Hi,
We have 21 MW GE GTG with compressor and turbine on the same shaft in our industry with variable Inlet guide vanes (56 is minimum position of IGV).

We have following data at different MW load of GTG (with IGV position in degrees and Compressor discharge pressure in bar):


In the above cases, compressor RPM is same and IGV opening is also same, then why does Compressor discharge pressure is changing?

 

@shubham07377,

This is normal and to be expected. Yes, the axial compressor speed is constant when the generator is synchronized to a grid with other synchronous generators and their prime movers (because speed and frequency are directly related).

To increase the electrical load being carried by the machine (the generator and its prime mover--the gas turbine) the fuel flow-rate into the machine has to be increased. Increasing the fuel flow-rate into the combustors increases the pressure inside the combustors as the fuel is being combusted--thereby increasing the "backpressure" on the axial compressor discharge. The axial compressor has to work a little harder to force the same amount of air into the combustors at a constant speed and IGV angle.

Also, while not a significant amount of the total mass flow into the combustor the fuel flowing into the combustors must be at a higher pressure than the pressure inside the combustors, and when it ignites and combusts, again, that raises the pressure inside the combustors against which the axial compressor must overcome to keep air flowing into the combustors (for combustion as well as cooling and dilution).

So, again, the phenomenon of increasing CPD as load is increased (by increasing the fuel flow-rate into the combustors) is normal and to be expected. It would be unusual if the CPD didn't increase, or decreased, as the machine was loading.

When the turbine is tripped (emergency shutdown) the compressor discharge pressure decreases extremely quickly--because the fuel flow-rate has been stopped, which causes the flame in the combustors to be extinguished (because at any time the mass flow-rate of air through the machine is more than three times what's actually required just for combustion (the "excess" air being used to cool and dilute the hot gases as they enter the turbine section)) which immediately decreases the back-pressure against the axial compressor which reduces CPD. Usually the IGVs will revert to the minimum angle if they aren't already at the minimum angle and will close as the machine speed decreases, and the machine speed will decrease also because there is no work being extracted from the gases entering the turbine section when there is no fuel being combusted. BUT, the initial and almost immediate effect of a sudden shut-off of fuel is for the CPD to decrease, because the pressure in the combustor has decreased.

Good question though! And at least you recognize that the machine speed is constant when generating power. Most people think that because the machine accelerates when fuel flow-rate is increased during start-up that after synchronization the machine speed increases as the machine is loaded--which just ain't true for a well-regulated grid, or for a well-functioning machine governor (control system) powering load(s) when not synchronized with other machines. (Again--generator frequency and speed are directly related--increase one and the other increases, and decrease one and the other decreases.) But, again, most people just ass-u-me that speed changes when load changes (which, again, just ain't true). Some also think that's why CPD increases when the machine is being loaded. And, still some people even believe the (false) description that when operating under Droop Speed Control the machine speed will decrease as the machine is loaded...! (Yes; the textbooks and reference books are mostly all wrong when they describe Droop Speed Control this way--it is true, but only for a limited set of circumstances and it's NOT normal, and most textbooks and reference books don't properly state the conditions for when the description is true, they just make it read as if it's always true--and it's not always true.)

Anyway, good question; I hope this helped explain why CPD increases when load is increased--and, conversely, whey CPD decreases when load is decreased!

 

I am Not sure if we have Inlet Bleed heat valve or Not? @
shubham07377