Abnormal Speed


Thread Starter



One of the GTG was shut down for CI. while coming down speed stayed at 90% (3240 RPM) for 5 to 7 minutes and GTG trip. Looked into the Mark VIe trip log. Following are the finding:

L94X1=1 during normal shut down. Breaker opened at 0.5 MW (ok). FSR was 11 and at about this time L4=0 and FSR was ramped down to minimum. P2 pressure was 241 and ramped to 260 or so. Trend shows L4=0, L94X1=1, FSR=minimum. SRV position negative GCV position negative. During entire time FSR=0. Fuel flow was 2.3, P2 was 260, GCV/ SRV negative but speed 3240 RPM. After sometime GTG tripped.

Questions: when normal shut down at what time L4 drops out? Check all constants and found to be right. I might have aggregated time but GTG was at 3240 for may be 1 or 2 minutes or there was no smooth ramping down on speed. IT is Frame 7EA and marl Vie control.

I appreciate valuable info especially from CSA.


Things have really changed at your site, haven't they?

From your post it seems the unit was tripped (L4 @ "0") and yet it was still running....

When did this occur? After a maintenance outage when the "gas valves" were calibrated?

Which fuel was the unit running on when it was shut down?

Did the fuel valves get replaced at your site? There was talk of electric valves....

A unit shutting down while burning natural gas should be able to decelerate to less than 20% speed on a normal fired shutdown before L94T picks up to drop out L4. (Have a look at L4--the only two things which can drop out L4 are L4T and L94T--and L94T is the fired shutdown logic, and you can work backwards from that to see what drives that--it's usually 30- or 30 seconds after the unit decelerates below 20% TNH, L83RB I think).

And, you should be able to pull up historical data from past shutdowns to see when the unit normally "flames out" (below 20% speed).

Usually, when a unit "stall" during shutdown it's because the fuel flow wasn't reduced sufficiently for some reason. Possibly a Control Constant change (Minimum FSR affects both start-up and shutdown in the twisted GE control scheme.) Or, an LVDT calibration went awry, or a fuel valve didn't close sufficiently. Or, the wrong fuel was flowing when the unit was shut down....

Hope this helps!

(How things have changed....)
Thanks for prompt reply I will look into it and also try to match other unit-trip with this one to see if any constants are copied same or wrong way. but I did check constants with old Mark IV for shut down FSR and found to be ok.