When gas turbine GE 9E start up on gas fuel fail to ignite or flames fluctuate after firing and tripped several times even, we decided to isolate the AA booster comp. Before firing which find the unit running well with stable flames.
Is that true or not of booster isolation during starup on gas fuel?
The most common causes of flickering flame indications, and/or loss of flame trips during firing/warm-up/acceleration and/or failures to ignite when starting on gas fuel are:
--changes in the caloric content of the gas fuel (new supply/field)
--improperly calibrated gas fuel control valve LVDTs (NOT the SRV, but the GCV or IGCVs (Independent Gas Control Valves), if so equipped
--dirty inlet air filters and/or axial compressors
--improperly calibrated IGV (Inlet Guide Vane LVDTs)
--Torque Adjustor Limit Switches improperly adjusted
Firing and warm-up are what's called "open loop" control--there is just a valve position setpoint/reference and the gas control valve goes to that position and the hope is that the fuel make-up hasn't changed since commissioning AND that the LVDT calibration is reasonably good AND that the inlet air filters are reasonable clean AND that the IGV LVDTs are calibrated properly and that the Torque Adjustor Limit Switches are adjusted correctly. Valve position is related to fuel flow, and so many things have to be just so for the proper fuel flow-rate to occur during firing and warm-up. And, air flow is also critical during firing; too much air or too little air can both cause problems with flame stability.
The Torque Adjustor controls the amount of torque being applied to the turbine through the torque converter. Many older torque adjustors use limit switches to control torque transmission and many limit switches are not properly adjusted--which can cause acceleration during warm-up to be higher or lower than desired, both of which can lead to problems when warm-up is complete. Also, the rate at which the torque converter guide vanes are opened during warm-up can also play havoc with flame stability once warm-up is complete--again, especially if the acceleration is too high.
Finally, the setting of the Minimum FSR value during firing and warm-up can also be problematic, especially if FSR gets cut back to minimum when warm-up expires and acceleration is to begin. This usually happens when the acceleration during warm-up is too fast, and if the Minimum FSR Control Constant is too low, well, there can be flame instability.
Dual fuel machines (those with gas and liquid capability) require a Booster AA Compressor for help in atomizing liquid fuel during starting. Most of the GE-design turbine control systems start and run the Booster AA Compressor when starting on gas or liquid fuel, and it usually doesn't have much of an effect on starting reliability or flame stability. If you are saying that when you stop or prevent the Booster AA Compressor from running when starting on gas fuel the flame stability is better, I would say the problem isn't caused by the Booster AA Compressor--it's the result of one, or more, of the issues listed above.
Fuel make-up does change over time as new gas fields are brought on line and other are taken off line. All natural gas is NOT the same, and neither is all LNG (Liquefied Natural Gas). And, neither are all liquid fuels, for that matter.
Also, if fuel nozzles have changed over time, that can affect fuel flow-rates also. Most fuel nozzles are tested and come with documentation as to flow-rate, and are put in as a matched set with particular attention paid during installation so as not to put the nozzle with the highest flow-rate next to the one with the lowest flow-rate (which can cause higher than normal exhaust temperature spreads immediately after re-start).
Is the Booster AA Compressor absolutely required during starting on natural gas fuel? Probably not, but one thing it does provide is protection against the back-flow of hot combustion gases into the AA nozzle passages and AA manifold around the axial compressor. If you have flame instability during starting and high exhaust temperature spreads during starting that means there are imbalances in the pressures in the various combustors and that can cause undesirable back-flows and possible circulation through manifolds.
Best to find the root cause(s) of the flame instability and starting reliability and leave the Booster AA Compressor running when starting on gas fuel.
Hope this helps!!!