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Our BFWP HP discharge is 170 bar, but just after the hp feedwater control valve is 120 bar and valve opening is %39. can anybody please explain the reason why does the BFWP HP discharge is so high.

If it would be 130bar and hp feedwater control valve opening would be %85 than doesn't it would be enough.

regards..

HP BFP discharge pressure is the pressure developed by the pump is high (170bar in your case). So in the upstream of the control valve the feed water pressure will be same (170bar). But in the control valve throttling action will take place & the down stream of the control valve the feed water pressure will be less (130 bar in your case)that depends on the boiler Drum pressure. This pressure will be always above the drum pressure.

So due to the throttling action of the feed control valve pressure is dropping & this pressure is depends on the drum pressure.

The Boiler Feed Water Pump is designed for high capacity than the rating of the Boiler. So high pressure at the BFP discharge will give you the margin in the flow/pressure.

In the same way boiler feed water control valve are also designed for higher flows than the boiler rating. Normally at rated boiler load the feed water control valve will be around 50%.The remaining valve opening is meant for higher flow demands & to take care of control valve internals chocking etc.

Hope i cleared your doubt.

why does boiler feed pumps have such high pressures.

well , it is pretty straight forward , the BFP pressure is determined by

a. The operating drum pressure
b. Pressure drop in tubes due to fluid flow

well , the first point is obvious , if you are operating a boiler at 90 Kg/cm2 drum pressure , to force water into it you need more than 90kg/cm2. The second consideration is how much water you need to force into the drum. fluid flow through pipes always produce a pressure drop. This calculated with the help of the D'Arcy-Weisbach Equation for Pressure and Head Loss. for a given flow rate , pipe dia , density , length of piping and a friction co efficient you can find out the pressure drop in the pipes. this also need to be overcome by the boiler-feed water pressure. then there are some considerations on pressure loss on bends , temperature correction etc. then you add a factor of safety to the pressure head. now adding up all these

BFP pressure required = drum operating pressure + pressure drop in tubes ( due to bends , density variation , fluid flow , friction , gravity etc) + safety margin

as you will see , the BFP is designed to overcome the maximum possible fluid flow at the maximum design operating condition. This much pressure is not required for normal operation , thus to control the fluid flow you have a control valve. this in effect reduces the pressure and controls the water flow into the boiler.

hope i have given you a head start , you can get pretty much these information from any power plant engineering text , i would recommend

power plant engineering - black and vetach ( advanced and gives good design details for steam plant operation)
power plant engineering - PK nag ( pretty basic , good for beginners :) )

Thanks for Anil and Process Value

appreciated for your helps and supports,
also extra thanks for process value for suggesting two great books.
I Have already bought them from amazon, hope they will be as useful as you:)

regards..

Good to know that you followed up on my advice and brought the texts :). always glad to help out.

Mxromen, is your boiler feedwater pump steam turbine driven or electric motor driven?

Electric motor driven.
If extra information needed, I'll be glad to supply..

regards..

Can you please provide the technical specifications for electric motor that you are using for feedwater pump.

thanxs,
GS