GE MS5001P Overspeed trip on fuel gas startup

We are trying start one turbine GE MS5001P Heavy duty dual fuel, but it had tripped several times by overspeed protection.
The machine was working on liquid fuel because not fuel gas availability, now we are trying start it on fuel gas but the speed turbine up over FSNL (100.3%) setting and continues until it trips by "Protective module overspeed trip - HP".
We had made next actions:
SRV servovalve was replaced.
SRV/GCV blocks was checked.
Autocalibrate was made.
In addition we did start on liquid fuel and tried fuel transfer to gas and the turbine trip too.
The last trip log display is attached to this post.
We please your help in order to fix this trouble
Thanks in advance



Hmmm.... Well it seems the trip occurred at 12:36:34nn, because usually the Mark V Trip Log records three points after the trip (three horizontal rows), and shows a couple of alarms after the trip as well.

Here are some snippets of information which might help describe that statement and what was going on at the same time:



I don't have access to a true editor at this writing, but there's something very odd going on with the IGVs, because they open to 81.8 DGA (or more) and then close back to approximately 57 DGA after the trip--and seem to hold the 56.8 DGA (57 DGA) at the time of the trip. The IGV current (CAGV) seems to be a little out of whack, too.... I would expect to see values of -2% or -3%, not -1%.

It's also pretty odd that the unit tripped at 108% TNH (or thereabouts) and then it did overshoot a little after the trip, but it didn't get to 110%--which is the typical <P> core overspeed trip setpoint (older machines used to use 112.5% TNH, or the approximate value that could be achieved with the Berg jumpers on the TCEA cards).

I'm wondering if there isn't something amiss with the speed signals, or possibly the fuel BTU content has greatly increased. Or, something is forced that shouldn't be.... What logic signals are forced?

Also, what are the other Process Alarms that are present during starting and acceleration? What are the Diagnostic Alarms that are present during starting and acceleration and when the unit gets to FSNL?

I know I'm asking for a lot of information, but I don't think the Trip History has enough information to be useful. And, some of the information seems to be questionable.

I would recommend running VIEW2 during the next start and collecting data on TNH1 and TNHOS and TNR and FSGR and FAGR and FSG and FAG at a minimum. Also, CSGV, CSRGV, CAGV and TTXSP1, TTXSP2, TTXSP3.

I know you said an AutoCalibration was performed--but did anyone visually verify the indicated LVDT position (scaled feedback) on the HMI actually matched the actual physical position of the Gas Control Valve? (But, really, when the unit hits 14HS and transitions from Acceleration Control to Droop Speed Control it shouldn't make any difference if the LVDT calibration is correct or not--because at that point the GCV should be moving to try to make TNH equal to TNR.

So, what is TNR? Why is TNR continuing to increase after the unit reaches FSNL? These are the questions inquiring minds want to know. (Those and what logic signals are forced, what Process- and Diagnostic Alarms are present during starting and acceleration to FSNL.)

Anxious to hear the requested information. There have been many threads on where the use of VIEW2 has been detailed. And I think it's also documented in the Maintenance Manual (GEH-5980) and there is help for the function at the command line (type VIEW2 /? and press ENTER and there will be a help screen).
Thanks CSA for you soon response.
Another important things are:
1.- During start sequence the FSR control switched from ACC to TEMP because exhaust temp reaches 560 C
2.- The time spent for the turbine from start command to overspeed trip was below 10 minutes, but normally is close to 15 minutes until FSNL.
When we could start the machine we will collect more information for share
Thanks and regards

Potentially a very good piece of information that the time from START to trip is 10 minutes instead of 15 and the exhaust temperature was so high the Mark* switched from FSRACC to FSRT--and yet the unit continued to accelerate, eventually tripping.

Because usually when a unit goes on Temperature Control during acceleration the rate of acceleration (TNHA) slows way down and causes the time to reach FSNL (Full Speed-No Load) to increase. In your case the opposite occurred--the time to reach FSNL DEcreased and, in fact, the unit exceeded rated speed.

You also mentioned that when you attempted to change from liquid fuel to gas fuel at FSNL the unit again increased in speed and tripped.

This would appear to be some kind of problem with a significant increase in the BTU content of the fuel OR a problem with the fuel valves (either the SRV or GCV). It shouldn't be the GCV because it should be trying to control speed--or exhaust temperature--and yet the turbine speed continued to increase.

Looking at FSR in the Trip History it's clear it stopped increasing at 16.03% and yet speed continued to increase. Again, this points to either a significant increase in the BTU content of the fuel OR the P2 pressure is MUCH higher than the Mark* thinks it is. Is it possible there is a problem with the calibration of the P2 pressure transmitter or that somehow the scaling of the P2 pressure transmitter inputs(s) got changed?

If the gas fuel pressure upstream of the GCV were higher than it should be--and increasing (which it would be because TNH was increasing)--then the speed would increase even though the GCV was staying at a fixed position because the exhaust temperature was so high.

Furthermore, if the P2 pressure was higher than it should be or the Mark* thought it was during the fuel transfer at FSNL the unit would again overspend uncontrollably.

Methinks--based on the information provided--there is a problem with the actual P2 pressure versus what the Mark* thinks the P2 pressure is. Either the P2 pressure transmitter calibration is way off or the input scaling is incorrect.

OR the BTU content of the gas fuel is suddenly much higher than it has been in the past.

I'm unable to read the Trip History file very easily on my smarter-than-me phone so I can't easily look at the value of FSGR but I seem to remember it wasn't outrageously high nor was the P2 pressure (14.91 barg, if I recall correctly at the time of the trip). BUT, if the ACTUAL P2 pressure was higher than the Mark* believed it to be then the amount of fuel flowing through the GCV would be higher than expected and the exhaust temperature would be higher than expected and the speed would be higher than expected.

Please add TNHA and TNHAR and TTXM to the list of data points to be collected using VIEW2 during the next start attempt. AND--verify both the P2 pressure transmitter calibration and P2 pressure transmitter input scaling!

If the gas fuel supply has changed recently (from a different field or supplier, or the unit burns LNG and the gas comes from a different field or supplier) it's also possible the BTU content of the gas fuel is suddenly much higher (which could happen because there's a lot we don't know about the unit).

Looking forward to hearing back from you on what you find!!!

Today I was able to import the csv portion of the Trip Display into MS-Excel and do some more analyzing than I was able to on my smarter-than-me phone. It's pretty common, though not desirable, for many units to reach temperature control during acceleration and for the FSR to be limited during that period. Based on what I saw today, FSR did appear to be limited at about 15.83-15-93% for a period while the exhaust temperatures were around 540 deg C, but that soon after that when TNH increased and the IGVs started opening the exhaust temperature did decrease and FSR did increase--slightly--to 16.03% and held pretty firmly at 16.03 for the remainder of the failed start attempt.

I have to believe either the fuel gas BTU content has changed dramatically, OR the P2 pressure transmitter calibration is way off--and the P2 pressure the Mark* is sensing is much less than it actually is, causing higher-than-normal flows through the GCV which is holding steady for a good period of time at 16.03%. I wonder if the MIN FSR value for your unit isn't 16.03%.... And, I wonder if the GCV LVDT calibration might also be slightly suspect. If the GCV were physically open more than indicated (meaning the scaled LVDT feedback was less than the actual position) then more fuel that normal would be flowing through the valve.

It's hard for many people to believe--but the Mark* for decades just didn't calculate actual gas fuel flow-rate and compare that to a gas fuel flow-rate reference. The Mark* relies on good, accurate GCV calibrations and proper P2 pressures and sets a GCV position reference--and if the LVDT calibration is good (meaning the indicated feedback (the scaled LVDT feedback) is close to the actual valve position ("stroke")) then the flow through the valve at a known good position should be equal to a value determined during factory configuration and that should correspond to desired operation. Until just recently, gas fuel flow-rate was only monitored and used for wet low NOx injection reference calculations--NOT for load- or speed control. And many people just have a very difficult time understanding that the digital Mark* isn't really all that sophisticated. There are any air/fuel calculations or references or measurements--that the fuel injection systems on some inexpensive automobiles and some motorcycles are more sophisticated than the Mark* in that regard.

I'm not a big believer in the veracity of the data from Trip Displays; I just don't think it's all that reliable (at least that's been my experience). It's a good indication of what's happening, but not always very accurate.

But something is definitely amiss with the unit--as you noted, when you started on liquid fuel and switched to gas fuel everything should have been just fine. But, if I understand correctly the unit also tripped on overspeed during or shortly after the fuel transfer, so that would suggest that something is allowing more fuel than should be flowing (such as incorrect P2 pressure sensing or scaling), or the BTU content of the gas fuel has changed dramatically.

Again, we are looking forward to hearing what you discover--and getting good, hard, actionable data to analyze to prove--or disprove--what may (or may not) be happening!