Premix Mode operation on exhaust temperature control occurs after approximately 70% load. By lowering the allowable minimum IGV angle, exhaust temperature control operation and Premix Mode can be extended to lower loads down to approximately 40-50% load.

At our site during load reducing it has been multitimes observed that GT did change mode to Extended Lean Lean IBH did not open. Very few times IBH opened and Premix mode remained intact approximately 50% load.

Can any one tell me what are conditions for IBH Operation? What is impact of system frequency on its operation, did low frequency limit/restrict the IBH to remain close? and what is the impact of climate/atmospheric temperature with IBH?

Because IBH is like mystery for me. Totally dependent on IBH while load shedding. and most of the times reduce load to remain in Lean lean mode to avoid extended lean lean, can not fulfill the demand of load center because IBH do not open while reducing load.

What are exactly conditions for IBH opening to support the premix mode on lower load operation?

 

Dear SB,

typically the operation of IBH based on the position of the inlet guide vanes. This assumes that you are operating some design of dry low Nox machine. The purpose of the inlet guide vanes and IBH is to reduce total air mass flow through the compressor at reduced loads. This allows the unit to stay in low emissions mode, and also maximizes the exhaust temperature for use in combined cycle mode.
Typically the position of the IBH valve is proportional to the IGV position, higher IGV position, no IBH, reduced IGV position, increased IBH valve.

You mention a problem with system frequency which may be causing issues. If you are in an area with erratic system frequency, especially low frequency dips, then possibly this may be causing you to drop out of DLN mode. If frequency drops quickly, the IBH and IGV's may not be able to react quickly enough. If compressor pressure ratio drops quickly due to a dip in frequency, this can cause the unit to return back to lean-lean mode. Typically this occurs because the low system frequency causes a quick drop in CPD pressure, which changes the pressure drop in the combustor area, mainly in the area of the combustion liner where the venturi is. This pressure drop allows the flame burning in the secondary zone to travel backwards into the primary zone and relight the primaries. The control system sees this and fires the spark plugs to assure that flame is established in all primary zones, and the unit falls back into lean-lean mode.

I know I am not explaining this very well, but there is a lot to cover to fully understand this system and its proper operation.
Trying to state it simply. As IGV's close down due to reduced load, the IBH should start opening. This usually occurs around 62 degrees of IGV. There should be no atmospheric conditions that stop the IBH valve from opening. By opening the IBH valve it reduces compressor air mass flow in two ways. One is by sending compressor air back to the inlet, reducing the compressor pressure ratio by "bleeding" off CPD air. Secondly the warmer air in the inlet makes the compressor less efficient since warmer air has less mass. This is all done to reduce air in the combustor area as load is reduced so the unit does not flame out due to a lean blow out.

In your situation it sounds like system frequency may be causing issues that the turbine is not fast enough to handle.
I hope this helps some, its a very interesting but complicated subject.