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Compressor Pressure Discharge
Compressor pressure discharge, how does it change with MW?
By abdullah on 11 May, 2019 - 7:19 am

why when the gas turbine generates 120 MW or generates 50 MW, the compressor pressure discharge changes? In the same time shaft speed is constant (3000) rpm.

Sincerely
Abdullah

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The short answer may be that less power needed = less fuel = less air needed to burn it. The inlet air is cut back up front, (IGV's), or re-routed by way of extractions, (inlet bleed heat / anti icing function).

Build a few trends, if possible, regarding the same, and you may discover just where the "extra" or "un-needed" air is going. Emissions? Exhaust temperature control? Anti icing? Blade cooling? Machine type & application?

Kurt

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Abdullah,

At 50 MW the IGV's are likely near the minimum angle and at 120 MW they are near or at maximum angle (you didn't identify which model gas turbine you have). Therefore the airflow through the gas turbine is significantly less at 50 MW. Also, at low IGV angle you may have inlet bleed heat in operation, maybe even compressor bleed valves open, both of which would lessen compressor discharge pressure. The IGV angle is the biggest influence.

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This assuming that you have IGV Temp. Control on. If IGVs are fully open, bleed valves closed and no bleed heat, you will still get the same effect. What do think causes it then?

>This assuming that you have IGV Temp. Control on. If IGVs
>are fully open, bleed valves closed and no bleed heat, you
>will still get the same effect. What do think causes it
>then?

I think changing axial compressor discharge pressure is also a function of fuel flow and combustion. As fuel flow increases and more fuel is added to the combustor the pressure in the combustor increases, which increases the back-pressure on the axial compressor. Axial compressors are unusual beasts and always want to have flow through the compressor when running at rated speed--so to maintain flow the axial compressor discharge pressure increases when the back-pressure on the unit (in this case from the increased pressure in the combustor due to increased fuel flow).

Actually, it's a LOT More complicated than that. Gas turbines operate on the Brayton cycle which sort of says that the pressure is "constant." I'm told by some very educated people that there are pressure waves introduced when fuel flow into the combustor changes and that causes knock-on effects which cause the axial compressor discharge pressure to change as fuel flow changes. It all takes too much maths for me--but I did get these highly educated people to agree that increasing fuel flows cause increased axial compressor discharge pressures (and decreasing fuel flows cause decreased axial compressor discharge pressures)--in other words, that the net effect is to cause the axial compressor discharge pressure to change as fuel flow changes.

I would probably be a better engineer if my math skills and my knowledge of thermodynamics was better than it is--but being close to retirement my math skills are what they are and my knowledge of thermodynamics is what is is, and neither of them is going to improve any time soon (without a LOT of study on my part, which I'm probably not going to do given impending retirement).

Primarily, axial compressor discharge pressure (CPD) is a function of both IGV angle and fuel flow-rate when the unit is at rated speed.

By shahabas on 13 May, 2019 - 1:03 am
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Hi sir,

The IGV opening is propotional to the load requirement. if the load increases the the opening of igv increases, that's why the compressor discharge pressure increases (air flow to compressor increases).

Why the turbine shaft speed remains same because after synchronization with customer grid, the load will increase up to base load. There is a relation between pressure P2 (interstage pressure between SRV and GCV) and turbine shaft speed. The load increases proportionally; the gcv valve will open to give more fuel to burn to keep the turbine speed constant. That time p2 pressure decreases so correspondingly the SRV will open to maintain the p2 pressure constant. So that's why at 50MW and 120MW the turbine shaft speed remains same.

I hope you understand.

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shahabas,

>The IGV opening is proportional to the load requirement. if
>the load increases the the opening of igv increases, that's
>why the compressor discharge pressure increases (air flow to
>compressor increases).

IGVs are modulated for various reasons--NONE of them are directly related to the load being carried by the generator. It is most often related to the gas turbine exhaust temperature, some times related to the speed (more on that below). But, I cannot recall any IGV control scheme that used load as the reference or the feedback. Even simple cycle IGV control uses gas turbine exhaust temperature as the controlling parameter for IGV modulation. I always forget the temperature at which the IGVs start opening (700 deg F or 900 deg F), and as the unit load increases and gas turbine exhaust temperature would tend to increase the IGVs are opened to maintain the gas turbine exhaust temperature setpoint (either 700 deg F or 900 deg F--I can never remember), until the IGVs are fully open at their maximum operating angle (and gas turbine exhaust temperature is either 700- or 900 deg F). And, then as the gas turbine is loaded further, the IGVs remain in their maximum operating position.

For combined cycle operation (IGV Exhaust Temperature Control) the IGVs are held closed (at their minimum operating angle) until the gas turbine exhaust temperature reaches the calculated exhaust temperature reference (limit) and then as the gas turbine is loaded and the exhaust temperature would tend to increase the IGVs are slowly opened, until they are at their maximum operating angle, at which time the unit is at or very nearly at Base Load.

For units with DLN combustors, the IGVs are usually modulated, again, based on exhaust temperature--but that's because GE knows what the air flow is for each angle and what they're really trying to do is control air flow to control fuel/air mixture and maintain flame stability. But, the control scheme is based on gas turbine exhaust temperature.

>Why the turbine shaft speed remains same because after
>synchronization with customer grid, the load will increase
>up to base load. There is a relation between pressure P2
>(interstage pressure between SRV and GCV) and turbine shaft
>speed. The load increases proportionally; the gcv valve
>will open to give more fuel to burn to keep the turbine
>speed constant. That time p2 pressure decreases so
>correspondingly the SRV will open to maintain the p2
>pressure constant. So that's why at 50MW and 120MW the
>turbine shaft speed remains same.

This is just SO WRONG. The SRV (Stop/Ratio Valve) has two functions--its most important one is the gas fuel stop valve. Its second function is to control the P2 pressure--the pressure upstream of the GCV (Gas Control Valve), or GCVs as the case may be--as a function of turbine speed. Because the GCV position can change during acceleration or deceleration and when loading or unloading that can have an effect on P2 pressure also, which the SRV control scheme must compensate for.

The reason the turbine-generator (and axial compressor) shaft speed doesn't change when the generator breaker is closed and the turbine is producing electrical power is because the generator is SYNCHRONIZED to the grid and, normally, the grid frequency is (or should be) stable. Speed and frequency are directly related, and when a synchronous generator (which is the type used for GE-design heavy duty gas turbine-generator applications) is connected to--synchronized with--a grid of any appreciable size the speed of the generator is controlled by the frequency of the grid. And, one of the most important aspects of an AC (Alternating Current) grid is its frequency--and that's what grid operators are trying very hard to control.

The SRV has absolutely ZERO affect on turbine-generator shaft speed when the unit is synchronized to a grid of any appreciable size. None. Because the turbine-generator shaft speed is (or should be) constant when synchronized to a grid and producing electrical power the P2 pressure reference is (should be) constant. You are correct, shahabas, that the GCV (or GCVs) will open or close as the unit is loaded and unloaded and that will cause the P2 pressure to change (to drop as the GCV(s) is(are) opened, and to increase as the GCV(s) is(are) closed), and the SRV control scheme will open or close the SRV to try to maintain the actual P2 pressure equal to the reference (which should be constant). But, that has nothing to do with actual turbine-generator shaft speed--which is being "controlled" by grid frequency when the generator breaker is closed and the unit is synchronized to the grid.

The reason the SRV controls P2 pressure is two-fold. During starting and acceleration, when gas fuel supply pressure can be 250 psig for smaller turbines and as high as 350 psig for larger turbines, if a single valve were used to control the gas fuel flow-rate into the turbine when only approximately 3-6 psig are required at the fuel nozzles the single control valve would only be "cracked" open (very minimally open) to drop the pressure from 250-350 psig to 3-6 psig. And, it would be very difficult for all but a few types of control valves to control the pressure and flow-rate to the fuel nozzles when the upstream pressure was so high (250-350 psig). So, the SRV is used to decrease the gas fuel supply pressure upstream of the GCV(s) to something around 30 psig--which makes it much easier for the GCV(s) to control the pressure and flow-rate to the nozzles during firing and acceleration.

Second, when the gas fuel pressure upstream of the GCV(s) is constant when the unit is at rated speed and producing electrical power the flow through the GCV(s) is proportional to valve position (valve position is called "stroke" in GE-speak). LVDTs (Linear Variable Differential Transducers) can be used to provide valve position (stroke) feedback to the turbine control system. And, when the unit is at rated speed the flow through the valve will vary linearly as the valve is moved.

So, the SRV acts as the gas fuel stop valve, and controls the pressure upstream of the GCV(s) to aid in starting and acceleration and to help make flow through the GCVs proportional to position (stroke) when the unit is at rated speed and producing electrical power. But, it has NOTHING to do controlling turbine-generator shaft speed. It uses actual turbine-generator shaft speed when calculating the P2 pressure reference--but it does not actually control speed.

I hope you understand.

By shahabas on 14 May, 2019 - 1:40 am

hello Mr.CSA

I answered as per Abdullah question.

How the exhaust temperature increases please explain it?

from FSNL to base load...

P2 pressure is interstage pressure between SRV and GCV OK that should be constant, and also TNH should be constant 3000/5000rpm as per machine model. The shaft speed making constant by giving fuel by GCV only, so that pressure drop will rectified by SRV. Once synchronize need turbine shaft need to produce more torque so the GCV will open proportionally to keep the shaft speed constant. SRV will balance the p2 pressure.

SRV is fuel valve. all person know who are working with GE gas turbine. so they know it will shut off 1st if any trip happened. OK so no need to say that in this question.

after synchronization
fuel consumption = MW

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shahabas,

>How the exhaust temperature increases please explain it?
>
>from FSNL to base load...

Gas turbine exhaust temperature increases because fuel flow-rate increases. And when fuel flow-rate changes, the load changes.

BUT, the IGVs are not adjusted based on load--they are adjusted based on gas turbine exhaust temperature. Load will be what it will be when the fuel flow-rate changes, and exhaust temperature "may" change when fuel flow-rate changes. Depending on the mode the IGVs are being operated in (Simple Cycle, or IGV Exhaust Temperature Control OFF; or, Combined Cycle, or IGV Exhaust Temperature Control ON), the exhaust temperature can be controlled by the position of the IGVs--up to a limit. But, load is not used in any calculation of IGV position I have ever seen in a GE turbine control system. Gas turbine exhaust temperature: yes. Load: no.

If the unit is operating at 75% of rated load (let's say the unit was rated at 100 MW; 75% of rated load would be 75 MW), and the unit was being operated in Droop Speed Control Mode with IGV Exhaust Temperature Control ON, the IGVs would probably be somewhere in mid-stroke (somewhere between 57 DGA and 84 DGA), and the exhaust temperature would be up near 1100 deg F (the typical maximum allowable exhaust temperature for most non F-class machines). If the load was increased, that would mean the fuel flow-rate would have been increased--and that would mean the gas turbine exhaust temperature would tend to increase. BUT, IGV Exhaust Temperature Control would sense the increase in gas turbine exhaust temperature and would open the IGVs a little bit to keep the gas turbine exhaust temperature at it's maximum allowable limit (1100 deg F in our example). Yes, the IGVs would move as the load changed--but they're NOT moving BECAUSE the load changed, they're moving because the gas turbine exhaust temperature changed as the load changed when the fuel flow-rate changed. It's not the load change that's making the IGVs move, it's the resultant exhaust temperature change that's making the IGVs move (when IGV Exhaust Temperature Control is active). [DLN combustor-equipped machines have IGV Exhaust Temperature enabled by default--and, typically it cannot be disabled by the operators, and it SHOULD NOT be disabled because of potential damage to the hot gas path parts.]

When a synchronous generator coupled to a prime mover (ANY prime mover) operating in Droop Speed Control mode is synchronized to a grid of any appreciable size, the generator is LOCKED into synchronism and its speed is fixed (controlled) by the frequency of the grid it is synchronized to. There is a very simple formula that explains the relationship between speed and frequency:

F=(P*N)/120

Where: F=Frequency (in Hertz)
P=Number of magnetic poles of the generator
N=Speed of generator (in RPM)


The formula can be re-arranged to solve for any of the variables; for example, if you want to know the speed of a two-pole synchronous generator synchronized to a 50.0 Hz grid:
N=(120*F)/P

N=(120*50.0)/2=6000/2=3000RPM


No matter how much torque is applied by the prime mover to the generator rotor (as long as the excitation is normal) magnetic forces inside the generator keep the generator rotor LOCKED INTO SYNCHRONISM with the magnetic field of the generator stator. (We all know how strong magnets can be, even very small magnets will repel each other very strongly when similar poles are held close to each other, and when released the magnets will usually very quickly realign themselves so that opposite poles attract each other and "stick" together, and have to be pulled apart. This is what is happening inside synchronous generators--simple magnetic forces of attraction--keeping the magnetic fields of the generator rotor (and the shaft of the prime mover driving the generator rotor) locked into synchronism with the apparently rotating magnetic fields of the generator stator (when the generator breaker is closed and AC (Alternating Current) is flowing in the generator stator windings). If the frequency of the grid changes, the speed of the generator rotor will change--and so will the speed of the prime mover coupled to the generator rotor. ALL synchronous generators obey the formula above when synchronized to a grid of any appreciable size, and ALL generators rotate at the speed defined by the grid frequency and the number of poles of the generator rotor. It's not possible for 50.0 Hz to be coming out of a plug receptacle on the wall in a house or business with some generators running at 48.9 Hz and some generators running at 50.3 Hz and one generator running at 51.2 Hz, and a couple of generators running at 49.8 Hz. There's no "smoothing" or "averaging" thing-a-ma-bob on grids that allows multiple generators running at different frequencies to supply 50.0 Hz to all users of the grid power.

The SRV has nothing whatsoever to do with the speed of the prime mover, which is coupled to the synchronous generator. Whether or not the generator breaker is closed or open. The SRV is simply controlling a pressure that is a function of turbine shaft speed--which, when the generator breaker is closed is a function of grid frequency. Full stop. Period.

You implied that the SRV, in its secondary function, was used to control shaft speed. The SRV control P2 (interstage) pressure as a function of shaft speed. And, when the generator breaker is closed, shaft speed is a function of grid frequency. When the generator breaker is not closed, shaft speed is a function of GCV position--and when the generator breaker is open the turbine control system is either trying to control acceleration rate (during starting and run-up to FSNL (Full Speed-No Load)), or Droop Speed Control (when at FSNL and during synchronization, or shutdown control (after the generator breaker is opened during a normal, orderly, fired shutdown. And during all those times, the SRV control scheme is monitoring turbine shaft speed and calculating the desired P2 pressure reference (setpoint), and is adjusting the SRV position to make the actual P2 pressure equal to the P2 pressure reference. Which is a function of turbine shaft speed. It's NOT controlling turbine shaft speed--it's using turbine shaft speed to calculate the P2 pressure reference, and the SRV position is adjusted as necessary to make the actual P2 pressure equal to the P2 pressure reference (which is calculated based on actual turbine shaft speed). The P2 pressure is what's being controlled by the SRV, based on a P2 pressure reference that's calculated from turbine shaft speed.

When the synchronous generator breaker is closed and the unit is synchronized to the grid, turbine shaft speed is determined using the formula above, because of the magnetic forces at work inside the generator keeping the generator rotor spinning at a speed that is a function of the grid frequency.

Even if the fuel to the turbine was completely shut off and the generator breaker remained closed--the generator rotor would continue to spin at the speed proportional to the grid frequency per the formula. The generator would be drawing amperes from the grid to keep the generator--and the turbine--spinning at the speed defined by the grid frequency (known as "reverse power"), and the generator actually becomes a motor at that point (known as "motorizing the generator). There are relays to protect against this by detecting reverse power and opening the generator breaker.

By the same token, if the GCV went full open and gobs of fuel were admitted to the combustors, if the unit didn't trip on exhaust overtemperature the generator and turbine would still keep spinning at the speed defined by the formula above--and gobs of amperes would be flowing out of the generator, generating LOTS of heat in the generator windings.

Even if the SRV went fully open allowing the P2 pressure to greatly exceed the P2 pressure reference (presuming the gas fuel supply pressure remained constant), the unit speed would not change if the generator breaker were closed. The turbine control system would sense the change in exhaust temperature (because of the extra fuel flowing through the GCV in the beginning) and would close the GCV to try to limit the exhaust temperature, or trip the turbine if it couldn't (probably on exhaust overtemperature).

The SRV controls P2 pressure simply to make the GCV position linear with respect to flow through the valve when the unit is at rated speed. And, because when the unit is producing electricity at rated speed the speed is constant (or should be--because the grid frequency should be constant), the designers chose to use speed as the reference for the P2 pressure--constant speed results in constant P2 pressure. Using speed also has the added benefit of greatly reducing P2 pressure during firing and acceleration--negating the need for expensive valves for controlling fuel flow-rate at very low pressures and flows.

And, yes--when at rated speed if the GCV opens or closes the immediate effect on P2 pressure will be to decrease or increase, respectively--and the P2 control scheme will sense the change in pressure (not speed--pressure) and will move the P2 to the position required to return actual P2 pressure to the P2 pressure reference. But, the change is, as you said, not because speed changed but because actual P2 pressure changed. If the grid frequency changed, that will cause the P2 pressure reference to change, and the turbine control system will cause the SRV to change position to make the actual P2 pressure reference equal to the new P2 pressure reference--which will also have somewhat of a knock-on effect on GCV position ultimately. But when grid frequency returns to normal, the P2 pressure reference will return to normal and the SRV will move to maintain the normal P2 pressure reference. (When gas fuel supply pressure changes while the grid frequency is normal, that will also cause P2 pressure to change--and the turbine control system will have to move the SRV to maintain the P2 pressure equal to the P2 pressure reference.)

But, the SRV does not control speed. Its position is controlled by the speed of the shaft, but it does not control the speed of the shaft. And IGVs are not modulated (positioned; controlled) by load.

Hi,

I agreed with you, but am posted about the turbine shaft speed control only. Am not post about the generator speed. In the case of generator speed there is no role for the GCV it is fully by f=(p*n)/120.

Frequency is making the speed constant.

For IGV and exhaust thermocouple relationship, you gave me great knowledge.

thank you so much sir