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Exhaust Temperature Control Reference Limit
Does exhaust temp control reference (TTRX) have reference value at simple cycle and another reference values at combined cycle?
By Mohamed zaki on 27 April, 2018 - 1:01 am

We work at GE gas turbine frame 9fa. Does TTRX and firing temp and IGV has different reference value at simple operation and combined cycle operation?

And Does IGV control exhaust temp at simple cycle or only in combined cycle?

Mohammed zaki,

Unfortunately, the answer to your question is not simple. And it's made even more complicated by the unit you work on.

GE-design F-class turbines can be configured in MANY different ways, AND they can also exhaust into MANY different types of HRSGs (Heat Recovery Steam Generators, or boilers) to produce steam for a steam turbine and/or a nearby process (such as a refinery). This can--and does--complicate things greatly.

Most F-class turbine are also equipped with DLN (Dry Low NOx) combustion systems. And most of these Dry Low NOx combustion systems use the IGVs to control air flow-rates at part load conditions (less than Base Load). Further, many F-class DLN combustion systems utilize Inlet Bleed Heat, a very poor name for the function it provides which is to protect the axial compressor when IGV angles are below compressor design minimums (which is necessary with DLN combustion systems operating at low- and part load conditions).

There are also HRSGs which have maximum inlet temperatures which cannot be exceeded in order to protect the HRSG internals. This requirement is often programmed into the GE turbine control system when operating in combined cycle mode (exhausting into the HRSG). And, we don't know anything about the HRSGs at your plant.

So, there are simply too many unknowns for us to make any definitive statement or provide any concrete answers to your questions. Presuming the HRSGs don't have any inlet temperature limits and that the unit have DLN combustions systems my best guess would be that, no, there would not be any difference between simple cycle and combined cycle IGV scheduling. But, that's just a guess based on past experience. And, the GE engineers at Belfort, France, that have the responsibility for programming Frame 9 machines (9Es and 9Fs and 9FAs) have been known to do some very atypical things with their turbine control schemes.

I wish the response could have been better, but with all the possible variables it's really impossible to provide an answer which is applicable to your site without knowing a LOT more about the turbines and the HRSGs. And, with the Belfort complication, it's even more difficult to make a definitive statement about the units at your site.

>And Does IGV control exhaust temp at simple cycle or only in
>combined cycle?

As for your last question, on units equipped with DLN combustion systems the IGVs are programmed in such as way as to maximize exhaust temperature at part load while controlling air flow in order to maintain stability in the combustors. GE knows what the air flows are when the IGVs are being used to maximize exhaust temperatures and uses that information to schedule (program) IGV operation to optimize combustor stability during part load operation (so as not to exceed optimum air/fuel ratios, which are NOT usually measured or monitored or even calculated by the turbine control system). Technically speaking, the IGVs don't "control" exhaust temperature, but rather they are controlled "by" exhaust temperature--they are held closed as much as possible during part load operation and open when the exhaust temperature reaches the maximum for that operating condition as fuel is added to the unit to increase load.

It's kind of difficult to explain without graphs and charts and drawings, but the IGVs are being used to limit air flow, and when they limit air flow that results in higher than normal exhaust temperatures (higher than if the IGVs were NOT being used to limit air flow at part load). So the miminum opening of the IGVs is determined by the maximum allowable exhaust temperature, and as fuel in increased during loading of the unit the exhaust temperature would tend to increase but the IGVs are opened to maintain an exhaust temperature limit based on current operating conditions.

Again, a very difficult thing to explain without graphs and charts and drawings. The IGVs of machines with DLN combustors aren't being used to control exhaust temperature; the position of the IGVs is a function of exhaust temperature--at part load. The exhaust temperature can never be allowed to exceed the exhaust temperature reference limit (TTRX) for any given operating condition. And, as fuel is added which would tend to increase the exhaust temperature the IGVs are then opened to reduce or maintain the exhaust temperature equal to the exhaust temperature reference limit. And, this is all done to limit air flow into the combustors so as to maintain stable combustion in the combustors and not blow the flame out or have too rich a flame (excessive fuel with respect to air).

Hope this helps! And, for F-class machines with exhaust bypass stacks that can run in simple cycle mode, usually the IGV schedule (program) is the same as in combined cycle mode when exhausting into an HRSG. UNLESS the HRSG can't tolerate the maximum GT exhaust temperatures in which case the turbine control system is programmed to limit exhaust temperature (which also limits load).

And, then you add the GE Belfort factor into the mix which can change anything and everything--or nothing.

Isn't this fun?