating cylinders.

The first-stage valve changes the small electric signal into a hydraulic force that precisely positions the piston of the second-stage valve.


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Servo Valves

The second-stage valve meters hydraulic pressure to and from the actuating cylinder. Movement of the single-acting piston actuator is opposed by a spring in the gas control valve.

The last two components in the fuel gas control system are the pressure gauges and Speedtronic control system. The pressure gauges are discussed next.


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Pressure GaugesThree gas pressure gauges are installed in the fuel gas supply line.

The upstream gauge measures fuel gas pressure entering the stop/speed ratio valve.

The middle gauge measures fuel gas pressure between the stop/speed ratio valve and the gas control valve.

The downstream gauge measures the pressure of the gas that has been metered to the fuel gas manifold and fuel nozzles.

Next we will discuss the Speedtroniccontrol system as a component of the overall fuel gas control system.


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Speedtronic Control SystemThe positions of the stop/speed ratio valve and gas control valve are controlled by an electrical signal from the Speedtronic control system.

The signal causes the electro-hydraulic servo valve to send oil to or release oil from the hydraulic cylinder that actuates the stop/speed ratio valve and gas control valve.

A signal from the position sensors (LVDTs) tells the Speedtronic control system that the valve is in the correct position or that a position change is needed.

We will now take a more in-depth look at the components of the Speedtronic control system.


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Speedtronic Control Components

The Speedtronic control system is a microcomputer control system that provides analog and digital signals to control and protect gas turbine operation.

The primary operating parameters of a gas turbine are start-up, temperature, and speed. All are controlled by regulating fuel flow to the engine. Contd.


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Turbine operations are sensed and used as feedback signals to the Speedtronic control system.

The Speedtronic control system is also equipped with protective devices as a backup to the main system.


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Control Loops

The Speedtronic control system consists of three major control loops:

· start-up and shutdown· speed· temperature

The output of these control loops is connected to a minimum value select logic circuit.

Contd.


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The minimum value select logic circuit interfaces the speed, temperature, and start-up control output signals to FSR for fuel control.

Only the control segment (e.g., start-up, speed, or temperature) calling for the lowest voltage output is allowed to pass the gate to the fuel control system as controlling FSR voltage. FSR control is the command signal for fuel.


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Start-Up & Shutdown Control Loops Proper speed sensing is an important part of the start-up and shutdown sequence control of the turbine.

The graphic shows the speed sensors used on G.E. gas turbines:· zero-speed detector· minimum-speed relay detector· accelerating relay speed detector· high-speed relay detector Contd.


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The following is a basic description of how these sensors function.

If speed is zero, permissive logic allows clutch engagement and the cranking sequence for turbine start-up is initiated.

The zero-speed detector provides a signal when the turbine shaft starts rotating.


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Start-Up & Shutdown Control Loops During the shutdown cycle, the zero-speed detector provides a signal to permit the ratchet gear, or turning device, to be placed in service in the cool-down sequence.

A minimum speed detector indicates that the turbine has reached the minimum firing speed before ignition.

The acceleration speed relay indicates that the turbine has reached approximately 40% to 50% speed in the acceleration cycle.

The high-speed sensor indicates that the turbine is at operating speed and that the accelerating sequence is complete.


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Temperature Control LoopThe next main component discussed is the temperature control loop.

The purpose of the temperature control loop is to limit the turbine firing temperature by regulating fuel flow.

The actual firing temperature is most difficult to measure and generally is not measured. Contd.


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Exhaust temperature is measurable and is proportional to the firing temperature.

Thermocouples mounted in the exhaust provide temperature feedback proportional to the firing temperature.

Air is more dense on cool days, causing the firing temperature to increase for a given speed.


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Protective System

The increased firing temperature improves turbine efficiency, but the control system must prevent overfiring the machine. This is accomplished by the control system lowering the temperature control point.

Protection systems are also provided to prevent abnormal conditions that can damage the turbine.

Contd.


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These control and protective systems are independent systems that back up the primary control systems.

The protective systems will trip the machine when overspeed or over-temperature trip conditions occur.

The over-temperature system protects the gas turbine against possible damage caused by overfiring. It is a backup system that operates only after failure of the temperature control loops.


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Supertonic Control: Basic Function

The basic function of the Speedtronic control system is described next.

Under normal operating conditions, the temperature control system limits increases in fuel flow when the firing temperature limit is reached.

Contd.


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If the system malfunctions, exhaust temperature can exceed control limits. If a malfunction occurs, the over-temperature protection system provides an over-temperature alarm before it trips the gas turbine.

Speed control software changes the FSR based on the difference between the actual turbine generator speed/load and the speed/load reference set point.


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Speedtronic Control: Basic Function

When the generator breaker is closed, on a power grid, speed is held relatively constant, or synchronous. Fuel flow in excess of that necessary to maintain full speed/no load will increase power generator output capabilities instead of increasing turbine speed.

Isochronous controls hold turbine speed steady during load changes.

When there is a difference between turbine speed and setpoint, the electronic controls increase or decrease the FSR until there are no error signals.


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Liquid Fuel System: Purpose

The purpose of a liquid fuel system is to deliver metered quantities of fuel, at the correct pressure, to the engine.

Gas turbine liquid fuels are liquid hydrocarbons similar to kerosene. Almost any combustible fluid can be used for turbine fuel, although high viscosity fuels present special problems.

The figure shows a typical liquid fuel system. Liquid fuel, or fuel oil, is stored in a storage vessel and routed to the gas turbine.



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Liquid Fuel System ComponentsThe main components of a typical liquid fuel system are as follows:

· fuel storage tank· fuel oil pumps· pressure switches· fuel filters· manifold· nozzles· fuel control valve· solenoid-operated valves· control systemA fuel storage tank is discussed first.


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Fuel Tank Storage & PSVThe purpose of a fuel storage tank is to store fuel oil received from an outside source. Storage tanks usually store enough fuel for 24 hours of gas turbine operation. They are sometimes called day tanks.

Fuel storage tanks are often equipped with the following devices:

· relief valve· pressure controls· pressure indicator· temperature indicator· level indicators· level controls and/or alarms The relief valve (PSV) is a safety device

designed to prevent excessive pressure in the tank.


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Pressure, Temperature, & Level Controls

In the figure, the storage tank is protected against overpressurization by pressure controls. These controls consist of a pressure transmitter (PT), controller (PIC), actuator (PY), and control valve (PV).

Operators can monitor the fuel oil storage tank pressure at local pressure indicators (PI).

A local temperature indicator (TI) and a level gauge (LG) are also provided.

Another level gauge, usually a sight glass, is installed on the boot of the tank.


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Level Control & Shutoff Valve

A level transmitter (LT) signals fuel level information to the DCS level indicator (LI). The level indicator contains alarms for high level, low level, and low-low level. The low-low level alarm is usually accompanied by a unit shutdown.

In the figure, the storage tank shutoff valve (MOV) is motor operated. The valve is opened or closed by a hand switch (HS).

The next components discussed are the fuel oil pumps.


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Fuel Oil Pumps & Hand SwitchesThe purpose of fuel oil pumps is to deliver fuel oil, under pressure, to the gas turbine fuel control system. Fuel oil pumps are started and stopped by a local HOA hand switch (HS). HOA means hand/off/automatic.

When the hand switch is in the H position, the fuel oil pump is under manual control. When in the O position, the pump is off. (Contd.)


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When both pump switches are placed in the A position, the running pump becomes the lead pump and the other pump is in auto standby.

If the lead pump fails when operating with both pumps in auto, the standby pump will start automatically with no interruption in fuel supply.


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Fuel Boost Inlet Pump

There are two types of liquid fuel system pumps:

· fuel boost inlet· high pressure fuel

The purpose of a fuel boost inlet pump is to raise the inlet fuel pressure to the pressure that is required for proper fuel system operation.

(Contd.)


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The boost pump is installed upstream from the low pressure duplex fuel filters.

An electric boost pump is a rotary, positive-displacement, gear, motor-driven pump. The pump takes fuel from the low pressure liquid fuel supply and delivers it to the fuel system inlet at the required pressure.


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Hp Fuel Pump, Pressure Switch, & Gauge

The high pressure fuel pump is a gear-type, positive displacement pump and may either be engine-driven or electric motor-driven.

The low fuel pressure switch senses fuel pressure to the high pressure fuel pump. This switch initiates engine shutdown if fuel pressure decreases below the setpoint. A pressure gauge, with the switch, indicates the inlet fuel pressure.


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Fuel Filters

Fuel oil flows from the fuel oil pump to the gas turbine via the pump discharge header. In the figure, a filter removes solid particles from the fuel oil.

In a liquid fuel system, filters are of two types:

· low pressure duplex filters· high pressure fuel filter

The duplex filter assembly incorporates two parallel-mounted filters equipped with a selector valve, filter check valves, and a differential pressure switch.


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Fuel Filters

Each filter contains two replaceable filter elements with a 10-micron nominal rating, connected to the fuel system through the control valve so that fuel flow may be directed through either filter. This arrangement serves two purposes:

· servicing of the inactive filter during engine operation· manual transfer to the clean filter without engine shutdown (Contd.)


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The high pressure fuel filter is installed in the fuel line between the high pressure fuel pump and the fuel control valve. It contains a replaceable filter element rated at 40 microns nominal.

The fuel manifold is discussed next.


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Fuel ManifoldThe purpose of a fuel manifold is to divide a single fuel supply into several outlet streams.

A liquid fuel manifold usually has an inlet and outlet orifice called a boss. There is an inlet boss and an outlet boss for each fuel nozzle.

Injector tube assemblies are connected to the fuel outlet bosses and carry the fuel to the fuel nozzle.

A liquid fuel manifold is sometimes called a fuel flow divider manifold. It incorporates inlet and outlet fuel connections for manifold-to-injector tube assemblies that carry the liquid fuel to the fuel nozzles.


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Fuel ManifoldSolar gas turbines may also have an air assist manifold. Air assist manifold-to-injector tube assemblies carry fuel-atomizing air to the fuel nozzles.

The fuel and air mixture ratio is the weight of combustor primary air in relation to the weight of the fuel. A specific proportion of air is needed for efficient operation.

Liquid fuel flows from the fuel manifold to the fuel nozzles, which are discussed next.


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Fuel NozzlesFuel nozzles are located in the inlet of the combustor.

The purpose of fuel nozzles is to deliver highly atomized fuel in a controlled spray pattern in the combustors.

Fuel nozzles are of three types:

· simplex· duplex· air blast


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Simplex Fuel NozzleThe simplex fuel nozzle has a small orifice that provides one spray pattern.

The simplex nozzle has a set of vanes, called flutes, that give a swirling motion to the fuel. This motion reduces the axial velocity of the fuel and provides better mixing of fuel and air.

Duplex fuel nozzles are:· single line· dual line


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Single Line Duplex Fuel Nozzle

The single line duplex fuel nozzle receives fuel at one inlet port.

A flow divider in the nozzle distributes fuel through two spray orifices. The inner orifice sprays at a wide angle. The outer orifice opens at a preset pressure and sprays the primary fuel.

(Contd.)


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The higher volume and higher pressure fuel flow from the outer orifice narrows the spray pattern so that fuel does not touch the combustion liner.

Single line nozzles also use a spin chamber for each orifice. This chamber provides efficient fuel mixing and fuel-air residence time over different fuel pressures.

The head of the fuel nozzle usually has air holes that provide some primary air for combustion. This air also cools and cleans the nozzle head and spray orifices.


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Dual Line Duplex Fuel Nozzle

The dual line duplex fuel nozzle is similar to the single line except it does not have a flow divider to separate primary and secondary fuel.

The duplex fuel nozzle has a fuel inlet port for each spray orifice. The dual line duplex nozzle contains flutes and cooling air orifices.


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Air Blast Fuel Nozzle

The air blast fuel nozzle enhances the atomization process and produces finer fuel droplets. This nozzle is more effective during start-up when low fuel pressure causes atomization problems.

By using a high velocity airflow, air blast nozzles atomize the fuel more completely than can be accomplished by only pressurized fuel.

(Contd.)


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A cone-shaped, atomized spray pattern provides a large fuel surface of very fine fuel droplets. This pattern optimizes mixing of fuel and air and ensures the highest heat release from the fuel for more completecombustion.

The most desirable flame pattern occurs at higher compressor discharge pressures. During start-up and other off-rated speeds, flame length increases because of low compression.


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Compression Ratio

Recall from a previous lesson that a gas turbine engine must maintain its rated compression ratio for efficient operation.

Compression ratio is the amount of discharge pressure in pounds per square inch absolute (psia) over suction pressure in psia.

As compressor discharge pressure increases, fuel pressure to the engine also increases. As the engine nears rated speed, fuel flow is regulated according to load requirements.

Fuel flow as it controls turbine speed is discussed next.


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Fuel Control Valve: Purpose & ComponentsTurbine speed is controlled by the fuel control valve.

The purpose of a fuel control valve is to:

· provide the correct air/fuel ratio to the combustion section· regulate fuel flow to control engine speed and exhaust temperature

The main components of a liquid fuel control valve are:

· fuel metering valve· fuel topping actuator solenoid· Pcd bleed valve


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Solenoid Operation ValvesThe fuel valve assembly is mounted on the fuel control assembly. The assembly is an explosion-proof junction box, on which are mounted four solenoid-operated valves.

These valves and their functions are described next.

The two-way, normally closed bypass valve connects fuel flow from the fuel control valve to a return line leading to the fuel filter inlet.

The valve remains closed during start-up and operation. Upon engine shutdown, the bypass valve opens.


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Two-Way Fuel ValveThe two-way, normally closed fuel valve operates in conjunction with the bypass valve during the start-up cycle and normal engine operation.

The fuel valve opens during the start-up sequence when the bypass valve closes. This directs metered fuel flow to the fuel nozzles.

During engine shutdown, the valve closes.


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Torch & Purge ValvesThe two-way, normally closed torch valve functions for only a short time during engine start-up.

Ten seconds after the engine reaches 15% speed, the valve opens, fuel is directed to the igniter torch assembly, and ignition occurs.

At a predetermined engine temperature, the valve closes and combustion is self-sustained.

The normally closed purge valve is a two-way valve connecting the fuel supply line to the fuel nozzles and to a return line.

During start-up, the valve opens. The purge valve closes when the engine reaches 15% speed.


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Fuel Valve Linkage AssemblyOn engine shutdown, the purge valve again opens to permit drainage (purging) of the liquid fuel lines. The valve closes as engine speed drops below 15% speed.

The linkage assembly is the interconnecting device between the fuel control valve and the electrohydraulic servo actuator.

The assembly consists of a linkage rod and rod ends. One end of the rod is attached to the servo actuator output shaft, and the opposite end is connected to the fuel control valve fuel metering lever.

Movement of the servo actuator results in repositioning the fuel metering valve.


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Fuel Topping Actuator Solenoid

The fuel topping actuator solenoid operates on input signals from the electrical control system to decrease fuel flow during the start-up cycle if turbine temperature exceeds a preset limit.

When energized, the fuel topping actuator solenoid reduces the metering valve open position.


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Pcd Bleed Valve

The Pcd bleed valve is a solenoid-actuated, normally open, three-way valve mounted on the liquid fuel control valve housing.

The Pcd bleed valve operates on input signals from the electrical control system.

The purpose of the Pcd bleed valve is to receive or vent Pcd from or to the acceleration limiter.


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Pcd & Turbine Speed MeasurementDuring engine operation, the valve closes and Pcd is admitted to the acceleration limiter through an orifice. Upon engine shutdown, the valve opens and vents the acceleration limiter.

An increase or decrease in the turbine load causes a corresponding change in the turbine speed.

Turbine speed can be measured by the following means:

· flyweight governor· magnetic sensors

The flyweight governor is discussed next.


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Flyweight GovernorA flyweight governor consists of a pair of weights, called flyweights, a tension spring, and a governor rod.

The governor rod rotates, and centrifugal force moves the flyweights apart.

Movement in the flyweight position also repositions the governor rod. The rod connects the action of the flyweights to the throttle valve, which controls fuel flow.


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Magnetic Sensors & Main Fuel Actuator

Magnetic or pickup sensors consist of a permanent magnet, wrapped with a coil in a sealed case. The sensors are mounted around a gear wheel on the gas turbine rotor shaft.

Each sensor generates an electrical signal proportional to engine speed. A signal is sent each time a gear tooth passes under the sensor.

( Contd.)


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Modern gas turbines use an electric or electronically controlled main fuel actuator to control turbine speed.

The main fuel actuator controller can be programmed to maintain a constant gas producer turbine speed, a constant power turbine speed, or a constant compressor discharge pressure, depending on the requirements of the system.


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Probes & Sensors

The controller receives signals from the:

· magnetic proximity probes or sensors

· exhaust gas temperature sensors

Shaft speed may be monitored by magnetic proximity probes positioned near the shaft. These are called "key phasors." These probes emit an electromagnetic field that fluctuates each time the key phasor slot passes the probe. One fluctuation in the circuit equals one revolution of the shaft. (Contd.)


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Shaft rpm is compared with speed setpoints by the main fuel actuator controller. The controller then increases or decreases fuel flow until the desired speed is reached.


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Temperature Sensors

Exhaust gas temperature is the most critical of all gas turbine engine operating parameters. Exhaust gas temperature sensors send signals to the main fuel actuator controller. The exhaust gas temperature is monitored by several thermocouples, which signal temperature information to the engine temperature controller.

(Contd.)


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Several different terms are used to describe exhaust gas temperatures:

· Turbine Inlet Temperature (TIT): temperature is monitored upstream of the turbine wheel(s)

· Interstage Turbine Temperature (ITT): temperature is taken at an intermediate position between multiple turbine wheels

· Exhaust Gas Temperature (EGT), or Turbine Outlet Temperature (TOT) is taken downstream of the turbine wheel(s)


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Lesson : 19 Liquid Fuel System Ops

Introduction

The preceding lesson explained liquid fuel system components and their function.

This last lesson on gas turbine fuel systems provides information about the operation of a liquid fuel system as a complete system.

A Solar gas turbine liquid fuel system is used as an example.


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Liquid Fuel System: Purpose

The purpose of a liquid fuel system is to deliver clean liquid fuel to the engine in correct volumes at the correct pressure.

The delivery of the fuel is controlled by a microcomputer control system. This system monitors all phases of engine operation, from start-up through shutdown.

(Contd.)


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The rest of this lesson describes the operation of a typical liquid fuel system during start-up, run, and shutdown sequences.

Operation of the liquid fuel system during the start-up sequence is described first.

NOTE: The start-up operation is for a local, manual initiation after all pre-start conditions have been satisfied.


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System Operation: Start-up Sequence1) The start-up sequence of the fuel system is initiated when the start switch is pressed.

2) The load/speed sensing control unit (governor) is energized.

3) A signal is transmitted to the electrohydraulicservo actuator. The actuator retracts and moves the fuel control linkage toward the maximum fuel position when servo oil pressure builds up.

This action moves the metering valve lever from the minimum fuel stop position. This also allows the acceleration limiter to progressively enrich the fuel and air mixture in accordance with the acceleration schedule.


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System Operation: Start-Up Sequence

Engine temperature control is offset during start-up. The impending high temperature alarm and high turbine temperature shutdown setpoints are temporarily increased approximately 50°F.

During the start-up sequence when the engine is operating between 0% and 15% speed, the liquid fuel purge solenoid valve is energized and remains open until 10 seconds after 15% engine speed is reached.

The electric liquid fuel boost pumps are energized, if used.


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System Operation: Start-Up Sequence

The air assist solenoid-operated shutoff valve is energized (opened). As the engine accelerates, the fuel pressure, Pcd, and engine oil pressure increases.

If 15% engine speed is not reached in 30 seconds after starters begin to crank, engine shutdown is initiated and FAIL TO CRANK malfunction is indicated.

The run sequence of fuel system operation is discussed next.


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System Operation: Run Sequence

At 15% engine speed plus 10 seconds:

1) The Pcd bleed valve opens and begins to act on the acceleration limiter.2) The purge valve closes.3) The torch valve and the fuel valve open.4) The electric motor-driven main fuel pump starts, if used.5) The ignition relay and ignition exciter are energized. Spark plug starts firing.6) Fuel flows through the torch valve to the torch. (Contd.)


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7) Torch fuel is atomized by air assist pressure and is ignited in combustion air.

8) Metered fuel from the fuel control valve flows through the fuel valve and the torch bias relief valve to the fuel nozzles.

This fuel flow is then atomized by air assist pressure.

The torch flame ignites the fuel and air mixture from the fuel nozzles, beginning combustion.

The engine continues to accelerate. When engine temperature reaches setpoint, approximately 350°F, the torch valve and ignition are de-energized.


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If turbine temperature has not reached the setpoint in 25 seconds after attaining 15% engine speed, engine shutdown is initiated and IGNITION FAIL malfunction is indicated.

If high pressure fuel pump suction pressure is lower than the setpoint of the low fuel pressure switch, approximately 7 psig, 25 seconds after attaining 15% engine speed, engine shutdown is initiated and LOW FUEL PRESS malfunction is indicated. (Contd.)


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After light-off occurs, turbine temperature increases rapidly. If temperature exceeds setpoint while accelerating to 90% engine speed, engine temperature control warning IMPENDING HIGH ENGINE TEMPERATURE isinitiated.


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Fuel topping solenoid valve is energized.

Fuel flow is reduced to topping flow until turbine temperature decreases to normal. The fuel topping solenoid is then de-energized.

The fuel topping solenoid operates with on-off action if the over-temperature condition persists until 90% engine speed is attained.

If the temperature topping circuit malfunctions, a further temperature increase will activate a backup shutdown circuit.


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When engine speed reaches 66%, the engine start system and the atomizing air assist shutoff valve are both de-energized.

Fuel atomizing air is then supplied by Pcd.


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System Operation: Run SequenceWhen engine speed reaches 90%:

1) The electronic load/speed controller (governor) takes control of the electrohydraulic servo actuator and positions the fuel control linkage to accelerate to operating speed.

2) Offset setpoints are transferred to normal operating values for engine temperature control. (Contd.)


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3) The topping control circuit is de-energized.

4) The temperature shutdown timer is armed.

5) Fuel is metered to the engine according to the demand of the electronic control system, basedon load, speed, or temperature.

(Contd.)


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6) Engine speed increases to operating speed.

7) The following events take place if a turbine over-temperature condition occurs when the engine is operating above 90% speed:

8) At approximately 1,155°F, the engine shutdown timer is de-energized.

9) After a 5-second delay, allowing for transient over-temperatures, HIGH ENG TEMP alarm is indicated.

10) At approximately 1200°F, engine shutdown is immediate and HIGH ENG TEMP is indicated and engine shutdown is initiated.


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System Operation: Shutdown Sequence

Shutdown of the fuel system operation occurs in sequence when the stop switch or emergency stop switch (Local Panel) is pressed.

When start/run relays are de-energized:

1) The postlube timer relay begins to time out.

(Contd.)


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2) The pre/postlube pump motor is energized.

3) The purge valve opens to purge the fuel system until engine speed decreases below 15%.

4) The fuel bypass valve opens to the filter outlet line.

5) The Pcd bleed valve opens to cutoff Pcd air and vent the fuel control valve.

6) Fuel valve closes to fuel injectors.

7) Control power to the electronic load/speed sensing control is stopped, and the governor is deactivated.

(Contd.)


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8) Main electric fuel pump, if used, and fuel boost pump are both de-energized.

9) When the fuel supply to the engine is cut off, combustion stops and the engine begins to decelerate.

10) When engine speed decreasesbelow 15%, the purge valve and the fuel bypass valve close.

11) The engine coasts to a stop.


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System Operation: Shutdown Sequence

55 minutes after the stop switch is pressed:

1) The postlube timer relay times out.

2) The pre/postlube oil pump is de-energized.

3) The postlube cycle is complete.

4) The master control switch can be turned off and the engine is ready for restart.


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Introduction

Preceding lessons have discussed gas turbine engine components and operation, oil systems, and fuel systems. This series of lessons will describe gas turbine engine starting systems.

This lesson presents information about a pneumatic starting system, its components, and operation. A Solar Centaur pneumatic starting system is used as an example.

Lesson: 20 Pneumatic Starting System


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Pneumatic Starting System: PurposeThe purpose of a gas turbine engine starting system is to provide power to:

· rotate the turbine shaft to starting speed

· assist the turbine to self-sustaining speed after combustion occurs

Most gas turbine engines are started by starter power input to the main accessory gearbox. The gearbox is connected to the turbine rotor and compressor.

Any gas under pressure may be used as a power source for a pneumatic starting system.


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Starting System: ComponentsNatural gas must meet the manufacturer's specifications. A typical pneumatic starting system requires approximately 2600 scfm.

The main components of a Solar Centaur starting system are as follows:· gas inlet strainer· pilot gas filter· solenoid-operated pilot valve· starter motor shutoff valve· lubricator· starter motors

The gas inlet strainer is discussed first.


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Starting System: Gas Inlet StrainerThe gas inlet strainer is located at the gas turbine skid upstream of the shutoff valve.

A shutoff valve is located downstream of the strainer to shut off the gas supply to the starter motor.

The strainer is a Y-shaped fitting that houses a removable cylindrical "strainer" screen.

A pilot supply line branches off from the main gas supply to the starting system. This line provides pneumatic pressure to the solenoid-activated pilot valve.


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Starting System: Pilot Gas Filter

A pilot gas filter assembly, consisting of a 10-micron filter and a pressure-reducing orifice, is installed in the pilot supply line upstream of the pilot valve.

The filter prevents foreign matter from entering the solenoid-operated pilot valve.

The pressure-reducing orifice creates a pressure drop in the pilot supply line. This pressure drop ensures that the pilot valve will operate properly in the event of excessive gas pressure.


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Starting System: Pilot ValveThe pilot valve is a three-way, solenoid-operated valve that is powered by 24-volt DC from the electrical system and actuated by the control system.

The pilot valve opens and closes the starter motor shutoff valve.

When the solenoid is de-energized, pilot gas pressure closes the starter motor shutoff valve.

When the solenoid is energized, pilot gas pressure is vented, allowing the shutoff valve to open.

A pilot relief valve protects the filter and pilot valve from excessive pressure.


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Starting System: Starter Shutoff Valve

The pilot-actuated starter motor shutoff valve controls gas flow from the supply line to the two starter motors.

The starter motor shutoff valve is installed upstream of the lubricator.


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Starting System: Lubricator

A lubricator, located downstream of the starter motor shutoff valve, injects lubricating oil into the gas flow.

The purpose of the lubricator is to provide atomized lubricating oil to the starter motor vanes.

A sight dome is used to check oil flow. In addition, the lubricator bowl has an oil level sight glass.

(Contd.)


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On Solar Centaur gas turbines, two vane-type, pneumatically operated starter motors are mounted on the starter adapter housing, located on the reduction gear.

The motors transmit starting power to the engine through a common overrunning clutch and shaft.


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Starting System: Starter MotorThe figure shows a starter motor and the clutch, called a sprag or sprag clutch assembly. The clutch in this installation is in the housing that is mounted on the engine gearbox drive.

The pawls , driving or holding links of a ratchet that permit motion in one direction only, are forced inward by small springs to engage the sprag clutch ratchet. At a preset engine speed, the pawls are thrown

(Contd.)


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outward, disengaging the drive shaft assembly from the sprag clutch ratchet. The sprag clutch ratchet and starter gear train coast to a stop and the drive shaft assembly containing the pawls continues to rotate at engine gearbox speed.

The operation of the pneumatic starting system is explained next.


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Starting System: Operation

The operation of the pneumatic starting system is automatically actuated by the gas turbine control system panel.

The control system provides automatic starting and sequencing as the gas turbine is accelerated to operating speed.

During acceleration and operation, the control system monitors the gas turbine and driven equipment, such as a generator or compressor.

If an operating malfunction occurs, the control system identifies the nature of the trouble and may initiate an emergency shutdown.


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System Operation: Start Sequence

The following start sequence is not a start-up checklist. Before a gas turbine start is attempted:

· A thorough prestart inspection must have been completed.· Prestart conditions must be satisfied.

When the operator presses the START pushbutton:

· The RUN indicator lamp illuminates.· Start/run control system relays are energized, and the start circuit is initiated.· The pre/postlube pump starts, and prelubrication begins.


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System Operation: Start Plus 15 Seconds

The following events occur 15 seconds after the START button is pressed:

· The prelube timer times out, and the start relay remains energized up to 66% of engine speed.

· The solenoid-operated pilot valve is energized, and the starter motor shutoff valve opens.

· Starter motors begin to crank and accelerate the gas turbine.


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System Operation: Turbine At 15% Speed

When gas turbine speed reaches 15%:

· Purge timer relay inhibits fuel while purging the machine.· Ignition begins when fuel is admitted 10 seconds after 15% speed. (Purge timer times out.)· Lightoff occurs and combustion begins.· The engine continues to accelerate with assistance from the starter motors.


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System Operation: Temperature @ 350°F

When engine turbine temperature reaches 350°F:

· Ignition system is shut off.

· Pre/postlube pumps stop when engine-driven lube oil pump pressure reaches 35 psig.


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System Operation: Engine Speed @ 66%

When engine speed reaches approximately 66% or self-sustaining speed:

· The starter clutch overruns.· The start-relay and start circuits are de-energized.· The solenoid-operated pilot valve is de-energized.· The starter motor shutoff valve is closed by pilot pressure.· Acceleration continues.

The start cycle is complete at approximately 66% engine speed.


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Hydraulic Starting System: Purpose

The purpose of a hydraulic starting system is to supply high pressure hydraulic fluid to a hydraulic starter motor. This motor is mounted on the accessory gearbox.

The hydraulic starting system module and its associated piping, valving, and instrumentation is mounted on a self-contained skid.


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Hydraulic Starting System: Components

The main components of a hydraulic starting system are as follows:

· electric motor· hydraulic pump· hydraulic fluid reservoir· hydraulic starter motor (engine mounted)· filter elements


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Electric Motor

The purpose of the electric motor is to drive the hydraulic pump.

The size of the electric motor depends on the capacity of the hydraulic start module.

Larger engines require greater starting power.

A typical 55 gpm hydraulic starting system that operates at 5000 psig uses a 200 horsepower electric motor.

The motor normally operates on 480 volt, 3-phase AC and rotates at 1800 rpm on 60 hzor at 1500 rpm on 50 Hz.


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Hydraulic Pump

The hydraulic pump is an axial-piston, variable-displacement or variable-volume pump. This type of pump automatically increases or decreases the volume of fluid flow to limit output pressure.

A small charge pump is mounted on the head of the hydraulic pump.

The purpose of the charge pump is to prime the system and to ensure that air is purged from the system.

The charge pump is driven through the main hydraulic pump.


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Axial-Piston PumpAn axial-piston pump consists of several small reciprocating pumps in a common cylinder block and housing.

The head, which contains an inlet port and an outlet port, is attached to one end of the pump housing.

The cylinder block contains an odd number of cylinders (usually seven) equally spaced from the center.

Three cylinders are always connected to the inlet port, and three cylinders are always connected to the outlet port. One cylinder is located between the ports.


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Axial-Piston PumpThe cylinder block and pistons rotate inside the pump housing.

The pistons have connecting rods that are fastened to a swash plate by ball joints.

The drive shaft of the axial-piston pump rotates the swash plate and cylinder block so that the pistons move back and forth in the cylinder block, creating the pumping action.

Piston movement is almost overlapping, which results in a constant flow of hydraulic fluid.


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Hydraulic Pump: Pumping Action

The figure shows the pumping action of one piston as the cylinder block makes one revolution inside the pump housing.

When the piston is at point 1, it has just started its inward movement and is only partly open to the inlet port.

(Contd.)


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At points 2 and 3, the piston is moving inward and the space in the cylinder is being filled with hydraulic fluid.

At point 4, the piston is at the bottom of its stroke and it is not connected to either port at this time. This movement from point 1 to point 4 represents the first half of a revolution.


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Hydraulic Pump: Pumping Action

During the last half of the revolution, the piston moves outward toward the head.

The hydraulic fluid that filled the cylinder during the first half of the revolution is now forced out through the outlet port as the piston returns to point 1 to start another cycle.


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Variable Displacement Piston Pump

A variable-displacement piston-type pump provides a means for varying the length of the pumping stroke.

The length of the pumping stroke determines the volume of hydraulic fluid that is discharged to the system.

Pump stroke length is varied by changing the angle of the swash plate.

( Contd.)


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The angle of the swash plate is controlled by two different means:

· A solenoid selects high or low flow operation.

· An internal compensator limits output pressure to 5000 psig by reducing the swash plate angle to reduce cylinder displacement.


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Hydraulic Starting System: Operation

The typical hydraulic start system is a closed loop system with all return lines to the hydraulic fluid reservoir. The reservoir usually stores 40 to 50 gallons of hydraulic fluid.

Automatic temperature controls maintain hydraulic oil temperature between 50°F and 60°F.

( Contd. )


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Alarms and shutdown switches are provided for high and low hydraulic oil temperatures and low liquid levels.

During starting system operation, the charge pump takes suction from the hydraulic fluid reservoir through a suction strainer.


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Fluid from the charge pump flows through a charge pump discharge filter and then to the main hydraulic pump. The charge pump provides over 300 psig to the main pump suction.

The main hydraulic pump provides up to 55 gpm at 5000 psig to the hydraulic starter motor.

High pressure fluid lines are installed between the starting system module and the hydraulic starter motor.


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The hydraulic starter motor is similar to the hydraulic pump.

The motor is variable displacement with a swash plate. The power cylinders are arranged axially around a shaft. The angle of the swash plate on the shaft is controlled by a speed-sensing mechanism.

( Contd. )


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At speeds up to 2500 rpm, the swash plate is held at its maximum angle, and the pistons are working through their full stroke.

At speeds over 2500 rpm, the swash plate angle decreases. At about 4500 rpm, the swash plate is almost square to the shaft.


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The flow through the motor is equal to cylinder displacement times rpm. The flow gradually increases as the motor speed increases up to 2500 rpm. Flow then remains more or less constant up to cut-out speed (speed increasing, displacement decreasing).

The starter converts high pressure fluid energy to shaft torque, which rotates the engine.

( Contd. )


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The fluid discharges from the starter to the low pressure side of the system (about 150 psig) and returns through a filter to the supply pump inlet.

Solenoid control valves are sequenced and controlled by the main unit control panel.


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When a start is initiated, the system accelerates the gas generator from static conditions to self-sustaining idle speed.

The hydraulic starter motor has an overrunning clutch, which disconnects the starter shaft from the gas turbine during engine operation.


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IntroductionThis lesson is the last in the series about gas turbine starting systems. Previous lessons provided important information about pneumatic and hydraulic starting systems.

This lesson focuses on the diesel starting system, its components, and operation.

The figure represents a typical diesel starting system.

The lesson begins with the purpose of a diesel starting system.

Lesson: 22 Diesel Starting System


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Diesel Starting System: Purpose

The purpose of a diesel starting system is to provide power to start the turbine axial flow compressor.

A diesel starting system performs three primary functions during the start cycle:

· start the turbine roll (breakaway from standstill)· accelerate the gas turbine to firing speed· assist the gas turbine to self-sustaining speed

A diesel starting system has three main components. Each is described in the information that follows.


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Diesel Starting System: ComponentsThe main components of a diesel starting system are:

· diesel engine

· torque converter

· hydraulic ratchet system

The three components are graphically represented in the figure. The diesel engine is discussed first.


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Diesel EngineA diesel engine is the primary component of a gas turbine diesel starting system. It is used to rotate (crank) the gas turbine for start-up.

A diesel engine is an internal combustion engine that converts the heat of fuel into work in the cylinders of the engine.

Diesel engine operation is based on the reciprocating (upward and downward) movement (stroke) of a piston in a cylinder.

( Contd.)


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In a two-cycle engine, the combustion cycle is completed in each cylinderduring one revolution of the crankshaft, which is one piston stroke.

The diesel starting engine for G.E. Frame 5 gas turbines is a 12 cylinder, two-cycle engine rated at 2300 rpm.


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Two-Cycle Diesel Engine

In a two-cycle engine, four events occur during one piston stroke. The events are:

· scavenging (air intake)· compression (fuel injected)· power (fuel ignition)· exhaust

The action taking place during each of these events is shown in the figure.

The next component discussed is the torque converter.


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Torque Converter

The torque converter is a hydraulic device that transmits the torque (turning force) of the diesel engine to the gas turbine through the ratchet jaw clutch.

The hydraulic ratchet's jaw clutch couples and uncouples the torque converter and diesel engine from the gas turbine.

Rotation of the hydraulic turbine causes the output shaft of the torque converter to rotate. This turning force (torque) is transmitted to the gas turbine through a starting clutch.The third component of the diesel starting system is the hydraulic ratchet assembly. The purpose of this component is discussed next.


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Hydraulic Ratchet AssemblyThe purpose of the hydraulic ratchet assembly is to assist the starting device to begin the rotation of a gas turbine rotor at breakaway.

The ratchet system also rotates the gas turbine rotor during the cooldown cycle, after gas turbine shutdown.

The ratchet is controlled partially by the starting control system.

The diesel starting system has several auxiliary systems that are essential to its operation.

These auxiliary systems are discussed next.


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Diesel Starting System: Auxiliary Systems

A diesel engine has the following auxiliary systems:

· cooling· lube oil· air· fuel· starter control

The first system discussed is the cooling system.


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Cooling Systems

A diesel engine uses one of two types of cooling systems:

· radiator and fan· heat exchanger

Both systems use a centrifugal water pump to circulate coolant and a thermostat to maintain operating temperature.

The radiator and fan cooling system is discussed first.


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Cooling Systems: Radiator & FanIn a radiator and fan cooling system, the coolant is drawn from the lower section of the radiator by the water pump.

The coolant is then forced through the lube oil cooler, into the cylinder block, and up through the cylinder heads.

When the engine is at normal operating temperature, the coolant passes through the thermostat housings into the top part of the radiator.

In the radiator, coolant temperature is reduced by heat exchange with the airflow through the radiator created by the fan.


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Cooling Systems: Radiator & Fan

When starting a cold engine or when the coolant is below operating temperature, coolant flow is restricted by the thermostat.

A bypass tube permits water recirculation in the engine during warm-up.

The heat exchanger cooling system is discussed next.


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Cooling Systems: Heat Exchanger

In a heat exchanger cooling system, the coolant is pumped through the heat exchanger by the water pump. The coolant then passes through the engine oil cooler, cylinder block, cylinder heads, exhaust manifolds, and thermostat housings.

A bypass tube permits coolant recirculation when the thermostat is closed. When the thermostat is open, coolant returns to the heat exchanger for cooling and recirculation.As coolant passes through the core of the heat exchanger, the coolant temperature is lowered by heat exchange with raw water. Raw water is supplied by a raw water pump from an outside source


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Lube Oil System The diesel engine lube oil system ensures positive lubrication at all times.

This lube oil system has the same purpose and components as other lube oil systems discussed in previous lessons.

Diesel engine lube oil provides full pressure lubrication for all main, connecting rod, and camshaft bearings; piston pins; gear train; and drive gear. Lube oil also cools the piston.


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Air System

The purpose of a diesel engine air system is to provide air for scavenging (removing) exhaust gases from the engine cylinders and for combustion.

In a two-cycle diesel, a charge of air is forced into the cylinders by the blower(s). Each cylinder is filled with fresh, clean air.

( Contd.)


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The air charge thoroughly sweeps all of the burned gases out through the exhaust valve ports.The air helps to cool internal engine parts, particularly the exhaust valves.

The diesel starting fuel system is discussed next.


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Fuel System

The main components of the diesel engine fuel system are:

· diesel fuel tank· fuel charge pump· canister tank· main fuel pump· filters· fuel header· fuel injectors

(Contd. )


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A diesel fuel tank is built into the base of the gas turbine.

The fuel charge pump is driven by the diesel engine camshaft. This pump supplies fuel from the base tank to a small canister tank mounted on the side of the engine.


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Fuel SystemThe main fuel pump takes suction from the canister tank. It pumps the fuel under pressure through the fuel filter to the fuel header.

The main fuel pump is a positive displacement gear pump that is attached to the blower.

The fuel header delivers fuel to the fuel injectors. The fuel is filtered through filter elements in the injectors and then atomized through small spray tip orifices into the combustion chamber of each cylinder.


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Control System

This section discusses the starter control system.

The primary diesel controls are:

· starter

· speed control system

· stop mechanism

· electronic logic


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Control SystemThe starter is a DC electric starter motor. The starter motor is attached to the diesel engine block.

Lead-acid storage batteries provide electric power for starting.

Diesel engine speed is controlled by the variable speed governor. A hydraulic actuator positions the governor speed control lever. A small, engine-driven pump supplies engine oil for actuator operation.

When the accelerating solenoid is energized, the governor lever is driven to the high operating speed position.


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Control System

When the solenoid is de-energized, the lever is returned to the idle position.

The diesel engine stop mechanism is a solenoid connected to the governor.

When the stop solenoid is energized, it activates the shutdown mechanism, shutting off fuel and stopping the engine.

Electronic logic in the gas turbine control panel provides automatic sequencing of the starter and control solenoids for normal diesel engine start-up, for normal and emergency engine shutdown, and for exercising and testing the engine.


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Control System

To protect the diesel engine, electronic logic also monitors the starting clutch position, the engine lube oil pressure and the engine speed.

Alarms and emergency shutdowns are initiated for out-of-limit operating conditions.

If the diesel engine is tripped when the throttle is in the full open position, the throttle does not reset.

The next diesel engine restart will be made with the throttle set at full power. The throttle lever should be manually reset before the next start attempt.The next section discusses the operation of a diesel starting system.


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Diesel Starting System: Operation

The following describes the operation of a diesel starting system. The procedure assumes the gas turbine is off cooldown, in a ready to start condition with all pre-start-up checks complete.

When START is selected:

· starting clutch is engaged

· starting motor is energized to start the diesel engine


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The diesel engine idles through a 2-minute warm-up cycle.

The acceleration solenoid is energized to accelerate the diesel engine to max speed for gas turbine breakaway with hydraulic ratchet assistance.


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Operation

Following gas turbine breakaway:

· acceleration solenoid is de-energized and the diesel engine slows to approximately 1900 rpm

· acceleration stop solenoid controls diesel engine speed until the gas turbine is sequenced through the warm-up cycle

· acceleration stop solenoid is de-energized· acceleration solenoid is energized to accelerate the diesel engine to maximum rpm for acceleration of the gas turbine


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Operation

At diesel engine speed of 3000 to 3400 rpm:

· clutch automatically disengages· acceleration solenoid is de-energizedAt this time, the diesel engine:· returns to idle speed· idles through a cooldown period· stops when the stop solenoid is energized


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IntroductionPrevious lessons discussed gas turbine components and operation. This lesson provides information on the enclosures that house the gas turbine and its driven and auxiliary equipment.

A gas turbine/generator set is used as an example through out the lesson.

A basic gas turbine/generator set consists of the gas turbine, gearbox, and generator with complete self-contained operating systems.

Lesson : 23 Enclosure


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Enclosure: Purpose

The purpose of an enclosure is to protect the gas turbine/generator set from environmental elements, to improve appearances, and to reduce noise, to meet local area classifications, and to provide an easier means for fire protection and containment.The enclosure discussed in this lesson is designed for outdoor installation and high wind loads.The enclosure is divided into compartments by bulkheads (walls).Each compartment contains lighting, access doors, and, when needed, removable panels for inspection and maintenance.


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Ventilation System: PurposeA ventilation system is provided when a gas turbine/generator set is enclosed.The purpose of an enclosure ventilation system is to minimize temperatures in the turbine and generator compartments.

Enclosure ventilation systems include:· air inlet· airflow· exhaust

Information about the air inlet system is presented first.


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Ventilation System: Air InletThe purpose of an enclosure air inlet system is to:· take in air for ventilation of the enclosure· treat the quality of inlet air to make it suitable for turbine use

The main components of an enclosure air inlet system are:· inlet screens· weather louvers· filters· ducting


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Ventilation System: Air Inlet Components

Inlet screens are installed upstream of the weather louvers toprevent entry of birds, leaves, twigs, paper, and similar objects. The screens must be kept free from the accumulation of this debris to ensure free airflow.

( Contd.)


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Incoming air passes through weather louvers where moisture is removed. These louvers provide a moisture "trap" using the inertia of the water droplets as the means of separating the droplets from the incoming air.

The filters remove foreign particles from incoming air. Filters should be checked regularly. Dirty filters can result in overheating of the equipment in the enclosure.


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Some enclosures have an air inlet. Others take ventilation air from the gas turbine air inlet system.

The duct exterior walls and inlet support structure are either stainless steel or carbon steel with multiple coats of protective paint.

Information about an enclosure's airflow system is next.


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Ventilation System: Airflow Components

The purpose of an enclosure airflow system is to:

· minimize temperature in the enclosure compartments· minimize hazards in the event of a fuel system failure

The main components of an enclosure airflow system are as follows:

· dampers· fans

The purpose of dampers is to control the airflow in the enclosure. Air inlet and outlet dampers are normally open. They are closed by gravity to provide an airtight enclosure when the fire protection system is activated.


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Ventilation System: Airflow Components

Inlet dampers are opened by airflow when the compartment fan is operating.

Outlet dampers are held open by pressure-operated latches. The latches must be manually reset after the damper is released.

Fans provide ventilation by drawing air through the air intake and exhausting it to the atmosphere. The purpose of fans is to increase circulation in the enclosure. Fans may be installed in either the inlet or the exhaust duct.An enclosure exhaust system is discussed next.


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Ventilation System: Exhaust Components

The purpose of an enclosure exhaust system is to return the heated air to the atmosphere.

The main components of an enclosure exhaust subsystem are as follows:

· fans· dampers· screens· louvers

Some enclosures have a cooling air fan installed in the exhaust or outlet duct. The outlet dampers are the fire dampers. One or more screens is installed in the outlet ducting to prevent entry of foreign matter.


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Ventilation System: Exhaust Components

The exhaust ducting for some enclosures contains manual dampers that are adjusted to control ventilating airflow through the enclosure.

Louvers may also be installed to prevent the entry of moisture when the unit is shut down.


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Fire Detection System: PurposeThe purpose of a fire/rate-of-rise detection system is to detect a fire or serious heat conditions in the gas turbine/generator set enclosure.

An enclosure fire/rate-of-rise detection system must have the following characteristics:

· reliable detectors in the correct locations· means to test the system· effective maintenance and testing procedures


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Fire Detection System: RequirementsFire/rate-of-rise detection systems should meet the following requirements:

· initiate an immediate alarm on fire or excessive conditions

· provide an indication that a fire has been extinguished and another indication if the fire re-ignites

( Contd. )


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· be durable and able to resist environmental damage

· incorporate an accurate and effective testing system to ensure system integrity

· operate without special electrical equipment and require a minimum of power


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Fire Detection System: Types

The following types of fire/rate-of-rise detection systems are discussed in this lesson:

· thermal switches· thermocouples· thermistors· pneumatic circuits· optical detectors· gas detectors


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Thermal Switches

Thermal switches are either:

· single wire· two wire

The single-wire thermal switch fire/rate-of-rise detection system has heat-sensitive thermal switches located at points in the enclosure where temperatures are likely to be highest.

The switches contain a pair of contact points that are normally open. The contact points close at a preset temperature.

When the switch heats up, the heat-sensitive arms with the contact points expand. The expansion is in the direction opposite the electrical terminal.


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Thermal Switches

The sliding piston moves to the end of the switch, the points close, and the thermal switch completes an electric circuit for the alarm switch.

When the circuit is complete, the alarm switch initiates an alarm signal. The switch automatically resets when it has cooled.


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Thermal Switch: Single Wire Circuit

The figure shows a single-wire circuit in which 24V DC is applied to both paths of the circuit. If an overheat temperature or a fire occurs which closes one of the five thermal switches, a path to ground is completed through the circuit.

With this type of loop arrangement, an open circuit can occur and the system will still provide protection at each of the five surveillance points.

The test circuit tests the entire loop and will indicate an open circuit in the power input lead of the loop. A short circuit in the loop will cause a false fire warning indication.


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Two-Wire Thermal Switch

The two-wire thermal switch fire/rate-of-rise detection system remains functional with either an open or a short circuit.

The two-wire bi-metallic thermal switch operates the same way that the single-wire thermal switch does. The only difference is that an electrical lead is connected to both arms of the thermal switch.


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Thermocouples

Thermocouples are temperature sensing devices, primarily used in temperature indicating systems, such as exhaust gas temperature or turbine outlet temperature.

(Contd.)


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A thermocouple is the junction of two dissimilar metals that generate a small electrical current that varies with the difference between the temperature of the hot junction and the cold junction.

The dissimilar metals can be any combination of metals or alloys that will produce the required results such as iron-constantan (Type J) or Chromel-Alumel (Type K).

The complete thermocouple circuit consists of a "cold" junction, a "hot" junction, electrical leads made from the same material as the thermocouple, and a galvanometer-type indicating instrument


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Thermocouples

The point where the two dissimilar metals are joined that will be most exposed to the heat of a fire is called the hot junction.

The cold junction, sometimes called the reference junction, is enclosed in dead air space between insulation blocks.

( Contd. )


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A metal cage or sheath protects the thermocouple without hindering free movement of air to the hot junction.

Some thermocouple element wires are surrounded with magnesium oxide to prevent vibration damage to the wires. Magnesium oxide also enhances heat transfer between the medium being measured and the measuring junction


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Thermocouples

A typical thermocouple is installed in a protective well or cage. Other thermocouple cages have several passages that allow air (or gas) to enter the protective cage and surround the elements.

A thermocouple fire detection system has a different response to the thermal switch system. (Contd.)


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In a thermocouple system:

· If the temperature increases rapidly, the thermocouple produces a voltage because of the temperature difference between the hot junction and the cold junction.

· If both junctions are heated at the same rate, no voltage is generated and no alarm signal is initiated.


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Thermistor

A thermistor (thermal resistor) is a resistive circuit component. When cool, a thermistor has high resistance to current flow. As the temperature of a thermistorincreases, its resistance decreases.

A thermistor fire/rate-of-rise detection system is a continuous loop system that usually surrounds the surveillance area.

(Contd.)


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Single-wire and two-wire systems are commonly used on large gas turbines.

In the single-wire system, the outer case provides the ground potential. In the two-wire system, the second wire provides the ground.


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Thermistor

In both systems, the power lead is insulated from ground. The single-wire system uses an insulator of ceramic beads that are coated with a substance called eutectic salt. The two-wire system uses a thermistor material to insulate the wires.

Each of these materials loses electrical resistance when heated.

In the fire/rate-of-rise circuit diagram shown in the figure, 24V DC is supplied to the hot lead through an alarm relay coil.

When cool, the insulation does not allow current flow between the hot lead and ground.


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Thermistor

When a fire condition heats the insulator material, it loses electrical resistance and a path is complete from the hot lead to ground.

The thermistor system, like the thermal switch systems, automatically resets when cooled.

A pneumatic fire detection system is discussed next.


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Pneumatic Fire Detection System

The pneumatic fire detection system uses a gas-filled tube as a detector. The detector is produced in various sensor tube lengths and alarm temperatures.

The gas expands when heated. When setpoint temperature is reached, the gas pressure is sufficient to overcome the check valve and gas flows from the detector tube to the right side of the diaphragm.

This flow forces the diaphragm contacts to the left onto the alarm contacts, which energizes the alarm circuit.

The gas returns to a low pressure after the heat source is removed.

The check valve arrangement and diaphragm force the gas back into the tube, ready for another operation.


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Optical Fire Detection System

The detection systems discussed so far detect temperature increases. The optical system detects changes in the light spectrum inside of the enclosure.

Optical flame detectors use infrared and ultraviolet detectors to receive direct or reflected rays from a source of flame or heat. ( Contd.)


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When a fire or hot object is detected in a compartment, the detector signals the controller, which powers the fire alarm. A fire signal is generated only when a fire is detected by both sensors.

Three optical fire detectors are shown in the figure, two in the turbine compartment and one in the generator compartment.


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Optical Fire Detection System

When two optical detectors are installed in one compartment, they are usually cross-zoned or use a voting system.

Both detectors must agree that a fire condition exists before the fire monitoring system will initiate action


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Gas Detection SystemThe gas detection system should prevent the possibility of a gas-fueled fire, which is considered potentially the most likely and the most dangerous. However, should a fire occur it will be detected by one or more of the sensors.

Enclosure fire detection system operation is discussed next.


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Fire Detection System: Operation

When fire is detected by any of the sensors, the fire control system performs the following functions:

· The alarmed zone is indicated.

· All ventilation is shut down.

· An audible alarm is initiated, specifying the zone. (Contd.)


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· External audible and visual alarms are initiated.

· An emergency shutdown signal is relayed to the gas turbine/generator control system.

· Generally, but not always, after a time delay to allow personnel to evacuate the enclosure (usually 10 seconds), the fire extinguisher agent is discharged.


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Fire Detection System: Operation

The alarms will continue until they are manually switched off.

The fire extinguishing agent must be recharged and the detection/protection system reset before the gas turbine can be restarted.


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Gas Detection: Purpose & Components

The purpose of a gas detection system is to detect the presence of combustible gas in a gas turbine/generator set enclosure.

The main components of a gas detection system are as follows:

· sensors· detection circuit· protection systemSensors are discussed first.


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SensorsTwo or more sensors are installed in the gas turbine compartment and one or more in the generator compartment near the divider wall where the drive shaft or coupling penetrates the wall.

Two sensing elements are used in a sensor.

One element is calibrated to detect a low combustible gas concentration of 15% or 20% L.E.L. L.E.L. is the lower explosive limit.

The other element is calibrated to detect a high combustible gas concentration of 60% L.E.L.


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Sensors

The sensors detect the combustible gas concentration with air at the lower explosive limits (L.E.L.).

A 5% methane concentration with air is the lowest concentration that can be ignited. A 15% methane concentration is the highest concentration that can be ignited. Lower or higher concentrations cannot be ignited.

The 5% methane concentration with air is 100% of the L.E.L. An alarm that occurs at 20% L.E.L. actually indicates a combustible gas concentration of 1%.

The detection circuit is discussed next.


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Detection CircuitMost gas detection systems are calibrated to initiate an alarm when the gas concentration in the enclosure reaches 15% to 20% L.E.L.

This is the low L.E.L. alarm.

For a low L.E.L. alarm, the combustible gas detection system usually initiates both an audible and a visual alarm.

Enclosure ventilation fans that are not already running are started.

The fans continue to operate until the gas detection/protection system is reset and the alarm indication is cleared by the operator.


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Detection Circuit

The gas detection system is normally provided with readout indicators.

These indicators allow the operator to observe the presence and concentration level of any combustible gas that may be inside the enclosure.

The protection system is discussed next.


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Protection System

A high L.E.L. alarm is initiated at 60% L.E.L. A 60% L.E.L. is a 3% combustible gas concentration. On some gas turbine/generator sets, a 60% L.E.L. alarm initiates the following actions:

· immediate gas turbine/generator shutdown, if operating

· immediate shutdown of all operating enclosure ventilation fans


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· immediate closing of all ventilation/isolation dampers

· immediate activation of the audible warning device inside and outside the enclosure

After a short time delay, usually 10 seconds to allow personnel to exit the enclosure, fire extinguishing agent discharges in both the gas turbine and generator compartments.


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Extinguisher Systems: Purpose

The purpose of a fire extinguishing system is to discharge concentrations of fire extinguishing agents into the enclosure compartments.

The two systems discussed in this lesson use either Halon or CO2 as the extinguishing agent.

A Halon fire extinguisher system is discussed first.

NOTE: Halon is being phased out worldwide. New systems will use CO2.


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Halon Extinguisher System

Halon is a colorless, odorless, non-corrosive, and electrically non-conductive gas. After discharge, it leaves no residue and does not require clean-up.

Halon is a chemical compound that inhibits combustion by reacting with oxygen in the air, so that oxygen is suppressed and no longer able to support combustion.

The figure represents a typical halonextinguisher system.


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Halon Extinguisher System: Components

The main components of a Halonfire extinguisher system are as follows:

· cylinders· valve assembly· pressure switches· discharge pipes and nozzles· heat (thermal) and fire (optical) detectors· audible alarm (horn)· monitor controller


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Cylinders

Halon is stored in cylinders, on or near the gas turbine/generator package. The number of Haloncylinders in a system depends on the volume of the protected area.

A minimum of two cylinders is required. One cylinder supplies the initial discharge, and the other provides the extended discharge. Any signal which initiates Halondischarge also trips the turbine.

(Contd. )


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When Halon is discharged into a compartment, the initial discharge is at arapid rate The reason for a rapid rate during initial discharge is to build an extinguishing concentration as quickly as possible.

This is followed by an extended discharge at a slower rate to maintain the extinguishing concentration and minimize the possibility of re-ignition of combustibles exposed to hot metal surfaces.


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Cylinders

Halon cylinders are manufactured in different sizes from 18 to 600 pounds. The cylinders are purged with nitrogen and then filled with Halon.

Halon cylinders are held in the upright position by mounting brackets.

The cylinders should be located in an area that does not receive direct sunlight because Halon is extremely temperature sensitive. Halon cylinders should not be exposed to temperatures above 130°F.


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Cylinder Valve Assembly

A valve assembly is an integral part of each Halon cylinder. The valve is made of brass, which makes it corrosion resistant.

The valve usually contains a differential piston. Differential piston operation allows the valve to discharge Halon when actuated manually, electrically, or pneumatically.


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Cylinder Valve Assembly Operation

Halon pressure from the cylinder enters the valve assembly and rises through a bleed port that contains a ball check valve.

Halon pressure acts on both sides of the piston with equal pressure in areas A and B in the figure. This pressure keeps the Halon in the cylinder because the piston seals off the discharge port. ( Contd.)


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When the vent valve is actuated, there is a large pressure difference between the top (B) and bottom (A) areas of the valve. When the ball check valve closes the bleed port, Halon cannot flow to the top of the piston and the pressure in areas A and B are no longer equal


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Cylinder Valve Assembly Operation

The valve is actuated by venting pressure from the top of the piston through the vent valve.

When the pressure is vented, there is no downward force acting on the piston. The piston moves upward, the discharge port is opened, and Halon is released into the system.

The release of Halon is initiated by valve actuators. The valve actuators are described next.


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Types of Cylinder Valve Actuators

Different types of valve actuators are used to initiate Halon discharge, depending on the environmental and system design. They are:

· electrical/latching· pneumatic· manual-local override· manual-cable pull

The figure shows various types of valve actuators. Each is described in the information that follows.


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Electrical & Pneumatic Valve Actuators

The electrical/latching valve actuator is installed on top of the Halon valve assembly. The actuator contains a metal pin that pushes the vent valve off its seat when actuated. The electrical/latching valve actuator operates as an electric solenoid.

The pneumatic valve actuator may be mounted on the vent fitting of the Halonvalve assembly or on an electrical valve actuator. ( Contd. )


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When mounted on the Halon valve vent fitting, the pneumatic valve actuator provides direct actuation of the vent valve.

When mounted on an electrical actuator, the pneumatic actuator closes a switch that causes the electrical actuator to operate.


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Manual-Local Override Actuator

The manual-local override actuator is installed when the system requires manual Halonrelease at the cylinder. This actuator is mounted either directly on the valve assembly vent fitting or indirectly on an electrical actuator. A pneumatic actuator is incorporated so that the manual-local override also functions as a pneumatic actuator.

The manual-local override actuator is operated by pulling the ring pin and depressing the palm button or by providing a minimum of 30 psi pneumatic pressure to the inlet port.


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Manual-Cable Pull Actuator

The manual-cable pull actuator, mounted on the valve assembly, can be used as the primary means of Halon release or as an alternate type of manual-local override.

The manual-cable pull actuator is connected to a remote manual pull station by a wire rope.

If the ring pin is removed from the manual-cable pull actuator, pulling the handle of the remote manual-pull station actuates the cable pull actuator to release Halon from the cylinder.

The next component discussed is pressure switches.


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Pressure Switches

Pressure switches are installed in Halon fire extinguisher systems to monitor the pressure in the storage cylinders and in the discharge piping.

In the figure, low pressure switches (PSL) monitor the pressure in the two Haloncylinders. If the pressure in a cylinder decreases to 185 psig, the switch iniates an alarm.

The high pressure switch is identified as PSH in the figure. When the Halon pressure in the discharge manifold reaches 150 psig, the high pressure switch signals the control system that a cylinder has discharged.


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Pressure Switches

Most fire extinguisher control systems are programmed to discharge another cylinder if the high pressure signal has not been transmitted within a certain period, usually 5 seconds.

The components used to discharge the extinguishing agent are discussed next.


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Discharge Pipes & Nozzles

Discharge pipes and nozzles are used to discharge extinguishing agents.

Discharge pipes carry the fire extinguishing agent from the storage cylinders to the discharge nozzles.

The nozzles discharge the agent into the protected zones. The nozzles are placed to ensure a concentrated discharge in all parts of the protected zone.


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Detectors & Alarm

Heat and fire detectors were explained in the two preceding lessons. If an overheat or fire condition is detected in a gas turbine/generator enclosure, the detectors initiate an alarm signal to the control system.

The thermal (heat) and fire (optical) detectors are shown in the figure.

( Contd.)


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An audible alarm (horn) is installed at each enclosure fire-protected zone, or one horn for each major piece of equipment. This alarm, along with a flashing red light or beacon, is activated by the control system when a fire is detected in the enclosure.

When an alarm is activated, it will continue until it is manually switched off by the operator.


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Monitor Controller

The last component discussed in the Halon fire extinguishing system is the fire monitor controller. This controller works in conjunction with other gas turbine control systems. The controller uses a microprocessor system to monitor signals from sensors installed in the enclosure.

An example of a monitor controller is shown in the figure. The controller illustrated is typical of a monitor controller for an optical fire detection system with two independent types of detectors. A signal is sent to the controller only when both detectors sense a fire simultaneously.


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Monitor ControllerThe controller monitors up to eight zones. One LED (light emitting diode) is provided for each zone. An LED blinks when a fire is present in the corresponding zone.

One LED is also provided for each voting zone. A lit LED signals that the voting zone is actuated.

The Test/Accept push-button disables an energized alarm or initiates a manual test of a selected detector.

The inhibit LED is illuminated in the Test or Reset mode to indicate that the controller outputs are inhibited.


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Monitor ControllerWhen the lamp test switch is depressed, all LEDsand digital displays illuminate. In the reset mode, the lamp test switch indicates a complete system reset.

The mode switch is used to select Normal, Reset, or Test mode. The power LED illuminates when power is applied to the fire detection system.

The fault LED illuminates on fire detection system malfunction or when the controller is in the Reset or Test mode.

The system status display provides a numerical code to report the system status.


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Monitor Controller

The zone digital display identifies the zone and the fire detection system conditions in that zone.

The select push-button selects the desired detector for testing.

Halon and CO2 fire extinguisher systems are very similar. A brief discussion of the CO2 system follows.


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CO2 Extinguisher System: DescriptionA CO2 fire extinguisher system has the same basic components as the Halonsystem. The CO2 components are shown in the figure. A CO2 system also operates in the same manner as a Halon system.

CO2 (carbon dioxide) fire protection systems extinguish fires by reducing the oxygen content of the air in the compartment.

( Contd. )


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The oxygen content is reduced from an atmospheric normal of 21% to less than 15%, an amount that will not support combustion.

To reduce the oxygen content, a quantity of CO2 equal to or greater than 34% of the compartment volume is discharged into the compartment in 1 minute.


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CO2 Extinguisher System: Description

A CO2 fire protection system supplies CO2 from either low pressure storage tanks or high pressure cylinders to a distribution system.

This system transfers the CO2 to discharge nozzles located in the various compartments of the gas turbine package.

Low pressure storage tanks maintain saturated liquid CO2 at 300 psig and 0°F with a refrigeration system.

( Contd.)


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Actuator ArrangementA CO2 system may be actuated by several different methods.

Pilot-operated selector valves in the CO2 discharge valve are opened when solenoid valves are energized, when the pull lever is actuated, or when another cylinder is discharged.

The solenoid valves are actuated by an electric signal from the fire detectors. The system may also be manually actuated by switches located on the electrical control cabinet or by manual valves located in the control cabinet.

Actuation of the system, either automatically or manually, will trip the turbine.


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Discharge Systems: InitialTwo separate discharge systems are used in a CO2 system:

· initial discharge · extended discharge

Within a few seconds of actuation, sufficient CO2 flows from the initial discharge system into the compartment to rapidly build up the CO2 concentration.

( Contd.)


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This concentration is maintained by the gradual addition of more CO2 from the extended discharge system, compensating for compartmentleakage.

CO2 flow rate is controlled by the orifices in the discharge nozzles.


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Discharge Systems: ExtendedThe orifices for the initial discharge system permit a rapid discharge of CO2 to quickly build up an extinguishing concentration.

Orifices for the extended discharge system are smaller for a relatively slow discharge rate.

By maintaining the extinguishing concentration, the likelihood of a fire reigniting is minimized.

( Contd.)


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In addition to their use of similar components, both the Halon and CO2 extinguisher systems operate the same.

The discussion about extinguisher systems continues with a description of the operation of a typical fire extinguisher system.


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Extinguisher Systems: Operation

The figure shows the components found in a typical fire extinguisher system. In this system the first line of defense against fire is provided by the optical detectors.

Optical detectors usually have a response time of less than 5 seconds and an 80 degree field of vision. ( Contd.)


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When more than one optical flame detector is installed in an enclosure, a voting system is usually programmed into the fire monitor controller.

With a voting system, an alarm signal from one optical detector will initiate an alarm but no other action.

An alarm signal from one optical detector and a thermal detector or from two optical detectors is needed to initiate turbine shutdown andextinguisher release.


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Thermal detectors are installed in a gas turbine enclosure as a redundant detection system to the optical detectors. Thermal detectors initiate an alarm when the temperature in a gas turbine enclosure reaches 450 degrees F.

The sequence of events that occurs when a fire is detected is described next.


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Extinguisher Systems: OperationWhen a fire is detected, an alarm signal is sent to the fire monitor/controller and other gas turbine control systems.

This signal initiates the fire extinguisher agent discharge, alarms, and shutdowns.

In a typical fire extinguishing system, the following sequence occurs:

· Audible and visual alarms start.· All electrical equipment is de-energized.· All enclosure vent fans stop.· Gas turbine/generator shuts down.


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Extinguisher Systems: OperationAfter 5 to 10 seconds (time delay), extinguishing agent is released by actuation of the cylinder valves.

Extinguishing agent flows through the valve into the piping.

The high pressure switch signals the control systems that the extinguishing agent has been discharged.

Extinguishing agent pressure unlatches the ventilation dampers in the turbine and generator.


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Extinguisher agent is discharged into the turbine and generator compartments.

After the initial discharge, the extended discharge starts and maintains compartment flooding for approximately 10 minutes.

After emergency shutdown and the fire is extinguished, the following should occur before inspecting the space for fire damage:

· Allow the machine to cool down.· Thoroughly ventilate the enclosure.


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The fire and heat detectors must be cleaned and tested.

The fire extinguisher system must be serviced, refilled and reset.

The ventilation dampers must be latched to restore the fire protection system to service.


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Theory of Electrical Power Generation

Electrical current is the flow of electrons (e-) through a conductor. A conductor is a substance that transfers an electrical charge.

The force that causes the electrons to flow is called electromotive force (emf).

The two basic components of a simple battery are a cathode and an anode.

(Contd.)

Lesson : 27 Principles of Power Gen


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Theory of Electrical Power Generation

The solution reacts with the anode and cathode causing current to flow through the conductor.

The electric current that flows from a battery is known as direct current (DC).

Direct current flows through the conductor in only one direction.

DC current is used to supply electricity to electrical circuits in battery-operated equipment, such as radios and flashlights, DC motors, lights, etc.


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Electrical Circuit ComponentsA basic electrical circuit consists of:· power supply · load · conductor

The power supply provides emf. Examples of power supplies are batteries and generators. The term emf is also called potential or voltage.

The load is the resistance to current flow in the circuit. Examples of loads include motors, light bulbs, and heaters.

The conductor is used to carry the electrical current to the load.


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Electrical Circuit MeasurementThe emf is measured in units of volts using a voltmeter. The symbol for volts is E.

Resistance is measured in units of ohms using an ohmmeter. The symbol for resistance is R.

The current is measured in units of amps using an ammeter. The symbol for current is I.

If two of the quantities are known, the other quantity can be found by rearranging the following equation:

I=E/R


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Electrical Circuit MeasurementVoltage is always measured across an element. For example, to measure the voltage across the load in the figure, the voltmeter is placed in parallel with or across the load.

To measure the current flowing in the circuit, the ammeter is inserted into the circuit. This is an example of placing the ammeter in series with the load.

(Contd.)


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The resistance can be calculated after the voltage and current are measured. For example, if the voltage is 24 volts and the current is 6 amps, the resistance of the load is 4 ohms.

Information about the theory of magnetism is presented next.


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Magnetic Fields

Electricity flowing in a circuit creates a magnetic field.

The figure shows the effect of a magnetic field on a compass needle.

The needle aligns itself parallel with the magnetic field lines created by the current.

This alignment is caused by the force of the magnetic field. (Contd.)


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The strength of the magnetic field is proportional to the amount of current flowing through the conductor.

If the current is increased, the strength of the magnetic field increases.

If the current is decreased, the strength of the magnetic field decreases.


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Magnetic Fields

The strength of a magnetic field can be increased by winding a conductor around a core, forming a coil.

The magnetic field strength increases as the number of windings increases.

The direction and amount of current flowing through the coil determines the magnetic pole and strength of the magnetic field.


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Electromagnetic Induction

Just as current flowing through a conductor creates a magnetic field, a magnetic field can cause current to flow through a conductor.

This principle is the basis for a simple generator. The conductor is moved through a magnetic field, and current flow is induced.

This is known as electromagnetic induction.

The current is produced only when the conductor is moving in a direction that is perpendicular (90°) to the magnetic field.


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Magnetic Field Flux LinesThe magnetic field consists of flux lines that always point from the north pole of a magnet to the south pole.

The conductor moving through the flux lines causes current to flow.

A simple generator consists of a single-loop conductor called a coil.

To create current flow, the coil must move through the field in a perpendicular direction. (Contd.)


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When the coil is in a vertical position, no current flows through the conductor.

The current flow is maximum when the coil is in the horizontal position.


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Generation of AC CurrentElectricity is generated as the coil rotates through the magnetic field.

At position 1, no current is generated. As the coil rotates to position 2, the current increases to a maximum output.

The current output returns to zero as the coil is rotated to position 3. As the coil is rotated to position 4, the current reverses direction to a maximum negative value. ( Contd.)


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As the coil continues rotating and returns to position 1, the current output returns to zero.

This is how alternating current (AC) is generated.

The current and voltage alternates between a positive and negative value.


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AC Frequency

The AC frequency is the number of complete cycles generated per second. The unit of measurement for frequency is hertz, abbreviated Hz.

A complete cycle occurs when the current moves from zero to a maximum positive to zero to a maximum negative and back to zero. A complete cycle in one second equals one (1) Hz.

Common frequencies generated are 50 Hz and 60 Hz.


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Calculating FrequencyThe generators discussed so far are two-pole generators. A two-pole generator has one magnetic field with one north pole and one south pole.

Remember that each cycle completed in one second is 1 Hz. A generator with four poles completes two electrical cycles for each mechanical revolution.

The output frequency of any generator can be determined using the following equation:

f=p(rpm)/120

Where f is the output frequency, P is the number of poles, rpm is rotational speed of the generator shaft.


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Frequency & Shaft RPM

Comparing the required speed for 60 Hz and 50 Hz generators in the figure shows that the shaft speed can be reduced by increasing the number of magnetic poles.

A 60 Hz generator with two poles has to turn at 3600 rpm. If the number of poles is increased to eight, the shaft speed is reduced to 900 rpm.

( Contd.)


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Three-Phase Generator

Three-phase current is generated by three coils on the same shaft.

A three-phase generator has three coils, also known as windings. Each coil carries the current produced as it passes through the magnetic field. The coils are located at 120°increments around the shaft.

( Contd.)


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This arrangement produces three waveforms 120° out of phase. The generator has one terminal for each phase, A, B, and C, and a neutral terminal, N.

If the neutral terminal is brought outside of the generator, the voltage across any terminal (A, B, or C) to the neutral terminal is the phase voltage. The voltage between any two of the three line terminals (A, B, or C) is called the line voltage.


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Three-Phase Generator Connections

Three-phase generator outputs are connected two different ways:

· WYE configuration

· DELTA configuration

In the WYE connection, one side of A, B, and C, is connected to the neutral terminal.

(Contd.)


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In the DELTA connection, one side of A is connected to B, the other side of A is connected to C, and the other side of B is connected to C.

The power generated by either a wye or delta connected generator is the same, but the voltage and current relationship varies.


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Generator-Control: Transformer

The voltage produced by either a wye or delta connected generator is often different from that needed by the load. A transformer is used to regulate the voltages.

For example, a motor that requires a 480 volt source cannot be driven by a 24,000 volt supply. In this situation, a transformer steps down the voltage.

A transformer uses electromagnetic induction to regulate the voltages.


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Generator-Control: Transformer Coils

A transformer has two coils, a primary coil and a secondary coil. The primary coil is the supply voltage. The secondary coil is the induced voltage.

The primary coil has current flowing through it from the generator.

(Contd.)


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This creates a magnetic field that induces current to flow in the secondary coil.

Transformers only work with AC because of the current flowing into, then out of, the primary coil determines the voltage output in the secondary coil.

The fluctuating magnetic field produces the same result as moving a conductor in a magnetic field.

The ratio of the number of windings in the primary coil to the number of windings in the secondary coil is the electromagnetically induced voltage.


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Step-Up Transformer

Based on the ratio of windings, transformers are one of two types:

· step-up· step-down

A step-up transformer has fewer windings in the primary coil than in the secondary coil. It increases the voltage.

(Contd.)


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Step-Down Transformer

A step-down transformer has a greater number of windings in the primary coil than in the secondary coil. It decreases the voltage.

For example, if the primary coil has 100 turns at 110 volts AC and the secondary coil has 10 turns, the secondary coil has 11 volts AC across it.

The transformer is a step down transformer because it steps down the voltage produced by the generator.


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Generator Control: Rectifying Diodes

Sometimes it is necessary to extract DC power from an AC generator.

In a generator, the main generator field and the exciter field both require DC voltage.

This requirement is achieved with rectifying diodes.

(Contd.)


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Rectification is the process of turning AC voltage into DC voltage.

Diodes are electronic parts that allow current flow in only one direction. This allows only positive voltage to flow.

The voltage regulator (transformer) and diodes are very important to basic generator operation, which is described next.


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Generator: Components & OperationThe basic industrial generator consists of three generators in one, all driven by the same shaft:

· permanent magnet generator· exciter generator· main power producing generator

The three generators work together to produce power for industrial loads.


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Permanent Magnet GeneratorThe permanent magnet generator produces AC current. The current is rectified to DC using diodes.

The DC current is then used to supply power to the exciter generator.

However, the current first flows through a voltage regulator that adjusts the amount of voltage applied to the exciter.


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Exciter Generator

The exciter generator is connected to the main generator field.

Current from the exciter generator flows through the main generator field.

Remember the current flowing from the exciter generator to the main generator field is controlled by the voltage regulator.

Diodes are also used in the exciter generator to supply the main generator field with DC current.


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Main GeneratorThe main generator field induces current to flow in the main generator windings, supplying power to the loads in the area, such as motors.

The output of the main generator windings is sensed by the voltage regulator.

If the voltage sensed is low, additional voltage is supplied to the exciter generator. (Contd.)


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If the voltage sensed is high, less voltage is supplied to the exciter generator.

In this way, the main generator output is controlled.


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Generator Components

The components of a typical generator are shown in the figure. These components are:

· frame· rotor· bearings· stator

Information about the generator frame is presented first.


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FrameThe generator frame has two purposes:

· house the internal parts of the generator· support the weight of the rotor and stator plus any rotational or vibrationalforces

The frame is finished with a coating, usually paint, that provides protection against corrosion and the environment.

(Contd.)


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A frame is usually of one piece construction and includes the generator case.

The case is designed with air passages to allow cooling air to circulate into and out of the generator.


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CaseAs shown in the figure, the case contains:

· rotor

· bearings and seals

· generator field windings

· stator

The generator rotor is discussed next.


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RotorThe generator rotor is housed within the generator case and is supported at each end by bearings.

One end of the rotor is the drive end and the other is the exciter end.

Mounted on the rotor are:· cooling fans· main generator field windings· PMG magnets· exciter windings. (Contd.)


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The purpose of the rotor is to rotate the field windings to move magnetic lines of force through the windings of the stator core.

The moving field induces emf in the stator windings. The rotor assembly is dynamically balanced to minimize vibration.


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Bearings

The purpose of the bearings that support the rotor ends is to minimize friction associated with the rotating shaft.

Damage to bearings usually occurs as a result of high temperatures. Temperature sensors are installed in the bearings to provide an alarm or automatic shutdown if overheating occurs.

(Contd.)


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The generator lube oil system, discussed in detail in the next lesson, provides lubrication of the bearings and removes heat created by friction.

The rotor windings are the main generator field. This field causes electromagnetic induction to occur in the stator windings.


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Stator

The stator contains the stator windings, or armature.

The stator is made out of segmented plates called laminates.

The purpose of the stator is to generate the AC voltage that is conducted to the electrical circuit.

(Contd.)


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Recall from the preceding lesson that a three-phase generator has three separate stator windings.

One end of each phase is connected to a terminal for circuit connection. The other end of each phase is connected to a common neutral terminal.


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Lube Oil System: Purpose

The purpose of a generator lube oil system is to provide a reliable supply of clean, cool lubricating oil to the generator bearings.

These bearings are located on both ends of the rotor shaft. The bearings are pressure lubricated, sleeve-type bearings.

(Contd.)

Lesson : 29 Lube Oil System


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A reliable lube oil system is critical for generator operation. If the lube oil supply to the bearings is stopped during operation, the bearings and rotor shaft journal will be severely damaged.

Lube oil must be supplied to the generator bearings before and during start-up, during operation, while the generator is coasting to a stop, and during the cool-down cycle.


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Lube Oil System: Components

The main components of the generator lube oil system are shared with the gas turbine and gearbox lube oil systems. Information on these components was presented in the gas turbine lube oil system lessons. Recall the main components are: · lube oil reservoir· lube oil pumps

- engine-driven main- auxiliary or pre/postlube- emergency

· oil cooler(Contd.)


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. oil filters· instruments and controls· piping

If needed, return to the lessons covering these components and review the information before continuing on in this lesson.

This lesson continues with information on the generator lube oil system's instruments and alarms.


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Instruments & Alarms

The figure shows typical instruments and alarms associated with a generator lube oil system.

A well-monitored and protected generator lube oil system incorporates a relief valve, pressure switches and indicators, temperature instruments, and oil flow sight glasses.


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Pressure Relief Valve

The first protective device in the generator lube oil system is the relief valve (PSV).

The relief valve is generally set at 35 psig or lower. If lube oil pressure exceeds this value, the relief valve opens to return the excess oil to the lube oil reservoir.


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Switches, Transmitter, & IndicatorThe next protective devices are the low pressure switch (PSL), the low-low pressure switch (PSLL), and the pressure transmitter (PT). These devices initiate signals to the gas turbine generator control system.

If lube oil pressure decreases to 20 psig, the PSL transmits an alarm signal to the control system. If lube oil pressure continues to decrease, the PSLL initiates an alarm and a shutdown signal at 12 psig. ( Contd. )


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The PT signals lube oil pressure to the control system for operator readout. Lube oil system pressure may be observed at the local pressure indicator (PI).


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Temperature InstrumentsA temperature element (TE) monitors lube oil temperature in the oil header to the generator. This device initiates an alarm if lube oil temperature reaches or exceeds 160°F.

If lube oil temperature reaches 190°F, a shutdown signal is transmitted to the control system. Lube oil from the generator oil header is separated into two equal flows to the bearings. Oil flow into each bearing is controlled by an orifice in the supply line. ( Contd. )


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In the figure, each bearing has two temperature detector elements. These detectors are imbedded in the bearings to measure the temperature of the bearing metal.

The TEs initiate an alarm at 197°F and shut down at 203°F. These TEs may also signal the fire monitor/controller to discharge the fire extinguisher agent.


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Generator Cooling System: Purpose

The purpose of a generator cooling system is to cool the generator internally.

It accomplishes this cooling through an open or a closed cooling air circuit.


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Generator Cooling System: ComponentsThe main components of a generator cooling system are similar in purpose and function to those of an enclosure cooling system.

These components are:

· inlet filter· inlet duct· heavy duty axial flow fan(s)· internal air passages· exhaust ducts· heat exchangers (generators with closed air circuits)· temperature measuring devices


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Inlet Filters & Duct

Inlet filters are housed in the inlet duct. Their purpose is to prevent foreign particles from entering the generator.

During generator operation, an increase in both cooling air and stator temperatures could indicate generator overheating. The overheating may be caused by fire, clogged air filters, ventilation fan failure, dirty air inlet screens, or blocked air passages in the generator.

High temperature accompanied by vibration indicates the possibility of plugged rotor cooling air holes or broken shaft cooling fan.

If vibration is not indicated, dirty air inlet filters is probable.


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Heavy Duty Axial Flow Fans

The figure shows the location of the heavy duty axial flow fans. The fans are mounted on the rotor shaft and provide cooling air to the stator core.

Internal air passages and the exhaust duct are discussed next.


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Internal Air Passages & Exhaust Ducts

The figure shows the air flow through the internal passages and ducts. Note in the figure that the space behind the stator core is divided into six compartments. The first, third, fourth, and sixth compartments are open to the generator air exhaust ducts.

(Contd. )


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Internal Air Passages & Exhaust Ducts

Internal generator cooling involves cooling the stators and the rotors. The stator core has radial cooling ducts at intervals along the core. The radial inward flow of air over sections of the stator provides cooling airflow over the stator core.

The rotor is cooled by airflow under the rotor endcap, past the endwinding, and through axial cooling passages between the main winding slots.

(Contd.)


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Heat Exchanger

Some site conditions such as severe desert conditions, salty atmosphere, contaminated environment or hazardous area classification require a closed cooling air circuit.

In a closed cooling air circuit, the hot exhaust air from the generator is cooled in an exchanger loop and then returned to the generator.

(Contd.)


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Make-up cooling air for the generator's closed air circuit is provided by the pressurized generator enclosure.

Air cooling is accomplished by a water-to-air or air-to-air heat exchanger.


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Heat Exchanger

The heat exchanger is usually mounted in a steel housing on top of the generator, but it can be designed and positioned as needed.

A common arrangement places the heat exchanger on the roof of the generator enclosure with ducting to and from the heat exchanger.


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Resistance Temperature DetectorsGenerators include resistance temperature detectors (RTDs). These devices measure the following generator component temperatures:

· stator winding· enclosure· exhaust duct· bearings

RTDs in smaller generators only measure winding temperatures.

An RTD is similar to a thermocouple. Both devices measure temperature and are similar in appearance.

The main difference between an RTD and a thermocouple is that dissimilar metals in a thermocouple generate an electrical signal and an RTD requires an external source of DC power.


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Resistance Temperature DetectorsAn RTD consists of a resistance probe in a DC circuit. The probe contains a temperature-sensitive nickel wire that is wound around a mica core contained in a metal outer casing.

The probe is installed in a strategic location to monitor stator core temperature.

RTD temperature readouts are displayed at the gas turbine generator control Video Display Unit (VDU).

The control system initiates an alarm when the level of the signal from the RTD reaches the calibrated setpoint.


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Resistance Temperature DetectorsLarge generators usually have nine RTDs to monitor the stator core temperature. Three RTDs are installed at strategic locations in each of the three stator windings. Two of these RTDs are active, and one is a spare.

The small to medium size generators usually have three active RTDs and three spare RTDs. Temperature readout and protection are provided by the control system in the turbine control room.

(Contd. )


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In the system shown, the circuit provides an alarm when one of the stator RTDs senses stator temperature in excess of 270°F. The system initiates a shutdown when the stator RTD level of temperature increases to 290°F.


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Resistance Temperature Detectors

The generator air inlet and air outlet plenums contain separate RTDs to monitor generator air temperature.

The inlet air circuit initiates an alarm when the temperature reaches 115°F and a shutdown at 150°F.

The air outlet circuit activates an alarm at 200°F and shuts down at 220°F.


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Cooling System Operation

Generator cooling system operation is through either an open or a closed cooling air circuit.

Open cooling air circuits are usually used on small 750 KW generators up to 30,000 KW G.E. generators.

Cooling air is drawn into both ends of the generator.

(Contd.)


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Air enters the generator through two screens:

· one screen is located on the bottom of the exciter cover

· the other screen is located below the bearing on the drive end of the generator

The air is circulated by heavy-duty axial flow fans mounted on the generator shaft.


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The exciter and PMG are cooled by air flowing through the exciter cover. In the generator, air flows along the rotor, through rotor cooling slots, and through the stator core via ducts and air passages.

The airflow absorbs some of the heat that is produced by generator operation.

During generator operation, rotor and stator temperatures are designed for even distribution. After flowing through the generator the heated air is discharged to the atmosphere through the generator air exhaust duct.


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Large generators have cylindrical rotors with limited heat dissipation surface. Forced air ventilation through the generator must be used to remove the heat.

Generator heat must be constantly removed to prevent winding damage. Airflow through the generator picks up heat from the generator windings and gives up the heat to circulating water or air in the heat exchanger.

Cooling air is forced through the generator by two axial flow fans mounted on the rotor shaft. Aluminum alloy fan blades are set in slots in a steel ring.


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Some of the cool incoming air is ducted to the generator exciter, which is also equipped with an axial flow cooling fan.

A generator ventilation system is discussed next.


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Generator Ventilation System: Purpose

The purpose of a generator ventilation system is to pressurize the generator enclosure to prevent entry of any leaking fuel gas and to provide cooling airflow through the enclosure.


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Ventilation System: ComponentsThe main components of a generator ventilation system are similar in purpose and function to those of a turbine enclosure ventilation system.

These components are:· inlet air filter· air inlet duct or exhaust fans· air inlet duct with fire damper· air exhaust duct with fire damper· L.E.L. gas detectors and fire detectors


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Ventilation System: Operation

In the operation of a typical gas turbine/generator enclosure ventilation system, two generator enclosure vent fans draw air from the atmosphere through an inlet air filter.

Vent fan operation pressurizes the enclosure.

The figure shows a typical gas turbine/generator enclosure. (Contd.)


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Ventilating air circulates through both sides of the enclosure and exits to the atmosphere via exhaust ducts.

During normal gas turbine/generator operation only one of the ventilation fans is operating. The other one serves as a standby unit.


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Each fan is provided with a back flow damper and a flow switch to verify airflow to the enclosure.

If flow is lower than the setpoint, the control system will energize the standby ventilation fan and initiate an alarm.

The generator is equipped with air exhaust ducts with fire dampers. These dampers close if a fire occurs in the enclosure.

The fire extinguishing agent is introduced when the generator and gas turbine are tripped by a fire or gas detection.

The release of the extinguishing agent closes the dampers.


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Gas and fire detectors are mounted on the walls of the generator compartment.

Gas detectors are used to detect the presence of combustible gas at a preset L.E.L. (Lower Explosive Limit). When the preset limit is reached an alarm is initiated.

Fire detectors are used to detect a fire or overheating condition in the generator compartment. An alarm is initiated immediately when a fire or overheating condition is detected.


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Voltage Regulator: Purpose & Function

As discussed in a previous lesson, generator output voltage is determined by the voltage applied to the generator field.

Generator output voltage is controlled by the exciter, which is controlled by the voltage regulator.

( Contd.)

Lesson : 31 Generator Control


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The purpose of the voltage regulator is to match the output voltage of the main generator with the voltage of the distribution system.

The voltage regulator senses the output voltage of the generator and adjusts the voltage applied to the field of windings.


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Synchronization System: Purpose/Types

Another protection system of the main generator is the synchronization system.

The purpose of the synchronization system is to ensure that the generator output is in phase with that of the distribution system.

Synchronization is one of two types: manual or automatic.


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Synchronization System: Manual

Manual synchronization is done through monitoring the synch lamps and the synch scope. These devices allow the operator to match the frequency and the phase of the generator and the distribution system.

The operator matches the generator frequency and phase with that of the distribution system by adjusting the fuel flow (speed) of the turbine.

After matching the frequency and the phase, the operator ensures that the generator voltage is the same as the distribution system voltage. The main circuit breaker is then engaged.


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Synchronization System: Automatic

Automatic synchronization first compares the two frequencies. It then signals the fuel system to adjust the turbine speed to match distribution system frequency and phase.

After the generator and the distribution system are synchronized, automatic synchronization matches the voltage and initiates an automatic close signal to the main circuit breaker.

The generator voltage must match the system voltage to minimize electrical arcing across the circuit breaker.Electrical protective relays are discussed next.


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Electrical Protective Relays: Synch Check

Electrical protective relays protect generator circuits against high and low voltage and current. The protective relays discussed in this lesson are as follows:

· synch check· generator differential· reverse power· loss of excitation· generator time overcurrent (Contd.)


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. over/under frequency· overvoltage· undervoltage· overvoltage ground relay

Protective relays follow a standard numbering system that is used for all generators.

The synch check relay, number 25, confirms that the generator issynchronized with the distribution system. It prevents circuit breaker operation when the generator is not synchronized with the distribution system.


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Generator Differential Relay

The generator differential relay, number 86 or 87, protects the generator stator windings, conductors, and switchgear.

Relay 86 or 87 confirms that all of the current flowing into one end of the stator windings flows out of the other end. If not, a fault exists that allows current to flow to ground.

If a fault occurs, the 86 or 87 relay initiates a signal to trip the generator.


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Reverse Power RelayAnother protective relay is the reverse power relay, number 32.

The purpose of this relay is to prevent the generator from becoming a motor powered by the distribution system if the turbine shuts down and the generator is not disconnected from the distribution system.

Relay 32 senses the flow of current relative to the voltage. If the generator is motored, this relay trips the main circuit breaker and the gas turbine fuel shutoff valves.


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Loss of Excitation & Time OvercurrentThe next relay discussed is the loss of excitation relay, number 40. The purpose of this relay is to detect the loss of excitation to the generator and disconnect the generator from the system.

The generator time overcurrent relay, number 51V, protects against excessive current in the circuit. If an overcurrentcondition occurs, the 51V relay delays for several seconds based on the level of overcurrent to allow external circuit breakers time to operate. (Contd.)


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Without this relay, a faulty motor could shut down the distribution system.

This relay allows time for the motor's circuit breaker to trip first, allowing the generator to continue operation. If the overcurrent condition continues after the time delay, the 51V relay will trip the generator breaker.


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Over/Under Frequency & Voltage RelayThe over/under frequency relay, number 81, detects both over and underfrequency operation. These conditions indicate generator overspeed or underspeed.

Relay 81 is a backup for turbine underspeed and overspeed.

The overvoltage relay, number 59 and the undervoltage relay, number 27, protect the generator if the automatic voltage regulator fails.

Over/undervoltage conditions may occur routinely during normal operation. Therefore, these relays have a built-in time delay.


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Over Voltage Ground RelayThe overvoltage ground relay, number 59G, senses an overvoltage condition from the neutral leg to ground.

This condition can occur only if one or more of the three generator windings is leaking current to ground.

Vibration is a problem with all rotating equipment and can cause damage or equipment malfunction. Monitoring systems are used to detect vibrations, which is our next topic.


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Vibration Monitoring: PurposeVibration monitoring is important to preventing damage to the gas turbine generator.

The purpose of the vibration monitoring system is to detect vibration and shut down the generator if excessive vibration occurs. Protection against excessive vibration increases the life of the generator and reduces maintenance costs. (Contd.)


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The vibration monitoring system consists of vibration sensors and a vibration monitor.

The vibration sensors detect the vibration and relay this information to the vibration monitor.

The monitor initiates an alarm and a shutdown if the vibration exceeds preset levels.


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Gas Compression: Purposes

The purposes of gas compression are:

· store energy· reservoir maintenance· gas transmission· increase storage capabilities· liquid production· gas lift

We begin with storing energy.

Lesson : 32 Principle of Compression


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Store Energy

Air tools such as jackhammers and impact ratchets are operated by compressed air. These tools work by changing potential energy into kinetic energy.

When a gas is compressed and put in a "receiver tank", the potential energy is stored.

The figure shows potential energy and kinetic energy. The balloon is full of air under pressure and is not moving. This is an example of potential energy or energy at rest and stored energy.

Once the balloon is released, potential energy is changed to kinetic energy or energy in motion. The pressure and volume start decreasing, as the mass of the balloon and remaining air moves away.


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Reservoir Maintenance

Compressed gas is used in reservoir maintenance to keep sufficient gas pressure in the reservoir to maintain production levels.

A reduction in gas pressure occurs naturally in reservoirs. As oil is produced, gas in the reservoir has a larger space to occupy. This decreases the reservoir pressure and reduces oil flow to the surface.

(Contd. )


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Another way reservoir pressure is reduced is, as oil is extracted from the reservoir, gas molecules become entrained in the oil. When the oil and gas mixture is produced from the wellhead, the amount of gas in the reservoir is reduced and reservoir gas pressure begins to decline.


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Gas TransmissionCompressing a gas allows its transmission through a pipeline. The gas will always move from a higher pressure to the lower pressure.

The balloon mentioned previously contains air that is at a higher pressure than the outside air.

When the stem of the balloon is released, the higher pressure air inside moves to the lower pressure air outside.

If the stem of the balloon were attached to a tube with the opposite end opened to atmosphere, the air would move down the tube.


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Increased Storage Capacity

Compressing gases increases storage capabilities. When gases are compressed, their volume is decreased. This decrease in volume allows more scf(standard cubic feet) of gas to be stored in the same size storage tank.


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Production of Liquids

Compressing gases can also produce liquids. Normally, oxygen is a gas. The molecules are relatively far apart.

Compressing the oxygen forces the molecules closer together. The oxygen will become a liquid if compressed to a high enough pressure. Many gases will liquify if they are compressed to a high enough pressure.


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Gas LiftCompressed gas is also used in gas lift. Compressed gas is sent down into the earth to force oil to the surface.

The gas carries the crude to the surface where it is then separated.

Gas injection increases the gas specific density which allows it to move to the surface at the same downhole pressure.

The next section of this lesson explains the operating principle of centrifugal gas compressors.


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Operating PrincipleCentrifugal compressors operate on the principle of centrifugal force. This principle is illustrated in the figure.

When a ball on a string is swung around in a circle, a centrifugal force requires that the string be held or the ball will fly off.

If a long string is used and is slowly let out, the circle gets bigger and bigger.

When gas in a compressor is forced to move in a circle, centrifugal force causes it to move away from the center of the circle at high speed.


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Centrifugal Compressor: Components

The centrifugal compressor changes the kinetic energy of the moving gas into potential energy by forcing the gas to slow down, which increases the pressure.

Centrifugal compressors are generally used in applications that involve large volumes of gas with compression ratios up to 3.5.

(Contd.)


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The figure shows the gas flow in a basic single-stage centrifugal compressor. The components of a gas compressor include:

· eye

· impeller

· volute


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Eye, Impeller, & Volute

Gas enters the eye of the impeller and travels outward as a result of centrifugal force developed by the spinning impeller.

The gas is forced into the volute, which collects the gas and directs the flow to the compressor discharge nozzle.

The gas begins to slow as it moves from the impeller to the volute.

An important factor in the operation of a centrifugal compressor is compression ratio, which is discussed next.


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Compression RatioThe figure shows the pressure rise across the compressor.

The discharge pressure is equal to the suction pressure plus the pressure rise across the compressor stage. If the suction pressure is increased, the discharge pressure also increases.

The ratio of the discharge pressure to the suction pressure is the compression ratio.

(The discharge pressure divided by the suction pressure).


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Compressor Drivers

The compressor requires a driver to spin the impeller.

The driver can be a gas turbine, steam turbine, gas or diesel engine, or an electric motor. The figure shows a typical gas turbine-driven compressor set.

The next section of this lesson explains multistage centrifugal compressor operation.


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Compressor Operation

As mentioned previously, the discharge pressure is equal to the suction pressure plus the pressure increase across the compressor stage.

The pressure can be increased significantly if the discharge from one compressor is fed to the inlet of another compressor.

The discharge pressure of the second stage equals the suction pressure of the first stage plus the pressure rise across both stages. This is an example of tandem compressors.


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Compressor Operation

Multistage compressors use multiple impellers to achieve a greater pressure increase than is possible using a single impeller.

A four-stage centrifugal compressor is shown in the figure. This compressor has four impellers. Each impeller in the compressor is known as a compression stage.

The impellers are separated by diaphragms. Diaphragms direct the flow from the outlet of the impeller to the eye of the next impeller.


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Lesson : 33 Compressor Components Compressor Components & OperationThe components described in this lesson are:· casing· impeller· shaft · bearings· seals· guide vanes· diffusers· couplingsThe compressor casing is discussed first.


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Compressor Casing

The purpose of a compressor casing is to house the internal parts, hold the pressure created by the impeller, and direct the flow toward the discharge. The figure shows a single-stage compressor.

The rotating impeller accelerates the gas, which is then directed to the gas outlet nozzle.


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Vertically Split Casing

To allow access for maintenance, the compressor casing is split either vertically or horizontally.

In a vertically split casing, the bolts holding the end cover to the casing are in a vertical line.

The vertically split casing is called the barrel and the internal components are called the bundle.


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One advantage of a vertically split casing is that the seal formed by the end cover and casing is not subject to high pressure. Therefore, vertically split casings are used for high pressure applications.

Another advantage of a vertically split casing is that it can withstand higher temperatures. The casing expands uniformly when it is exposed to the heat of compression, which reduces the stress on the case.


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Horizontally Split Casing

A casing can also be split horizontally. An advantage of this design is easier maintenance. The internal components are easily accessible when the top half of the casing is removed.

Because the seal on the horizontally split casing is subject to high pressure, it is not as suitable for high pressure applications.


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ImpellersThe purpose of impellers is to increase the velocity of the incoming gas, which is then converted into an increase in pressure and temperature. This increase in velocity occurs as the impeller rotates.

The gas enters the center of the impeller, known as the eye. It is accelerated outward and collected in the volute. The pressure increase results from decreasing the volume of the gas.


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Impeller DesignsImpellers are classified by their design and manufacturing method.

Impellers are open, semi-closed, or closed.

Some types of impellers are shown in the figure.

Impellers are dynamically spin balanced to reduce vibration.

Balancing RPM varies with the size of the impeller.


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Compressor Shaft & Rotor AssemblyThe compressor shaft is machined to tight tolerances and then dynamically balanced. Various components are added to the shaft, and the complete unit is called the rotor assembly.

The illustration shows a typical seven-stage rotor assembly. The rotor must be balanced at each stage of completion to prevent vibration. Vibration during operation can cause severe damage to the compressor.

Rotor shaft components include:· sleeves· impellers· balancing drum or piston

Sleeves are fitted onto the shaft. Impellers are attached to the sleeves to prevent wear to the shaft. This arrangement also allows removal of the impeller without damaging the shaft.


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Mounting Methods

Various methods are used to attach impellers to the sleeves. One method uses a key that prevents the impeller from turning independently of the shaft.

Another method is called shrink fitting. With this method, the impeller is gradually heated and allowed to expand. The impeller is then slipped over the sleeve on the shaft and allowed to cool. Once cool, it contracts and forms a tight friction fit onto the sleeve. This same procedure is used for fitting the sleeves to the shaft.


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Rotor Thrust

Compressors increase the pressure of the gas. As a result, the outlet pressure of each stage is higher than the inlet pressure. These forces produce a thrust on the rotor. The resulting force tends to move the rotor toward the lower suction.


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Balancing Drum

A balancing drum is used to compensate for the thrust. A balancing drum is a solid metal cylinder with a labyrinth seal that is attached to the rotor shaft behind the last impeller.

High pressure discharge gas exists on a very small area on one side of the drum. Suction pressure is piped to the full face area on the other side. This arrangement produces a counteracting force that reduces the total thrust.


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Load Bearings

Two types of bearings are used in compressors: load and thrust bearings. Load bearings, also called main or journal bearings, support the weight of the rotor and prevent radial movement.

The load bearings have no rotating elements. The lubrication system provides a film of oil between the bearing and the shaft. This oil film prevents metal-to-metal contact and reduces wear.


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Thrust Bearings

Thrust bearings prevent axial movement of the shaft caused by thrust forces.

A thrust collar is fitted to the shaft and rotates with the shaft. The thrust bearing is stationary with the casing.

Pads tilt on the leveling plates. The leveling plates tilt-a result of lube oil hydraulic forces caused by aligning the pads with the thrust collar pad. The plates ensure that every shoe carries an equal load.


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Seals

Seals prevent most of the gas from leaking into the atmosphere and leaking between stages. The types of seals discussed in this module include the following:

· labyrinth· segmented ring· mechanical· liquid film

(Contd.)


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The simplest type of seal is the labyrinth seal. The basic labyrinth seal has teeth machined into a metal ring attached to the casing. The labyrinth seal can also be an interlocking seal with teeth machined into the shaft and into the ring as shown in the figure.


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Labyrinth Seal: Buffer Gas System

Because all labyrinth seals leak, a buffer gas system is installed to direct the leak. The figure shows this system.

Buffer gas is injected at a higher pressure than the gas on the other side of the seal.

The buffer gas system directs the leaking process gas to a collection point instead of letting it leak into the atmosphere.


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Segmented Ring Seal

The segmented ring seal uses several segments of carbon held against the shaft by a compressive spring band. Carbon is used because it does not readily wear the shaft.

Several rings are combined to provide the seal.

Because different gases have different properties, the number of rings required varies.


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Mechanical Seals

Mechanical seals rely on contact to provide sealing.

One element of the seal is stationary, (stationary face) and the other moves with the rotor (rotating face).

To reduce wear, a carbon ring is placed between the two metal surfaces.

Lubrication is provided to reduce the friction.


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Liquid Film SealThe liquid film seal is common in large compressors.

The liquid film seal is similar to the mechanical seal except that space is purposely left between the seal faces.

Oil at a slightly higher pressure than the gas is forced between the seal faces to prevent gas from escaping to the atmosphere.

Guide vanes and diaphragms are discussed next.


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Guide VanesThe purpose of guide vanes is to re-direct the flow of gas entering the compressor and through the compressor.

An example of an adjustable inlet guide vane is shown in the figure.

Inlet guide vanes direct the incoming gas in the desired direction of flow.

The guide vanes for interstage impellers are not adjustable and are built into the diaphragm.

Interstage guide vanes direct the flow of gas into the next impeller eye.


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Multi-Stage Compressor

A multi-stage compressor has two or more impellers on a single shaft. The shaft includes sleeves, impellers, balance drum, etc., and is called a rotor. These are the parts that rotate inside the casing.

The figure shows a four-stage and a two-stage compressor. The stage components permit easy changing of parts. Stages can be removed or changed to meet changing operating requirements.

Impeller rotation and centrifugal forces maintain the gas flow through the compressor.


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Compressor Surge

A condition called surge occurs when the flow of gas being discharged is less than the gas flowing into the suction.

This condition causes areas of high pressure in the compressor where the gas flow stops.

Severe damage can occur if this condition is allowed to continue.


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Diaphragms

The impellers on the rotor shaft are separated by diaphragms.

Diaphragms divide the individual stages in a multistage compressor.

Diaphragms convert velocity to pressure and are sometimes referred to as diffusers. Additionally, diaphragms redirect the gas flow evenly into the following impeller suction eye.

Couplings are the last of the compressor components to be discussed.


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Couplings: Purpose

The purpose of couplings is to connect the driver to the compressor. Couplings are also used to connect gearboxes to the driver and to the compressor.

Couplings provide a flexible connection to reduce compressor and driver vibration and wear.


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Couplings: Types

Three types of couplings are discussed in this section:

· spring grid coupling

· gear coupling

· flexible disc coupling


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Spring Grid Coupling

The figure shows a spring grid coupling.

The inner hubs are connected by a spring that fits in slots on each inner hub.

The outer hub contains lubrication to reduce friction and to keep the internal components clean.


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Gear & Flexible Disc CouplingLike the spring grid coupling, the gear coupling has two inner hubs and two outer hubs.

The coupling is usually connected to the driver and compressor by a keyway connection.

The flexible disc coupling has two hubs. One hub attaches to the driver, and the other hub attaches to the compressor.

The flexible discs are situated between the hub and the coupling torque tube.


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Lesson : 34 Compressor Lube Oil Lube Oil System: Purposes

The purposes of compressor lubrication are to:

· reduce friction between moving parts

· remove heat generated by friction

· maintain cleanliness by removing contaminants


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Primary Purpose

The primary purpose of the compressor lube oil system is to reduce friction between moving parts. It does this by providing a film of oil between the moving parts.

As long as the oil film is unbroken, any friction in the compressor is fluid friction instead of metal-to-metal friction.

For example, oil pressure lifts the journal of a shaft off the bearing on which it is resting. As the shaft rotates, a layer of oil prevents the journal from touching the bearing.


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Lube Oil System: Components

There are five main components of the lubrication system:

· reservoir· pumps· coolers· filters· control system

The first component discussed is the reservoir.


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Reservoir

The purpose of the lube oil reservoir is to store lube oil and provide a reliable supply of lube oil to the lube oil pumps.


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Main & Auxiliary Pumps

The purpose of pumps is to provide pressure to force the oil through the coolers and filters and onto the moving surfaces. The lubrication system may include three pumps:

· auxiliary· main· emergency DC

Before operating the compressor, the moving surfaces must be oiled to prevent damage and wear during start-up.

An auxiliary pump provides the oil pressure before start-up. It helps the main pump maintain the oil pressure until the compressor is at operating speed.


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Auxiliary & Emergency PumpsThe auxiliary pump also supplies lube oil to remove heat and to provide lubrication during and after shutdown.

The emergency pump starts if the main and auxilary pumps fail. It provides the minimum amount of lube oil to the bearings as the turbine shuts down and cools.

Emergency pumps are driven by an alternate power source, separate from the main and auxiliary pumps.


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Oil Coolers

The purpose of oil coolers is to remove heat passed to the lube oil from the compressor.

The oil carries heat away from the compressor. The heat is removed from the oil when the oil is routed to the oil cooler.

Because heat can cause decomposition of the oil, oil temperatures must be carefully controlled by automatic temperature regulators.

Two coolers are generally provided so that maintenance can be done on one while the other is in service.


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Oil Filters

The purpose of oil filters is to remove particles that are suspended in the oil. As the oil circulates through the compressor, it collects foreign matter and carries it away with the oil that is returned to the lube oil reservoir.

Any foreign matter in the oil is removed by the filtration systems so only clean oil is circulated to the compressor.


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Filters & CoolersBackup lube oil filters and coolers are provided as redundancy. These are duplex systems.

The filters are equipped with either pressure gauges or differential pressure gauges to determine the pressure drop across them.

An excessive pressure drop indicates fouling. The fouled filter must be taken out of service and lube oil routed through the standby filter. Proper venting of the standby filter is required prior to placing it in service.


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Control SystemThe last area of the lube oil system is the control system. This system is discussed in greater detail in the next lesson.

The lube oil control system has various sensors that provide shutdown or alarms if the system is operating outside predetermined parameters.

The figure shows some of these sensors, including a low pressure switch, differential pressure across the filters, lube oil temperature, pressure switches, and lube oil reservoir level.The seal oil system is discussed next.


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Seal Oil System: Purpose

The purpose of the compressor seal oil system is to provide lubrication for seal surfaces and to prevent gas leakage.

Gas enters the suction end of the compressor at a required suction pressure and leaves the discharge end of the compressor at higher pressure.

(Contd.)


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Seals prevent gas from leaking in two places:

· into the lube oil system that lubricates the bearings

· out of the casing

In the first case, seals are located between the bearings and the impellers.

In the second case, seals are located between the bearings and the casing.


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Seal Oil System: ComponentsLike the lube oil system, the seal oil system also contains pumps, coolers, filters, and a control system.

These components perform the same functions in the seal oil system that they do in the lube oil system.

A typical seal oil system is shown in the figure.

A seal oil system provides seal oil at a pressure slightly higher than the gas pressure that is being sealed.


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Seal Oil System: Labyrinth Seal

A special labyrinth seal is used at the discharge end of the shaft. This seal allows some gas, called buffer gas, to leak. Leakage of buffer gas reduces the seal pressure.

Gas pressure outside the labyrinth seal is only slightly greater than the suction pressure. Leaked gas is piped to the seal area on the suction side of the compressor.

The same oil pressure is applied at the suction and discharge seals. This is called reference pressure.


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Seal Oil System: Head Tank

The compressor shaft is supported by bearings on each end. These bearings must be lubricated with clean lube oil.

Compressed gas must not be allowed to leak into the bearing lube oil. Therefore, the seal oil pressure must always be higher than the reference pressure.

( Contd.)


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A method used to ensure a higher seal oil pressure during an emergency shutdown is the head tank.

The head tank is pressurized by reference pressure during normaloperation. The head tank also contains seal oil at a certain level. This provides a liquid head and reference gas pressure.


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Seal Oil System: Degassing TankThe seal oil system allows mixing of seal oil and gas. The figure shows that some seal oil flows toward the gas and some flows away.

The uncontaminated oil flows back to the reservoir. The oil that leaks through the inner seal is mixed with the gas and drains to the drain pots. The contaminated seal oil must be separated from the gas in drain pots and degassing tank.


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Seal Oil System: Degassing Tank

A slight amount of oil leaks through the inner and the outer seals. The oil that leaks through the outer seal is eventually returned to the reservoir.

Some recovered seal oil is contaminated with gas making it unsuitable for reuse. This contaminated seal oil must first go to the degassing tank before returning to the reservoir.


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Lesson: 35 Compressor Cooling Control System: Purpose

The purpose of a compressor control system is to provide protection against malfunctions and control the following functions during compressor operation:

· equipment operation, such as oil temperatures, vibration levels, and oil pressures

· compressor performance, such as discharge pressure and flow rate

( Contd. )


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Control System: Level of ControlCompressor control systems vary from the simple to the complex. The levels of control in this lesson include:

· manual control· sequence control· process variable measurement and regulation· decision-making control· optimizing controlInformation about manual control is presented first


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Manual Control

Manual control is the simplest level of control. Manual control depends on the actions of the operator.

In this level of control the operator:

· sets valve positions· starts and stops pumps· makes all adjustments to the process

The next level of control is sequence control.


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Sequence Control

Sequence control uses an automated system and operating instructions. The instructions are listed in the order required to perform a certain task to reach a desired result.

The sequence may begin with the operator pushing a button in a remotely located control room.

( Contd.)


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Measurement & Regulation ControlA more complex level of control is process variable measurement and regulation.

At this level of control, process variables are measured and the system regulates the process based on these measurements.

Warning lights and audible alarms inform the operator when process variables have exceeded set limits.

This level of control has little operator interface for adjustments to optimize the process.

Information about the decision making level of control is covered next.


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Decision-Making Control

Decision-making control (PLC) adjusts the process and makes decisions based on compressor performance feedback.

Examples of this level of control are automatic start-up and shutdown of compressors based on header pressure.

( Contd. )


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Optimizing Control

Optimizing control is the most complex control. This control system measures the process performance and makes adjustments to reach an optimal level.

Optimizing control measures changes in process variables, predicts the resulting changes to the process, and adjusts the process to reach a predetermined level of performance. (Contd.)


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Control System: ComponentsControl system components and instruments include the following devices:

· sensing· monitoring· protective· sequencing· regulating· optimizing

Information about each component follows.


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Sensoring & Monitoring DevicesSensing devices are the most basic control system elements. The figure shows a partial list of common sensing devices.

These devices only provide information to the control systems. They do not make adjustments.

Sensing devices provide information to the monitoring devices.

Monitoring devices include gauges, lights, and distributed control systems. Monitoring devices provide detailed information about operating conditions


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Sensoring & Monitoring DevicesProtective devices include shutdown and alarm devices. If operating conditions are sensed to be at, or in excess of, an emergency shutdown setpoint, the equipment is automatically shut down.

Start-up and shutdowns must be done in a particular order to ensure safety of personnel and prevent equipment damage. Sequencing devices perform the necessary steps to accomplish start-up and shutdown.


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Protective & Sequencing DevicesRegulating devices receive information from sensing devices and adjust the process to achieve a certain setpoint.

Regulating devices such as the anti-surge controller make adjustments based only on the current measurement and do not predict trends, as does an optimizing system, which is discussed next. (Contd.)


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Optimizing devices are the most complex devices in a control system.

Optimizing systems can look at one variable or can be connected to a computer-based system that controls the entire process.

The programmable logic computer (PLC) is an optimizing device.

Compressor capacity control is discussed next.


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Regulating & Optimizing Devices

Compressor capacity control depends on variables such as the following:

· compression ratio· operating speed· percent of design flow· inlet guide vane adjustment

The figure shows a set of operating curves. ( Contd. )


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The left side of the graph shows the surge line. Compressor surge occurs when there is an insufficient flow of gas through the compressor for a given differential pressure or compression ratio.

The compressor will be damaged if it continuously operates in the surge region.


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Compressor Capacity Control

The right-hand side of the graph shows stonewall. Stonewall occurs when the speed of the gas is equal to the speed of sound.

At stonewall, the gas within the compressor is moving as fast as physically possible (critical flow).

Most centrifugal compressors have an operating curve similar to the figure.

( Contd.)


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The operating curves show compressor capacity and compression ratio at different compressor speeds.

Capacity varies with compressor discharge pressure at a constant suction pressure.

Compression ratio is used to control compressor capacity because it is related to both suction pressure and discharge pressure.


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Safe compressor operating limits are between the surge region on the left and a maximum capacity limitation (stonewall) on the right.

For the compressor operating point shown in the graph, only 80% of the design gas flow rate is available.

To use the graph, determine the percentage (%) speed and compression ratio, then move down to find the capacity. At this speed and ratio this is the expected throughput (80%).

The compressor will not operate at 100% speed, 100% compression ratio, and 80% capacity.


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As you recall, the compression ratio is the discharge pressure in psia divided by the suction pressure in psia.

When compression ratio increases, the discharge pressure increases for a set suction pressure and/or the suction pressure decreases for a set discharge pressure.


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The only way to operate the compressor at 100% compression ratio and 80% capacity is to slow the compressor to a speed between 95% and 100%.

Speed control is needed to hold a constant compression ratio at different gas flow rates, or differential pressure control is needed to hold a constant compression ratio at different speeds and flows.

At any given operating speed, the compression ratio depends on the amount of flow through the compressor.


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Centrifugal compressors are either constant speed or variable speed.

The figure shows various methods of adjusting capacity for both types of compressor.

The capacity of a constant speed compressor can be adjusted by throttling either the suction or the discharge. Throttling a valve creates a pressure drop.


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Compressor Capacity Control Methods

As seen in the graph, a reduction of flow causes an increase in the compression ratio.

To prevent this increase in compression ratio, the discharge pressure can be reduced or suction pressure can be increased to provide the desired condition using the throttling valve.


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Compressor Capacity Control MethodsCompressor capacity control can also be maintained through recycle. Recycle controls the capacity by providing enough recycled gas to achieve 100% of design flow.

For example, if the flow is only 68% of design, the recycle valve may open to allow the remaining 32% of design flow to recycle from discharge to the suction side of the compressor.

Note that the driver energy required is 100% of design, but the gas flow rate is 68%. As a result, the system efficiency is sufficiently decreased.


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Another method of controlling capacity is adjusting the inlet guide vanes. Inlet guide vanes direct gas flow to the first impeller.

The vanes can be moved manually or automatically to change gas flow.

Compressor capacity is reduced by moving the vanes to restrict the flow of gas into the first stage impeller.


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Variable speed compressors change the speed to control capacity.

Throttling is used in variable speed compressors when speed adjustment cannot accomplish the desired capacity.


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Control systems provide for either local or remote starting and stopping, automatically monitored operation, and automatic malfunction shutdown.

Sequence control uses an automated system to follow a set of instructions.

Control system elements include sensing devices, monitoring devices, protection devices, sequencing devices, regulating devices, and optimizing devices.

Compressor surge occurs when there is an insufficient flow of gas through the compressor for a given compression ratio.


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Fuel Gas Control System: Purpose

The purpose of a fuel gas control system is to provide required fuel flow and pressure to the engine. It uses specially designed components to accomplish this function.

A simplified view of the fuel gas control system is shown in the figure.

(Contd. )


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The diagram on the following page shows a complete fuel gas control system with the main components highlighted. Each of the highlighted components and its operation are discussed in the following pages.


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Fuel Gas Control System: ComponentsThe main components of the fuel gas control system are:

· strainer· gas supply pressure switch· stop/speed ratio valve assembly (stop/ratio valve and gas control valve)· fuel gas pressure transducer· fuel vent solenoid valve· four linear variable differential transformers (LVDT) position sensors· two electrohydraulic servo valves· three gas pressure gauges· Speedtronic controlsThe strainer is discussed first.


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Strainer

The purpose of the strainer is to remove foreign particles from the fuel gas before it enters the stop/speed ratio valve assembly.

A blowdown connection on the bottom of the strainer body is used for periodic cleaning of the strainer screen.

The next fuel gas control system component discussed is the supply pressure switch.


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Gas Supply Pressure Switch

The gas supply pressure switch is installed in the gas piping upstream of the stop/speed ratio valve.

This switch initiates an alarm when fuel gas pressure decreases below the setpoint.

Downstream of the gas supply pressure switch is the stop/speed ratio valve and gas control valve assembly. This assembly is discussed next.


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Stop/Speed Ratio & Gas Control Valve

The combination stop/speed ratio and gas control valve assembly contains the following independent valves:

· stop/speed ratio valve· gas control valve

Both of these valves are actuated by hydraulic pressure through servos which receive the signal from the Speedtronic control system (discussed later in this lesson). The stop/speed ratio valve has

two functions:· a stop valve· gas pressure regulation


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As a stop valve, the stop/speed ratio valve shuts off fuel gas flow during normal and emergency shutdowns.

The hydraulic dump valve, control oil, and servo valves control the action of the stop/speed ratio valve as a stop valve.

The hydraulic dump valve is located between the electrohydraulic servo valve and the actuating cylinder.

( Contd. )


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When control oil pressure is low, a spring moves an internal spool to the "dump" position.

When hydraulic pressure is removed, a closing spring on the stop/speed ratio valve plug closes the valve.

Fuel gas flow to the gas control valve and gas turbine is stopped.


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Date post:	11-Nov-2014
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