Nice discussion.. taken from..
http://control.com/thread/1232811875
The primary purpose of the Stop-Ratio Valve (SRV) is to serve as the gas fuel stop valve. The secondary purpose is to regulate the fuel pressure upstream of the Gas Control Valve (GCV), particularly during starting and acceleration, but also during rated speed operation.
Consider the typical situation where the gas fuel supply pressure is approximately 15-20 barg. Note that the gas fuel pressure required at the gas fuel nozzles in the turbine during starting is only approximately 0.2-.035 barg. If there were just one control valve, unless that control valve was a very special control valve (which translates into a very expensive control valve) it would only be open a couple of mm, at best, and the ability of that valve to reduce and control the downstream pressure from 20 barg to 0.2-0.35 barg would be very severely limited. Establishing flame and controlling fuel flows during acceleration would be very difficult with many types of single control valves.
So, the SRV serves to drop the pressure upstream of the GCV to about 2.0 barg (the exact value is site-specific!) during starting, and increases the pressure as a function of turbine speed, until the unit reaches rated speed (Full Speed-No Load, FSNL, for a generator drive application). At that point the gas fuel pressure reference is 100% (since the speed is 100%).
With a significantly lower pressure upstream of the GCV uring firing (establishing flame in the combustors) the GCV can be opened to a position where it's much easier to control the flow through the valve (and the downstream pressure); the same is true for acceleration of the unit to rated speed.
Now, when the turbine is synchronized and loaded, the GCV opens to increase fuel flow to the combustors. This increased fuel flow tends to cause the pressure upstream of the GCV to drop, but the SRV will open slightly to maintain the pressure reference as the GCV is opened.
As load is reduced, the GCV closes and the pressure upstream of the GCV tends to increase but the SRV closes slightly to maintain the pressure reference as the GCV is closed.
So, the pressure downstream of the GCV will change as the unit is loaded and unloaded and started and accelerated (and decelerated). But the SRV will move to whatever position it needs to in order to maintain the required pressure reference upstream of the GCV. The SRV regulator is *NOT* a position loop; it is a pressure loop with position control for indication and stability. The SRV moves to whatever position is required to try to maintain the pressure upstream of the GCV equal to the pressure reference (which is a function of turbine speed).
Also, if the gas fuel supply pressure were to increase during steady-state turbine operation the pressure upstream of the GCV would tend to increase, but the SRV will close slightly to maintain the pressure equal to the reference. The opposite is true if the gas pressure decreases. I've been at many sites where the SRV is at 100% stroke (full open) because the gas fuel supply pressure is much lower than it should be, and usually the power output of the turbine is also lower than desired (because it can't get enough fuel).
Finally, on a more typical style control valve if the upstream pressure is known and/or is controlled to a specific value or range then the flow through the control valve is more linear with respect to stroke and can be controlled by controlling the position (stroke) of the control valve. That's another really important benefit of the SRV/GCV system. (Note, that on most GE-design heavy duty gas turbines the Speedtronic does *not* monitor or control either the gas fuel flow-rate or the pressure downstream of the GCV! This reduces the complexity of the control system required to control gas fuel flow, but also puts some requirements on the system that is used to control gas fuel flow. That's where the flow-versus-stroke relationship becomes important.)
Now, there are single valves which could have been used, but back when GE was first developing their heavy duty gas turbines those valves were extremely expensive and very difficult to interface with. GE probably decided to use two more typical valves in the staged configuration in order to reduce cost as well as to reduce the complexity of the control system.
Good question; hope this helps!
