Parallel Operation of Pumps

Were any changes made to any other part of the system 2 weeks ago? Even if it seems unrelated.

Do you have a separate feed pump going to boiler? If so, this could be faulty,causing back pressure to condensate pumps.

From the sounds of it, the problem lies somewhere between your condensate pumps and the boiler.

This sounds obvious, but have you checked to make sure all valves upstream of pumps are fully open?

We've to shut down the turbine and condenser vacuum breaks but we've checked the values of vacuum they are same as previous.

No seprate pump for boiler

You are talking about the problem at discharge of pump please elaborate as according to us the drop in amperes, flow and pressure are result of suction problem between pump and condenser.

Yes all valves at upstream of pumps are fully opened

Sorry, I misread, you said pressure decreasing.

With pressure decreasing and rpm increasing and impellers and pump checking out ok, the only thing I can think of is air getting to the pumps somehow, but I can't imagine how. Hopefully, someone can offer some insight.

Do these pumps have strainers? Possible sediment buildup.

You said the water is clean. Eliminating the possibility of a clog, but you need to visually inspect strainers to truly eliminate that option.

Kramarat is asking the most important question. I have about 15 years of power plant experience and I will say that when problems arise the #1 question to ask is "What has been done recently?" (maintenance, valve line-ups, operations, etc).... It's important to fully explore this because nearly every time it points to the answer.

Most of what you are describing sounds like the normal operation for a pump being throttled. That is, if a pump is pumping at some point on the curve and we throttle down the discharge, the pump will pump less (and the condenser level will rise), the lower flow means less energy (amps go down). The only thing that doesn't fit this scenario is the pressure is decreasing.

What is happening to the boiler level?

From the description of symptoms it sounds almost like normal operation to me. Boiler level might start a little low, system calls for water, pumps pump, boiler level nears setpoint, system calls for less water, control valve throttles more, flow/amps go down, condenser backs up. Is it possibly just a water management issue? Without more details its hard to say...

The only other thing is that being common to BOTH pumps it is unlikely a problem with either pump, it's something common between them such as a closed suction valve, etc.

Go back to Kramarat's question... What happened 2 weeks ago??

GA

I would add for clarity:

1- If the condensate temperature is different from before, or has increased from what it used to be, it might become near to the evaporation point at the pump. This could be due to the cooling circuit of the condenser (Which makes the vacuum...) having a problem (?). Then Cavitation will occur. This can explain the lower flow volume, lower pressure and the speed increase of the turbine (Steam control compensating...).

The electric motor speed will change marginally due to less power demand but remains within the nameplate value (Asynchronous) or if Synchronous motor, does not change.

Therefore: either cloging or cavitation but still at suction. Check .

Not knowing what your elevation is I assume that your vacuum is low. Any idea why?

I know it is kind of basic, but are your recirculating valves wide open?

It sounds as though the pumps are working so hard as to make the condensate boil at the pumps' inlets. If so, don't do it for too long as the cavitation caused by it will damage the impellers longer-term.

Try modulating the pump speed on the basis of the level in the condenser catch pot; presumably there is an instrument, otherwise no-one would notice the level going up.

How old are the pumps? and when did you last check impellers for cavitation?

Hi

Reading what you've said about the actions of both pumps when running is the way a submersible pump act if

either the inflow has a problem

the pump has a problem namely the impellers AND diffusers are worn.

I know you've said you have checked the pumps, have you put them on a test bench and compared the test results to the pump curve?

If you have NEW spare pumps, change out the existing ones to see if that makes a difference.

Not knowing how old, the duty time and the history of the pumps it could be just old age. Do you have a amp chart for the motor driven pump that might indicate a gradual drop in running current? Over time as the impellers wear, the running current drops as the efficiency drops even if you have super clean water, also the impeller is affected if its NOT running in its operating range, something else to check. Do you record the discharge pressure of each pump, if so look back at the charts

While I think about it have you checked the RPM of the motor? A drop in the motor RPM will change the pump curve, reducing the pump flow rate.

As you have only checked the motor driven pump check the supply frequency and voltage. For the Turbine pump, I cannot explain how the increase in RPM reduces the flow rate or why the RPM increases, it must be a large increase in RPM for you to mention it. How much does it increase by, what should the RPM be?

If new pumps don't solve your problem, then you don't have a pump problem and you need to look at something else and maybe take up the suggestions given to you in this forum

As for improvements, lets see what your further investigation finds first and how you make out with the good answers you've received so far.

If the hotwell or condenser was opened up for any work to be done, there is always the possibility that some construction material may have been overlooked when the doors were closed back up on the system. My first check would be to drain the hotwell and look for any obstruction between the hotwell and the suction side of the pumps.

Also, if the water level in the hotwell is allowed to get low, a vortex can develop at the suction of the condensate pumps. Normally, raising the level of the hotwell should reduce the vortex but not always. Perhaps you can temporarily shut off your motor driven pump to allow water to fill in the void caused by the vortex.

When you first start up to fill your deaerator, overfill the hotwell and closely monitor the hotwell level until you get the deaerator level and boiler level to normal levels.

Ron

Dear all!

Thanks for replying.

We've resolved the problem, there is a demin water valve at suction line for start up, we've tightened its glands and RTV sealent was applied at its bonnet. Now the problem has been resolved.