Frame 5 gas turbine tripped on hydraulic oil low pressure. On cranking Hyd. oil pressure was 0. Hydraulic supply pump, check valve and relief valve were replaced. This solved the problem. However no observation found in pump.

We have two GE frame 5 gas turbines MS5001P. Recently one machine was shutdown on hydralic oil low pressure. Hydraulic Supply pump is Delavan axial piston pump with standard pressure compensator (VPR3).On cranking hydraulic oil pressure was zero. we replaced the hydaulic supply pump, check valve and relief valve(mounted on hydralic supply manifold). on start up pressure was restored to 60 Kg/cm2 which was increased to 80Kg/cm2 by releief valve adjustment.
Latter no observation found in pump i.e. shaft, rotor, plungers and plunger springs were OK.

Please immediately advise what can be reason of hydraulic oil pressure dropping to 0 suddenly.

best regards

 

The Hydraulic System Pressure should <b>NOT</b> be set using the <b>relief valve</b>. The <b>relief valve</b> (VR21, if I recall correctly) is there to protect against failure of the pump pressure compensator. If you are adjusting VPR3 above the setting of the relief valve and then using the <b>relief valve</b> to set the Hydraulic System Pressure, then there is too much flow through the Hydraulic Pump (through the <b>relief valve</b>, which should only operate if the pump pressure compensator fails).

The setting of the <b>relief valve</b> is checked and adjusted by temporarily increasing VPR3 until it opens (relieves) and observing the pressure at which it operates, adjusting it if necessary. Once the <b>relief valve</b> is adjusted (or found to be adjusted) properly, VPR3 is adjusted back down to set the normal, running, specified Hydraulic System Pressure.

If the turbine has an AC motor-driven Aux. Hydraulic Pump (not necessarily your Frame 5P, but many other Frame 5s and Frame 6Bs and Frame 7Es and Frame 9Es) if the <b>relief valve</b> is used to set the Hydraulic System Pressure then it will usually be found that the current being drawn by the Aux. Hyd. Pump Motor is at or above rated nameplate value, sometimes causing the thermal overload relay of the motor's starter to actuate.

There is normally very little flow through a hydraulic system when the unit is operating at steady-state conditions. The fuel valves and the IGVs are in a stable (steady-state) condition and so there is little to no flow through the hydraulic system, except for some small through the Air-Bleed Check Valve (VAB1, if I recall correctly). Only when the fuel valves and/or IGVs are being asked to move is there any substantial flow through the hydraulic system.

The Hydraulic Pump compensator adjusts for the reduced flow at steady-state conditions while maintaining pressure, and allows for a flow increase while still trying to maintain pressure (a Hydraulic Accumulator, if present) helps with very large, sudden flow increases).

If you have some documentation from the OEM/packager that tells you to do otherwise, please cite the specific instruction that tells you to use the <b>relief valve</b> to set the Hydraulic System pressure.

 

Thanks for your reply.

OEM instructions tell to set VPR3 below VR21 (VPR3 1275PSIg and VR21 1350 PSIg). What I understand from your reply is that in our case VPR3 was set above VR21 and in fact VR21 was maintaining system pressure and perhaps VR21 failed causing hydraulic oil dumping into console. Should we inspect VR21 after dismantling?

Currently we are keen to find component causing hydraulic oil pressure to drop to 0 so that we can prevent its recurrence.

 

If you are so keen to find the "culprit" and you have already examined the pump(s) and found them to be working properly (does that include the pressure compensator mechanism?), and you also replaced the relief valve(s) and the air-bleed check valve(s), then it would seem logical to continue examining the relief valve(s) and the air-bleed check valve(s) until you find the component which failed.

The people at Delavan have always been very helpful whenever I've contacted them for assistance (from anywhere in the world). They provide a lot of pumps to GE and it's licensed packagers and are pretty familiar with the application. Perhaps they could help with your search, but be sure to provide them the same information you provided here: That you were using the relief valve and not the pump pressure compensator to set the hydraulic system pressure. They may have some insight into the possible failure mode of the pump or the pressure compensator.

You would do well to properly set the pump pressure compensators and relief valves on the running turbines while you continue your search.

Please let us know what you find.

 

Thank you very much for your assistance.
In our turbine we only have accessory gearbox driven hydraulic supply pump. The pressure compensator mechanism was also disassembled and inspected thoroughly. Now we will carefully inspect VR-21 and air bleed and check valve. Then I will inform you