We are commissioning a GE MS5002C gas turbine mechanical driver. At 20 % crank speed, the hydraulic oil pressure developed by main hydraulic pump is 77 bar against the required 80-85 bar. The question is what should be the hydraulic oil pressure at 20 % crank speed? If the pressure at this speed is not adequate, what should be the probable cause?

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What have you done to try to troubleshoot the problem, and what were the effects of your efforts?

Is this a new unit undergoing commissioning for the first time? If this is a new unit, are you certain all of the valves in the Hydraulic Supply system are in the proper positions? Is there any significant discharge from the air-bleed check valve? (There shouldn't be anything more than a slight flow or drip.) The compensator on the pump should be used to adjust pump discharge pressure--when the unit is at rated speed! The relief valve is ***NOT*** to be used to set the Hydraulic Pump discharge pressure.

Or, is this a "re-"commissioning of a unit after a maintenance outage? If so, was the Hydraulic Pump replaced or refurbished during the outage? Are you certain the valves in the Hydraulic Supply system are in the proper positions?

Has anyone tried adjusting the compensator of the hydraulic pump?

If this is new unit under commissioning, I would suggest waiting to adjust the pump discharge pressure (with the pump compensator) until the unit reaches rated speed. Also, make sure the relief valve is not relieving; it's ***NOT*** to be used to set the pump discharge pressure; only to protect the system against overpressure?

If there is a hydraulic accumulator on the unit, are the valves in the proper positions and is the charge in the accumulator at the proper pressure?

Are there any flushing blocks in place of any servo-valves or the liquid fuel stop valve (if present)?

There is likely sufficient pressure to start the unit in order to get to rated speed to adjust the pump discharge pressure. That is, if the valves are in the proper positions and there is no flow through the relief valve/air-bleed check valve.

Lastly, are the Hydraulic Filters properly valved in?

Lots of questions, all of which require answers to continue trying to help with your problem.

 

You haven't said whether the pressure is OK at higher speeds or FSNL. If you haven't run up to there yet, take a look at the main pump relief valve (usually VR-21 from memory) or the pump PCV (VPR3) and be sure that they are set correctly. Do the VR first of course, take a look at your P & IDs.

 

If you can tell me a few things first, I may be able to help. Do you know the make and model of the pump in question? Do you know the age of the pump in question? Pumps have a power curve that they operate under, if you are under RPM or over RPM your efficiency will be greatly reduced (not to mention pump life). On a side note, why are you trying to generate pressure while cranking instead of running?

CB

 

well normally till 95% speed the AHOP will run producing sufficient pressure. how exactly did you get the MHOP pressure to be 77 bar at crank speed??. did you force off the AHOP? never done that before but perhaps i will try it at next startup . but my guess is that MHOP alone will not produce such pressure at 20% speed. either way the 77 bar you are getting at 20% speed is sufficient enough, a little less but sufficient.

 

There are hundreds (literally) of GE-design heavy duty gas turbines around the world which do not have an AC motor-driven Auxiliary Hydraulic Oil Pump.

For these turbines, they usually crank the unit to obtain Hydraulic Oil Pressure for purposes of stroking devices which use electro-hydraulic servos and to calibrate the LVDT feedback from those devices. Some sites have fashioned some tubing set-ups to use the DC motor-drive Hydraulic Ratchet Pump to provide hydraulic pressure for stroking/calibrating. But, this method does not provide full pressure, and it has other potential problems as well (the filtering of the oil from/through the DC ratchet pump isn't usually very good; the DC Hydraulic Ratchet Pump motor usually isn't rated for the "continuous" operation that it's subjected to when stroking/calibrating; etc.).

For many smaller GE-design heavy duty gas turbines (Frame 6B, Frame 5, Frame 3) an AC motor-driven Auxiliary Hydraulic Oil Pump was an option that the Customer had to purchase. Some have been retrofitted with Aux. Hyd. Oil Pumps over the years; many have not.

I will say it again (and again, and again, and again, and again, and ...): Every GE-design heavy duty gas turbine is not the same as every other GE-design heavy duty gas turbine. Yes, they all compress air, burn hydrocarbon fuels, produce torque, and exhaust hot combustion gases--but they all do so with slightly different auxiliaries and packaging, which affects the operating sequence(s) and maintenance procedures to some degree or another.

 

What you should do is run the unit to FSNL and if the pressure is not what you expect, you must regulate the pressure at the hydraulic pressure regulating valve (I think is VR-21 o VR-22). and if this is not the solution, please check the coupling of the MHOP at the accessory box. Another thing you can do is regulate the pressure at the same MHOP turning the bolt provided for pressure regulation.

 

VR-21 and -22 are <b>NOT</b> to be used to set the Hydraulic system pressure. They are <b>relief</b> valves to protect the hydraulic system against overpressure.

If they are used to set the system pressure the flow through the respective hydraulic pump will be excessive and is a cause of early pump failure.

The compensator on the pump is used to set the hydraulic system pressure.

To check and set the relief valve one needs to increase the pump compensator setting above the relief valve setting, make any adjustment to the relief valve setting, then reduce the pump compensator setting to desired system pressure.

 

I wouldn't try and adjust the pump regulator at FSNL unless you need to, it's very sensitive and it is easy to trip the machine. It can be adjusted perfectly well at cranking speed and will be OK at FSNL