We have at power plant 16 units (GE 7E). Sometimes we faced problem in hydraulic system in some unit (4 or 5 units) such as:
- gas hydraulic pressure low
- Gas fuel hydraulic pressure low
- 88 HQ running
During annual maintenance we did the following inspections:
1- main hydraulic
2- replace main hydraulic coupling
3- auxiliary hydraulic pump (88HQ)
4- 88HQ motor and breaker
5- relief valves for main and auxiliary hydraulic pump
6- hydraulic filters
7- 20FG solenoid
8- 63HG1 -63HG2
9- oil leak inside pipes (lube oil tank)
10- check valves for main and auxiliary hydraulic pump
After annual maintenance we start the unit. At full speed no load 3600 RPM, we adjust hydraulic at 1500 PSI to make sure the main hydraulic pump work Correctly, give the desired hydraulic pressure, and be the auxiliary hydraulic pump standby. We succeeded in some units.
After one or two months later, the auxiliary hydraulic pump running again! And some units tripped by Gas fuel hydraulic pressure low or 88HQ overload!
Note: accumulator did not work 10 years ago. But strange in this subject, some units of the same frame work without problems for six consecutive months (peak season)
Note: type of fuel, Gas
Also, please tell whether hydraulic accumulators are working or not on the machines with hydraulic issues.
> Please describe how you are adjusting the hydraulic pumps.
We have two relief valves in hydraulic system, one for auxiliary hydraulic pump, another for main hydraulic pump by relief valves you can increase or decrease hydraulic pressure. we adjust auxiliary hydraulic pump at 1480 psi. after unit reach to full speed no load, we adjusting main hydraulic pump pressure by relief valve. if the gauge response that mean the main hydraulic work. if it didn't, we change relief valve by new one and try again. Sometimes we succeed, and sometimes we did not
- We open some covers for main lube oil tank during adjusting process to make sure there is no oil leak inside hydraulic pipes or check valves passing
- 63HQ (pressure switch) check and Recalibrate
- In the units that have this problem, we have replaced main hydraulic pump with coupling by new one. we succeed in some of them, but unfortunately after one or two months later 88 HQ running again.
Interesting thing about this issue is that it is not happening on just one machine. It does not seem like an issue with the pumps. This points towards some common culprit for all of these machines. Can you please verify following things:
1. The quality of oil used is matching the OEM requirements?
2. How often is the Oil tested for moisture and viscocity?
3. What is the condition of Hydraulic Oil filters after this scenario?
1. What is the pressure reading on Local PG at the discharge of Main Hydraulic pump when 88HQ is running.
2. Monitor the Lube oil system pressure in the machines with this issue. A disturbance in the LO pressure can also cause performance issue on Hydraulic pumps.
Exactly as suspected.
It is improper engineering practice to use a relief valve to set a system pressure. A relief valve's purpose is to protect the pump and/or system from over-pressure--NOT to control pressure. (There is only one exception to this rule that occurs on many GE-design heavy duty gas turbines--and that is on a gear-type Main L.O. Pump, where because the pump is a positive displacement pump fear-type pump there is no choice but to use a relief valve for pump discharge pressure control. BUT--BOTH the pump AND the relief valve and the pump coupling are rated for the flow and power requirement.)
The axial piston, positive displacement pumps typically used on GE-design heavy duty gas turbines have internal pressure compensators which are to be used to set hydraulic system pressure. The Device Summary document provided with GE-design heavy duty gas turbines CLEARLY states the hydraulic pump pressure compensator is to be set equal to the specified hydraulic system pressure and the hydraulic relief valves are to be set st a slightly HIGHER pressure setpoint. This is to protect the system against over-pressure by relieving flow.
BUT, the hydraulic pumps--AND their couplings, AND the AC Motors used to drive the Auxiliary pumps are NOT designed to continuously provide the extra flow which occurs when a relief valve is continuously relieving trying to control/maintain hydraulic system pressure.
Find and read the unit-specific Devise Summary document and the Hydraulic System P&ID and you will see the internal pump pressure compensator setpoint is to be less than the relief valve setting which will keep the system flow-rates within normal ranges, will prevent AC motor overloading and keep pump couplings from failing.
Here's a link to a recent thread about how to properly set the hydraulic system pressure and the relief valves, also for long, reliable pump, coupling and motor life:
Finally, just as important to hydraulic system operation as properly setting system- and relief valve pressure is the proper maintenance and charging of the hydraulic accumulator(s). They can help the hydraulic system easily provide rated press- and flow in the event of a high-flow event, such as a sudden change in IGV position. But if the bladder isn't properly charged (usually to half of typical system pressure) AND they are properly valved-in (the block valve is to be open, and the bleed (drain ) valve closed. If both valves are open there will be excess flow through accumulator which could cause problems with the pump couplings failing prematurely, and/or the AC Auxiliary Hydraulic Pump Motor is alarming in overload condition.
Any of this sound the least bit familiar to your situation? (A lot of it does to many of us.)
Please write back to let us know how you fare in resolving the issues.