We are operating GE Frame 6 Gas turbine ( Gas only) . We had gas turbine ventillation duct gas detection in 2 sensers. We inhibited and checked for LEL & Methane . Report was negative. Later we identified cross fire outer tube gasket leak.
GT was shut down, gasket replaced, gas senser replaced, its cable checked. After restarting after 4 hours again gas senser detected gas . Lab analysis reports negative. But 6 of the cross fire outer tube flange leak was observed. GT is running with sensers inhibited.
Thermal imaging indicates 15 deg C increase in the crossfire outer tube .

Can someone share any similar experience/ guide this situation.

From my experience typically outer crossfire tube leaking does not contain methane gas since the air blowing out is from the hot CDC (combustion discharge). Hot air is discharged and pulled up to the vacuum blowers thats extracting hot air out of the compartment. Since most Haz gas sensors can only handel about 250degs F of heat before degradation, they are typically the first tell tail that you have hot air leaking our of the unit.
I've seen sporadic false LEL's which resulted in a sensor failure due to hot air leaking out of the crossfire tubing. Since the packing is normally replaced during an outage when the combustion hardware is removed, replacing it insitsu is difficult and requires splitting the packing ring. Once there is an air gap in the packing rings even the slightest hot air leakage will accelerate its failure and blow it out. I would recommend upgrading to crossfire bellows modification which uses a flexitallic gasket and expansion bellows as opposed to a graphite packing type with clearance gaps. If you have high LEL, and it is determined true, there is more than likely leaks on the gas flex lines and associated flanges. best bet for detecting leaks is your typical rag on a stick.. Good luck!

Rob

Thank you Mr Stevens for an intelligent technical diagnosis and a prompt reply.

We were also thinking in the same line. We had adopted many methodology to identify the leak or any after combustion. We had used the explosive meter, we had analysed the GT turbine compartment air at defferent locaton with gas chromatograph, we had used thermal imaging and also pyrometer to identify varying range of gas and temperatures. In none of the methods we could find even traces of gas or oxydation tendency.
Based on these we had deciding to run the machine till M.I which is due in september 2010

We had similar experinces and upon further study it turned out that the leaks sufraced after water washes. Apparently water wash effulent would make its way from the CDC to the outer crossfire packing rings and the graphite would get wet. Upon startup the hot air would just blow them out. Be careful with pushing too much water into your unit which could flood the drains and water will make it way up the CDC and out towards the FWD combustion can areas. Watch out for those extra rinses and not fully opening all WW drains. thanks

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O&M Manager-BSME, MSME, P.E.

We currenly have a worseneing cross-fire tube leak on a Fr6B. We have known of leak for last two months only (suspect developing over last year if connected to high compartment temps). Unsure if during last offline water wash if it tripped on hi spread or other. It would have been back mid 2009.

Hoing to hold out until next month.

aksunhil, what did you find in your Sept MI?

Sounds like you need to upgrade to the Crossfire Bellows mod which uses spiral wound gaskets instead of those crappy graphite rope/packing.

Are the crossfire bellows available for frame 6 MS6001B machines? I wa under the impression it was only for F-technology machines.

good question. We have 7Fa and 6B's on site and will ask our GE sales rep to see if the bellows are available on the 6B.