Re:Positive Displacement pump (Main oil pump) in gasturbine not delivering required pressure.

[Rajuuma]

Hello Everyone,

I would like to know some suggestion from experts to problem i am describing here in post.

In one of the site i was working with client they have MS-5001 (Frame-5) GE gas turbine.During the starting of gas tubine when turbine had reached 85% of the speed auxillary pump has to be stopped and main oil pump on the auxillary gearbox has to develop requied pressure of 7.5 Bar.But even turbine has reached FSNL,auxillary pump was running and MOP was not developing required pressure of 7.5 Bar.I have tried force stop auxillary lube oil pump that was motor driven through control panel.But due to low pressure developing by Main oil pump,auxillary pump was immediatly coming in line as it is in auto mode.

From mechanical end later we shutdown the machine drained the lube oil tank reservoir and we calibrated pressure control valve (PCV) on downstream end of main oil pump in workshop with pressure set point of 120 PSI and found PCV was poping off at lower pressure at less that 10 PSI.New PCV was changed calibrated before assembly at lube oil tank and an non return valve has been checked and found okWe have inspected Main oil pump and found all internal are ok.Priming was done before turbine startup.When turbine was started Auxillary gearbox Main oil pump has generated pressure and auxillary pump was switched off automatically.

But After two days from proble rectified on Main oil pump again turbine was tripped due to load gearbox Load side end bearing high vibration and it was false alarm.When every thing was reset and started turbine same problem that was before i.es MOP was not developing pressure and auxillary pump is running at FSNL.


Kindly suggest me any cause.I am attching P&ID for referance.If any details required i can share in written formate.


Regards,
Rajuuma

 

[ivymike]

if the internals of a positive displacement pump are fine, then pressure is set by the downstream resistance to flow. Your PCV is one method of reducing downstream restriction to limit pressure - and it may well be malfunctioning again. Alternatively, something downstream may be allowing too much flow (leaking, etc).

On your diagram, there are several pressure measurement locations. I assume that PI-1 is the one you've primarily been discussing here? Are the others also reading low (if not, then gauge 1 is likely bad).

Which icon represents your pressure control valve? Looks like VR-1 and VPR-2 could each play a role?

 

[Rajuuma]

The Icon which represent pressure control valve is VR-1 for main oil pump control pressure valve and VPR-2 is bearing header pressure control is not a problem i mean header pressure is good as auxillary lube oil pump is running with main oil pump to compensate operating pressure of lube oil.

Gauge was correct.For information PI-1 was gauge on panel outside the machine compartment.However even if it is fauly does show correct value through calibrated transmitter data on the control panel logic (Mark-vi).
we have already checked under lube oil tank internal piping.VR-1 valve was new one and was calibrated in workshop at 80 PSI and assembled on lube oil skid 5 Days ago.

 

[fegenbush]

Here is my low pressure in hydraulic system checklist (abridged):
1. Check for leaks.
2. Check pressure relief valve(s).
3. Check actuators (includes loads).
...
...
...
87. Check pump.

In my experience, positive displacement pumps are rarely, if ever, the problem.

 

[racookpe1978]

Your mechanics (the re-assembly team) may have left out one (or more!) a orifice plates.

What piping mods did you do on the lube oil piping?
Do you absolutely know that all of your bearings are getting full-rated flow? They must be getting some oil, or they would have burned out. But, are they getting the same flow they were before the outage?
Do you have a seal oil system that may be blowing by oil or re-cycling oil somehow, or are you using a air cooled generator?

 

[Rajuuma]

They are no orifice plate as per P&ID.
We have not made any piping modification.We had visual inspection of lube oil tank at time inspected NRV and changed pressure control valve on down stream of PCV.
We are getting good rated flow for bearing header pressure always.Load with an aircooled generator.

 

[ScottyUK]

Rags left in the LO tank from cleaning, ingested by main LOP intake? This has never happened at a site I've worked at. Honestly! ;-)th