Hydraulic Oil Pump

-We have a frame 5 gas turbine with a mechanical only-accessory gear box driven hydraulic pump(90 bar).

-The turbine tripped 3 times(within 6 years) before due to the failure of the coupling of the pump.

-What is root cause of coupling failure and what are the recommendations of pump inspection or coupling replacement?
 
Ahmed,

The typical cause for these positive displacement, axial piston pumps with built-in pressure compensators is that the hydraulic system pressure was set using the relief valve instead of using the pump's pressure compensator. The relief valve setting (per the Device Summary) is to be HIGHER than the hydraulic system pressure setpoint so that in the event the pump discharge pressure is too high relief valve will protect the pump and the other components (including tubing and fittings).

If there is flow through the relief valve under normal operating conditions then either the relief valve has been used to set the hydraulic system pressure, OR the pump's built-in compensator has been misadjusted or has failed. There should be NO flow through the relief valve under normal operating conditions (with the unit at rated speed, or even at CRANK or FIRE speed). There should be a slight "weeping" (very small flow) through the air bleed of the manifold near the relief valve (as it's purpose is to try to vent any air in the system and it's constantly venting).

If there is too much flow through the hydraulic pump under normal circumstances then the pump will take more energy from the Accessory Gear--through the coupling (usually a nylon "Nylint" coupling)--and that will cause the coupling to fail. And, if the relief valve is constantly relieving this is too much flow through the hydraulic system which the pump has to supply.

The proper procedure for setting the hydraulic system pressure is to ensure the relief valve is NOT relieving, then use the pump's built-in pressure compensator to raise the hydraulic system pressure up to the relief valve's setpoint (possibly something like 91 0r 91.5 barg, for example, if the hydraulic system pressure is to be 90 barg), then adjust the relief valve until it just starts to relieve (the pressure shouldn't drop too much if the compensator is working correctly) and set the lock-nut on the relief valve's adjusting screw.

Then reduce the hydraulic system pressure to rated (90 barg in this example) using the pump's compensator and set the lock-nut on the compensator's adjusting screw and replace the acorn nut.

For those reading this post with a gear-driven Main hydraulic pump and a AC motor-driven Auxiliary Pump, the procedure is the same--for each pump. Set the Auxiliary Pump's relieve valve and pressure compensator first, when the unit is NOT running, and then, while at FSNL set the Main pump's relief valve and pressure compensator. Too much flow through the Auxiliary pump can cause motor overload alarms for the Aux. hydraulic pump motor, and can even lead to broken Auxiliary hydraulic pump motor couplings.

But, it's <i>flow</i>--not pressure--that causes the coupling of these pumps to fail (or a seized pump--but that' doesn't happen very often).

It's also advisable, for those GE-design heavy duty gas turbines with hydraulic accumulators, to make sure the hydraulic accumulators are properly charged (usually at one-half the system pressure setpoint), and are properly valved-in (the block valve(s) should be open, and the bleed valve(s) should be closed during normal operation).

Replacement should be when necessary upon inspection, which should be done during every maintenance outage (CI, HGPI, Major). If the coupling is worn (per the coupling manufacturer's recommendations), it should be changed.

Again, it's excessive flow (which can also be caused by improper valve positions for the hydraulic accumulator(s), if used for the machine--if the both the block and bleed valves are open then that will also cause excessive flow through the hydraulic system), or some mechanical problem with the pump (too much drag or worn bearings, etc.) that causes the power to be drawn from the prime mover (the Acc. Gear, or the Aux. pump motor) that causes the couplings to break.

Hope this helps!!!
 
Dear Ahmed and dear All,

>But, it's <i>flow</i>--not pressure--that causes the
>coupling of these pumps to fail (or a seized pump--<b>but that'
>doesn't happen very often</b>).<

Hydraulic power is defined as flow multiplied by pressure (Power = Q x P). Both pressure and flow should be adjusted. But maybe the problem is just mechanical ?!

In most cases, alignment errors or too small coupling clearances lead to rapid coupling wear, may cause noise, vibration and may damage your hydraulic pump.

On GE Frame 5, a quill shaft accommodated inside of a hollow shaft is commonly used through a sleeve coupling to drive the main hydraulic pump. The quill shaft is a little bit long regarding its diameter and torsional forces may bend/wear it which leads to misalignment and/or clearances reduction (or too much clearances).

When replacing pump coupling you must respect all clearances on both pump and gearbox sides then be sure that there is not misalignment.

Regards
Karim
 
Bkarim55,

>Hydraulic power is defined as flow multiplied by pressure
>(Power = Q x P). Both pressure and flow should be adjusted.

In most cases, hydraulic system pressure is to be maintained at a constant value, so power is primarily equal to flow. Presuming pressure is 5 to be maintained at a constant value (which is what the internal pressure compensator is designed to do--maintain pressure as long as flow does not exceed the range of the pump's design), the power consumed by the pump is primarily a function of flow.

>But maybe the problem is just mechanical ?!

Absolutely! That's not outside the realm of possibility at all. I've seen people have to "shave" one side of the Nylint coupling because it was just a little too thick for the installation. And some couplings can withstand more misalignment than others. Hopefully the mechanical department takes good care when installing the new coupling to ensure the alignment is within the coupling's specifications.

Again, my experience is that excessive flow caused by improper setting of hydraulic system pressure using the relief valve instead of the pump's internal compensator is the biggest contributor to premature and/or repeated hydraulic pump coupling failures. Most failures can be traced back to a recently completed maintenance outage when "calibrations" were performed on field devices and instruments--including the hydraulic pump relief valves. And, I generally don't hear of continued problems, unless the pump eventually fails mechanically a short time later (sometimes just a matter of hours).

Finally, many GE-design Frame 5 heavy duty gas turbines don't have hydraulic accumulators, which can help to reduce pressure pulsations from the pump discharge which can be pronounced when the relief valve is constantly relieving.

Anyway, you made some good points; thanks!
 
S

Siluman Mark Vie

Ahmed,

Which frame 5 are you referring to? Ms5001PA or MS5002C/D?, If MS5001PA, There are several cases the coupling failure. If you can check the coupling, There are 3 different colors of 'Star' coupling which indicated the capability to absorb the torque. The green one is supposed to be the tougher one, as well the hub is not easily to tighten due to space which able to move axially during operation. You need to retain the hub especially the one connected to Shaft#4 by using another locking thru shaft end axially.
 
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