Drop in control oil to 55 PSI, and Nozzle goes to current suicide during loading

Dear All,
Machine : GE MS5002C
Control system: Mark VIe TMR

we just completed Zero overhaul
1- Control oil pressure drop to 58 PSI
2- Nozzle current suicide @ 2.5 degree if we increase the LP speed fast
- during start up the control oil pressure stay at 110 PSI.
- at LP speed 95%, auxiliary pump stopped and pressure dropped to 90%
- Control oil pressure gradually decreasing to 58 PSI within 2 hours
- Lube oil pressure steady at 150 PSI
- Hydraulic pressure steady at 1220 PSI
- during calibration the nozzle acting perfect and no deviation between the Ref and position feedback
- during the startup the current suicide happened and gap between Ref and feedback exist and the LP speed go back to 70%
- we should increase the LP speed slowly to avoid current suicide
Action taken so far:
- checked and replaced the Lube oil filters
- replaced the hydraulic filters and hydraulic GCV filter
- checked the Dump solenoid but no significant leak in inlet pipe
- replaced the Nozzle Servo with new one
- checked the Nitrogen Accumulator the pressure was 350 PSI and increased it to 800 PSI
- full calibrate the NGV Servo and LVDTs

Do you have any idea what possible causing this issue?
Note: the turbine now online and loaded, LP speed at 98%

1) When does the control oil pressure decrease--is it just before and during the nozzle problems, or is it just something which was noticed after the maintenance outage?

2) What is the normal control oil pressure for this machine (in other words, what was it before the maintenance outage)?

3) What Diagnostic Alarms are present on the I/O Pack which powers the nozzle servo (when there are problems with starting and suiciding)?

4) Was a servo valve polarity check performed after the new nozzle servo was installed?

5) Was anything changed in the I/O configuration of the nozzle servo valve output as part of the re-start or troubleshooting, and, if so, what was changed and what was the as-found value and what was the as-left value?

You need to be looking at GEH-6721, specifically the section for the I/O Pack which drives the nozzle servo, and look to understand the causes of a suicide. Usually, it's something like excessive servo current or a large discrepancy between position reference and position feedback--both of which usually point to a problem with the hydraulic actuator and/or nozzle ring having mechanical binding. It could also be that one or both of the nozzle LVDTs are failing or there is a wiring problem (loose terminations).

But, you need to check and troubleshoot and resolve the Diagnostic Alarms associated with the I/O Pack which powers the nozzle servo with an understanding of what conditions result in a suicide of the servo-valve output.

Finally, the Mark VIe calibration function DOES NOT calibrate the servo--it ONLY calibrates LVDT feedback. Nothing else. Full stop. Period. If one changes ONLY the servo on a servo-operated device with LVDTs for position feedback it IS NOT necessary or required to use the Mark VIe calibration function on the device, because it does nothing to or for the servo and changing the servo does nothing to change either the length of travel (stroke) of the hydraulic actuator or its stops or the LVDT feedback. "AutoCalibrate" ONLY calibrates LVDT feedback--that's all. Nothing else. Nada. Niente. Zilch. Nothing.

"AutoCalibrate" And, the manual positioning feature along with the testing functions ('Verify Current' and 'Verify Position') can be used to test the servo functionality (in an indirect manner) and the LVDTs (in an indirecty manner, also), but "AutoCalibrate" does NOTHING to the servo or the servo output gain or anything to do with the servo. The manual positioning feature of "AutoCalibrate" is also used to check the servo output polarity being applied to each of the servo coils, but that's it.

[I realize this goes against everything you've probably seen or been told, but all tribal knowledge is not truth. And I'm very confident you didn't learn to use "AutoCalibrate" from any manual or procedure that said it needed to be performed after changing a servo valve. (Actually, I'm pretty confident you didn't learn to use "AutoCalibrate" from any manual or written procedure.) That's not a criticism of you, but rather, the supplier of the turbine.]
Really appreciate your interacting with my thread
1- it start decreasing from 110 to 90 when the auxiliary pump stopped, then it start decrease gradually to 55 within 2 hours.
2- it was 100 to 90 PSI, now pressure 60 to 52 PSI
3- the alarm only- Nozzle not following alarm because the deviation between the reference and feedback more than 2 degree.
4- yes we already checked the Servo Coils polarity and resistance and tested coil by coil.
5- I will go back to overhaul report to check if any change in configuration.

as I said the current suicide was because the discrepancy between the reference and position feedback.

I don't know what a Zero overhaul is (I'm not familiar with two-shaft maintenance terminology), so I don't know what was done.

And without access to the Control Oil and L.O. P&IDs I'm not 100% sure where the Control Oil comes from--though it usually comes from the Main L.O. Pump and Aux. L.O. Pump discharges. If the pressure is dropping after the Aux. L.O. Pump shuts down and the unit is only running on the Main L.O. Pump (and the Main L.O. Pump is driven by the Accessory Gear), then one has to wonder if the Main L.O. Pump discharge pressure is also lower than normal but not low enough to cause a low pressure alarm or the Aux. L.O. Pump to start and run. USUALLY the Accessory Gear-driven Main L.O. Pump is a positive displacement pump and there is a pressure relief valve used to set the pump discharge pressure (if I recall correctly). It's one of the times when a pressure relief valve is used as a pressure regulator (not really a proper application for a pressure relief valve--but it's worked fine for decades!). It could be that the relief valve is failing or is not adjusted correctly.?.?.? Or, there's a leak somewhere that's causing an excessive flow which the relief valve can't properly deal with (it can't shut off flow enough to maintain pressure). I would be looking at what piping was disturbed when the maintenance outage was occurring if it seems to be a leak.

If there's a problem with actual position versus reference position it's either:

a) the hydraulic actuator and/or the ring gear linkage is binding
b) there is a worn area on the inside of the hydraulic actuator cylinder that is allowing excess oil to pass and forcing the servo to have to have more current than normal to get enough oil to flow to move the piston
c) I have seen actuator seals improperly installed and very quickly cause scoring on the piston which leads to binding of the piston
d) LVDT issues.

If it's the latter, you should be able to see the feedback from both nozzle LVDTs using ToolboxST (I'm presuming there are two LVDTs...?). You can use the manual positioning feature of "AutoCalibrate" to move the LVDTs and I think it will plot the feedback from both LVDTs, and you can save and compare the plots and see if there's a region where they diverge for a distance which could be causing the problem. (AFTER and AutoCalibrate is performed on a particular set of LVDTs (such as the second-stage nozzle guide vanes) one can use the 'Verify Position' and 'Verify Current' features to automatically stroke the device while monitoring LVDT position and servo current. If I recall correctly, 'Verify Position' causes "AutoCalibrate" to put out a constant current to move the device in one direction, then a constant current to move it in the other direction, while plotting LVDT feedback (from both LVDTs if there are two). What you should see is a pretty straight line for each LVDT when moving in one direction, and a pretty straight line for each LVDT when moving in the other direction (it should look like a mountain peak with straight sides for each LVDT, in other words). If it does that then the LVDT output (feedback; input to the Mark*) is probably good and neither one is failing or failed. If you see jagged edges on either plot then something is probably wrong.

Using 'Verify Current' the Mark* will put out whatever current is required to make the LVDT feedback change at a constant rate--and plot the current necessary to do so. If everything is okay what you should see is a pretty flat line as the device moves in one direction, a slight bobble as the current changes polarity, and then a pretty flat line as the device moves in the other direction. But, if there's any mechanical binding or any leakage inside the cylinder then you will see large bumps in the current required to keep the LVDT feedback changing at a constant rate.

I think the hydraulic dump valve setting is listed in the Device Summary--that would be the pressure at which the dump valve would prevent flow to the servo/actuator. If the Control Oil pressure you're seeing is around that pressure it might be the problem--especially if the suicide is occurring around the time the Aux. L.O. Pump is shutting down....

This USUALLY isn't a Mark* problem--even though the Mark* is doing the alarming and it seems like it's that damn Mark* that's causing the problem. It could even be a loose screw or terminal on one of the LVDT wires somewhere along the LVDT circuits causing the problem (though that seems unlikely, stranger things have happened--I guarantee it! sometimes it's just a vibration thing at a certain point when accelerating at a certain rate...). But, it's usually not a Mark* issue (but one time out of several hundred it is). If the LVDTs were disconnected during the maintenance outage, I would suspect a wiring issue (loose terminal crimp; loose screw; incorrect wiring order; etc.).

Hope this helps! Please write back to let us know how you fare and how you resolve the problem! (Also, if you need more help it would be great to know how many LVDTs are on the NGV actuator and if the Mark VIe is SIMPLEX or DUAL Redundant or TMR.)
Another thought just occurred to me, I believe the Aux. L.O. Pump has a check valve in it's discharge, and the flapper of the check valve has a hole drilled in it (not that the hole has anything to do with the problem--it just shocks some people when they see it! and they think there's a problem because the flapper has a hole in it!). Sometimes that flapper can get stuck and not close properly, and if that's happening then that could be the source of the "leak" the Main L.O. Pump relief valve can't keep up with.

Probably not, but just a thought. My money's actually on an LVDT wiring issue or a worn LVDT stator or a bent LVDT armature (the piece that slides up and down in the stationary part (the stator). If the armature is bent and scratches the inside of the armature it can damage windings inside the LVDT (the armature is supposed to slide STRAIGHT up and down inside the stator and not touch the inside of the stator at all). This would definitely show up when using the 'Verify' tests. (And it also usually shows up as a warning message when performing an "AutoCalibration"--only most people don't pay attention to alarms like that.)

Anyway, just a thought.

Please write back to let us know how the troubleshooting is going (though you probably can't shut the machine down right away for troubleshooting); just don't forget to write back when you do. (Let us know what you do--and, most importantly, what the results of your troubleshooting were/are!)

Can you be specific about how you test the polarity of the servo currents being applied to the NGV servo valve coils? It doesn't have to be specific, I just want to know you're NOT using the procedure usually supplied in the Control Specification, and if you are can you tell me what is says to do? (For a couple of decades the procedure for checking servo coil polarity (beginning with the Mark V turbine control system) was not correct. And, they just kept copying and pasting the same procedure for Mark VI and Mark VIe. And, it said, in a nutshell, to test with only one servo output connected to its coil and then stroke the device in steps using the Manual Position feature of "AutoCalibrate" and if the action was "jerky" and not "smooth" the servo polarity was backwards and needed to be changed. That's a pretty subjective procedure--subject to a lot of different, and widely varying, interpretations. AND, the Mark VI, and even the Mark VIe, servo outputs when being changed using the Manual Position feature of "AutoCalibrate" are jerky to begin with because they only usually change at a very slow rate, once per second, instead of the 100 Hz that is used during normal operation. So, no matter what one tries when using the Manual Position feature of "AutoCalibrate" the device movement is going to appear jerky (at least to me--and compared to normal operation).

Several past threads on Control.com have the proper procedure for testing servo current polarity; it's not difficult though it does take a little time to determine which I/O terminal board and which screws the servo outputs to the device are terminated on and then establishing L.O. and Hydraulic pressure, and sometimes Trip- or Control Oil pressure, too, as required by the system. But, it's a very necessary step and test--because I have seen new, in-the-factory-box Moog servos, which all have the same color coil leads, have the leads connected differently (opposite of the servo which was removed). If one just uses the same color leads at the same terminals as the colored leads of the servo being removed its possible that one or more of the servo coils will be reversed--hence the need to test polarity to be sure it's correct before starting the unit.

I don't really think this is your problem--but it could be. Quite often problems like these are the result of more than one cause or stimulus or condition.
we have a concerns regarding the NGV Actuator cylinder, and Dump solenoid, so we are planning to change them in next shutdown.
Main L.O pump pressure is good and Keep the same pressure as Aux L.O pump was doing. also we run the Aux L.O to support the Main L.O but found no big difference in Control oil pressure only 8 PSI increase while the L.O pressure increased from 150 to 180 PSI with both of pumps online.
we already checked the position manual verification for LVDTs and found there response is ok with only about 0.5 degree discrepancy between the NGV flag and the LVDT reading in Mark VIe. both LVDTs reading was matching.

we didn't use current verification during our test. we will do it in next shutdown.
LVDTs never disconnected during major overhaul.

I don't think we have a problem with Aux L.O check valve flapper, as we already rqn the pump to check the integrity of Main L.O Pump the control oil pressure still low.

I will keep you posted whenever found a solution.

Thank you very much for the feedback! Feedback is the most important part of the Control.com threads.

We await the status of your troubleshooting.
To close the above case.
We found the drop in control oil pressure was caused by LP overspeed dump line. We blinded it and Control Oil pressure back to 100 PSI. Now NGV is moving up to 0.5 degree without any interruption to the load.