Feed water system

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In a boiler feed water system following sub systems are involved.
  1. FST
  2. Booster pump
  3. Boiler feed pump 
  4. Drive of boiler feed pump (motor or turbine)
  5. High pressure feed water heater
  6. Feed regulating station / feed control station
  7. Economiser
FEED WATER SYSTEM



Feed Storage Tank
    Water is pumped from feed storage tank to the boiler or boiler drum with the help of boiler feed pump. there are two or three boiler feed pumps are installed in a single unit. this is a high pressure pump. the discharge pressure of this pump is greater than that the boiler drum pressure due to which water in flow in to the boiler drum.
    Boiler feed pump is a special type of pump. this is a multistage pump. generally a multistage pump is called as a turbine type pump.

    FST(Feed Storage Tank) is a part of Deaerator which reserve store the treated condensate water. fst may be an integral part of deaerator or it may be installed bellow the deaerator tank.The FST is installed at a specific height to provide the NPSH to the boiler feed pump.

    The treated water is a medium to the boiler called as boiler feed water and this water is free from any dissolved gases and oxygen.
    The working pressure and temperature of boiler feed water system is very high.
    Heat released by the burning of fuel is supplied to the boiler feed water which supplied to the boiler. So finally feed water is converted in to the steam in the boiler after receiving the heat.

Boiler feed pump
It is a multistage centrifugal pump.
It generate higher pressure against the boiler drum pressure.
The boiler feed pump drive of two types
Electric motor driven boiler feed pump.
Turbine driven boiler feed pump.
Boiler feed pump always keep hot stand by. If any pump failure occurs then standby pump auto start.
Suction pipe of boioer feed pump is connected th the discharge of booster pump.Booster pump suction line is connected to the heat Storage tank.
A suction isolation valve is connected before booster pump. Also a suctio strainer is installed in between suction valve and booster pump. For monitoring the differential pressure a DP monitoring instrument is installed in suction in let and out line.

In a unit feed water balance is as given 
Feed water flow = boiler evaporation rate+ boiler blow down.
Feed water flow = fresh makeup water + return condensate water + incoming drip to Deaerator+ steam supplied to Deaerator.

Generally in a plant approximate 70 to 80 % of condensate return.

Diagram 

Deaeration of water in Deaerator 
Mechanical deaeration takes place in to the Deaerator
In Deaerator all dissolved gases are removed. For best quality chemical treatment is carried out.

Why deaeration is carried out
to remove oxygen carbon dioxide and other non condensable gases in coming from condensate water.
To heat the incoming condensate water at an optimum temperature before supplying to the boiler.

Treated boiler feed water prevent the corrosion in the boiler.
Complete oxygen removal is not possible by mechanical deaeration alone.
With the help of deaerator oxygen concentration in feed water is reached upto 7 ppb or 0.005 cc/l and zero free carbon dioxide.
this oxygen is further removed by the chemical treatment with the help of oxygen scavengers. For oxygen removal oxygen scratches are you which are prepared in separate chemical tank and it dosed in the suction of boiler feed pump.
This provide complete protection of the boiler against oxygen corrosion.
 In condensate water oxygen concentration is high. 
Oxygen entered at the following points.
LP turbine gland
Valve gland connected to condensers.
Leakage through condenser flanges.
Makeup water additions (as CST is open to atmosphere)
Air leakage to vacuum breaker.

FST is a fully pressurized tank innwhich large quantity of treated condensate water is stored.in smaller system it Deaerator and feed storage tank are integral part.for increasing the temperature of feed storage tank water steam is supply. When steam is supply to FST highly noise is created.
As hot water contain less oxygen for carry.
After downstream of FST ammonia and hydrazen ase added.
A water level indicator is fitted to the FST for measurement of water level.





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