We have 120 MW Combine cycle power plant having Steam Turbine Generator (STG) 40 MW capacity BHEL Supplied.
The present Governing system is Eloctro Hydralic turbine controller.
We observe hunting in Main Oil Pump (Connected at turbine shaft) discharge pressure from 7.2 Kg/Cm2 to 6.4 Kg/Cm2. Normal pressure remain before this is around 8 Kg/Cm2.
Same way we have two Auxilary oil pump (AC Supply) and One Emmergency Oil Pump (DC Supply) connected with present oil system. All these three pump remain off and start at low main oil pressure at different pressure setting. Use for emmergency perpose. This three pumps are submerse type.
All pumps are connected in same Oil Tank having capacity of around 6300 ltr.
At the time of hunting we have check the DC Emmergency oil pump but there is no oil discharge pressure and also motor getting low ampere. same way we have checked both the Auxilary Oil pump but same way both generating pressure of around 1 Kg/cm2 instead of 8 Kg/cm2.
We have checked tank oil level found normal. checked quality of oil found normal.
There no disturbance observed in STG Output MW and governing valve.
Presently only main oil pump running with dischage pressure variation as mentioned.
From the details, you have mentioned it is extremely difficult to ascertain the cause since the information is very scarce. Prima-facie what seems is that some valve is returning oil back to the main tank (NRV or PRV) or there is some hunting from pump end
For a thorough diagnosis, P & ID of the system would be required
And last, you have a problem with lube oil and confusing with the control/hydraulic oil system
Check the location of pressure measurement instrument, hope they are upstream the NRV and immediatly after the pump discharge. It is important to note, where you are observing the pressure hunting or low pressure.
For me it seems like you have totally three positive displacement pumps(two auxiliary & one emergency pump).
Incase of positive displacement pumps there will be two pressure control valves(PCV) & two pressure safety/relief valves(PSV). First, check the settings of the pressure safety valve, if found correct check the setting of PCV that is close to the pump discharge line. If there is any problem with this PCV (seems like the problem is with this PCV)then check the regulating line that is connecting to this valve. A through checking might require with this PCV.
And, before you say that the oil quality is good, make sure you have reached till the bottom of the oil tank to confirm any dirt accumulation. Anyhow this doesnt seem to be a problem.
If the pumps are centrifugal type, then there will be two orifices (one each at pump discharge), check the size of these orifices.