Feedwater System

transcript

Feed Water System

Presentation outline

• Feed Water System

• Boiler Feed Pump

• BFP constructional details

• HP Heaters

• Feed Regulating Station

• Drip System

Components of feed water system

• Feed Water System The main equipments coming under this system are:

• Boiler Feed Pump : Three per unit of 50% capacity each located in the '0' meter level in the TG bay.

• High Pressure Heaters: Normally three in number and are situated in the TG bay

• Drip Pumps : Generally two in number of 100% capacity each situated beneath the LP heaters.

• To give the required pressure to the

Feed water before entering into boiler

• Horizontal barrel type multi stage pump located at Zero meter height.

Boiler Feed Pump

BoosterPump

MotorMain Pump

Suction Strainer

S S for Main Pump

Feed WaterFrom D/A

HydraulicCoupling

To HPH. Recirculation to FST

General Arrangement of BFP

BOILER FEED BOILER FEED PUMPPUMP

NDE BEARING DE BEARING

SOLE PLATE

(BASE PLATE)

FOUNDATION BOLT

KEYSLEEVE

Booster Pump

To Ensure a Net Positive Suction Head (NPSH) for

Main Feed Pump

Saves a Costly Main Pump due to any damage from

Cavitation.

Typically a single-stage, centrifugal pump

Recirculation :

To maintain the Continuous flow of feed water when

the valve to Economizer is Closed

Components of BFP

Suction Strainers

To Protect Booster pump and main pump against

Catastrophic failure due to entry of coarse particles

Hydraulic Coupling

Used to Transmit power from motor to pump as per the

Load requirement

Components of BFP

NPSH is an acronym for Net Positive Suction Head. It shows the difference, in any cross section of a generic hydraulic circuit, between the pressure and the liquid vapor pressure in that section.NPSH is an important parameter, to be taken into account when designing a circuit : whenever the liquid pressure drops below the vapor pressure, liquid boiling occurs, and the final effect will be cavitations : vapor bubbles may reduce or stop liquid flow.In pump operation, two aspects of this parameter are called respectively NPSH (a) Net Positive Suction Head (available) and NPSH (r) Net Positive Suction Head (required), where NPSH(a) is computed at pump inlet port, and NPSH(r) is the limit NPSH the pump can withstand without cavitating.

• Considering the circuit shown in the picture, in 1-1 NPSH is :

• NPSH = P0 + H - Y - Vt• (to be solved with

coherent measuring units), where Y is the friction loss between 0-0 and 1-1, and Vt the liquid vapour pressure at the actual temperature in section 1-1.

Booster Pump Major damage in BF pump is from cavitation

or vapour bounding.

Provides positive pressure at the pump suction.

CONSTRUCTIONAL FEATURES

OF BOILER FEED PUMP

Horizontal, Multi Stage, Barrel Casing, Single Suction, Radial Flow.

High Efficiency. Fully Cartridgised construction. Stiff Shaft design. Thermal shock capability. Hence no warm up. Balance Brum and tilting pad Thrust Bearing for Axial

thrust. First Stage Impeller erosion life : 40,000 hrs (minimum) Shaft sealing by Mechanical Seals. Compatible materials for rotating and stationary parts.

DESIGN FEATURES OF BFP

BFP CARTRIDGE ASSEMBLY COMPRISES OF THE FOLLOWING SALIENT PARTS :• SHAFT• IMPELLERS• DIFFUSERS• RING SECTIONS• SUCTION GUIDE• DISCHARGE COVER• BEARING HOUSINGS• BEARING BRACKETS• JOURNAL BEARINGS• THRUST BEARING• MECHANICAL SEALSIN OTHER WORDS BFP CARTRIDGE IS A COMPLETE PUMP EXCEPTING BARREL (PUMP CASING).

BOILER FEED PUMP CARTRIDGE

SPARE BFP CARTRIDGE OFFERS THE FOLLOWING ADVANTAGES

IN CASE OF A BREAK DOWN OF RUNNING PUMP, SPARE CARTRIDGE CAN REPLACE IT TOTALLY.• DOWN TIME FOR CHANGE OVER WILL BE MINIMUM (ONE OR TWO SHIFTS)• SPARE CARTRIDGE RESTORES THE ORIGINAL EFFICIENCY OF THE PUMP

ADVANTAGES

BFP BARREL & CARTRIDGE

BFP CARTRIDGE

BFP WITH COMMON FOUNDATION FRAME

BP ASSEMBLY WITH BASE FRAME

BOOSTER PUMP TUBING

BFP SEAL COOLER PIPING

COUPLING ASSEMBLY

MECHANICAL SEAL

• The use of mechanical seal reduces the loses of feed water in the stuffing box to a minimum.

• Working ability of the feed pump increases. • Cooling of stuffing box space should be perfect by the use

of mechanical seal.• Cooling is carried out by the circulation of water between the

stuffing box space and the cooler. Even after stopping the pump stuffing box cooling should be continued as its cooling circuit is different from the seal cooler.

• Coolers are designed to keep the stuffing box space temperature below 800C.

It consists of two highly polished surfaces, one surface connected to the Shaft and the other to the stationary part of the Pump.

• Both the surfaces are of dissimilar materials held in continuous contact by a spring.

• These wearing surfaces are perpendicular to the axis of Shaft.• A thin film of working fluid between these faces provides cooling &

lubrication.

MECHANICAL SEAL

SEAL HOUSING It houses the Mechanical Seal.

• They support the Pump Rotor.• They keep the Shaft or Rotor in correct alignment with

stationary parts under the action of radial and axial loads.

• They are of two types :

• Line Bearings

• Thrust Bearings

BEARINGS

• They give radial positioning to the rotor.• They are of two types :• Antifriction Bearings• Sleeve Bearings

Line Bearings

They locate the rotor axially & take residual axial thrust.

• They are fitted in the NDE Bearing Housing.• They have 8 white metal lined tilting pads held in a

split Carrier Ring positioned on each side of the Thrust Collar.

Thrust Bearings

• They house Journal Bearing at the DE side and both Journal & Thrust Bearings at the NDE side.

• These are in the form of cylindrical castings split on the horizontal Shaft axis, located one each at DE & NDE sides of the Pump.

BEARING HOUSINGS

BEARING HOUSING ASSEMBLY

A DETAIL VIEW OF BEARING HOUSING ASSEMBLY

It houses the hydraulic components of Pumps.• It prevents the leakage and guides the liquid in a

proper direction.• It is closed by Suction Guide at it’s suction side and

Discharge Cover at it’s discharge side.

PUMP CASING

• It guides the fluid from suction pipe to the eye of the Impeller.

• It closes the drive end of Pump Casing and forms the suction annulus.

SUCTION GUIDE

It closes the NDE of Pump Casing and forms the balance chamber.

• It is closed by NDE Water Jacket and Mechanical Seal Housing.

• A Spring Disc is located between the last stage Diffuser and the Discharge Cover Balance Drum Bush.

DISCHARGE COVER

It rotates the mass of fluid with the peripheral speed of its vane tips, thereby determining the head developed or the Pump working pressure.

IMPELLER

• It converts Kinetic energy of the fluid into Pressure Energy.

DIFFUSER

It consists of Ring Sections located one to another.• Each Ring Section houses one Impeller and one

Diffuser.• Ring Sections along with Diffusers form the passage

of liquid from the Impeller outlet of one stage to the Impeller inlet of the next stage.

RING-SECTION ASSEMBLY

• It consists of Shaft, Impellers, Balance Drum, Thrust Collar, rotating parts of Mechanical Seals and the Pump Half Coupling.

• It is dynamically balanced.

ROTATING ASSEMBLY

Balancing Device

• A small portion of the feed water in the order of about 10% which is not calculated to the guaranteed delivery capacity is taken off from the space behind the last impeller for the operation of the automatic balancing device to balance the hydraulic axial thrust of the pump rotor.

• The purpose of the balancing device is to take up thrust pressure in a similar way as the thrust bearing.

BALANCE DRUM

• The Balancing Chamber is connected either to the Pump suction or to the Deaerator, thus the back pressure in Balancing Chamber is slightly higher than the suction pressure.

• 95% of axial thrust is balanced by the Balancing Drum.• Residual axial thrust is taken by Thrust Bearing.• Provision of Thrust Bearing is recommended for varying

condition of Head and Flow which affect the axial thrust.

BALANCE DRUM

Lubricating System

• All the bearings of boiler feed pump, pump motor and hydraulic coupling, are force lubricated.

• In addition to the lubrication oil is used in hydraulic coupling for the BFP scoop control also.

• AOP supplies the forced oil for the lubrication as well as for the scoop control.

• Working oil and lub oil coolers are used for cooling the oil.

CONSTRUCTIONAL FEATURES

OF BOILER FEED BOOSTER

Horizontal, Single Stage, Double Suction, Axial Split Casing, Radial Flow.

Double Suction Impeller for minimum NPSHR.

Shaft sealing by Mechanical Seals.

Compatible materials for stationary and rotating parts.

DESIGN FEATURES OF Booster Pump

Turbine Driven Boiler Feed Pump

• The single cylinder turbine is of the axial flow type. The live steam flows through the emergency stop valve and then through the main Control Valves (5 nos. (Nozzle governing) These valves regulate the steam supply through the turbine in accordance with load requirements.

• The journal bearings supporting the turbine shaft are arranged in the two bearing blocks. The front end-bearing block also houses the thrust bearing, which locates the turbine shaft and takes up "the axial forces.

TDBFP• There are 14 stages of reaction blading. The

balancing piston is provided at the Steam admission side to compensate the axial thrust to the maximum extent. Since the axial thrust varies with the load, the residual thrust is taken up the thrust bearing. The leak off from the balancing piston is connected back to the turbine after 9th stage.

• The turbine is provided with hydraulic and electro-hydraulic governing system. A primary oil pump is used as a speed sensor for hydraulic governing and Hall Probes are used as a speed sensor for electro hydraulic governing.

• Whenever steam is drawn from the cold reheat line or auxiliary supply, steam flow is controlled by auxiliary control valve. During this period the main control valves (4 nos.) will remain fully opened and the bypass valve across it will remain closed. (Bypass remains closed for a short period when changeover from IP steam to CRH takes place).

• The steam exhaust from the BFP- Turbine is connected to the main condenser and the turbine glands are sealed by gland steam.

HP Heaters

• These are regenerative feed water heaters operating at high pressure and located by the side of turbine. These are generally vertical type and turbine bleed steam pipes are connected to them.

• HP heaters are connected in series on feed waterside and by such arrangement, the feed water, after feed pump enters the HP heaters. The steam is supplied to these heaters form the bleed point of the turbine through motor operated valves. These heaters have a group bypass protection on the feed waterside.

To FRS

From BFP Discharge

Water side of HP Heaters

Group Bypass scheme• In the event of tube rupture in any of the

HPH and the level of the condensate rises to dangerous level which may harm the turbine blades, Group bypass protection device diverts automatically the feed water directly to boiler, thus bypassing all the HP heaters from feed water side as well as from steam side.

HP Heaters

• Feed water flows through the tube spirals and is heated by steam around the tubes in the shall of the heaters.

• These heaters are cylindrical vessels with welded dished ends and with integrated, desuperheating, condensing and sub cooling sections.

• The internal tube system of spirals is welded to the inlet and outlet headers.

• Both feed water and steam entries and exits are from the bottom end of the heaters.

HP Heaters

• Following fittings are generally provided on the HP heaters :

• Gauge glass for indicating the drain level.• Pressure gauge with three way cock.• Air Vent cock.• Safety valve shell side.• Seal pot.• Isolating valves.• High level alarm switch.

DrainCooling

Condensing Desuperheating

Tube Surface

Extraction Steam

Heat Transfer Pattern in H P Heater

Feed Water

Operation of Unit With Heater out of Service

1. Increased Steam flow through remaining section of the turbine.

2. For Given Throttle flow Turbine output increases.

3. Turbine cycle Heat Input Increases because of low Feed Water temperature.

4. Turbine & Plant Heat rate are Poorer.

Feed Regulating Station

• In order to ensure security of supply it has been considered necessary to provide a multiple feed-water regulating valve arrangement and on 200 MW units there is a 100% regulating valve. In addition there are two smaller regulating valves for low load conditions which can also be used for standby duty.

From HPH

• The three-element feed-flow regulating system has been developed to meet the requirements of the modern boiler. The essential factors in obtaining the fine degree of regulation required are the steam flow from the boiler, the feed water flow to the boiler and the water level in the boiler drum.

• To maintain the rate of feed-water input in correct ratio to the steam output under all conditions of loading, the system measures feed flow and steam-flow and the meter readings are balanced against each other by means of a differential linkage.

Drip/Drain system

• The bleed steam from the turbine does the work of reheating in the regenerative heaters and gets condensed. This condensation is termed as drip/ drain. Drip from the Heaters is further utilized for reheating.

• HP heaters drip is cascaded from the HPH6 to HPH5 and then to deaerator.

• LPH drip is cascaded from high pressure to low pressure heaters and ultimately to the condenser via condenser boxes.

THANK YOU

Feedwater System

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