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BFWP (Boiler feedwater pump) pressure
why does is BFWP HP discharge is so high after feed water control valve

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

By J. Moszynski on 21 July, 2013 - 3:10 am

Any centrifugal pump is dependent on one thing only. Back Pressure. If a machine is rated 500 GPM at 500 PSIG, then the system is required to provide the back pressure equivalent to the 500 PSIG design point. If you are overloading a driver, that means your machine is probably operating to the right of BEP, and even possibly operating close to the "break", because your system does not have the requisite back pressure, which means the performance drops off the curve, and overloads the motor as well as creating an unstable NPSHR situation. The final result of this will be a damaged pump. You need to artificially induce back pressure via a valve or an orifice, and get the pump back onto its acceptable operational "window" of performance.

The converse is also true. Throttling a pump will cause it to operate to the left of BEP. with a corresponding increase in discharge pressure.

please remember that a centrifugal pump is a self regulating device, dependent only on the system back pressure. Hope this helps.

By Rangarajan Desikachar on 24 March, 2014 - 8:15 am

It seems BFWPs in power plants are always designed for considerably excess pressure to cater for the future and throttled to meet current requirements. Can we conclude that there is a clear advantage in using VFDs (variable frequency drives) to vary the speed of the electric motor (BFWP driver) instead operating at fixed speed and throttling?

@J. Moszynski

The first condition which you mentioned typically occurs during a boiler startup, when the FRS valves & deaerator level & pressure controller valves are manually operated. But since this slight motor overloading is just a temporary phenomenon, I have not observed the damages which you quote. Typically, the standby HP BFP will take auto start if flow demand is high enough. And one rarely approaches NPSH requirements during startup, due to low FW parameters. But of course, if this occurs during normal operations, the running BFP will trip on low deaerator level [NPSH protection].

Again, in the second condition, sustained operation is needed to cause any observable change in discharge pressure. This condition happens during pump changeover, but then it is a balance between discharge flow & re-circulation flow.

@Rangarajan Desikachar

This really depends on your boiler design. For example, you could have a high pressure economizer design & subsequent settings for economizer safety valves. In such a case, if you are going for a VFD or gearbox reduction or a hydraulic coupling [scoop]or pump stage reduction, you may have to change the economizer safety valves. Also, you may have to go for a heat transfer analysis for your economizer to see whether due to a lower economizer pressure & higher temperature rise of feedwater, whether there is a possibility of boiling in the tubes [the economizer tubes might not have been designed to handle 2-phase flow]. If such is the case, you may have to change the economizer tubing as well. Again, the same may also apply for your evaporator tubes/water wall, depending on the location of your FRS valves. Also, there is the possibility of reduction in flexibility of drum level & pressure control due to increased response time [your FRS is now just a dummy, your BFPs have to respond]. So, whether or not you could go for a VFD should be considered after all these aspects.

Can you explain to me a question:

In my power plant, there are 2 BFP for 1 Unit and they are connected together.

So I confused, when 2 pump in operated, if differences pressure appear between 2 pumps in discharge line. How can reduce this differences pressure?

This is a common enough occurrence when you replace am existing old BFP with a new one. The new pump is likely to be more efficient with higher discharge pressure and flow than an older one. What happens in such a case is that the more efficient pump is likely to take a larger share of the load, ie it will supply more flow and it will draw a higher motor current. In normal cases, this difference is not likely to cause any problem but you should check for

1) the running motor current with respect to the rated value

2) the differential temperature between the pump discharge & suction

3) the current suction & discharge flows of the pump.

However, if the difference in discharge pressure is sufficiently high [10 ksc or more], you may need to take some action. The very first thing to do would be to clean/replace the suction side strainer of the pump. If there is still significant difference between discharge pressures, then one would have to throttle the suction or discharge valve [i.e. reduce the opening of the suction or discharge valve of the pump which you wish to unload]. If you have a manual suction valve and a motorised discharge valve, then it would be better to adjust the suction valve. However, your pump might have a trip condition on suction valve not open. In such a case, you would need to shut down the pump, dismantle & re-install the limit-switch of the suction valve, after adjusting the valve opening so that it will sense valve open at the adjusted value. This is a trial-and error method and may require a number of start-ups & shutdowns. The same can be done to the discharge valve also but you would need to operate the valve actuator manually from local for proper adjustment.

While you could do these, the best approach would be to replace the other older pump as well from the OEM, so that you can have a better flow balance as well as operational flexibility.

High pressure feedwater heaters

http://mda139.net/feedwaterheat/functional-needs.html