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Low lift oil pressure - GE 7FA
7FA experiencing lift oil issues

We have currently experienced low lift pressure on start-up of our GE 7FA unit. The pressure switch 63QB-1 switched and provided the low lift oil pressure alarm just as the unit was rolling up - approximately 75RPM.

We have verified the switch and cross checked against a pressure transmitter we have installed on the lift oil circuit.

Reviewing the trend data the lift is approximately 3050 psi on turning gear and as the unit starts to roll up it dropped to 2650 psi at 75 RPM and recovers to 3050 psi at approximately 400 rpm. This is the first time we have experienced this issue.

Any feedback would be appreciated.

By Hashem Alkhasawneh on 9 June, 2012 - 5:12 am

you can increase it by adjust the pressure from the manifold that what containing (Auxiliary hydraulic supply pump pressure relief valve VR22+ Hydraulic system air bleed valve VAB1+ Hydraulic pump check valve VCK3-1), just Just tiet by allen key

Moderator's Note: could not figure out what "tiet" should be.

Hashem Alkhasawneh,

If the Jacking Oil (Hydraulic Oi) Pressure is good at the beginning of the START procedure, but it drops during the procedure then recovers, how does increasing the pressure solve the problem?

Also, one should NEVER set the Hydraulic System Pressure with VR21 or VR22. They are relief valves whose purpose is to protect the system against overpressure. The Hydraulic System Pressure is to be set with the pump-mounted compensator, which is adjusted with an Allen wrench after removing the "acorn" nut and loosening the locking nut.

If the relief valve(s) is(are) used to set the Hydraulic System Pressure there will be too much hydraulic system flow because the relief valve will dumping oil to maintain pressure. There is actually very little to no flow in the hydraulic system of a GE-design heavy duty gas turbine because when the unit is operating at steady-state conditions and all the valves and IGVs are at positions equal to their references there is no hydraulic oil flowing to any of the actuators through the servo-valves--none.

I find this is one of the major problems with units experiencing instability of gas valves and IGVs: the improper use of the Hydraulic System Relief Valves to set Hydraulic System Pressure. In fact, some Auxiliary Hydraulic Pump AC motors will draw excess current because of the abnormal flow-rate through the relief valve which most sites just ignore by increasing the TOL (Thermal Overload) adjustment of the motor starter.

This is a curious problem, but not one that can be solved by increasing the Hydraulic System Pressure to overcome a temporary decrease in pressure.

It would be helpful to know when this problem started? After a maintenance outage? Does the unit use the locking (by key) jacking oil pressure adjustment mechanisms at the turbine bearings? If so, have they been adjusted recently?

In short, what--if anything--changed just prior to the start of this problem?

I've always been curious why Jacking Oil Pressure is not required during operation of the Emergency L.O. Pump during a unit trip on loss of station AC mains. It's just as important during coastdown as it is during start-up. I believe Jacking Oil (pressure/flow) is only really required during cooldown (Turning Gear) operation and during low-RPM cranking such as during starting. Certainly it's helpful during other periods but not critically required as it is during cooldown and low-RPM operation. But, that's only my personal opinion and I've never discussed the critical need for Jacking Oil with any design engineers.

CSA,

I agree with your comments regarding adjusting the relief valves. This is not something we would contemplate as this could create a dangerous situation.

All that being said discussing it here it appears we have another issue (most likely related) with our pumps. The pump discharge pressure has been swinging approximately 100 psi when on cool down control (turning gear) - this would be the situation where the pump is required to produce the maximum flow as we are supplying the lift oil in addition the trip oil supply is being dumped to drain. This being the case we isolated the trip oil supply to each valve one at a time (we had installed isolation valves years ago) and watched the discharge pressure - it was noted the pressure stabilized. This is leading us to believe we may be running the pump on the edge of the curve or actually on run out?

To answer your original questions (1) The jacking oil pressure adjustment mechanisms at the turbine bearings have not been changed. (2) The unit has not been through a recent outage. (3) Speaking with our Technicians this problem has existed for some time.

We are going to review the system and gather more data - current draw on the motor, review the piping arrangement, etc. I will keep you posted on what we find.

GRJS,

Thanks for the feedback and information.

While it's conceivable that the Hydraulic Pump could be running on the edge of its curve, most of the pumps I have seen used for hydraulic supply on GE-design heavy duty gas turbines have been positive displacement pumps. These pumps are equipped with built-in pressure compensators, meaning they will automatically adjust the pressure output in response to changes in flow-rate. Could it be possible the compensator needs rebuilding or replacing? Are you experiencing the same problem with both pumps?

I have had occasions to call the US manufacturer of the hydraulic pumps used on many GE-design heavy duty gas turbines; they have always been extremely helpful and informative.

It is also conceivable that the pumps may not have been sized properly for the condition when they are supplying Jacking Oil flow AND Trip Oil flow while on cooldown, but GE is usually pretty conservative with these kinds of things, erring on the side of too much flow rather than too little.

You say the "problem" has existed for some time. Which problem? The one you originally posted about in this thread (the loss of Jacking Oil pressure during acceleration)? Or, the 100 psi pressure fluctuations during cooldown operation? 100 psi out of 3000+ psi is pretty hard to observe with gages or even pressure transmitters.

Have you checked the charge of the Hydraulic Accumulators? If you are experiencing the same problem with both pumps it could be the accumulator charge, or valving. It's common for the Hydraulic Accumulator valves not to be properly set.

If the accumulator valves are properly set (block valves open; bleed valves closed), there should have been a special hose provided with the unit for checking/charging the accumulators. Read all the available instructions, including any stickers on the tops of the accumulators near the fittings, before connecting the hose. It's VERY common for pressure to be lost by improperly connecting the hose to the fitting on top of the accumulators. And, you should also have a proper source of nitrogen when checking the charge so it could be brought up to specification, if necessary, or if the charge is lost inadvertently.

Please do write back with the results of your investigations.

We have had an issue with fluctuating lift oil pressure pretty much since the units were commissioned in 2004. We have seen swings as large as 400psi.

We had tried replacing several components in the hydraulic block and we would see a temporary improvement but the pressure fluctuations would always return. Repeating the hydraulic setup procedure has also helped temporarily. The recommendations from TIL1525-2 have also been followed.

More recently (early November), we had a technician from Mitten on site to go over our system. All components within the hydraulic block were replaced plus 2 new hydraulic pumps and a new bladder in the accumulator. The setup procedure was followed before placing the system back into service. Despite all of this effort, we have seen the problem return.

We had been able to stabilize the pressure by energizing the trip oil circuit. We could also correct the problem by isolating the oil supply lines to the IGV and fuel gas actuators.

KSB,

That's quite an effort; thanks for the information.

Do the pressure fluctuations occur when lift oil pressure is present but the shaft IS NOT being rotated by the turning gear?

Do the pressure fluctuations seem to be at all related to oil temperature?

Are the units gas turbine-generators or single-shaft STAG units?

Have you tried using the individual bearing pressure adjusters (usually locked into position with a key) to isolate one bearing at time when the shaft is not being rotated to see if one of the devices might be bad? How about two at a time to see if there is some harmonic pressure interaction between two devices?

Greetings,

We had same issue in ge7fa & we checked all lift oil pressure regulators before bearings and found one of them was passing more than other regulators. After changed the pressure regulator, our lift oil pressure became normal.