88HQ (Aux Hydraulic) Pump is always running

Hi everyone,
Hope all is well. I've been having this issue where a 88HQ (Aux Hydraulic) pump of a GE Frame 5PA turbine is always running/in operation and would never stop after we conducted a major overhaul. We tried calibrating and replacing 63HQ and also replacing the 88HQ but to no success. We also tried to purge the lines to make sure that no plugging is causing a reduction in Hydraulic pressure but still to no avail.
My next assumption is that the main hydraulic pump can't generate enough pressure, therefore we tried to replace the main hydraulic pump,which mitigates the issue for a while but with a finding that the pressure gauge reading (near 63HQ) indicates the pressure is consistently fluctuating. And after a couple of weeks, 88HQ continues to start again and would never turn off.
I was wondering if any of the members here had any similar experience and what would be the cause of the issue?
Even though 88HQ always operating is a minor issue and may not create further issue if left running, I prefer it to operate based on the operating philosophy as I'm not sure the pump is rated for continuous operations.

If there are other documents or info required, please inform me so I can provide it to the best of my capability

Thank you very much and best regards
Do you have accumulators in your hydraulic system, you should have. have they been checked for pressure, low pressure in accumulators will cause exactly your problem, the Aux pump will kick in on any sudden system change which "bumps" the system pressure and, if you look at your logic, once the Aux. hydraulic pump runs it will latch in and needs a manual intervention to stop it.
Many GE-design Frame 5 heavy duty gas turbines do not have hydraulic accumulators (for some reason; who knows). Even those that do have small accumulators, which are rarely checked and when checked are done so incorrectly. Using the proper charging hose and knowing how the Schrader-like valve works is very important. Also, it's quite common (unfortunately) for the block and bleed valves on an accumulator to be unmarked, and have the same size valve handles--adding to the confusion as to which should be open and which should be closed (the block valve should be open and the bleed valve should be closed).

Many GE-design heavy duty gas turbines have the same relief valve assembly and check valves for both the Main (Accessory Gear-driven) and Auxiliary (AC motor-driven) Hydraulic pumps. MANY people--MOST in fact--believe the hydraulic system pressure is to be set using the relief valve of one or both pumps. WRONG. Very often this leads to two things: near-constant operation of the Aux. Hyd. Pump, and Aux. Hyd. Pump motor overloads (which are usually disabled by technicians or the thermal overloads are changed out for higher ampacity ones or the 90-110% adjustor is just maxed out (at 110%), or some combination of all of the above. NOBODY EVER uses a clamp-on ammeter to see if the Aux. Hyd. Pump is drawing more or less than nameplate rated current (flow and pressure are directly related to current draw).

Each of the hydraulic pumps (they are usually the same--exactly, and can be interchanged most times (sometimes the mounting methods are slightly different) has a pressure compensator--which is what is SUPPOSED to be used to adjust the pump output, and hydraulic system pressure! To do this properly, one has to ensure that the relief valve setting is higher than the pump compensator setting (not the easiest of tasks, but not impossible either--one can usually see the relief valve relieving, and it SHOULD NEVER BE RELIEVING during normal operation--NEVER). Then one has to adjust the built-in pressure compensator of the pump to set the pump output pressure to normal system pressure.

Once that's complete, then one has to set the relief valve pressure to the value specified in the Device Summary. To do that one has to increase the hydraulic pump built-in compensator setting up to the relief valve setting in the Device Summary, then start decreasing the relief adjustor until it starts relieving. At this point, the lock-nut on the relief valve needs to be tightened and then the built-in pressure compensator adjusted to bring the hydraulic pump back to normal system hydraulic pressure.

To do this properly, one has to adjust the Aux. Hyd. Pump when the unit is NOT running, then when the unit gets to FSNL (or even while CRANKing) set the Main Hyd. Pump pressure compensator WITH THE AUX. HYD. PUMP OFF.

It's that simple--no rocket science involved. Zero. Use the built-in pump pressure compensator of each pump, without the other pump running, to increase system pressure to check or set the relief valve properly. Then reduce the built-in pressure compensator of each pump to set the pump discharge pressure to normal system pressure (as also defined in the Device Summary--for each pump).

This topic and the procedures for checking and setting the pump- and relief valve pressures have been written about several times before on Control.com, and can be found using the site's 'Search' feature.

Instructions for using the charging hose and gauge supplied with every accumulator can be found sometimes on an adhesive label on the top of the accumulator, sometimes in the manufacturer's literature provided in the Operations and Service Manuals, and most probably on-line (I have never looked, but most component manufacturers now post their instruction manuals on line).

You need to check the hydraulic pump relieve valves, air-bleed check valves (which should always have a little dribble ("weep") leaking from them (to get the air out of the lines), the built-in hydraulic pump pressure compensators and the accumulators. Many people when "filling" (really, just attempting to fill) the accumulators, don't actually admit any nitrogen into the accumulator because of improper use of the charging hose and gauge assembly. AND, making sure the accumulator block and bleed valves are in the proper positions is also VERY important (this is a little harder to do, but again--not impossible if one looks at the P&IDs and thinks through the process).

Please write back to let us know what you find!

The hydraulic systems on GE-design heavy duty gas turbines are usually very "static" systems--meaning, there is very little flow when the unit is operating at steady-state conditions. It's when the IGVs are moving quickly or the fuel valves are moving quickly (as when the grid/generator frequency is not steady) that hydraulic flow increases. For this reason, most Aux. Hyd. Pump motors usually draw 15-20% LESS than nameplate rated current when operating (when the Main Hyd. Pump is not running). BUT, when the Aux. Hyd. Pump relieve valve is relieving--OR the accumulator block and bleed valves are open, OR the hydraulic accumulator is not properly charged (or the bladder is broken)--then the motor current draw will usually be slightly above motor nameplate rating, because of the higher-than-normal hydraulic flow-rates. And, while one can't measure the energy drawn by the Main Hyd. Pump, the concept is the same.

Check your built-in pressure compensator settings, the relief valve settings, and the hydraulic accumulator valve positions and charge pressure (if the unit has a hydraulic accumulator), and report back what you find--and what effect this has on the pump operation.