Hey all,

I'm having a problem with a GE Frame 7EA gas turbine.  While running full speed no load, the exhaust spread becomes too large causing a "Combustion Alarm".  

-We dissamebled the two cans that would, taking into account the exhaust twist, be in having issues.  A loose secondary nozzle was found, the improperly installed gasket was replaced and the nozzle tightened.  

-No smoke was noted coming out of the stack.  I have included a trend of the last run that the alarm occoured in, it was at full speed no load.

-Everything seemed to be in a steady condition when suddenly the temperatures of 3 thermocouples drop causing the maximum differential to rise above the alarm limit.

-One of the thermocouples that contributed to the alarm also had an irregularity at the 6 minute mark on startup.

There is an attached powerpoint graph showing the trend from startup through shutdown (34.5 minutes).

Any ideas?

Thanks,
Carl

The three TC's drop at the same time, this could indicate two different problems.

1. Combustion hardware. Are the TC's adjacent in the exhaust stack?

2.Control system hardware. Look at the control drawing, could a single point failure, a card, a loose connection make this happen? Are all three TC's associated with the same control system hardware?  Review the ambient compensation, a loose/ wrong ambient compensation throws off TC signals.

If this were a 501 machine I would say that looks like the failure signature of a transition between the combuster and the power turbine developing a crack. I have some photos somewhere although I'll have to see if they're copyrighted before posting them.
  

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If we learn from our mistakes I'm getting a great education!
 

One thermocouple drops, I'd be checking thermocouples and/or wiring.  Three thermocouples drop simultaneously, I'd be checking cans and transition pieces and maybe even burner nozzles.  What are you burning?

I think that thermocouple you mentioned was telling you something significant during your acceleration run-up.  One or more of the cans was in trouble all the way.

What did you observe at higher power levels with these same TC's on previous runs?

rmw

Interesting point from rmw - on re-reading the OP I notice there's a reference to 'no smoke'. What are you running this machine on - distillate? If it's a liquid fuel machine then a blocked nozzle sounds like another possibility. Never seen one blocked on gas fuel but have seen more than a few on naphtha fuel.
  

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If we learn from our mistakes I'm getting a great education!
 

Have you solved the problem?

as you mention a "secondary nozzle" i understand you have a DLN 1 combustion system, otherwise you would not have such a nozzle.

as mentioned before if the three thermocouples are adjacent, then the problem is a can (most likely flaming out).

Have you checked exhaust emissions? an increase in CO will indicate poor combustion.

if one can is flaming out, perhaps the nozzles are not giving the correct pressure ratio.

give more info to troubleshoot.

saludos.

saludos.
a.