Boiler Feed Water System

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Boiler Feedwater Sy

S Shariq Asst. General M

Allied Energy Systems PS Shariq Ahmed


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INTRODUCTION

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Function

Boiler feedwater is water used to supply ("feed") a boiler to generate

steam or hot water. At thermal power stations the feedwater is usuallystored, pre-heated and conditioned in a feedwater tank and forwarded

into the boiler by a boiler feedwater pump

The function of the Feedwater System is to supply high-pressure water

the boiler during start-up, normal, and emergency operations.

The System automatically maintains the proper flow to and water leve

the boiler drum.

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Function

The Feedwater System also supplies water for Desuperheater Sprays thcontrol the Superheat steam temperature.

The feed water used in the steam boiler is a means of transferring heaenergy from the burning fuel to the mechanical energy of the spinningsteam turbine.

The total feed water consists of re-circulated condensate water andpurified makeup water.

The makeup water in a 500 MW plant amounts to perhaps 20 US galloper minute (1.25 L/s) to offset the small losses from steam leaks in the

system.

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Basic System Description

Feedwater is supplied by the Deaerators, which

also provides the necessary Net Positive SuctionHead (NPSH) to the Boiler Feed Pumps.

The Boiler Feed Pumps (BFP) must supply a

constant flow necessary to replace water in the

boilers that has been changed to steam.

The Boiler Feed Pumps (BFP) must also develop

the required pressure to overcome head and

drum pressure.

Deaerators

Boiler Feed Pu

Feedwater He

Feed Water Regu

Valves

Boiler Econom

Inlets

Feedwater System

Diagram

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Basic System Description

Boiler Feed Pumps on Units supply the

Feedwater System High Pressure Heaters and

Header.

The Feedwater header supplies heated, high-

pressure water, through level control valves, to

the boiler drums on Units

The Feedwater Header supplies Desuperheater

Sprays to control the steam temperature.

Deaerators

Boiler Feed Pu

Feedwater He

Feed Water Regu

Valves

Boiler Econom

Inlets

Feedwater System

Diagram

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System Flow Path

The Feedwater System starts at the

bottom of the Deaerator.

The Feedwater exits the bottom of th

Deaerator to the suction of the Boile

Feed Pumps (BFP).

The Deaerators are high enough to

supply the Net Positive Suction Head

(NPSH) to the Boiler Feed Pumps.

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System Flow Path

Boiler Feed Pumps, are motor driven

turbine driven pump.

The pumps can provide Feedwater to

of the High Pressure (HP) Feedwater

Heaters.

After the Feedwater is heated in the

heaters it is then sent through drum

control valves on the boilers and from

the drum level control valves to the

Economizer inlet valves.

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System Flow Path

The Feedwater System starts in the

Deaerator Storage Tank; water exits t

Deaerator Storage Tank and is high

enough to supply the NPSH (Net Posi

Suction Head) for the suction of the

Boiler Feed Pumps.

The Boiler Feed Pumps can also pum

water to any of the HP heaters and

boilers.

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System Flow Path

The Deaerator Storage Tank provides

NPSH to the Boiler Feed Pumps suctio

The Boiler Feed Pumps pump the

Feedwater through the drum level

control valve to the Low Temperature

Economizer.

There is a tie line between the Deaer

on Units.

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System Flow Path

With this tie line Feedwater from Uncan supply the Boiler Feed Pumps on

Unit and vice versa.

The Feedwater System also suppliesfeedwater to the De-Superheater Sprnozzles to cool the steam between th

465 pound steam system and the 165pound steam system.

The De-Superheater Sprays are also uto control steam temperatures from Boilers.

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System Flow Path

In the Feedwater System is a shell an

tube Feedwater Cooler, which uses

cooling water from the Cooling Towe

cool the feedwater.

This cooler takes a portion of the

feedwater from the High TemperaturFeedwater Heater off boiler, this wat

passes through the cooler and discha

the cooler feedwater to the Deaerato

Storage Tank. At the present time thi

cooler is not in service.

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SYSTEM MAJOR COMPONENTS

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System Major Components

The Feedwater System consist of the following major components:

Boiler Feed Pumps

Feedwater Heaters

Feedwater Regulation

De-Superheating Spray

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Boiler Feed Pumps (BFP)

Boiler Feed Pumps are motor driven and turbine

driven.

Although steam turbines cannot be used for

start-up unless a separate source of steam is

available for there operation.

With any pump, the pressure tends to fall as the

throughput increases rises, on the other hand

due to the effect of friction, the resistance

offered by the boiler system to the flow of water

increases as the flow rate increases.

Boiler Feed Pum

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Therefore the pressure drop across the valve will be highest at low flow

It is wasteful to operate with the pressure drop which is significantly

above that at which effective control can be maintained, both because

entails an energy loss and also because the corrosion of valve internals

increases with high pressure drops.

With fixed speed pumps there is nothing can be done about this but a

improvement can b made if variable speed pumps are used.

Variable speed pumps are more expensive but the increase in cost ten

to be offset by the operational cost savings.

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Such savings are increased if the plant operates for

prolonged periods at low throughputs and are most

apparent with the larger boilers.

From the engineers view point, variable speed

pumps are an attractive option because of the

following reason:

They enable the control system to be linearized over

a wide range of flows, leading to improved

controllability.Boiler Feed Pu

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All of the pumps are centrifugal pumps.

The pumps are equipped with recirculation valves.

When operating the BFP's for some time at

reduced capacity, much of the pump horsepower is

transferred into the liquid in the form of heat.

A recirculation line prevents the liquid in the pump

from becoming hot enough to vaporize and causing

cavitation and possible BFP impeller damage.

Boiler Feed P

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The recirculation lines are piped from the

discharge line of each of the BFP's prior to

the discharge check valve.

The recirculation flow is routed through a

manual block valve, air-operated valve,

orifice and another manual block valve to

the suction line of the BFPs.

Boiler Feed Pumps

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The recirculation valve automatically opens when the BFP discharge

decreases to approximately 45,000 pounds per hour flow to assure a f

above the minimum flow required. At 40,000 pounds per hour an alarm

sounded in the Control Room.

Each pump has a recirculation valve which recirculates water back to t

suction line.

Each pump has a recirculation valve which recirculates water back to t

condensate inlet of the Deaerator. These BFPs only supply feedwater t

Low Temperature Economizer inlet.

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Boiler Feed Pump Controls

All the Boiler Feed Pumps are controlled

and monitored from the DCS displays in the

Control Rooms.

Boiler Feed Pumps

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Feedwater Heaters

A feedwater heater is a power plant component used to pre-heat wate

delivered to a steam generating boiler.

Preheating the feedwater reduces the irreversibility involved in steam

generation and therefore improves the thermodynamic efficiency of th

system.

This reduces plant operating costs and also helps to avoid thermal sho

to the boiler metal when the feedwater is introduced back into the ste

cycle.

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Feedwater Heaters

Pressure classification

Low pressure heater

A heater located between the

condensate pump and either the

boiler feed pump or , if present , an

intermediate pressure (booster)pump. It normally extracts steam

from the low pressure turbine.

It normally extracts steam from the

low pressure turbine

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LP Closed Feed Water Heater

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Feedwater Heaters


Intermediate pressure heater

A heater located between the booster pump and the boiler feed pum

Usually the tubeside pressure is within 200-300 psi of the low pressur

heaters , and the steam is extracted from the intermediate pressure

turbine.

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Feedwater Heaters


High pressure heater

A heater located downstream of the boiler feed pump.

Typically, the tubeside design pressure is at least 1500 psig, and the st

source is the high pressure turbine.

With a similar purpose to the low pressure feed heaters, the high pres

feed heaters are the last stage of feedwater heating before the feedw

enters the boiler system at the economizer

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High Pressure Heater

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HP Closed Feed Water Heater

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Feedwater Heaters

Orientation

Vertical, Channel Up

These are the least frequently used.

Disassembly is by means of bundle removal.

If a subcooling zone is present, it must extend the full length of the

bundle, since the water must enter the bottom and exit at the top en

the heater.

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Feedwater Heaters

Orientation

Horizontal

Most heaters are of this configuration.

These are the most stable in regard to level control, although they occ

more floor space.

Disassembly is by means of either shell or bundle removal.

Most are floor mounted, although some are mounted in the condenseexhaust neck.

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Feedwater Heaters

Zones

Zones are separate areas within the shell in a feedwater heater

Condensing Zone

All feedwater heaters have this zone. All of the steam is condense

this area , and any remaining non condensable gases must be

removed. A large percentage of energy added by the heater occu

here.

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Feedwater Heaters

Zones

Zones are separate areas within the shell in a feedwater heater

Subcooling Zone

The condensed steam enters this zone at the saturation tempera

and is cooled by convective heat transfer from the incoming

feedwater.

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Feedwater Heaters


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Feedwater Heaters

Type of feedwater heaters

Open feedwater heater.

An open feedwater heater is merely a direct-contact heat exchanger i

which extracted steam is allowed to mix with the feedwater

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Feedwater Heaters


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Feedwater Heaters

Type of feedwater heaters

Closed feedwater heater.

Closed feedwater heaters are typically shell and tube heat exchanger

where the feedwater passes throughout the tubes and is heated by

turbine extraction steam

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Feedwater Heaters


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Feedwater Heaters

Feedwater Heaters are of the shell and tube, extraction steam type an

are flue gas type heaters.

Steam from the turbine extraction or from the steam header can supp

these heaters

The feedwater heaters are shell and tube, U-tube type heaters.

Feedwater enters the bottom of the heater and passes through the tu

picking up heat from the steam on the shell side of the heater; the

feedwater exits the top of the heater.

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Feedwater Heaters


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Feedwater Heaters

Feedwater Heater Controls

The inlet and outlet feedwater valves to the Extraction Steam Feedwater

Heaters on Units are manually operated valves.

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The objective of feed water control may seem simple : it is to supply

enough water to the boiler to match the rate of evaporation.

But doing so is a complex mission because of the following reasons:

There are difficulties even in the basic level drum measurements on which

control system depends.

Interactions occurring within the boiler system and the effects of these

interactions are smaller or greater at various points in the boilers load ran

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The Feedwater Regulation is controlled from the DCS (distributed cont

system) in the Control Rooms.

The Feedwater Regulation maintains a constant boiler drum level by

regulating feedwater flow to the boiler drums.

The system provides for drum level control from startup through thenormal full operating load range.

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Feedwater Flow is controlled by two (2) separate modes of control:

Single element The single-element system responds to the drum level to

control the level control valve. The level controller compares the drum lev

set point, set on the DCS to the actual drum level. The controller in AUTO

generates a signal, which regulates the level control valve to maintain the

drum level. In MANUAL the level is controlled by the operator.

Three-element The three-element system utilizes measurements of stea

flow, feedwater flow and drum level to control feedwater flow.

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Single Element Control

The level of water in the drum provides and immediate indication of the w

contained by the boiler.

If the mass flow of the water into the system is greater than the mass flow

steam out of it, the level of water in the drum rises and vice versa.

As the level of water in the drum rises , the risk increases of water being

carried over into the steam circuits.

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If the level of water falls there is a possibility of the boiler being damaged

partly because of the loss of essential cooling of the furnace water-walls

The target of feed water control system is to keep the level of water in the

drum to approximately the midpoint of the vessel.

The level of water is affected by transient changes of pressure within the d

and the sense in which the level varies is not necessarily related to the sen

in which the feed flow must be adjusted.

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The situation arises due the following effects:

Swell

Shrinkage

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Swell

Boiling water comprises a turbulent mass of fluid containing man

steam bubbles, and as the boiling rate increases the quantity of

bubbles which are generated also increases.

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Swell

The mixture of water and bubbles resemble foam, and the volum

occupies is dictated both by the quantity of water and by the am

of the steam bubbles within it.

If the pressure within the system is decreased, the saturation

temperature is also lowered and the boiling rate therefore increa

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Swell

As the boiling rate increase, the density of water decreases, but

the mass of water and steam has not changed , the decrease in

density must be accompanied by an increase in volume of the

mixture.

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g



Swell

By this mechanism the level of water in the drum appears to rise

phenomenon referred to as SWELL.

The rise of level is misleading, it is not indicative of the real incre

in mass of the water of system, which would require the supply o

water to be cut back to maintain the status quo.

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Swell

If the drop in pressure is the result of the steam demand sudden

increasing , the water supply will need to be increased to match increased steam flow.

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Shrinkage:

It is the opposite of swell.

It occurs when the pressure rises.

The mechanism is exactly the same as that for swell, but in the

reverse direction.

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Shrinkage:

It causes the level of water in the drum to fall when the steam flo

decreases, and once again the delivery of water to the boiler murelated to the actual need rather than to the possibly misleading

indication provided by the drum level transmitter.

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Shrinkage:

The effect of swell and shrinkage in addition to being determined

the rate of change of pressure, also depends on the relative size the drum and the pressure at which it operates.

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Shrinkage:

If the system pressure is low the effect will be larger than with a

boiler operating at higher pressure, since the effect of a givenpressure change on the density of water will be greater in the lo

pressure boiler than it would if the same pressure change were t

occur in a boiler operating at higher pressure.

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Shrinkage:

The simplest solution for these is implementation of a two elem

system since it is based on the use of two process measurementplace of the single drum-level measurements used.

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Two element system

A valve with a linear characteristic is employed in conjunction with atransmitter that produces a signal proportional to steam flow.

If the transmitter produces a signal which is equal to the steam flow at all

loads and if the flow through the valve is matched with this signal at every

point in the flow range, a controller gain of unity is will ensure that ,throughout the dynamic range of the system, the flow of water will always

equal to the flow of steam.

The correct gain of the controller can be determined from a knowledge of

swell and shrinkage effects within the boiler.

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Two element system

Theoretically better results can be obtained by carrying out tests to determthe swell effect at various points in the load range and introducing a non-

linear function within the level controller to compensate for the difference

across the range.

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Two Element System


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Three element system

In two element system it is assumed that the feed water valve characterislinear and the valve is sized to produce a fixed flow when it is 100% open.

However the flow through the valve depends both on its opening and on t

pressure drop across it.

In a feed water system, the pressure drop across the valve varies from inst

to instant, and the flow through it at any given opening will therefore vary

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The varying flow results in the level control becoming offset, to restore thesteam flow/ feed flow balance. This offset is undesirable since it needlessl

erodes the safety margin provided by the presence of the drum.

One method of correcting this error produced by the feed valve is the

addition of a third element to the system- a measurement of feed-water f

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Ways of implementing a three element system are:

1st Method

The output of the drum level controller is trimmed by a signal

representing the difference between the feed flow and the steamflow signals.

A gain block is introduced to compensate for any difference betw

the ranges of the two transmitters.

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1st Method

In most cases the steam flow and the feed flow signals will cance

out, and the drum level controller will be modulating the feed floto keep the level at the set point.

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Three Element System (1st Method)


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2nd Method

In this method a cascading control technique is applied.

The drum level controller compares the measured level signals w

set value and produces a bipolar output proportional to any erro

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2nd Method

This trims a modified steam flow signal , which is acting as the

desired value for a closed loop feed water controller.

As previously a gain block adjusts for any range difference betwe

the steam flow and the feed flow transmitters.

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Three Element System (2nd Method)


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Discrepancies between drum readings

Density errors- differences between installations can cause one instrumebe more affected by density factors than another.

Turbulence- standing waves exists around the downcomers, affecting som

measurement points more than the others.

Flashing off- differences in the geometry of the measuring systems can ca

some measurements to be more affected than the others by flashing off

during the pressure changes.

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Discrepancies between drum readings

Calibration- it is vital that all the transmitters are carefully and accuratelycalibrated, and that any density compensation is correctly set up.

Installation- errors or sluggish response can be the result of partial or

complete plugging of impulse lines, or imperfect blow down operations.

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In AUTOMATIC, the steam flow is the primary signal to the drum level

controller.

The drum level controller then compares drum level to drum level setpoin

on the DCS.

The output signal from the drum level controller is equal to steam flow if

actual boiler drum level is equal to boiler drum level setpoint.

An actual low boiler drum level is corrected by the drum level controller

generating an output signal greater than steam flow.

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In AUTOMATIC, the steam flow is the primary signal to the drum level

controller.

An actual high boiler drum level is corrected by the drum level controller

generating an output signal less than steam flow.

Thus, the output of the drum level controller is the desired Feedwater flow

which is equal to steam flow if the actual boiler drum level is at setpoint.

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SYSTEM OPERATION

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System Startup


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All system components that have been tagged out for maintenance ha

had tags cleared and removed.

All control and indication instrumentation on which maintenance or

calibration has been preformed has been returned to service. The

Instrument Air System is in service.

The system should be walked down to verify that all pump and motorlubrication are at their proper levels.

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System Startup


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That the systems valves have been placed in their proper startup posit

with all drains closed except where otherwise noted.

That all AC and DC electrical power supplies has been racked in and re

for service.

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Normal Operation

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All systems should be monitored for pressures, temperatures, flows, a

levels.

Pump and motor lubrication are monitored and filled as required.

Any abnormal conditions should be reported and corrective action tak

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System Shutdown

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The Feedwater System is normally in operation.

All pumps should be taken out of service and breakers racked out.

Close valving to isolate the system and open drain valves where system

needs draining.

Any part of the Feedwater System is to be worked on, proper clearanc

and tagging should be obtained.

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Boiler Feed Water System

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