Gas Turbine MS5001P shutdown procedures problem.

Dear Gents
I have a turbine (25MW) with Mark VIe control system(upgraded from Mark II at 2013).
- Recently only, I have a problem during stopping the turbine from FSNL.
- On FSNL, The FSR is about 20.8% and FSRSD is 100%.
-Directly after stop order, the FSRSD is switched to the current FSR value then logic signal L83SDL Becomes true which makes FSRSD drop to FSRMIN by the rate of (FSKSD[2]) until intercept with value (FSKMIND[3]) of the FSRMIN shutdown curve and the speed decreases until 85.5% and the bleed valve open and the guide van closes.
-The speed freezes at this value (85.5%) and the FSR at (18.6%) and still running for long time until I trip it manually by emergency stop.
Notes: FSKMIND[3]=18.75% at FSKMINN[3]=85% TNH
FSKMIND[2]=14.1% at FSKMINN[2]= 82% TNH
FSKMIND[1]=11.8% at FSKMINN[1]= 40% TNH
FSKMIND[0]=9.5% at FSKMINN[0]= 5% TNH

FSKMINU2 [1]=17.8%
- I don't know what makes the turbine speed to hold at this value?
- If I decreases the value of FSKMIND[3] slightly, Will the speed decrease below 85.5% and the turbine continue the normal shutdown sequence??

Appreciate your assistance.

One has to be careful with these Control Constants, because some of them are intertwined with the FSRSU scheme.... Lovely mess. Lovely.

How long has this problem been happening? Since commissioning? Since a gas valve refurbishment or calibration? (I'm presuming this is occurring on gas fuel; I'm not certain. Can you please confirm the unit is running on during shutdown?)

If the unit is using gas fuel, has the fuel supply changed in the last few years, and can the problem be related to an increase in fuel heating value/content?

It's probably likely that reducing the FSKMIND3 value will help with the shutdown. Unfortunately, the GE Mark* shutdown philosophy does not use a deceleration rate (the start-up philosophy on later Mark* turbine controls uses an acceleration rate reference, and adjusts fuel as necessary (as long as the exhaust temp doesn't hit the maximum allowable exhaust temperature) to make the actual acceleration rate equal to the acceleration rate reference, in RPM/sec, if I recall correctly). The GE Mark* shutdown logic just steps down FSR--with the expectation that speed will decrease. It usually does, but on some machines it hangs.

Unfortunately, if the unit is burning gas fuel, the GCV LVDT feedback calibration can also affect the issue if it isn't being done properly. If the fuel valve is more open than it is supposed to be then the amount of fuel flowing through the valve will be more than it should be. (MANY people are shocked and surprised to know for gas fuel, the Mark* does not have a gas fuel flow-rate reference--like it does for liquid fuel.)

Again, you can try dropping the FSKMIND[3] value, but just be careful that you don't muck up the start-up control--if it's working correctly. I don't have access to any Mark* files at this writing but I do know that if you make large changes to the shutdown curve, it can and does have knock-on effects on the start-up curve. So, just make slow and minor changes--say, the next time the machine is shut down and "stalls" at the 85% speed, decrease the value by no more than -0.2 (%), and see if the turbine speed starts to drop. If not, you have a few minutes (7 I think, or something like that) to try a couple more changes before the shutdown timer cuts off the fuel because it hasn't already been shut off because the unit was decelerating properly. So, you can try three or four or maybe more changes while trending turbine speed to see if it starts dropping and continues to drop to get to the next "step" decrease.

If you can get it to decelerate, the next thing is to pay attention to the next start-ups. So, the PROPER way to start this exercise is to create a trend of a good start-up so you have something to compare successive start-ups against to see if you have impacted it in any way. And, then compare that to the start-up after you've succeeded in getting the unit to decelerate (presuming you do).

Don't overlook fuel changes or GCV LVDT feedback calibration (if we're talking about gas fuel). If we're talking about liquid fuel, well the flow divider feedback scaling may not be 100% correct--if the problem has been ongoing since the upgrade to Mark VIe, if the fuel supplier hasn't changed or the fuel heating value hasn't changed.

There's a lot of factors to consider, and best not to just put all your efforts in one direction. First try to understand when the problem started occurring, then what changed around that time. Difficult to do sometimes, but, often after one expends a lot of time and effort troubleshooting something like this someone often says, "Oh, yeah--the guts of the GCV were changed a couple of years ago, and that's just about the time when this problem started happening!" Usually, after someone discovers an old outage report that says the GCV internals were changed, and it turns out the wrong kit was used. Or someone mis-calibrated the GCV LVDTs.

Hope this helps! Please write back to let us know how you progress on solving this problem. You've done a find job to this point. Let's get it finished--and share it with others in the process!
Dear CSA
Appreciate your efforts.
- This Problem has recently appeared since about two months only.
- No any activities have been done on the gas valves or the whole turbine.
- The gas fuel source is the same long time ago and has not been changed.
- The problem has appeared during operation on gas fuel due the unavailability of liquid fuel.
- Another thing that I forgot in the first post : the rate of speed drop from 100% to 85.5%(freezing point) is noticibily and comparatively slower,it takes more than two minutes.
- The first time this problem appeared(the turbine was running and loaded by about 10MW normally then we Synchronized the turbine to the bus and offloaded it then when I intended to stop it, the problem appeared.)
- I have doubts about the P2 pressure transmitter as if it is reading a lower value compared with the actual, SRV will tend to compensate for this pressure drop hence more gas fuel flow will pass and accordingly inhibit turbine speed drop.!
- But if this transmitter is unaccurate, Does turbine start up and loading loops performance be affected?? As I mentioned the start up and loading performance is OK.
- Appreciate your assistance and waiting for your kind response.

The plot thickeneth. The unit normally runs on liquid fuel--BUT, because of the inavailability of liquid fuel you are experiencing problems on gas fuel.

There is long-forgotten commissioning activity called fuel matching, which is supposed to help make FSR1 (Liquid Fuel FSR) and FSR2 (Gas Fuel FSR) have approximately the same heat content at the same FSR. So, when the unit is at FSNL on liquid fuel and FSR is, say, 19.43%, and the unit is on gas fuel at FSNL--also at approximately 19.43% (not usually exactly the same value of FSR, but close enough). And, when the unit is at or near Base Load, and FSR1 is 75.9% and FSR2 is 75.2% the unit is on CPD- (or CPR-) biased exhaust temperature control.

As I wrote, I would guess the liquid fuel flow divider feedback scaling is probably most likely not exactly 100%, And, if as you noted, the P2 pressure transmitter(s) are calibrated such that the P2 pressure output to the Mark* is higher than it actually is (in other words, let's say the actual P2 pressure was 242 psi, and the P2 pressure transmitter output to the Mark* was indicating 224 psi) then there would be more fuel flowing through the GCV than expected and that would cause the heat of the fuel to be producing more torque than it should be, which would causing the speed to "hang" or "freeze" and not decelerate.

If the transmitter was inaccurate, as described above, then during firing and acceleration the unit would probably establish flame very quickly, and it would probably accelerate quicker than it would on liquid fuel, and it might even cause FSR to be limited during acceleration because the exhaust temperature (TTXM) reached the maximum allowable exhaust temperature (TTRX). Now, this might slow down acceleration "artificially" and prolong the acceleration so that it didn't seem like it was taking longer.

Anyway, I think you're on the right track to solve the problem. Fuel matching is, again, a forgotten practive and a lost are, because it takes a few starts and stops to get it right. Note also that because for many start-up and shutdown sequences the Mark* uses many of the same Control Constant values, so that if you decrease FSKMIN[3] to get the unit to decelerate on gas fuel, it might decelerate much quicker on liquid fuel, causing the unit to lose flame in one or more combustors, resulting in the common and oft-ignored "Chamber Flamed Out During Shutdown" Process Alarm. (Which just means that flame was lost in one or more of the combustors with flame detectors before the unit automatically shut off the fuel, so the Mark* shuts off fuel to prevent large exhaust temperature spreads during shutdown. Not the greatest thing for the turbine, but not the worst, either.)
Fuel matching, even when it was being performed during commissioning as it should have been, was often cut short by the owner/operator of the turbine because it required several starts and stops to get it done correctly, especially if the liquid fuel flow divider feedback scaling was way off (and it could be and was from time to time). It's a painstaking process and if it wasn't done at your site, best let sleeping dogs lie, as they say in some parts of the world....

Please check the calibration of the P2 pressure transmitters, and let us know what you find.
Dear CSA.
Dear Gents.
Appreciate your time.
I've checked the P2 transducer (96FG) by injecting 8 bar air signal inside the gas valve (between SRV and GCV) while forcing 20VG solenoid closed, but the transducer output was about 2 bar.
- I dismantled the transducer and found it dirty by condensate and foreign particles then cleaned it.
- The tube to the transducer was slightly blocked by dirts then purged it.
- Reinstalled them and injected 8 bar again and the output of the transducer became (8bar)
- carried out no load test for the turbine successfully then manually stopped it and the turbine completed successfully the normal shutdown procedures.
- The problem is solved successfully.
- The root cause is that (96FG) was sending to the control system lower pressure value than the actual pressure hence the SRV was opening much than it should so excessive gas pressure was flowing through GCV and caused the turbine speed to drop very slowly and didn't latch to the FSRMIN shutdown curve and accordingly hold at 85.5% of TNH.

Again thank you very much.
I wish it will be useful for you.