During trip the machine was operating in Premix mode at 125 MW. In this mode only the Gas control valves PM1 and PM4 only will be in service and D5 valve will be in purge mode.

Finally the fault investigation revealed that the GT 9fa trip was a result of fluctuation in the gas control valve (GCV1). The servo for the GCV1 was replaced with the new one and the machine was started and running normal.

There are two things made me to confuse.

1). why the D5 valve got fluctuation when the machine is operating in premix mode. (I know it is almost at the juncture of Mode change over to PPM which brings Diffusion into service) {(around 125 mw and TTRF is about 1250. i don't know the exact value as i am working on field and not on desk)}. we observed no fluctuation in load and TTRF.

2). As already flame established in all the combustion chambers and machine running at 125 MW (I know two out of four flame scanners fail to detect flame leads to trip) how it will leads to machine trip on loss of flame? and during investigation by my shift charge it came to conclusion that becoz of fluctuation in the gas control valve (GCV1) it leads to trip. but how the fluctuation in GCV1-D5 have affect on loss of flame signal.

waiting for your valuable replies....

 

Sriram,

'Loss of Flame' occurs when a majority of the flame detectors fail to see flame <b>AND</b> there is no other trip condition detected which would cause the fuel to be shut off and flame to be lost. If there is no fuel, there is no flame.

I don't have any P&IDs for a Frame 9FA at this writing, so I can't relate valves to numbers, but if there was an attempt at a combustion mode change (from Premix to Piloted Premix) and the valves did not respond as desired because the servo-valve wasn't working properly and this caused an upset in fuel which caused a loss of flame, then this could be the result.

Any such 'Loss of Flame' trip from load would almost always be preceded by a sudden drop in load, so the data (from Trip Log or Trip History or Historian) should show this. As fuel is reduced the load is going to decrease and flame will be lost in various combustors not usually all at once. This means the exhaust temperature spreads will also be increasing in the time prior to the loss of flame.

Also, when flame is lost the "back-pressure" in the combustor decreases and this in turn leads to a drop in CPD just prior to a loss of flame.

I think you are correct in the thought that if the load were stable that would mean the fuel flow was stable so there should be nothing causing the servo-valve to make the actuator move the fuel valve. The exception to this is if the servo-valve spool piece was already "stuck" or "binding" at the stable fuel flow and there was already excess current flowing to try to maintain the load and suddenly the spool piece became unstuck then this could cause a problem, but if the Hydraulic Accumulators are working correctly the system should be able to survive an event like this at low load.

Unless, also, as you suggested the load was very near a combustion mode transfer and this was just the impetus needed to set a bad chain of events in motion that resulted in a loss of flame event.

All to often, servo-valves get improperly blamed. There is one contributor to control.com that has 40-50 servos that have been replaced and his company has purchased a servo-valve tester to try to determine if any of the servos had really failed or were still serviceable, and it would seem they might try testing a servo before replacement to see if it had failed (at least that's what it seems the test rig is going to be used for). We're still waiting to hear the results of the testing to see how many of the multitude of servos were truly failed, or how the test rig is being used to improve operations and reliability and troubleshooting.

The common cause of servo-valve failure is: bad oil. It has been proven that recent changes in turbine lube oil formulation, while improving the lubricating qualities of the lube oil, have not been good for those turbine systems that use lube oil for hydraulic systems and valve actuation (including IGVs). There is at least one manufacturer, BP-Castrol, that has formulated a turbine lube oil almost specifically for GE-design heavy duty gas turbines that has been proven to reduce and virtually eliminate lube oil quality problems leading to servo-valve replacement.

It just doesn't seem as if you have enough information (data) to be able to make your own analysis, though you do have some suspicions. Unfortunately, I don't believe there is any good "field" test of servo-valves to determine if they are good or bad. I have no experience with the test rig the contributor's company has purchased, so I can't say for sure. I know that there are any number of companies who refurbish electro-hydraulic servo-valves, but in my personal experience they aren't worth the money. (At one site we installed six refurbished servo-valves before we got one that would work properly! Fortunately, the site owner had a "fleet" of multiple turbines and had quite a number of servo-valves in Stores.)

Sometimes, one just has to chalk the experience up to whatever Management is saying. But, you ask valid questions and that's good. Suspicion and doubt are not always bad things, and it's these qualities that make for a good technician or operator. When you are able to have a look at information/data for yourself, you can then use these experiences and the knowledge and experience you have gained by watching others (and not every learning experience is good--the key is being able to distinguish good from bad) to make your own informed and experienced judgement. Sometimes, it's just best to sit back and watch and listen, and learn (sometimes the lesson is: how not to analyze or troubleshot an event).

Best of luck! Keep your head down, your eyes and ears open, and speak when spoken to (I need more practice with the latter...!).