GE 7EA machine was running at Base load @ 71 MW in Premix mode. Suddenly it's firing mode changed to Extended Lean-Lean mode. No appreciable alarm or event observed.

To change firing mode GT was deloaded to 9.9 MW to enable Primary mode and load was increased upto Base load but immediately it's firing mode changed to Extended Lean-Lean mode. Machine again deloaded upto 9.7 MW to enable Primary mode and loaded upto 69 MW just below base load. No abnormalities observed when machine is runing at speed control.

Please suggest probable causes and corrective action.

 

Dharmesh,
When did this problem start? After a maintenance outage? After a gas fuel servo-valve was replaced and/or LVDTs were re-calibrated?

The issue of primary zone re-ignition has been covered several times before on control.com. The basic causes of primary zone re-ignition are:

--fluctuations in gas fuel supply pressure resulting in fluctuating SRV position and P2 pressure, some source of ignition (hot "coke" deposits on fuel nozzles, for example, from oils or hydrocarbons entrained in the gas fuel);

--natural gas liquids condensing in the various pressure drops across the gas fuel valves and fuel nozzles (resulting from low gas fuel supply temperature, gas fuel temperature less than 50 deg F above gas fuel dew point);

--fuel splits very close to combustion stability limits for the given ambient conditions.

Recently one multi-unit site was experiencing a high number of primary zone re-ignitions on only one of the units. After much time and effort it was finally discovered that a #2 bearing drain valve had been left in the wrong position after an outage allowing lube oil to enter the combustion wrapper and get blown into the combustors creating hot carbonized deposits in the primary combustion zone leading to primary zone re-ignition.

It took several months to find a valve out of position, and then only after a combustion inspection with someone crawling around the bottom of the combustion wrapper. It had been noted that the lube oil tank of that particular had needed topping off frequently during the period they had been experiencing primary zone re-ignitions--after the fact. [Manual valve position is the most important factor in proper operation of GE-design heavy duty gas turbines, particularly those with DLN combustors. Many combustion problems and a lot of damaged combustion hardware can be traced to improper valve positions after maintenance outages, particularly after off-line water washing.]

If gas fuel valve servo-valves have been replaced without verifying servo current polarity, this could also be one cause of primary zone re-ignitions, especially if there are Diagnostic Alarms like FPRG_INT problems, or other gas fuel valve-related Diagnostic Alarms. What Diagnostic Alarms are active and present, and what Diagnostic Alarms are annunciated during the primary zone re-ignition event?

Lastly, it is not necessary to unload to primary combustion mode. It's only necessary to unload the unit until the unit is fully in Lean-Lean combustion mode (usually 50 deg F below the Lean-Lean-to-Premix transfer temperature).

Please write back to let us know what you find.