RAJGHAT POWER HOUSE

By:SAGAR CHANDELECTRICAL 5TH SEMESTERROLL NO.:322344CHHOTU RAM RURAL INSTITUTE OF TECHNOLOGYKANJHAWALA, NEW DELHI - 110081

2014

(FROM:-05 MAY 2014 TO 09 JUNE 2014)

ACKNOWLEDGEMENT

I would hereby like to express my profound sense of gratitude to G.M. (T)IPGCL & PPCL, Mr. Y.P. Arora for giving me the opportunity to carry out my

industrial training at Rajghat Power House under their experienced andhighly qualified staff. Equally thankful I am to Mr. Premraj Singh

(Manager,Electrical) for his invaluable guidance during the training period.

Thanks are due to Mr. Surendra Vishwakarma (Asst. Mgr., Electrical), Mr.Praveen Kumar (Asst. Mgr., Electrical), Mr. H.C. Pandey (Asst. Mgr., Electrical)

and Mr. R.K. Prajapati (Asst. Mgr., Cont. & Inst.) who despite of their busyschedule and workload, were able to find some time for us and imparted us

the knowledge that paved a way for better understanding of the fundamentalsand their applications alike.

I duly acknowledge the help, direct or indirect of the whole department andstaff members of the organization for providing all the facilities for the

training. The knowledge gained herein and the practical experiences learntwill be invaluable in the long run.

With day to day advantage of new technology the older machinery are beingreplaced by new machinery . Now it has not been the work of semi skilledpersons .It has opened a new horizon for degree holder engineers. But to dothe job properly a suitable training is needed.

The knowledge of entire system is must for an engineer to do thetrouble shooting in the quickest possible way so that the production does notget affected.

So for engineering the industrial training is playing a vital role indeveloping the practical knowledge. The industrial training is not merely anacademic requirement but a professional necessity too.

With the increasing demand and utilization of electricity an electricalengineer should be well versed or at least must be familiar with thegeneration ,transmission and distribution of electricity, at the same time mustbe capable of fault detection and elimination.

It is thus the responsibility of an electrical engineer to deal with thesophistication and make the maximum possible utilization.

PREFACE

S.NO

Title of Heading Page No.

01 About the company 06-06

02 Introduction of RAJGHAT POWER STATION 07-08

03SECTION-APOWER PLANT BASICS

09-09

04 Overview of thermal power plant 10-11

05 Steam 11-11

06 Rankine Cycle 12-13

07 Steam Turbine (Prime Over) 14-15

08 Impulse Turbine 15-16

09 Reaction Turbine 16-16

10 Turbo-Generator Arrangement 16-17

11 Bearings 17-18

12 Valves 18-18

13 The steam & water circuits 19-21

14 Deareator 22-22

15 Fuel air & Flue gas circuits 23-25

16 Generator 25-25

17Section-BControl & Instrumentation in Power Plant

26-26

18 Combustion and Draught Control 27-29

19 Fan Control 30-30

20 Flame Monitoring 31-32

21 Feed Water Control 33-37

22 Deaerator Control 37-38

23 Level Control 38-40

24 Steam Temperature Control 41-44

25 Turbine Control & Monitoring 45-48

26 LVDT 48-49

27 Accelerometer 49-50

CONTENTS

4

S.NO Title of Heading Page

No.

28 Velocity Transducers 50-51

29 Casing & Cylinder Expansion 52-52

30 505 Turbine Control 52-54

31 Communication 54-55

32 RTD 56-57

33 Thermocouple 57-58

34 Pressure Transducers 58-60

35 Level Measurement 60-62

36 Flow Meters 62-64

37 Automation of Process 65-65

38 PLC 66-70

39 Appendix-A 71-71

40 Appendix-B 72-72

41 Appendix-C 73-73

42 Conclusion 74-74

ABOUT THE COMPANYBeholding the grimness of power situation in Delhi and failure of all previous attempts in

restoring the quality of service to the citizens Delhi Electricity Board RegulatoryCommission (DERC) was constituted in May 1999 whose prime responsibility was to lookinto the entire gamut of existing activity and search for various ways of power sectorreforms. This was followed with a Tripartite Agreement which was signed by thegovernment of Delhi, DVB employees to ensure the cooperation of stakeholders in thisreform process. The tripartite agreement sent off very positive vibes to the people ingeneral as well as to the investor community about the sincere and hassle-free objectives ofpower reforms.

The Government of India on July 1, 2002, implemented the reforms by unbundling DVBinto six companies, one holding company, one generation company (GENCO), onetransmission company (TRANSCO) and three distribution companies (DISCOMS). Thegovernment handed over the management of the business of electricity distributions tothere private companies since July 1, 2002 with 51% equity with the private sector.

It was thus that IPGCL (GENCO) came into existence with the aim of meeting the powerdemands of a city which is the capital of one of the most populated countries of the worldand whose resources fall much below its demand and since then its contribution to thepower sector has been beyond the expectations as is evident from the current powersituation in the city. The following facts may summarize the success story of a never beforefundamental reform:

• 330 MW capacity Pragati Power Station was commissioned in the year 2002-03 andperforming excellently. Achieved 100.4% PLF during the month of Jan., 05 and88.27% PLF (Deemed PLF 95%) during the year 2004-05, Pragati PowerCorporation Ltd. paid dividends of Rs.17.5 Cr., for FY 2003-04 & Rs.14 Cr. for FY2004-05 as well as 2005-06.

• The performance of Indraprastha Gas Turbine Power Station which was 47.24% in2001-02 increased to 70.76% (deemed PLF 75.35%) in 2005-06. This is the bestperformance of the station in a year since its commissioning in 1985-86. The stationalso achieved highest ever generation in a day, 5.743 MU (84.86% PLF) on 26.12.05and highest ever generation in a month, 166.227 MU (79.23% PLF) in October,2005. The forced outages of the station have also reduced from 17.75 % to 5.2 %.• The overall performance of GENCO increased from 45.90% during the year 2001-02to 64.35% in 2005-06.

RAJGHAT POWER STATION

Two Units of 67.5 MW were installed in 1989-90 at Rajghat Power House as Replacementof old Units and the present generation capacity of this Station is 135 MW. Further, thestation has acquired the following certifications:

• ISO-9001:2000 for Quality Management• ISO-14001:2004 for Environment Management.• OHSAS-18001:2007 for Occupational Health & Safety Management

Rajghat Power House Specifications

1. Location: Rajghat

2. Type: Coal based thermal power plant

3. Installed Capacity: 2 x 67.5 MW = 135 MW

4. Land Detaila. Plant area: 40 acresb. Ash dump area: 24 acres

5. Cooling Watera. Source: River Yamuna

b. Cooling Method: Closed cycle with cooling tower

i.ii.

6. Fuel: Coala. Type: Bituminousb. Linked coal mines: Piparwarc. Gross calorific value: 3180-4280 kcal/kgd. Ash content: 35-42.2 %e. Sulphur content: 0.5 %f. Requirement

With design coal: 2000MT/dayWith actual coal: 2400MT/day

g. Stockyarda. Area: 72000 sq. mt.b. Capacity: 20000MT

7. Oila. Type: LSHS\LDOb. Nearest service outlet: Mathura-Shakurbasti oil depot

SECTION APower Plant Basics

1.OVERVIEW OF THERMAL POWERPLANT

Schematic of a thermal power plant

Power plants generate electrical power by using fuels like coal, oil or natural gas. A simplepower plant consists of a boiler, turbine, condenser and a pump. Fuel, burned in the boiler andsuperheater, heats the water to generate steam. The steam is then heated to a superheated state inthe superheater. This steam is used to rotate the turbine which powers the generator. Electricalenergy is generated when the generator windings rotate in a strong magnetic field. After thesteam leaves the turbine it is cooled to its liquid state in the condenser. The liquid is pressurizedby the pump prior to going back to the boiler.

Thus the main inputs required by a plant are:

• Coal: Should have high calorific value and low ash content• Water: De-mineralized water for steam generation• Air

In this chapter first steam and its properties are described which are essential to theunderstanding of the underlying principle of the thermal plant, the Rankine cycle which is

presented next.

SteamSteam power is fundamental to what is by far the largest sector of the electricity-generating

industry and without it the face of contemporary society would be dramatically different from itspresent one. We would be forced to rely on hydro-electric power plant, windmills, batteries, solarcells and fuel cells, all of which are capable of producing only a fraction of the electricity we use.

Steam is produced by boiling of water and it is achieved at atmospheric pressure at 100 degreeCelsius. Let us consider a quantity of water that is contained in an open vessel. Here, the air that

blankets the surface exerts a pressure on the surface of the fluid and, as the temperature of thewater is raised, enough energy is eventually gained to overcome the blanketing effect of that

pressure and the water starts to change its state into that of a vapour (steam). If the pressure ofthe air blanket on top of the water were to be increased, more energy would have to be

introduced to the water to enable it to break free. In other words, the temperature must be raisedfurther to make it boil. To illustrate this point, if the pressure is increased by 10% above itsnormal atmospheric value, the temperature of the water must be raised to just above 102 °C

before boiling occurs.The information relating to steam at any combination of temperature, pressure and other factors

may be found in steam tables, which are nowadays available in software as well as in the moretraditional paper form. These tables were originally published in 1915 by Hugh Longbourne

Callendar (1863-1930), a British physicist. Because of advances in knowledge and measurementtechnology, and as a result of changing units of measurement, many different variants of steamtables are today in existence, but they all enable one to look up, for any pressure, the saturation

temperature, the heat per unit mass of fluid, the specific volume etc.Steam becomes superheated when its temperature is raised above the saturation temperaturecorresponding to its pressure. This is achieved by collecting it from the vessel in which the

boiling is occurring, leading it away from the liquid through a pipe, and then adding more heat toit. This process adds further energy to the fluid, which improves the efficiency of the conversion

of heat to electricity.As stated earlier, heat added once the water has started to boil does not cause any further

detectable change in temperature. Instead it changes the state of the fluid. Once the steam hasformed, heat added to it contributes to the total heat of the vapour. This is the sensible heat plus

the latent heat plus the heat used in increasing the temperature of each kilogram of the fluidthrough the number of degrees of superheat to which it has been raised. In a power plant, a major

objective is the conversion of energy locked up in the input fuel into either usable heat orelectricity. In the interests of economics and the environment it is important to obtain the highest

possible level of efficiency in this conversion process.

\

Rankine Cycle: The Working Principle

The Carnot cycle postulates a cylinder with perfectly insulating walls and a head which can beswitched at will from being a conductor to being an insulator. Even with modifications to enableit to operate in a world where such things are not obtainable, it would have probably remained ascientific concept with no practical application, had not a Scottish professor of engineering,William Rankine, proposed a modification to it at the beginning of the twentieth century. Theconcepts that Rankine developed form the basis of all thermal power plants in use today. Eventoday¶s combined-cycle power plants use his cycle for one of the two phases of their operation.

The Rankine cycle in a steam-turbine power plant

In the system shown in figure, water is heated in feed heaters (A to B) using steam extractedfrom the turbine. Within the boiler itself, heat is used to further pre-warm the water (in theeconomiser) before it enters the evaporative stages (C) where it boils. At D superheat is addeduntil the conditions at E are reached at the turbine inlet. The steam expands in the turbine to theconditions at point F, after which it is condensed and returned to the feed heater. The energy in

the steam leaving the boiler is converted to mechanical energy in the turbine, which then spinsthe generator to produce electricity. The diagram shows that the energy delivered to the turbine ismaximised if point E is at the highest possible value and F is at the lowest possible value.

THERMAL EFFICIENCY

2. TEAM TURBINE (PRIME MOVER)

A steam turbine at Rajghat Power station

2.1 Raising steam

Steam is mostly raised from fossil fuel sources, mostly coal but also oil and gas, in a combustionchamber. Recently these fuels have been supplemented by limited amounts of renewable biofuelsand agricultural waste.

The chemical process of burning the fuel releases heat by the chemical transformation(oxidation) of the fuel. This can never be perfect. There will be losses due to impurities in thefuel, incomplete combustion and heat and percentage of the available energy in the fuel.

2.2 Working Principles

High pressure steam is fed to the turbine and passes along the machine axis through multiplerows of alternately fixed and moving blades. From the steam inlet port of the turbine towards the

exhaust point, the blades and the turbine cavity are progressively larger to allow for theexpansion of the steam.

The stationary blades act as nozzles in which the steam expands and emerges at an increasedspeed but lower pressure. (Bernoulli's conservation of energy principle - Kinetic energy increasesas pressure energy falls). As the steam impacts on the moving blades it imparts some of its

kinetic energy to the moving blades.

There are two basic steam turbine types, impulse turbines and reaction turbines, whose blades aredesigned control the speed, direction and pressure of the steam as is passes through the turbine.

2.2.1 Impulse Turbines

The steam jets are directed at the turbine's bucket shaped rotor blades where the pressure exertedby the jets causes the rotor to rotate and the velocity of the steam to reduce as it imparts itskinetic energy to the blades. The blades in turn change the direction of flow of the steamhowever its pressure remains constant as it passes through the rotor blades since the cross section

of the chamber between the blades is constant. Impulse turbines are therefore also known asconstant pressure turbines.

The next series of fixed blades reverses the direction of the steam before it passes to the secondrow of moving blades.

2.2.2 Reaction Turbines

The rotor blades of the reaction turbine are shaped more like aerofoils, arranged such that thecross section of the chambers formed between the fixed blades diminishes from the inlet sidetowards the exhaust side of the blades. The chambers between the rotor blades essentially formnozzles so that as the steam progresses through the chambers its velocity increases while at thesame time its pressure decreases, just as in the nozzles formed by the fixed blades. Thus thepressure decreases in both the fixed and moving blades. As the steam emerges in a jet frombetween the rotor blades, it creates a reactive force on the blades which in turn creates theturning moment on the turbine rotor, just as in Hero's steam engine. (Newton's Third Law - Forevery action there is an equal and opposite reaction).

The turbine employed at the station comprises of 47 stages one of which is impulse and the restare reaction stages.

2.3 Turbine-Generator arrangement

The plant employs a 2-pole synchronous generator with a synchronous speed of 3000 rpm. Theoutput frequency is determined by the relation

where N is the rotor speed in RPMf is the output frequency of generatorP is the number of poles of generator

The generator is air cooled and the bearings are cooled by oil which is supplied by various oilpumps EOP, JOP, BOP etc. The detailed description of the generator unit is provided inAppendix-C. The following figure shows the arrangement of turbine and generator set.

Overall turbine and generator arrangement

2.3.1 Bearings

Steam turbines are provided with journal bearings and thrust bearings. Journal bearings are ateach end of each rotor to support the weight of the rotor. One thrust bearing is provided for theentire steam turbine to maintain the axial position of the rotor.

Journal Bearings. The journal bearings are constructed of two halves that enclose the shaft. Theinside of the bearing adjacent to the shaft is lined with babbitt metal. Babbitt is an alloy of tin,copper, and antimony that has antiseizing qualities and a natural oiliness. The journal bearingsare oil-pressure lubricated. Oil flow is controlled to limit oil temperature rise to a set value.

Thrust Bearings. The thrust bearing consists of babbitt metal lined, stationary shoes that runagainst the rotor thrust runner. Shoes on both sides of the runner prevent movement in either

axial direction. The thrust bearing compartment is oil-pressure flooded with the oil introducednear the shaft and flowing outward by the centrifugal action of the runner. Oil flow is controlledto limit oil temperature rise to a set value.

2.3.2 Valves

Major control valves associated with the steam turbine and their operations are as follows:

Main Steam Stop (Throttle) Valves: The steam from the steam generator flows to the main

steam stop or throttling valves. The primary function of the stop valves is to provide backupprotection for the steam turbine during turbine generator trips in the event the main steam controlvalves do not close. The energy in the main steam and steam generator can quickly cause theturbine to reach destructive overspeed on loss of the generator load. The main steam stop valvesclose from full open to full closed in 0.15 to 0.5 s. The stop valves are also closed on unit normalshutdown after the control valves have closed.A secondary function of the stop valves is to provide steam throttling control during startup. Thestop valves have internal bypass valves that allow throttling control of the steam from initialturbine roll to loads of 15% to 25%. During this startup time, the main steam control valves arewide open and the bypass valves are used to control the steam flow.

Main Steam Control (Governor) Valves: The steam flows from the stop valves to the mainsteam control or governor valves. The primary function of the control valves is to regulate thesteam flow to the turbine and thus control the power output of the steam turbine generator. Thecontrol valves also serve as the primary shutoff of the steam to the turbine on unit normalshutdowns and trips.

3. THE STEAM AND WATER CIRCUITS

3.1 Steam Generation and use

The steam generation occurs in banks of tubes that are exposed to the radiant heat of combustion.The steam leaves the drum and enters a bank of tubes where more heat is taken from the gasesand added to the steam, superheating it before it is fed to the turbine. The superheater, comprisesa single bank of tubes but in many cases multiple stages of superheater tubes are suspended inthe gas stream, each abstracting additional heat from the exhaust gases. In boilers (rather thanHRSGs), some of these tube banks are exposed to the radiant heat of combustion and aretherefore referred to as the radiant superheater. Others, the convection stages, are shielded fromthe radiant energy but extract heat from the hot gases of combustion.

Schematic of a boiler

After the flue gases have left the superheater they pass over a third set of tubes (called theeconomizer), where almost all of their remaining heat is extracted to pre-warm the water before

it enters the drum.Finally the last of the heat in the gases is used to warm the air that is to be used in the process ofburning the fuel. The major moving items of machinery shown in the diagram are the feed pump,which delivers water to the system, and the fan which provides the air needed for combustion ofthe fuel (in most plants each of these is duplicated). In a combined-cycle plant the place of thecombustion-air fan and the fuel firing system is taken by the gas turbine exhaust.

3.2 Feed water-condensate cycle

Inside the plant, the steam and water system forms a closed loop, with the water leaving thecondenser being fed back to the feed pumps for reuse in the boiler. However, certain other itemsof plant now become involved, because the water leaving the condenser is cold and containsentrained air which must be removed.

Air becomes entrained in the water system at start-up (when the various vessels are initiallyempty), and it will appear during normal operation when it leaks in at those parts of the cyclewhich operate below atmospheric pressure, such as the condenser, extraction pumps and lowpressure feed heaters. Leakage can occur in these areas at flanges and at the sealing glands of therotating shafts of pumps. Air entrainment is aided by two facts: one is that cold water can holdgreater amounts of oxygen (and other dissolved gases) than can warm water; and the other is thatthe low-pressure parts of the cycle must necessarily correspond with the low temperature phases.

Steam and Water circuit

3.3 The Deareator

The deaerator removes dissolved gases by vigorously boiling the water and agitating it, a processreferred to as 'stripping'. The water entering at the top is mixed with steam which is risingupwards. The steam, taken directly from the boiler or from an extraction point on the turbine,heats a stack of metal trays and as the water cascades down past these it mixes with the steamand becomes agitated, releasing the entrained gases. The steam pressurizes the deaerator and itscontents so that the dissolved gases are vented to the atmosphere.

Minimizing corrosion requires the feed-water oxygen concentration to be maintained below0.005 ppm or less and although the deaerator provides an effective method of removing the bulkof entrained gases it cannot reduce the concentration below about 0.007 ppm. For this reason,scavenging chemicals are added to remove the last traces of oxygen.

Principle of a deaerator

4. THE FUEL, AIR AND FLUE-GAS CIRCUITS

4.1 Corner Fired Boiler: The plant employs corner fired (tangential) boilers in which the

burning fuel circulates around the furnace, forming a large swirling ball of burning fuel at thecentre. The burners are placed at five equidistant levels (named A, B, C, D and E) along theheight of boiler on all four corners. A certain degree of tilt can be provided to the burners todirect the fireball to a higher or lower position within the furnace, and this has a significant effecton the temperature of the various banks of superheater tubes, and therefore on steamtemperature.

4.2 Air Heater: It works as a heat exchanger and transfers the heat remaining in the exhaust

gases to the air being fed to the furnace. The reuse of heat in the exhaust gases results inimprovement of efficiency of the plant. However, the exchanger suffers from leakage of heat andperfect transfer of heat cannot be obtained.

Draught-plant arrangement

4.3 Pressurized Bowl Mill: The coal that is discharged from the storage hoppers is fed down a

central chute onto a table where it is crushed by a grinding rotor assembly and is ground down toa very fine powder (called pulverized fuel (PF)). Air is blown into the crushed coal and carries it,via adjustable classifier blades, to the PF pipes that transport it to the burners. The air that carriesthe fine particles of coal to the burners is supplied from a fan called a 'primary-air fan'. Thisdelivers air to the mill, which therefore operates under a pressure which is slightly positive with

respect to the atmosphere outside.

Pressurized bowl mill

For supplying the air to the mill, cool air and heated air are mixed to achieve the desiredtemperature. This temperature has to be high enough to partially dry the coal, but it must not beso high that the coal could overheat (with the risk of the coal/air mixture igniting inside the mill

or even exploding while it is being crushed). The warm air is then fed to the mill (or a group ofmills) by means of yet another fan, called a 'primary air fan'. It should be noted that the cooler ofthe two air streams is commonly referred to as 'tempering air' since, because it is obtained fromthe FD fan exhaust it may already be slightly warm, and its function is to temper the mixture.

GENERATOR

A steam turbine generator

In electricity generation, an electrical generator is a device that converts mechanical energyto electrical energy, generally using electromagnetic induction. The reverse conversion of electricalenergy into mechanical energy is done by a motor, and motors and generators have many similarities. Agenerator forces electric charges to move through an external electrical circuit, but it does not createelectricity or charge, which is already present in the wire of its windings. It is somewhat analogous to a

water pump, which creates a flow of water but does not create the water inside. The source of mechanicalenergy may be a reciprocating engine, a steam or gas turbine, water falling through a turbine orwaterwheel, an internal combustion engine, a wind turbine, a hand crank, the sun or solar energy,

compressed air or any other source of mechanical energy.

SECTION BControl & instrumentation in Power Plant

For the power plant to run and produce electricity, the equipment in each category must beplaced into operation to perform its intended function. The operation of the equipment must becoordinated to meet the demand of the electricity production process, and the production processmust be regulated so that the cycle and the equipment are operated within design conditions. Theplant control systems provide the necessary tools to enable the operators to orchestrate plantoperation for the reliable and efficient production of electricity. This section provides anoverview of the plant control system, its functions, and the type of control equipment used in theRajghat Power House.

5. COMBUSTION AND DRAUGHTCONTROL

In a fired boiler the control of combustion is extremely critical. In order to maximize operationalefficiency combustion must be accurate, so that the fuel is consumed at a rate that exactlymatches the demand for steam, and it must be executed safely, so that the energy is releasedwithout risk to plant, personnel or environment keeping in mind that the amount of energyinvolved in a power plant is considerable: in each second of its operation a large boiler releasesaround a billion joules, and in a process of this scale the results of an error can be catastrophic.

5.1 Load control strategy for pressurized mills

The approach is to provide closed-loop control of the primary-air flow, as shown. Here, becausethe system detects and immediately reacts to changes in PA flow, and adjusts the flow-controldamper to compensate, disturbances to steam production are minimized. Again, a feeder-speedsignal, representing fuel flow, is fed back to the master system to provide closed-loop correctionof speed changes, which would otherwise introduce disturbances to the steam pressure.

Closed loop control of PA flow

5.2 Mill temperature control

Control objective: It is very important that the temperature of the air in the mill should bemaintained within close limits. For many reasons, including inadequate drying of the coal,combustion efficiency will be reduced if the temperature is too low, while too high a temperature

can result in fires or explosions occurring in the mill.

Control Technique: The control technique involves mixing hot and cold air streams to achievethe correct temperature. Pressurized coal mills require the use of two dampers for this purposeone controlling the flow of hot air, the other the cold air.

Mill Temperature Control

5.3 Draught control

In a fired boiler, the air required for combustion is provided by one or more fans and the exhaustgases are drawn out of the combustion chamber by an additional fan or set of fans. The control ofall these fans must ensure that an adequate supply of air is available for the combustion of thefuel and that the combustion chamber operates at the pressure determined by the boiler designer.All of the fans also have to contribute to the provision of another important function--purging ofthe furnace in all conditions when a collection of unburned fuel or combustible gases couldotherwise be accidentally ignited. Such operations are required prior to light-off of the firstburner when the boiler is being started, or after a trip.

5.3.1 Maintaining the furnace draught

Apart from supplying air to support combustion, the FD fans have to operate in concert with theID fans to maintain the furnace pressure at a certain value. The heavy solid line of figure belowshows the pressure profile through the various sections of a typical balanced-draught boilersystem. It shows the pressure from the point where air is drawn in, to the point where the fluegases are exhausted to the chimney, and demonstrates how the combustion chamber operates at aslightly negative pressure, which is maintained by keeping the FD and ID fans in balance witheach other. If that balance is disturbed the results can be extremely serious. Such an imbalancecan be brought about by the accidental closure of a damper or by the sudden loss of all flames. Itcan also be caused by mal-operation of the FD and ID fans. The dashed line on the diagramshows the pressure profile under such a condition, which known as an 'implosion'. The results ofan implosion are extremely serious because, even though the pressures involved may be small,the surfaces over which they are applied are very large and the forces exerted become enormous.Such an event would almost certainly result in major structural damage to the plant.

Draught profile of a boiler and its auxiliary plant

5.3.2 Fan control

The throughput of two fans operating together can be regulated by a common controller or byindividual controllers for each fan. Although a single controller cannot ensure that each fandelivers the same flow as its partner, this configuration is much simpler to tune than thealternative, where the two controllers can interact with each other and make optimizationextremely difficult.

Controlling the windbox pressure

5.4 Binary control of the combustion system

So far, we have considered only the modulating systems involved with the combustion plant. Inpractice, these systems have to operate in concert with binary control systems such as interlocksand sequences.

The purpose of an interlock is to co-ordinate the operation of different, but interrelated plantitems: tripping one set of fans if another set trips, and so on. The purpose of a sequence system isto provide automatic start-up or shutdown of the plant, or of some part of it.

5.4.1 Flame monitoring

Monitoring the status of a flame is not easy. The detector must be able to discriminate betweenthe flame that it is meant to observe and any other in the vicinity, and between that flame and thehot surfaces within the furnace. The detector must also be able to provide reliable detection inthe presence of the smoke and steam that may be swirling around the flame. To add to theproblems, the detector will be required to operate in the hot and dirty environment of the burner

front, and it will be subjected to additional heat radiated from the furnace into which it islooking.

With their attendant BMS¶s, flame scanners of a boiler are vital to the safety and protection ofthe plant. If insufficient attention is paid to their selection, or if they are badly installed orcommissioned, or if their maintenance is neglected, the results can be, at best, annoying. Theproblems will include nuisance trips, protracted start-up of the boiler and the creation ofhazardous conditions that could have serious safety implications.

A flame scanner is a complex opto-electronic assembly, and modern scanners incorporatesophisticated technologies to improve flame recognition and discrimination. Although theelectronics assembly will be designed to operate at a high temperature (typically 65 °C), unlessgreat care is taken this value could easily be exceeded and it is therefore important to take allpossible precautions to reduce heat conduction and radiation onto the electronic components.

Typical flame scanner

5.4.2 The requirements for purge air

The purge air that is supplied to the scanner serves two purposes:

1. it provides a degree of cooling and2. it prevents dust, oil and soot from being deposited on the optical parts of the unit.

The air should be available at each burner, even if the burner itself is not operating. It shouldtherefore be obvious that the air used for purging should be cool, dry and clean, and that it shouldbe available at all times. Purge air can be obtained from the instrument-air supply, or it can beprovided by dedicated blowers. In some cases it is taken from the FD fan discharge. Each of

these is viable, provided the requirements outlined above have been thoroughly considered. It isalso important that the presence of the purge-air supply should be monitored and its losstransmitted to the DCS, because failure of the air supply could result in expensive and possiblyirreparable damage to the scanners. Modern scanners include self-monitoring circuits that willwarn of overheating. The scanner system should be fail-safe, as a failed system represents theloss of a critical link in the plant's safety chain. If it is overridden, the operator can become usedto operating without it in place, and such lapses can eventually create a severe hazard.

6. FEED-WATER CONTROL

6.1 Controlling the flow

Control Objective: To supply enough water to the boiler to match the evaporation rate for thewidest practical range of operation in a safe and cost-effective manner.

Difficulties:1. Measurements of drum level cannot be made easily.2. Interactions in the boiler system and uneven nature of these interactions.

Control requirement: The purpose of the drum is not only to separate the steam from the water

but also to provide a storage reservoir that allows short-term imbalances between feed-watersupply and steam production to be handled without risk to the plant. As the level of water in thedrum rises, the risk increases of water being carried over into the steam circuits. The results ofsuch 'carry-over' can be catastrophic: cool water impinging on hot pipework will cause extremeand localized stresses in the metal and, conversely, if the level of water falls there is a possibilityof the boiler being damaged, partly because of the loss of essential cooling of the furnace water-

walls.Therefore, the target of the feed-water control system is to keep the level of water in the drum atapproximately the midpoint of the vessel. Given this objective, it would appear that the simplestsolution would appear to be to measure the level of water in the drum and to adjust the deliveryof water to keep this at the desired value--feeding more water into the drum if the level is falling,and less if the level is rising. Unfortunately, the level of water is affected by transient changes ofthe pressure within the drum and the sense in which the level varies is not necessarily related tothe sense in which the feed flow must be adjusted. In other words, it is not sufficient to assumethat simply because the level is increasing the feed-water flow must be decreased, and vice versa.

This strange situation is due to effects known as 'swell' and 'shrinkage'. Boiling water comprisesa turbulent mass of fluid containing many steam bubbles, and as the boiling rate increases thequantity of bubbles that is generated also increases. The mixture of water and bubbles resemblesfoam, and the volume it occupies is dictated both by the quantity of water and by the amount ofthe steam bubbles within it. If the pressure within the system is decreased, the saturationtemperature is also lowered and the boiling rate therefore increases (because the temperature ofthe mixture is now higher in relation to the saturation temperature than it was before the pressurechange occurred). As the boiling rate increases, the density of the, water decreases, but since themass of steam and water has not changed the decrease in density must be accompanied by an

increase in the volume of the mixture.By this mechanism the level of water in the drum appears to rise, a phenomenon referred to as'swell'. The rise of level is misleading: it is not indicative of a real increase in the mass of waterin the system, which would require the supply of water to be cut back to maintain the status quo.In fact, if the drop in pressure is the result of the steam demand suddenly increasing, the watersupply will need to be increased to match the increased steam flow. 'Shrinkage' is the opposite ofswell: it occurs when the pressure rises. The mechanism is exactly the same as that for swell, butin the reverse direction. Shrinkage causes the level of water in the drum to fall when the steamflow decreases, and once again the delivery of water to the boiler must be related to the actualneed rather than to the possibly misleading indication provided by the drum-level transmitter.

If a slow change of steam flow occurs, all is well because the pressure within the system can becontrolled. It is when rapid steam-flow changes happen that problems occur since, due to swellor shrinkage, the drum level indication provides a contrary indication of the water demand.Following a sudden increase in steam demand, which causes the pressure to drop (and thereforethe drum level to rise), a simple level controller would respond by reducing the flow of feedwater. Equally, a sudden decrease in steam flow, which would be accompanied by a rise inpressure and an attendant fall in the drum level, would cause a level controller to increase the

flow of water. Both actions are, of course, in the incorrect sense.The effects of swell and shrinkage, in addition to being determined by the rate of change ofpressure, also depend on the relative size of the drum and the pressure at which it operates. If thevolume of the drum is large in relation to the volume of the whole system the effect will besmaller than otherwise. If the system pressure is low the effect will be larger than with a boileroperating at a higher pressure, since the effect of a given pressure change on the density of thewater will be greater in the low-pressure boiler than it would if the same pressure change were to

occur in a boiler operating at a higher pressure.

Control technique: Remembering that the basic requirement of a feed-water control system is to maintain a constant quantity of water in the boiler, it is apparent that one way of addressing theproblem would be to maintain the flow of water into the system at a value which matches theflow of steam out of it. The flow is controlled using a valve which maintains the rate of waterflowing through the valve at a figure which is directly proportional to the demand signal from thecontroller (i.e. if the demand signal varies linearly from 0 to 100%, the flow rate also changeslinearly between 0 and 100%). Such a valve is said to have a 'linear characteristic' and isemployed in conjunction with a transmitter that produces a signal proportional to steam flow.Used together, these two devices keep the parameters in step. If the transmitter produces a signalwhich is equal to the steam flow at all loads and if the flow through the valve is matched withthis signal at every point in the flow range, a controller gain of unity will ensure that, throughoutthe dynamic range of the system, the flow of water will always be equal to the flow of steam.However, the flow through a valve depends both on its opening and on the pressure drop acrossit. In a feed-water system, the pressure drop across the valve varies from instant to instant, andthe flow through it at any given opening will therefore vary. One method of correcting for theerror produced by the feed valve is the addition of a third element to the system-a measurementof feedwater flow.

Three-element feed-water control system

Here a cascade control technique is applied. The blocks are described below:

Item 1: Flow Transmitter for feed water flow rate

Item 2: Flow transmitter for steam flow rateItem 3: Level transmitter for drum levelItem 4: A gain block to adjust for any range difference between the steam-flow and feed-flowtransmittersItem 5: Drum-level controller (proportional only)Item 6: Error generatorItem 7: Closed-loop feed-water controller

6.2 Valves

A control valve consists of many components which may conveniently be considered as fallinginto one of two groups: the valve body and the actuator. The former is the part through which thewater flows and this flow is controlled by adjusting the resistance offered to the water. This isdone by moving the position of a plug in relation to its seat. The position of the plug is controlledby an actuator which acts via the stem.Figure shows a small-bore feed-water control valve body with a contoured trim (the 'trim' beingthe part of the valve which is in flowing contact with the water). The contour determines the

relationship between the position of the plug and the flow of water past it. The type of trim willbe dictated by the application, such as the need to minimize acoustic noise or cavitation, therangeability needed etc. In addition the trim design will determine the valve characteristic, whichis the curve relating the stem position to the rate of flow of water through the valve. This is an

important feature, since the characteristic determines the gain of the valve system, which formspart of the overall loop gain.

A typical feed-water control valve body

For a given opening, the flow through the valve will be determined by the delivery pressure ofthe feed pump and the resistance that the boiler pipework offers to the flow. To simplify the taskof selecting the correct valve size and characteristic, it is necessary to relate everything to adefinable set of conditions. This is achieved by determining what the flow through the valvewould be if a fixed differential pressure were to be maintained across it. This is termed theinherent characteristic of the valve.

Once the valve is operating on the actual plant, the position/flow relationship achieved inpractice will not match the inherent characteristic, because in the real world the inlet pressureand system resistance will vary, producing a pressure drop which is different from the value thatwas used to define the inherent characteristic. The pressure/flow relationship achieved in actualoperation is called the installed characteristic.

Inherent characteristics of valves

6.3 Deareater controlSteam admitted to the deaerator rises upwards past metal trays over which the water issimultaneously cascading downwards. As the water and steam mix and become agitated,entrained gases are released. The dissolved gases are vented to the atmosphere because the vesselis pressurised by the steam. The deaerator is situated in the water circuit between the dischargeof the condenser extraction pump and the inlet of the feed pumps. It will be evident that twocontrol functions are required by the deaerator:

• to maintain the steam pressure at the optimum value

• to keep the storage vessel roughly half full of water.

6.3.1 Steam pressure controlThe pressure of the steam entering the deaerator is maintained by a simple controller whosemeasured-value signal is obtained from a transmitter measuring the steam pressure in thedeaerator. The set value of the controller is fixed.

6.3.2 Level controlThe storage vessel provides a measure of reserve capacity for the plant. To achieve this functionthe level of water in it must be maintained at roughly the midpoint. This is achieved by means ofa level controller whose measured-value signal is obtained from a differential-pressuretransmitter or from capacitive probes which would normally be connected to tappings of anexternal water column which is in turn connected to the top and bottom of the deaerator storagevessel. If there were no losses in the system, the amount of water would be constant and the levelin the deaerator storage vessel would remain at the correct value set during commissioning.However, losses are inevitable (for example, due to leakages at pump glands or during soot-blowing or blowdown operations), and a supply of treated water must therefore be madeavailable. The deaerator level controller output adjusts the opening of a valve that admits thismake-up water to the condenser, as shown.

The make-up supply is conventionally fed into the system at the condenser. Figure shows thatinteraction between the level controllers of the deaerator and condenser is inevitable. Thesituation is made more complex because the condenser extraction pump has to be provided witha bypass arrangement to maintain a minimum flow through the pump at all times.In fact, the conditions which cause the deaerator level controller to call for more water to beadded to the system will also cause the condenser level to fall, and so the two systems do not actin opposite senses. Nevertheless they do interact, and care must be taken to minimise theinstability that is likely to arise.

Principle of deaerator level control system

a. Single element water level control, Only single sensor i.e. level sensor is used tocontrol the level

b. Double Element water level control

This uses two sensors level and flow sensors. It is better and precise than single elementcontrol.

Other Temperature, Flow and Pressure control is done by Feedback loop- This uses a singlefeedback loop which finds out error from the set point and finds out controller output.

Cascaded Feedback loop control

This uses two controller:

1. Master

2. Slave

The set point of slave controller is decided by the controller output of the master.

7. STEAM TEMPERATURE CONTROL

7.1 Temperature control

Control Objective: To maintain the temperature of steam at a precise value over the entire loadrange.

Control Requirement: The steam turbine requires the steam temperature to remain at a precisevalue over the entire load range, and it is mainly for this reason that some dedicated means ofregulating the temperature must be provided. Since different banks of tubes are affected indifferent ways by the radiation from the burners and the flow of hot gases, an additionalrequirement is to provide some means of adjusting the temperature of the steam within differentparts of the circuit, to prevent any one section from becoming overheated.

Difficulties: The long time constants associated with the superheater do not permit a simplecontrol strategy based on measuring the temperature of the steam leaving the final superheater,and modulating the flow of cooling water to the spray attemperator so as to keep the temperatureconstant at all flow conditions. This form of control would produce excessive deviations intemperature, and a more complex arrangement is required.Two time constants are associated with the superheater. One represents the time taken forchanges in the firing rate to affect the steam temperature, the other is the time taken for the steamand water mixture leaving the attemperator to appear at the outlet of the final superheater. Interms of temperature control it is the latter effect which predominates because, although changesin heat input will affect the temperature of the steam, a fast-responding temperature-control loopwill be able to compensate for the alterations and keep the temperature constant. It is the reactiontime between a change occurring in the spray-water flow and the effects being observed in thefinal temperature that determines the extent of the temperature variations that will occur.

Control technique: Another problem with a simple system is that it does not permit anymonitoring and control of the temperature to occur within the steam circuit--only at the exit fromthe boiler. These difficulties are addressed by the use of a cascade control system as shown.Since it is the temperature of the steam leaving the secondary superheater that is important, thisparameter is measured and a corresponding signal fed to a three-term controller (proportional-plus-integral-plus-derivative). In this controller the measured-value signal is compared with afixed desired-value signal and the controller's output forms the desired-value input for asecondary controller. (Because the output from one controller 'cascades' into the input of another,this type of control system is commonly termed 'cascade control'.) The secondary controllercompares this desired-value signal with a measurement representing the temperature of the steamimmediately after the spray-water attemperator.Because the steam temperature sensors used are subjected to the high pressures and temperaturesof the superheater, they have to be enclosed in substantial steel pockets. Even with the bestdesigns, pockets are usually slow-responding, with the result that any high-speed fluctuations inthe measured-value signal will be smoothed out and the resultant signal will be fairly stable. The

use of a derivative term is therefore easier than in, say, flow measurement applications wheresmall-scale but sudden changes in flow can occur. When rapid input changes are differentiated,the controller output changes by a large amount, and for this reason tuning three-term flowcontrollers for optimum response can become difficult. This is not a problem with thetemperature controllers used here, and the application of derivative action is viable.

Steam-temperature control with a single interstage spray attemperator

7.2 Spray Water Attemperator: The high-pressure cooling water is mechanically atomized intosmall droplets at a nozzle, thereby maximizing the area of contact between the steam and thewater. With this type of attemperator the water droplets leave the nozzle at a high velocity andtherefore travel for some distance before they mix with the steam and are absorbed. To avoidstress-inducing impingement of cold droplets on hot pipework, the length of straight pipe inwhich this type of attemperator needs to be installed is quite long, typically 6 m or more.With spray attemperators, the flow of cooling water is related to the flow rate and thetemperature of the steam, and this leads to a further limitation of a fixed-nozzle attemperator.Successful break-up of the water into atomized droplets requires the spray water to be at apressure which exceeds the steam pressure at the nozzle by a certain amount (typically 4 bar).Because the nozzle presents a fixed-area orifice to the spray water, the pressure/flow

characteristic has a square-law shape, resulting in a restricted range of flows over which it can beused (this is referred to as limited turn-down or rangeability). The turn-down of the mechanicallyatomized type of attemperator is around 1.5:1.The temperature of the steam is adjusted by modulating a separate spray-water control valve toadmit more or less coolant into the steam.

Mechanically atomised desuperheater

7.3 Temperature control with tilting burners

The burning fuel in a corner-fired boiler forms a large swirling fireball which can be moved to ahigher or lower level in the furnace by tilting the burners upwards or downwards with respect toa mid-position. The repositioning of the fireball changes the pattern of heat transfer to thevarious banks of superheater tubes and this provides an efficient method of controlling the steamtemperature, since it enables the use of spray water to be reserved for fine-tuning purposes andfor emergencies. In addition, the tilting process provides a method of controlling furnace exittemperatures.

8. TURBINE CONTROL ANDMONITORING

The steam turbine generator is controlled and monitored by several interrelated systems:

1. Turbine governor system: automatically controls turbine speed, acceleration, and load;

2. Trip system: provides protection through trips and ranbacks;3. Supervisory instrumentation system: provides past and present operating data through

parameter sensing, indicating, and recording; and4. Excitation system: controls generator voltage

8.1 Turbine Governor: The turbine governor system is a hybrid electrical-hydraulic system.This type of control system is significantly more advanced and preferable to the oldermechanical-hydraulic designs in that the linkages and cams of the mechanical designs have beenreplaced with electrical logic and fast-acting hydraulic servomotors. The hydraulic fluid issupplied to the stop and control valve servomotors by a high-pressure power pumping unit. Thefluid is flame retardant to minimize fire hazards in the event of a leak. The system also includescontrols and instrumentation.The turbine governor facilitates control of the turbine over the full operational range bypositioning the turbine control valves and the interceptor valves to control turbine speed, load,and throttle pressure.

8.2 Trip System: The trip system initiates protective tripping of the turbine by sensingpotentially damaging operating conditions. Typical parameters sensed for initiation of turbineprotective functions include the following:

• Over speed governor trip• Manual trip device• Generator trip• Generator protection trips (loss of coolant, high stator temperature, etc.)• High differential expansion• Solenoid trip• Turbine over speed• Thrust bearing failure• Low lubricating oil or hydraulic fluid pressure• Operator manual trip• Governor system protective trips• Condenser low vacuum trip• High exhaust temperature trip• High vibration trip

8.3 Supervisory Instrumentation System: The supervisory instrumentation system includesdevices to sense, indicate, and record parameters necessary to monitor the operation of themachine. The following parameters are monitored along with others:

• Shaft speed;• Governor or control valve position;• Shaft eccentricity;• Radial (X-Y) shaft vibration at all turbine, generator, and• Shell, rotor, and differential expansion;

• Shell and valve chest temperatures;• Water induction thermocouple temperatures;• Vibration phase angles;• Bearing metal temperature, including the thrust bearing;• Generator winding temperatures;• Generator gas temperatures;• Generator cooling water temperatures; and• Exciter temperatures.

These parameters are measured, recorded, displayed, and alarmed by hard-wired monitors in thesystem cabinets. High differential expansion, turbine over speed, and thrust bearing wear alarmsare provided to the trip system. Rotor vibration alarms are displayed in the main control room.

Turbine Supervisory Equipment Application

8.4 Sensors and Measurement Techniques

Turbine supervision is an essential part of the day-to-day running of any power plant. There aremany potential faults such as cracked rotors and damaged shafts, which result from vibration andexpansion. When this expansion and vibration is apparent in its early stages the problem canusually be resolved without any of the disruption caused when a turbine has to be shut down. Byappropriate trending of the various measurement points and the identification of excessive

vibration or movement, scheduled equipment stoppages or outages can often be utilised toinvestigate and resolve the failure mechanism.

8.4.1 The Eddy Current Proximity Probe

The principle of operation, as the name implies, depends upon the eddy currents set up in thesurface of the target material - shaft, collar, etc. adjacent to the probe tip.The Eddy probe tip is made of a dielectric material and the probe coil is encapsulated within thetip. The coil is supplied with a constant RF current from a separate Eddy Probe Driver connectedvia a cable, which sets up an electromagnetic field between the tip and the observed surface.Any electrically conductive material within this electromagnetic field, i.e. the target material,will have eddy currents induced in its surface. The energy absorbed from the electromagneticfield to produce these eddy currents will vary the strength of the field and hence the energizingcurrent, in proportion to the probe target distance. Such changes are sensed in the driver wherethey are converted to a varying voltage signal.The whole probe, extension cable and driver system relies for its operation on being a tunedcircuit and as such is dependent on the system¶s natural frequency. Thus each system is set up fora fixed electrical/cable length. Eddy probe systems are usually supplied with 2, 5, 9 or 14 metretotal cable lengths.

The probe types available are generally according to the API670 standard. Three main variantsare straight mount, reverse mount and disc type probes. The main difference between the straightand reverse mount is the location of the thread on the probe body and the fixing nut. Reversemount tend to be used exclusively with probe holders, while straight mount are the morecommon and are used on simple bracketry or mounting threads where adjustment to the target isachieved through use of the thread on the probe body in conjunction with a moveable lock nut.The maximum measurement range available on this type of probe is typically 8mm.The disc probe mounts the encapsulated coil on a metal plate with fixed mounting holes, makinga very low profile assembly with a side exit cable. Larger coils can be mounted on this plate; for

example, the 50mm diameter tip probe can provide a measurement range of beyond 25mm.However, care must be taken to ensure the target area is sufficient to obtain the required linearresponse. Note the relationship opposite ± between linear range, probe tip and target area.

Application: In rotating plant, the variations in shaft/bearing distance created by vibration,eccentricity, ovality etc. are measured by probes mounted radially to the shaft. When the target isstationary the measured voltage can be used to set the probe/target static distance. Shaft speedcan also be measured by placing the probe viewing a machined slot or a toothed wheel.

8.4.2 LVDT (Linear Variable Differential Transformer)

The LVDT is an electromechanical device that produces an electrical signal whose amplitude isproportional to the displacement of the transducer core. The LVDT consists of a primary coil andtwo secondary coils symmetrically spaced on a cylindrical former.

Schematic of an LVDT Core displacement characteristics

A magnetic core inside the coil assembly provides a path for the magnetic flux linking the coils.The electrical circuit is configured as above with the secondary coils in series opposition.When an alternating voltage is introduced into the primary coil and the core is centrally located,then an alternating voltage is mutually induced in both secondary coils. The resultant output iszero, as the voltages are equal in amplitude and in 180º opposition to each other. When the coreis moved away from the null position the voltage in the coil, towards which the core is moved,increases due to the greater flux linkage and the voltage in the other primary coil decreases dueto the lesser flux linkage. The net result is that a differential voltage is produced across thesecondary tappings, which varies linearly with change in core position. An equal effect isproduced when the core is moved from a null in the other direction but the voltage is 180ºdifferent in phase.

Application: The LVDT can be operated where there is no contact between the core andextension rod assembly with the main body of the LVDT housing the transformer coils. Thismakes it ideal for measurements where friction loading cannot be tolerated but the addition of alow mass core can. Examples of this are fluid level detection with the core mounted on a floatand creep tests on elastic materials. This frictionless movement also benefits the mechanical lifeof the transducer, making the LVDT particularly valuable in applications such as fatigue lifetesting of materials or structures.This is a distinct advantage over potentiometers which are proneto wear and vibration.The principle of operation of the LVDT, based on mutual inductance between primary andsecondary coils, provides the characteristic of infinite resolution. The limitations lie within thesignal processing circuitry in combination with the background noise.

Commercially available LVDT

8.4.3 The Accelerometer

The accelerometer is based on the electrical properties of piezoelectric crystal. In operation, thecrystal is stressed by the inertia of a mass. The variable force exerted by the mass on the crystalproduces an electrical output proportional to acceleration. Two common methods of constructingthe device to generate a residual force are compression mode and shear mode respectively. Aresidual force is of course required to enable the crystal to generate the appropriate response,moving in either direction on a single axis.An accelerometer operates below its first natural frequency. The rapid rise in sensitivityapproaching resonance is characteristic of an accelerometer, which is an un-damped single-degree-of-freedom spring mass system. Generally speaking, the sensitivity of an accelerometerand the ratio between its electrical output and the input acceleration is acceptably constant toapproximately 1/5 to 1/3 of its natural frequency. For this reason, natural frequencies above30KHz tend to be used.

Typical frequency response of an accelerometer

Although the piezoelectric accelerometer is a self-generating device, its output is at a very highimpedance and is therefore unsuited for direct use with most display, analysis, or monitoringequipment. Thus, electronics must be utilised to convert the high impedance crystal output to alow impedance capable of driving such devices. The impedance conversion electronics may belocated within the accelerometer, outside of but near the accelerometer, or in the monitoring oranalysis device itself.

8.4.4 Velocity Transducer

The velocity transducer is inherently different to the accelerometer with a conditioned velocityoutput. This device operates on the spring-mass-damper principle, is usually of low naturalfrequency and actually operates above its natural frequency. The transducing element is either amoving coil with a stationary magnet, or a stationary coil with a moving magnet. A voltage isproduced in a conductor when the conductor cuts a magnetic field and the voltage is proportionalto the rate at which the magnetic lines are cut. Thus, a voltage is developed across the coil, whichis proportional to velocity.

8.4.5 Absolute vibration

Absolute vibration monitoring is perhaps the primary method of machine health monitoring onsteam turbines. The type of transducer used is seismic (ie vibration of turbine relative to earth)and can either be a velocity transducer or an accelerometer.Vibration monitoring is nearly always in terms of velocity or displacement and can therefore beobtained by an accelerometer or a velocity transducer. Particular care needs to be taken whendouble integrating an accelerometer signal to provide a displacement measurement. Problemsusually occur below 10Hz when double integrating and 5Hz when single integrating. In thefrequency ranges normally monitored on steam turbines this is not a problem. Thesemeasurement issues can be reduced by integrating the signal at source rather than after runningthe signal through long cables (ie having picked up noise on route).Pedestal vibration is measured in the two axes perpendicular to the shaft direction where thebearing is under load, providing complete measurement coverage. In some instances the thrustdirection is also monitored depending on turbine configuration.

8.4.6 Eccentricity & shaft vibration

Eccentricity monitoring can be subdivided into shaft vibration and bent shaft monitoring. Bentshafts normally result when the turbine is stationary and thermal arching or bowing of the shaftoccurs or the shaft sags under its own weight. The Turbine is rotated slowly (barring) to preventthis happening or to straighten the shaft after it has occurred.One of the difficulties encountered when using shaft displacement transducers be they the eddycurrent probe or the older inductive probes, is the problem of ³runout´. Runout is the error signalgenerated by mechanical, electrical or metallurgical irregularities of the shaft surface.These errorsignals are generally of a low magnitude in comparison to the vibration signal and are often at amuch higher frequency.

8.4.7 Rotor differential expansion & shaft position

The eddy current probe, as well as providing ac vibratory information, also provides dcinformation of the probe to target gap. This makes it ideal for measuring rotor to casingdifferential expansion via a non-contact method.

Two probes monitoring expansion by observing a tapered collar

8.4.8 Speed - overspeed - zero speed monitoring

The eddy current probe as well as being used for shaft vibration and differential expansion canalso be used as a speed monitoring transducer. The eddy current probe gives a large voltageoutput, which is independent of shaft speed.

8.4.9 Casing & cylinder expansionThese techniques require a larger measurement range than can be offered through standardproximity probe equipment, the necessary probe target is also not easy to achieve. This is whereLVDTs are used to provide the expansion measurements required. A total range of 50mm

usually suffices and the various mounting options available with LVDTs makes installationstraightforward. The movement of the turbine pedestals on the cylinder sole plates is a relatively

easy measurement to make requiring an LVDT mounted on the turbine and the extension rodfixed or sprung onto the slides. The environment is not hostile although care must be taken to

prevent mechanical damage to the transducer.

LVDT monitoring cylinder casing expansion

8.4.10 Valve position monitoring

The Linear Variable Differential Transformer (LVDT) is ideally suited for valve positionmonitoring. In this type of application the LVDT is used to provide positional feedback to thegovernor control system to effect a closed loop system. The electrical properties of the LVDTtherefore play a key role in determining the system response. Linearity is obviously key as wellas a robust construction with flexible mounting options. AC type devices (as opposed to DC) areexclusively used for this type of application, which permit long cable runs and offset adjustmentwith gain control.

8.5 Woodward 505 Enhanced Digital Control for Steam Turbines

The 505 Enhanced controller is designed to operate industrial steam turbines of all sizes andapplications. This steam turbine controller includes specifically designed algorithms and logic tostart, stop, control, and protect industrial steam turbines or turbo-expanders, driving generators,compressors, pumps, or industrial fans. The 505 control¶s unique PID structure makes it ideal forapplications where it is required to control steam plant parameters like turbine speed, turbine

load, turbine inlet or exhaust header pressure, or tie-line power.The control¶s special PID-to-PID logic allows stable control during normal turbine operation andbumpless control mode transfers during plant upsets, minimizing process over- or undershootconditions. The 505 controller senses turbine speed via passive or active speed probes andcontrols the steam turbine through one or two (split-range) actuators connected to the turbineinlet steam valves. The plant employs one controller for each turbine.

DescriptionThe 505 control is packaged in an industrial hardened enclosure designed to be mounted within asystem control panel located in a plant control room or next to the turbine. The control¶s frontpanel serves as both a programming station and operator control panel (OCP). This user-friendlyfront panel allows engineers to access and program the unit to the specific plant¶s requirements,and plant operators to easily start/stop the turbine and enable/disable any control mode. Passwordsecurity is used to protect all unit program mode settings. The unit¶s two-line display allowsoperators to view actual and setpoint values from the same screen, simplifying turbine operation.

Turbine interface input and output wiring access is located on the controller¶s lower back panel.Unpluggable terminal blocks allow for easy system installation, troubleshooting, andreplacement.

System Protection

• Integral Overspeed Protection Logic• First-out Indication (10 individual shutdown inputs)• Bumpless transfer between control modes if a transducer failure is detected• Local/Remote Control priority and selection• Fail-safe Shutdown Logic

ControlThe following PIDs are available to perform as process controllers or limiters:

• Speed/Load PID (with Dual Dynamics)• Auxiliary PID (limiter or control)• Cascade PID (Header Pressure or Tie-Line Control)

Control SpecificationsInputs• Power: 18±32 Vdc, 90±150 Vdc, 88±132 Vac (47±63 Hz), 180±264 Vac (47±63 Hz)• Speed: 2 MPUs (1±30 Vrms) or proximity probes (24 Vdc provided), 0.5 to 15 kHz• Discrete Inputs: 16 Contact Inputs (4 dedicated, 12 programmable)• Analog Inputs: 6 Programmable Current Inputs (4±20 mA)

Outputs• Valve/Actuator Drivers: 2 Actuator Outputs (4±20 mA or 20±160 mA)• Discrete Outputs: 8 Relay Outputs (2 dedicated, 6 programmable)• Analog Outputs: 6 Programmable Current Outputs (4±20 mA)

CommunicationSerial: 2 Modbus (ASCII or RTU) Comm Ports (RS-232, RS-422, or RS-485 compatible)

Temperature

The hotness or coldness of a piece of plastic, wood, metal, or other material depends upon themolecular activity of the material. Kinetic energy is a measure of the activity of the atoms whichmake up the molecules of any material. Therefore, temperature is a measure of the kineticenergy of the material in question.

1. RTD ( Resistance temperature detector )

- The resistance of an RTD varies directly with temperature:

- As temperature increases, resistance increases.

- As temperature decreases, resistance decreases.

· RTDs are constructed using a fine, pure, metallic, spring-like wire surrounded by an insulatorand enclosed in a metal sheath.

· A change in temperature will cause an RTD to heat or cool, producing aproportional change inresistance. The change in resistance is measured by a precision device that is calibrated to givethe proper temperature reading.

The RTD used in RPH is Pt-100.

2. THERMOCOUPLE

Thermocouples will cause an electric current to flow in the attached circuit whensubjected to changes in temperature.The amount of current that will be produced is dependenton the temperature difference between the measurement and reference junction; thecharacteristics of the two metals used; and the characteristics of the attached circuit.

Heating the measuring junction of the thermocouple produces a voltage which is greater thanthe voltage across the reference junction. The difference between the two voltages isproportional to the difference in temperature and can be measured on the voltmeter (in millvolts). For ease of operator use, some voltmeters are set up to read out directly in temperaturethrough use of electronic circuitry.

Only J and K type of thermocouple is used in RPH.

Pressure Transducers

Bellows-Type Detectors

The need for a pressure sensing element that was extremely sensitive to low pressures andprovided power for activating recording and indicating mechanisms resulted in the developmentof the metallic bellows pressure sensing element. The metallic bellows is most accurate whenmeasuring pressures from 0.5 to 75 psig. However, when used in conjunction with a heavyrange spring, some bellows can be used to measure pressures of over 1000 psig.

Bourdon tube

The bourdon tube consists of a thin-walled tube that is flattened diametrically on opposite sidesto produce a cross-sectional area elliptical in shape, having two long flat sides and two shortround sides. The tube is bent lengthwise into an arc of a circle of 270 to 300 degrees. Pressureapplied to the inside of the tube causes distention of the flat sections and tends to restore itsoriginal round cross-section. This change in cross-section causes the tube to straighten slightly.Since the tube is permanently fastened at one end, the tip of the tube traces a curve that is theresult of the change in angular position with respect to the center. Within limits, the movement ofthe tip of the tube can then be used to position a pointer or to develop an equivalent electricalsignal to indicate value of the applied internal pressure.

Level Measurement

Ball Float

The operation of the ball float is simple. The ball floats on top of the liquid in the tank. If theliquid level changes, the float will follow and change the position of the pointer attached to therotating shaft.

Pressure head method

The differential pressure (_P) detector method of liquid level measurement uses a _P detectorconnected to the bottom of the tank being monitored. The higher pressure, caused by the fluidin the tank, is compared to a lower reference pressure (usually atmospheric). This comparisontakes place in the _P detector. Figure illustrates a typical differential pressure detector attachedto an open tank.

Flow meters

Differential Flow Transmitter

Orifice plates

· Flat plates 1/16 to 1/4 in. thick

· Mounted between a pair of flanges

· Installed in a straight run of smooth pipe to avoid disturbance of flow patterns

due to fittings and valves.

Venturi tube

· Converging conical inlet, a cylindrical throat, and a diverging recovery cone

· No projections into the fluid, no sharp corners, and no sudden changes in

Contour.

Dall flow tube

· Consists of a short, straight inlet section followed by an abrupt decrease in the

inside diameter of the tube

· Inlet shoulder followed by the converging inlet cone and a diverging exit cone

· Two cones separated by a slot or gap between the two cones.

Electromagnetic flow meter

The electromagnetic flow meter is similar in principle to the generator. The rotor of the

generator is replaced by a pipe placed between the poles of a magnet so that the flow of

the fluid in the pipe is normal to the magnetic field. As the fluid flows through this

magnetic field, an electromotive force is induced in it that will be mutually normal

(perpendicular) to both the magnetic field and the motion of the fluid. This electromotive

force may be measured with the aid of electrodes attached to the pipe and connected to a

galvanometer or an equivalent. For a given magnetic field, the induced voltage will be

proportional to the average velocity of the fluid. However, the fluid should have some

degree of electrical conductivity.

Ratio control is used to ensure that two flows are kept at the same ratio even if the flows arechanging. E.g. Air-Fuel Ratio where the proper ratio between air and fuel i.e. pulverized coalhas to be maintained.

Controller feed is adjusted according to the ratio of wild feed. The implementation is

shown above.

utomation of Processes

The main purpose of automation is to minimize human intervention to reduce the errors. Largeprocesses like power plant has large scope for errors.

The automation of industrial processes is carried by PLCs and SCADA.

The above figure shows the hierarchy setup of industrial automation.

The field devices are connected to the PLC and all the PLCs are connected to the SCADAserver based in control room.

The SCADA server is connected via LAN to ERP (Enterprise Resource Planning) and the datacan be accessed from any where on internet.

PLC (Programmable Logic Controller)

Digital electronic device that uses a programmable memory to store instructions and toimplement specific functions such as logic , sequencing , timing etc to control machine andprocesses.

Salient features

Cost effective for controlling complex systems.�

• Flexible and can be reapplied to control other systems quickly and easily.�• Computational abilities allow more sophisticated control.�• Trouble shooting aids make programming easier and reduce downtime.�• Reliable components make these likely to operate for years before failure.�

PLC HARDWARE

Many PLC configurations are available, even from a single vendor. But, in each of these thereare common components and concepts. The most essential components are:

Power Supply ±

This can be built into the PLC or be an external unit.

Common voltage levels required by the PLC (with and without the power

supply) are 24Vdc, 120Vac, 220Vac.

CPU (Central Processing Unit) ±

This is a computer where ladder logic is stored and processed.

I/O (Input/Output) ±

A number of input/output terminals must be

provided so that the PLC can monitor the process and initiate actions.

Indicator lights ±

These indicate the status of the PLC including power on, program running, and a fault. Theseare essential when diagnosing problems.

The configuration of the PLC refers to the packaging of the components.

Rack - A rack is often large (up to 18´ by 30´ by 10´) and can hold multiple cards.

When necessary, multiple racks can be connected together. These tend to

be the highest cost, but also the most flexible and easy to maintain.

INPUTS AND OUTPUTS

Inputs to, and outputs from, a PLC are necessary to monitor and control a process. Both inputsand outputs can be categorized into two basic types:

logical or continuous.Consider the example of a light bulb. If it can only be turned on or off, it is logical control. If thelight can be dimmed to different levels, it is continuous. Continuous values seem more intuitive,but logical values are preferred because they allow more certainty, and simplify control. As a

result most controls applications (and PLCs) use logical inputs and outputs for mostapplications. Hence, we will discuss logical I/O and leave continuous I/O for later. Outputs toactuators allow a PLC to cause something to happen in a process. A short list of popularactuators is given below in order of relative popularity.

· Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow.

· Lights - logical outputs that can often be powered directly from PLC output boards.

· Motor Starters - motors often draw a large amount of current when started, so they requiremotor starters, which are basically large relays.

· Servo Motors - a continuous output from the PLC can command a variable speed or position.

Outputs from PLCs are often relays, but they can also be solid state electronics such astransistors for DC outputs or Triacs for AC outputs. Continuous outputs require special outputcards with digital to analog converters. Inputs come from sensors that translate physicalphenomena into electrical signals. Typical examples of sensors are listed below in relative orderof popularity.

· Proximity Switches - use inductance, capacitance or light to detect an object logically.

· Switches - mechanical mechanisms will open or close electrical contacts for a logical signal.

· Potentiometer - measures angular positions continuously, using resistance.

· LVDT (linear variable differential transformer) - measures linear Displacement

Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is onor off. In other words, is the sensor connected to the first input on? How about the secondinput? How about the third... It records this data into its memory to be used during the next step.

Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time.Maybe your program said that if the first input was on then it should turn on the first output.Since it already knows which inputs are on/off from the previous step it will be able to decidewhether the first output should be turned on based on the state of the first input. It will store theexecution results for use later during the next step.

Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of the outputs. Itupdates the outputs based on which inputs were on during the first step and the results ofexecuting your program during the second step. Based on the example in step 2 it would nowturn on the first output because the first input was on and your program said to turn on the firstoutput when this condition is true.

Deadman SwitchA motor will be controlled by two switches. The Go switch will start themotor and the Stop switch will stop it. If the Stop switch was used to stopthe motor, the Go switch must be thrown twice to start the motor. Whenthe motor is active a light should be turned on. The Stop switch will bewired as normally closed.

APPENDIX ADesign\Rated Parameters

1. Generation: 1.62 (MU) max.2. Load: 67.5 (MU) max.3. Boiler steam pressure: 95 kg/cm2

Equipment Type Capacity Make

ID FanRadial Double Suc.

81 m3/sec BHEL

FD FanRadial Double Suc.

57 m3/sec BHEL

4. Boiler steam temperature: 540 deg C5. Opacity: 150 mg/Nm

6. Unburnt carbon in fly ash: < 1.0 %7. Unburnt carbon in bottom ash: < 3.5 %8. Feed water temperature after HPH: 236.85 deg C

9. Steam temp. at throttle valve before ESV:535 deg C10. Steam pressure ESV (kg/cm2): 86.49 kg/cm2

11. Vaccum: 695 mm12. Exhaust hood temp.: 44.25 deg C13. Circulating water rise of temp.: 8 deg C14. Fuel oil pressure (LSHS): 7 kg/cm215. Generator stator temp.: 100 deg C16. Steam flow before ESV: 257.5 T/hr17. Steam consumption per MW: 3.81 T/hr18. Turbine heat rate: 2232 kcal/kWh19. DM water conductivity: < 1.0 mho/cm20. DM water silica: < 20 ppb

21. Clarifier water outlet turbidity: 10 N.T.U22. Feed water conductivity: 6 m mho/cm23. Feed water silica: 0.02 ppm24. Boiler water conductivity: 30-70 mho/cm25. Boiler water silica: < 1 ppm

26. Moisture in turbine oil (at filter point):100 ppm27. Auxiliary consumption: 11.2 % per day (both units)28. Make-up water consumption: 624 kl/day (both units)29. Clarifier water inlet conductivity: 600-1700 m mho/cm

APPENDIX BDetails of Fans and Pumps used in the plant

Fans

B.F.PRadial Double Suc.

318.4 T/hr BHEL

C.E.P Centrifugal 283.2 T/hr KSB

PA FanRadial Single Suc.

18 m3/sec BHEL

Equipment Type Capacity (KW) Make

ID Fan Squirrel cage I.M. 400 BHEL

FD Fan Squirrel cage I.M. 400 BHEL

PA Fan Squirrel cage I.M. 190 BHEL

Bowl Mills Squirrel cage I.M. 260 BHEL

B.F.P Squirrel cage I.M. 2000 BHEL

C.E.P Squirrel cage I.M. 250 BHEL

Drives

APPENDIX CTurbine Specifications

Make: BHEL

Capacity: 67.5 MWExhaust Temperature: 44.25 deg CCritical speed: 1800 RPMGoverning: HydraulicEfficiency: 95 %Actual heat consumption: 3.85 kg/KWh

Generator Specifications

Make: BHEL- TARIType: TurboRated Capacity: 673.5 MW/ 84.375 MVAPower factor: 0.8 lagFrequency: 50 HzStator: 10.5 KV, 4639 ARotor: 303 V (max.), 601 ARotor S.C. ratio: 0.56Type of Cooling: Air

CONCLUSION:-During our visit to power plant station from June 07,2010 to July 24,2010.

The modernization of world as we see today would not become possibleif electrical power do not come into picture. Today each area of Science &technology is highly affected by the use of electrical energy. Electrical energy isthe only reliable form of Energy which is easily converted into any form of energywhether it is Mechanical, Chemical, Light, and Sound etc.

The electrical energy has been used not only in industrial areas but alsoin residential and commercial application as well. The reason behind the popularityof this form of energy is because it can be easily transferred to one place to anotherwith efficiency.

So at the outset I would like to conclude that there is no doubt withoutthe development of this form of energy, we would never achieved the faster paceof growth rate as today the world is growing and this has become possible only dueto the development of power system.

THANK YOU