Loss of flame trip on 9FA

W

Thread Starter

Windsor

We have four GE MS9001FA DLN2.0+ Gas-only 255MW gas turbines controlled by MarkV TMR Control System. We recently experienced two "Loss of flame" trips on the same unit (T1) during normal operation, and we are working on determining the root cause for that trip. LVDT error repeated itself for a long time before the first trip with the following alarm:

"TCQA LVDT position diff high REG#7"

Then loss of flame on cans resulted in trip.

361 27-AUG-2007 21:21:55.125 T1 L28FDA
FALSE EVENT Flame detector channel #1

362 27-AUG-2007 21:21:55.187 T1 L28FD
FALSE EVENT Flame Detected

363 27-AUG-2007 21:21:55.187 T1 L28FDB
FALSE EVENT Flame detector channel #2

364 27-AUG-2007 21:21:55.187 T1 L28FDC
FALSE EVENT Flame detector channel #3

365 27-AUG-2007 21:21:55.187 T1 L28FDD
FALSE EVENT Flame detector channel #4

385 27-AUG-2007 21:21:55.156 T1 0060
TRUE PALARM LOSS OF FLAME TRIP

After the first trip we just calibrated the LVDT#1 on PM4(G3) gas control valve.

Before the second trip in addition to the existing LVDT diff high alarm we received:

"TCQA LVDT regulator #7 feedback trouble"

alarm and we replaced the troubled LVDT. Following alarms occur before the second trip:

264 02-SEP-2007 19:05:24.125 T1 L28FDA
FALSE EVENT Flame detector channel #1

265 02-SEP-2007 19:05:24.125 T1 L28FDB
FALSE EVENT Flame detector channel #1

266 02-SEP-2007 19:05:24.187 T1 L28FD
FALSE EVENT Flame Detected

264 02-SEP-2007 19:05:24.187 T1 L28FDC
FALSE EVENT Flame detector channel #1

265 02-SEP-2007 19:05:24.187 T1 L28FDD
FALSE EVENT Flame detector channel #1

We suspect couple of problems that might cause the trip:

1) On second trip, LVDT #1 on PM1 goes bad, so valve could not follow the reference and the loss of flame occurs as a physical reality. LVDT #2 was working fine.

2) The idle time on <C> core of the unit T1 shows around 10 percent but the other three units indicate idle time around 22 percent. Right now there is no change on idle times.

3) We replaced the Reuter-Stokes flame detector module after the second trip but we are not sure if the module has gone bad. Is there a way to test the module in the workshop environment?

4) After the first trip, we removed all four flame detectors from the turbine and checked with light torch, and they function without any problems. So we put them back in. The cooling water supply was ok.

The unit is working for almost a week without any problems now. Any comments on understanding the reason would be most appreciated, thank you.

Regards,
 
B

Bob Johnston

Do you have any other Trip Log records? The flame detector alrms look like nothing more than loss of flame. For sure the LVDT fault could be the cause, but, if you have changed the LVDT, this should now be OK. The reason I would want to see the Trip Log is to see the sequence of events, did you lose fuel? If you are suspicious of the flame detectors, the level of exhaust temperature and fuel level at the time of trip should confirm or deny this.
 
Thanks for the link, but the unit is running now so we couldn't check the connections on the MkV panel. But when I check the OSM trip data, right before the trip, I saw FPG2 (Interstage Fuel Gas Pressure) increased from 404 psi to 464 psi and FSG3 (Position Feedback of G3 Gas Valve) changed from 61% to 128% (full open). As the ref is still 65% the valve wants to eliminate the error between the ref and feedback signal and closes. We are pretty much convinced that the trip couse is tracking problem and possible sudden closing of G3 valve and resulting loss of flame. But there are two LVDTs on G3 valve and the thing that I didn't get is why the valve fully closes if only one of the LVDTs malfunction? Could you please explain the working procedure of the LVDT's on MkV control system?

Regards,
 
The unit is working fine now but yesterday T2 (another unit) tripped with same alarms (loss of flame) but immediately we checked the LVDT on G3 valve and found it faulty. The moving rod was rubbing the inside of the outer case of the LVDT. We replaced it with a new one and calibrated and started the machine. We will check all LVDTs on other two GTs on the next planned outage.

Regards,
 
It's not really clear what the failure mechanism is at your site, but it seems that you are experiencing problems due to LVDT feedback going high, instead of low which is the normal failure mode. Usually, when an LVDT fails or experiences an open circuit, the feedback from the LVDT goes low (less than zero). The high-select function will "ignore" the extremely low signal, and presuming the other LVDT feedback is good, the unit will continue to run with the Diag. Alarm you noted in your original post. That's why the high-select function is used, because the predominant failure mode is for the feedback to be low or less than zero.

However, a number of sites with Mark Vs have experienced problems caused by LVDT feedback going high (greater than 100%), which causes the high-select function to close the valve in response to the higher-than-desired position feedback.

You should contact GE for assistance with this problem; re-connecting the wiring of the LVDT can prevent this condition, if it's what you're experiencing. The fact that you've experienced it on more than one machine seems to indicate this may be the source of your problem.

This is not a well-known phenomenon in Mark Vs, but it is documented and has been resolved by reconnecting the LVDT wiring. I think it was documented in something called a Mark V Newsletter, which was only distributed internally and not generally available to Customers or owners/operators.

If the G3 valve goes closed suddenly that will cause the P2 pressure to increase suddenly until the SRV "catches up" with the flow decrease.
 
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