We are having GE frame 6E gas turbine with Mark V control system.
we had been running our machine on Natural gas for last 8 years but because of change in scenario we have again moved to light naphtha as fuel.

since then we are facing high spread issues.
After taking sd we replaced the fuel nozzles with the reconditioned nozzles and again run the m/c, spread went upto 70 deg on 12 MW only.

Then we replaced the nozzles with brand new fuel nozzles, replaced both liquid fuel and purge air check valves, replaced flow divider with new calibrated one. again after starting, spread values were around 30 deg when the load was 25 MW, then after increasing the load to 30 MW, spread increased to 90 deg.

We may again have to stop m/c because of such high value.

but what may be the other reason for this which we can check and replace. Is there be any damages in the combustion chamber which may cause this or the exhaust thermocouples may be faulty?

 

What were the spreads before the change to liquid fuel?

If the exhaust T/Cs were working fine before this change it's likely they are still working fine. If the cold spot/hot spot moves with a change in load and IGV angle, it's not a T/C problem. If the cold spot/hot spot doesn't move with a change in load and IGV angle, then it could well be a T/C issue, or an air flow issue in the exhaust. Possibly a compressor bleed valve isn't closing fully for some reason?

Are you sure the AA Bypass Valve is closed? The AA pressure should be approximately 1.2 times CPD (absolute). If the AA pressure isn't sufficient that could be the issue.

Also, many times when a unit hasn't run on liquid fuel for a long time the Main AA Compressor can be not working very well, especially if the inlet air temperature wasn't maintained properly (presuming the unit has an AA Precooler). Moisture can condense in the AA Precooler shell and get pushed along into the AA Compressor which will start the damage process by reducing internal clearances and reducing AA compressor efficiency--which won't be noticed when running on gas fuel.

Short of that, how long has it been since the last maintenance outage?