Boiler Feed Pump Water Pressure

R

Thread Starter

rautpsrme

We have three boiler feed pump out of which one is stand by. When we decrease load on turbine feed water pressure drop suddenly and it remain for about 3 to 4 minute. we tried by starting stand by pump but it didn't works. All three pumps have common suction and common discharge. We have inspect suction filter of pump but it was ok. and while this going so motor current drop to no load current. please tell me why this is happening. this problem is for us first time in 8 years. I thought it may due to cavitation but from this much of years why it didn't happen.
 
I assume this is a straight steam plant with a fired boiler (rather than a combined cycle plant), so, since most of my experience is on combined cycle units, more help may be required. But, anyway, here's my input:

1. If everything worked fine for 8 years, what's different now? Any recent upgrades, maintenance or control system changes?

2. If the feedwater pressure is decreasing and there is nothing wrong with the pumps, it is likely that the water flow demand has increased with the load reduction, which would likely mean that the drum level (feedwater) control valve is being opened to supply the flow. This would most likely mean that the drum level has dropped significantly with the load reduction. Did you get any process alarms on low drum level?

3. How are you reducing turbine load? Is the load control from the steam turbine control or are you simply turning back the boiler firing? Rapidly decreasing the firing could easily collapse steaming in the evaporator which will rapidly decrease the drum level resulting in a high demand for feedwater flow.

So, I think you need to determine what is really causing the pressure drop, and not automatically assume the problem is on the feedwater supply side and maybe the trouble is on the load control side. Do you have process data from the load reduction period that show steam turbine inlet pressure, control valve position, drum level, drum pressure, feedwater control valve position, feedwater flow, boiler fuel flow, and any process alarms?
 
This reply might be helpful if your plant is a larger coal or natural gas fired boiler/steam turbine.

If the BFP's are supplied from a dearator, it might be possible that a fast and prolonged load drop could cause the dearator steam pressure to drop excessively resulting in flashing of the BFP suction line. The steam supply to the dearator should include a check valve to prevent the reverse flow of steam from the dearator to the extraction point. If the check valve is not holding, this could explain the difficulty.

Also, if hot DA water is used for process heating, and the feed for the process heating water is the same suction line as the BFP's, the flow of this process heating water helps circulate new water into the BFP suction line. If a sudden load drop occurs that causes a reduction of extraction pressure you want to get rid of the hotter water in the BFP suction line and replace it with the cooler water from the DA (that will now be normal due to the load drop). The process heating water flow helps to get rid of the hotter water. So if the process heating water system has been shut down, your unit might be more susceptible to flashing in the BFP suction line.
 
You might want to check out what happens to the boiler drum level when the load on the turbine generator is reduced. When the load is reduced the main steam header pressure might go up a little which creates a back pressure on the boiler drum and so for few minutes the drum level will show a bit low as the drum pressure increases which creates a down pressure on the water level beneath it. Its all three element control but still there is always an effect like this when noted very minutely. And so to maintain the drum level at the 50 percent setpoint the feedwater valves are opening suddenly which is causing the bfp discharge header pressure to drop. I still don't know how many boilers are there in your plant.But roughly assuming the more the boilers more the sudden flow.

By the way when this is happening is the drum level control in single element or three element?? Are the bfp's controlled in auto or manual??? Are they made to run through a VFD?? From how much amps to how much are the bfp's load coming to??
 
Top