Info: GE frame 9e, Dual fuel operation. Rated 110MW.
After completion of Combustion inspection, as normal practice we start up the unit on fuel oil, changeover test and test on start on gas operation. Initially, there were attempt 2 starts on fuel oil. Both failed with alarm only failure to ignite. Then the unit started on fuel gas, and it was all working fine, until FSNL and managed to changeover from fuel gas to fuel oil and back to gas. Then, they attempt to start again 2 times to start on fuel oil, same thing happened. Failure to ignite.
Nothing was one, out unit declared to operate only on fuel gas. The one day, the unit was requested to run up to base load operation. Somehow the GT behaves strangely the spread on the exhaust thermocouple is higher on the lower side, shown by TTXD 5 to 8. The spread 1,2,3 become higher and it is very near to the allowable spread limit. The unit could not reach base load, because it tripped at 100MW due to Exhaust temp spread high trip.
Can someone kindly helps to explain this issue?
The cause of the high exhaust temperature spread is most likely that something wasn't reassembled properly. Liquid fuel nozzles, AA tips, cross-fire tubes, etc. If the unit has DLN combustors it may be that the high energy igniter ("spark plugs") may not be inserted at the proper depth, or are getting wet during firing.
Have two (2) people monitor the fuel pressures at the liquid fuel flow divider during firing. One will move the selector valve handle and call out the pressures to the second who will be writing them down.
Monitor liquid fuel bypass valve position, liquid fuel valve servo output current, liquid fuel flow flow-rate reference and liquid fuel flow-rate feedback during firing using the trending feature of the Mark* version installed at your site. (Some Frame 9Es do not have LVDTs. If not, have a third person watching the liquid fuel bypass valve to check for stability.)