Control.com - Nerds in Control

Hello! CSA
I have one question here.. If Turbine speed is kept constant(5100rpm=50Hz india), Ambient temperature is constant(say25degC),
Inlet guide vane is constant(84DGA). ( I am running the GT on Isochronous mode independently-i.e. not connected to grid.) If plant load varies by 3-4 MW will there be any change in Air flow?

Actually i want to know that is there any change in Compressor Air flow at 30MW Preselect load(ambient 25degC & above condition ) and 30MW base load(ambient 28degC and above condition) ?If there is change in Air Flow then How?

If the turbine speed is constant and the IGV angle remains constant, then *air* flow through the unit can't change (the compressor is spinning at a constant speed and the IGVs are stable, right?).

But, if load is changing then *fuel flow* is changing which is changing the exhaust flow-rate, right? More fuel equals more exhaust flow; less fuel equals less exhaust flow, right?

Air flow through the compressor is usually measured by monitoring bell-mouth pressure drop and inlet static differential pressure, and they don't seem to vary much when the compressor speed is constant and the IGV angle is constant. CPD changes with fuel flow (load) even if the compressor speed is constant and the IGV angle is constant; the reason is because increasing fuel flow increases back pressure in the combustor (the pressure increase comes from the oxidation/burning of the fuel) and the axial compressor has to work harder to flow the same amount of air spinning at a constant speed. If the unit experiences a sudden trip while at load, what's about the first thing that happens? The CPD drops dramatically as the fuel stops burning and is not being admitted to the combustor. Also, to get fuel to flow into a combustor it has to be at a higher pressure than combustor pressure (which is only slightly less than CPD).

At least that's the way it was explained to me many years ago, and that's the way it's always seemed to work. I'm sure that air flow does change slightly as CPD increases, but I believe for all intents and purposes it's considered to be constant when compressor speed is constant and IGV angle is constant.

The earlier response presumes ambient temperature is constant; air flow through the unit does change with ambient temperature as ambient temperature changes throughout the day. Cooler air equals denser (is that even a word?) air equals increased air flow at the same compressor speed and IGV angle. Warmer air equals less dense air equals decreased air flow at the same compressor speed and IGV angle.

Thanks a lot, CSA.

There is an argument between our colleagues regarding this topic. I have explained in the same words what you explained, and I also have studied it by plotting so many trends/graphs with different data. (Argument was as the load increases or decreases, torque on the shaft increases or decreases and compressor draws almost 65 to 70% of the power, hence air flow varies with variation of torque also??? Not acceptable to me. I believe and studied that if rotary air compressor speed is constant and IGV opening is constant there is no question of changing air flow. Ok if inlet air temperature varies changes the mass flow, that is agreed.) Ok, thanks again.

When the IGV angle is constant and the axial compressor speed is constant, how can air flow be changing (other than by ambient temperature/density)? The only that's changing as load is changing is ... fuel flow-rate. More fuel equals more load; less fuel equals less load. The compressor load will change because the back-pressure in the combustor will increase as fuel flow is increased, but it's my understanding that air flow, for all intents and purposes, is considered to be constant.

Are your colleagues referring to experience and calculations, or supposition and transference ("it is so because it seems to be so")?

There is a saying: One can lead a horse to water, but one cannot make it drink. Sometimes, when it comes to knowledge, people can be similar.

Pick your battles so you don't lose the war. Winning every battle doesn't guarantee you'll win the war. (Maybe some leaders should consider, or have considered, this widely accepted fact.)

It's also wise not be drawn into other peoples battles, or wars.