FEED WATER SYSTEM

INTRODUCTION

• The source of feed water is FST into the feed water system, (in other words, we may say that FWS starts here).
• In FST water coming from Deaerator after deaeration process & stored in to feed storage tank (also initial make up line is provided used for pre filling of deaerator).
• Deaerator (& FST) installed at a height (normally 22-32 meter) provide NPSH to the Boiler Feed Pumps.
• Feed water is high pressure deaerated water supplied to the boiler from which steam is generated.
• Boiler feed pump develops very high pressure to supply the feed water to the boiler drum.
• After the steam is used in turbine then dumped to the main condenser.
• From the condenser it is then pumped to the deaerater & feed storage tank with the help of CEP.
• Feed water is free from gases like oxygen & carbon dioxide.
• The feed water is never open to the atmosphere.
• A condensate system is a low pressure feed heating system while a feed water system is high pressure feed heating system.
• Condensate system extract condensate water from hot well & delevered in to the deaerator.
• Feed water system supplies heated feed water to the boiler from deaerator.

FUNCTION OF FEED WATER SYSTEM

• To supply high-pressure feed water to the boiler during startup, normal, & emergency operations.
• To provide adequate flow of water to the boiler.
• Maintain the boiler drum level within operating range as load varied on boiler by regulating the feed water flow (for subcritical units).
• In once through boiler drum level control loop is not used.
• Feed water system also supplies feed water to the following purposes (for control the steam temperature) with the help of tapping lines.
  • Spray for HPBP.
  • Spray for MS temperature control.
  • Spray for RH steam temperature control.
  • Provide chemical dosing for scavengers the dissolved gases and increase the pH value of water.

PROCESS INVOLVED IN FEED WATER SYATEM

• BFP increases the pressure of feed water above boiler pressure .
• Feed heating included pre heating of feed water at various stages (In FWH & Economiser) before it enter in to the boiler.
• Its temperature is increases up to saturation temperature (Closed to saturation temperature).
• For heating of feed water high temperature extraction steam is used from turbine.
• Also use of economizer which utilizes the waste heat in a boiler flue gas to pre heat feed water before enters in to the boiler.
• This system automatically maintains the proper flow in the boiler drum in auto mode, (while in manual operation we can regulate proper flow & drum level).
• The BFP supply a constant flow to replace water in the boilers that has been changed to steam.
• The BFP must also develop the required pressure to overcome head & drum pressure.
• Ensure minimum flow of water through recirculation control of BFP when water is not utilized for boiler.
• Hydrazine & ammonia are added in to the suction of BFP line.
• Also an economizer recirculation line is given to prevent the starvation in economizer.

HEAT TRANSFER IN FEED WATER

SYSTEM MAJOR COMPONENTS

• FST
• Boiler Feed Pumps
• Feed water Heaters
• Feed Regulating station
• HPBP spray lines
• Ammonia & hydrazine dosing
• Suction line of BFP
• Discharge header of BFP
• Recirculation of BFP
• SH & RH spray line.
• Economizer

SYSTEM FLOW PATH

• Feed water System starts from FST.
• Booster pump take suction from FST & raise the feed water pressure as required for unit.
• The BFP discharges the high pressure feed water to the HPFWH
• (Where its temperature is increases up to saturation temperature, extraction steam from the turbine supplies the heating medium in HPFWH.
• After this water goes to Feed Regulating Station (FRS) or Feed Control Station (FCS) where its quantity is regulated as desired for boiler.
• After feed water heater water enters in to the economiser which is installed in side the boiler.
• In economizer heat is transferred from flue gas to water.
• If cool feed water entered into the economizer then temperature of the tubes reduces to a point at which the Sulphur oxides in the flue gas caused the gas side of the tubes to sweat and corrode.
• To overcome this problem feed heaters were installed, and these were often supplied by steam.
• A water line is goes for HPBP spray before HPFWH-5.
• Another pipe line is goes to MS & RH spray after HPFWH-6.

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