We are on the startup of GE gas turbine. The startup failed in “fail to fire alarm L30D13”. We noticed that the compressor discharge pressure (96CD) is not increasing along with the HP shaft speed. We use steam turbine for startup.

Someone can please advise my understanding is correct. If it is what could be the reason for the compressor discharge pressure not to increase along with the HP shaft speed.

 

Dear Ismayil,

we've just started one of our 6FA units. At 14HM (minimum firing speed) CPD is practically 0 bar. No problem firing the gas turbine.

I'm not an expert, but for me it's normal to see a relatively low CPD during startups until near rated speed, even though the compressor is speeding up. Think that the air flow that enters the compresor is limited by the IGVs (they are closed) plus the effect of the open compressor bleed valves.

Hope this helps.

 

Ismayil,

When did this problem start? After a maintenance outage? If so, what was done during the maintenance outage?

You say the start-up failed. At what speed?

Are you sure fuel was actually flowing?

What fuel are you starting the gas turbine on?

Do you know that the steam turbine control valve was working properly, and that the steam was at proper temperature and pressure for use in the steam turbine?

Why do you think using a steam turbine ("expander turbine") as a starting means would affect the operation of the unit?

A plot of axial compressor discharge pressure versus HP speed during start-up will show that, as for most axial compressors, compressor discharge pressure does not really start to increase until the unit gets above 80-90% speed, when the IGVs open to minimum operating position and the axial compressor speed is near rated. Axial compressors typically don't produce much pressure at low speed. So, at firing speed (which should be about 10% of rated speed or thereabouts) the axial compressor discharge pressure will likely not even be 1 barg. During starting (with any kind of starting means) the IGVs are closed until approximately 80% speed.

 

We also faced a similar issue after an outage. Finally it was diagnosed that CPD calibration was done with a incorrect range resulting in wrong value of CPD.