I want to know that why we keep pressure at upstream of Gas Control Valve (GCV) regulated by SRV. What would happen if SRV is not there before GCV? Why SRV regulates pressure as a function of turbine speed?
I have a GE MS5001 Gas Turbine installed and SpeedTronic Mark-V Control system.

 

Thanks for including the information about the turbine and control system at your site; though it wasn't really necessary to help with the answer to this question, more people should tell us about the unit and the control system when asking for help or information.

It would require a very expensive single valve (such as an equal percentage trim valve or something similar) to control the fuel flow through the valve during firing and acceleration (and shutdown). The GCV discharge pressure during firing and warm-up is probably less than approximately 0,2-0,34 bar (3-5 psi) during firing and warm-up, and with supply pressures of between approximately 18,7 bar and 22,1 bar (275-325 psi) a single valve would have to be very nearly closed to reduce the pressure and flow sufficiently to allow stable flame and controlled acceleration and shutdown. It would also probably require some more sophisticated gas fuel flow-rate measuring than is usually provided with GE-design heavy duty gas turbines.

Another benefit of controlling the pressure upstream of the GCV is that with known pressures upstream of the valve the flow through the valve at rated turbine speed is more proportional to valve position, so that for a 25% change in expected valve stroke during loaded operation a 25% change in load can be expected. What I'm trying to say is, that if the GCV travel during loaded operation is from 20% to 70%, no load to base load, a 25% change in valve position during loaded operation (from 20% to 32.5%, because 25% of the valve travel during loaded operation is 12.5% (valve travel during loaded operation in this example is 50% because the valve is at 20% at no load and 70% at rated load)) would result in an increase of 25% load. Or, an increase of 25% load would be accompanied by an increase of 12.5% in valve stroke (from 20% to 32.5%, because 25% of (70%-20%=50%) is 12.5%.

Who knows exactly why the designers chose to use turbine speed as the controlling reference for the SRV; it was either that or use a bunch of potentiometers (back then) to set references, and speed was a convenient, and relevant, parameter which could be measured and used to develop a reference. Also, when the unit was at rated speed the intervalve, or P2, pressure reference would be constant (for a generator drive unit being operated in droop speed control and connected to an "infinite" grid in parallel with other generators; don't you just love all the qualifications we have to make to be specific?).

By the way, did you realize that just as the gas fuel SRV position is a function of turbine speed, that accessory gear-driven liquid fuel pump output (pressure and flow) during starting and acceleration is mechanically "biased" in a similar fashion? So, that the Liquid Fuel Bypass valve doesn't have to be very nearly open (remember: it's a *bypass* valve which closes to increase fuel flow and opens to decrease fuel flow) to limit fuel during firing and acceleration, and shutdown. The bypass valve must go to a nearly closed (5-15% of stroke in most cases) to force some fuel through the liquid fuel check valves when the accessory gear-driven pump is at less than rated speed during starting, acceleration, and shutdown.

So, it's possible the designers of the gas fuel system recognized this fact of liquid fuel operation and just applied it similarly to the gas fuel system. Though I believe it was because to use a single gas valve to both reduce pressure *and* flow during staring and acceleration would have required a very expensive valve *and* accurate gas fuel flow-rate feedback and reference generation, which was more difficult and expensive in the analog control system days.

It should be clear that if the SRV weren't there, the GCV would have to be very nearly closed, probably only a couple of percent open, if that, to reduce the pressure from approximately 20 bar to approximately 0,3 bar during starting. To be able to have such a large pressure drop and properly control flows during starting and acceleration, either the valve would have be a very expensive design, or the control wouldn't be very reliable or stable.

 

Thanx a lot, CSA for the nice explanation.

I also believe that SRV controls inter-valve fuel pressure as a function of turbine speed because by virtue of this it can maintain the fuel-to-air ratio properly, since SRV allows for increasing fuel with increasing air flow as the turbine speeds up. More air is compressed into the turbine at higher speeds. Is this approach also correct?

Moreover you said that P2 pressure remains constant once turbine reaches its rated speed. I agree with this since P2 reference is given by:

P2 ref = Fuel Gas Pressure Ratio Gain * TNH + Fuel Gas Pressure Ratio Offset

But I had seen slight variations in FPG2 (the signal for P2 pressure in Mark-V) when unit operating at Part load Droop control mode.
Can you suggest what could be the reason for this??

Thanks and Regards

 

You can believe whatever you want about air-fuel ratios; it's really about reducing fuel pressure and maintaining known pressures upstream of the GCV. Gas turbines have varying air-fuel ratios during operation; can't be avoided. But, through a combination of GCV valve/plug selection, fuel nozzle sizing, IGV control, and cooling and dilution slot/hole sizing combustor head end slot sizing (this description is for non-DLN combustors), the air-fuel ratio is maintained in a range where stable flame and good combustion with little or no smoking is achieved.

I'm going to bet that when you saw the slight variations in FPG2 the frequency wasn't exactly stable.

It has also been reported that Mark Vs seem to have a little more oscillation in P2 pressure control than other Speedtronic turbine controls. P2 pressure regulation is a function of many things: GCV stability; frequency stability; servo condition (which is usually a function of L.O. condition); supply pressure stability; gas fuel supply strainer condition; SRV and GCV condition; actuator condition (particularly cylinder wall and sealing ring conditions). I see from another post today that you are saying there has been no maintenance for approximately 20 years on the SRV/GCV assembly. This could also be another part of the problem.