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Why Two Gas Valves (SRV GCV) in Frame 5 GT
What is the reason behind using two gas valves in the frame 5 gas turbine?
2 out of 2 members thought this post was helpful...

I just want to share and confirm an information that i have about using two gas valves (SRV and GCV) in the frame 5 GTs. The first valve SRV (speed regulating valve) regulates the infravalve pressure (between srv and gcv). This pressure depends on the hp speed. WHY? because the amount of fuel in the combustion chamber, how much is opened a valve (the valve stroke) is not enough. Because if the upstream pressure is high and the downstream pressure is low, just a little opening of the valve will let pass lot of fuel. At the contrary if the differential pressure across the valve is little, even if the stroke of the valve is big, not too much fuel will pass. as a result: The amount of fuel depends on how much the valve is opened and the pressure across the valve.

the second valve GCV (gas control valve) Once the pressure upstream is adjusted, the amount of fuel for the combustion chamber will be dependent only on the GCV opening. FSR is the signal 0-100% that indicates the opening of GCV and in consequence the fuel.

So i hope this is the right understanding. Correct me if I'm wrong.

3 out of 3 members thought this post was helpful...

Isulamu,

This topic has been covered before on control.com, but it has been several years.

The purpose of the SRV (Stop-Ratio Valve) is really two-fold. First, it is the gas fuel stop valve--or it was for decades, until newer gas system standards and regulations in some parts of the world require another stop valve upstream of the SRV.

The second function of the SRV is also two-fold--and that's the ratio portion of the name. You are correct, when the SRV is open it is always regulating the Intra-valve pressure--the pressure between the SRV and the GCV (called the P2 pressure). And it does this by using HP shaft speed (signal name TNH) as the reference, in the following formula:


FPRG = (TNH * FPKGRG) + FPKGRO

where FPRG = Fuel Pressure Reference-Gas (psi)
TNH = HP Speed (%)
FPKGRG = Fuel Pressure Reference Gas Control Constant-Gain (psi/%)
FPKRGO = Fuel Pressure Reference Gas Control Constant-Offset (psi)


So, let's look at what happens when the unit is firing, at say, 10% speed.

FPRG = (10% * 2.704psi/%) + -2.67psi = 27.04 - 2.67 = 24.37 psi

And let's look at what happens when the unit is at 100% speed (which it is, or should be, when producing electrical power at a constant frequency).

FPRG = (100% * 2.704psi/%) + -2.67psi = 270.4 - 2.67 = 267.73 psi

So, when the unit is firing at 10% speed, the SRV is limiting the pressure between the SRV and the GCV (the Intervalve Pressure, also called P2 pressure) to 24.37 psi. Now this is important, because when the unit is firing (trying to establish flame) the GCV (Gas Control Valve) position is usually something around 18-22%. If the pressure upstream of the GCV was 300 psi, well if the GCV was at 18-22% then TOO MUCH gas fuel would go into the combustors and there would be a big BANG when flame was established, and the exhaust temperature would be VERY high--too high.

In order to control the downstream pressure of the GCV to the 5-6 psi that is usually required during firing with 300 PSI upstream of the SRV the GCV would have to be nearly closed--and at that position it wouldn't be able to control pressure or flow very well at all.

So, the SRV, by reducing the pressure upstream of the GCV to a much lower value allows the GCV to operate in a more open position--which makes it easier to control flow and pressure to the nozzles during firing.

Now, as the unit accelerates to rated speed (100% speed), the SRV has to open to maintain the new pressure setpoint--and that also allows the GCV to remain in a position that makes it easier (and cheaper) to control the flow to the nozzles.

If you plot SRV position and GCV position and TNH during firing and acceleration, you will see that the GCV won't ever go much above 22-25% during acceleration, because the upstream pressure is increasing with speed and that increases flow through the GCV even though the GCV isn't opening much more than the firing position.

Now, let's look at what happens when the unit is synchronized to the grid and you load the unit. When you click on RAISE SPEED/LOAD the speed of the unit remains at 100% (because it's synchronized to the grid) but the turbine speed reference (TNR) increases. That increases the error between the speed reference (TNR) and the actual speed (TNH--which should be constant at 100%)--and that increasing error increases the fuel flow-rate to the turbine by opening the GCV to allow more fuel to flow to the nozzles, to try to increase turbine-generator speed. But, the speed can't change and the generator converts the extra torque (that would make the speed increase IF the unit were NOT synchronized to the grid) into amperes--and that's what makes the power produced by the generator (MW) to increase.

BUT, when the GCV opens while the unit is at 100% speed, the pressure upstream of the GCV would tend to decrease--but the P2 pressure formula above sets the pressure reference and so the SRV has to open to maintain the pressure upstream of the GCV. So, even though the unit is at 100% speed and the P2 (Intervalve Pressure) reference is constant (because the unit is at 100% speed), as the GCV opens to increase fuel flow to the unit to increase electrical power output of the generator the SRV position DOES NOT remain constant. It has to change as the GCV position changes in order to maintain the same pressure between the SRV and GCV (the P2 pressure, or the Intervalve Pressure).

Now, there's another benefit to keeping the pressure upstream of the GCV constant as load changes. And, that is that flow through the GCV is linear with position, meaning that for every 1% change in GCV position the fuel flow-rate will change be a constant amount over the entire load range of the unit, from 0 MW to Base Load.

So, the SRV serves many functions--it's the gas fuel stop valve, and it helps to reduce pressure upstream of the GCV during firing and acceleration which helps the GCV control flow easier (and cheaper--because a valve that can control pressure AND flow at the same time is VERY expensive--and flow through the valve is not proportional to valve position). And, when the unit is at 100% speed as it is when it is synchronized to the grid and producing electrical power, the SRV keeps the pressure upstream of the GCV constant so that flow through the GCV is proportional to stroke (linear with stroke).

The GCV is sometimes referred to as the "FSR Valve" and that's not a bad name. GCV position is or should be very close to FSR when running on gas fuel. And, the SRV allows that relationship to occur.

Now let's talk a little about the GCV. Let's say the GCV is at 20% stroke (position) when the unit is at 100% speed, just before synchronization. And, let's say the GCV is at 70% stroke (position) when the unit is at Base Load (full load). That means that the GCV changes position by 50% between zero load and full load. And that means that if the unit is rated at 25 MW, that for every 2% change in GCV position (above 20% position and while the unit it synchronized to the grid) the the load will change by 1 MW. This relationship is constant over the load range of the unit because the SRV is maintaining the P2 pressure (the Intervalve Pressure) constant while the unit is loading and unloading.

The SRV is a very important valve--and it's not a valve that maintains a particular position. It's reference is pressure--so it will go to whatever position it needs to in order to be able to maintain the pressure reference that's determined by the above formula. And, even though it's reference stays constant when the speed is constant at 100% the SRV will move to whatever position it needs to in order to maintain a constant P2 pressure (Intervalve pressure) upstream of the GCV.

If the gas fuel supply pressure changes, the SRV has to change also to maintain the P2 pressure reference. It's a very busy valve.

So, I hope this helps your understanding of the SRV and the GCV. Basically, the type of single valve that could do everything the SRV and GCV do as a pair would be VERY expensive, and flow through the valve over the range from starting (firing) and acceleration and loading would NOT be directly proportional to valve position (stroke). Which would make the control scheme much more difficult--or it would have back in the 1950s when GE-design heavy duty gas turbines were being developed. (Nowadays, it would be a piece of cake for a single--but still expensive!--valve to control the fuel flow to a gas turbine.)

You were very close with your explanation, but hopefully the above will help clarify your understanding.

By the way, that bit about the error between TNR and TNH--that's Droop Speed Control! And, between zero load and Base Load, all GE-design heavy duty gas turbines are operated on Droop Speed Control when synchronized to a grid with other prime movers and generators. Droop speed control is what allows multiple prime movers and generators to work together in a stable manner to supply an electrical load (lots of motors and lights and televisions and computers and computer monitors) that is larger than any single prime mover and generator could supply by itself.

Again, hope this helps!

CSA,

thank you very much for this explanation. Glad that i was right, but you've added some more important informations.

thnx again.

To CSA,

Sir i just want to know that what will happen if GCV gets full open? As in my point of view when GCV opens, its inlet pressure decreases which is maintained by SRV. So in order to maintain P-2, SRV may get full open due to which turbine can overspeed. Is this right?

And 2nd question is about SRV loop. Please explain about SRV Position control loop.

1 out of 1 members thought this post was helpful...

MSF,

First of all, the GCV should NEVER get 100% open under normal operating conditions (with proper gas fuel supply and P2 pressure, and with gas that meets the BTU content used for choosing the fuel valve internals (plugs and seats) and fuel nozzles (orifices)). The turbine designers choose valve internals and fuel nozzles such that the maximum opening (on the coldest day and with the turbine in a new and clean condition with clean turbine inlet filters and a low exhaust back pressure and with fuel gas at design specifications) will be approximately 75-85%. The power output of the unit would be above rated with a very low compressor inlet temperature and new and clean conditions and fuel gas at design specifications. At rated ambient conditions (ambient pressure; ambient temperature and humidity) the GCV would probably only be about 70-75% open with fuel gas that met design specifications. (When a turbine is being purchased, the purchaser must supply the fuel gas particulars, BTU content, methane, etc., to the supplier/packager, and that's what's used to choose fuel valve internals and fuel nozzles in order to have the fuel valves operate in a controlling region (not full open, and not full closed).)

Usually, if the GCV goes full open it's because there's some problem with gas fuel supply pressure/flow. Either the gas fuel supply pressure isn't correct, or there is some blockage (filters; strainers) that is preventing proper flow from getting to the SRV/GCV. The operator is trying to make as much power as possible, but the gas fuel supply pressure/flow is too low--and the SRV will usually go to 100% before the GCV goes to 100%, but it's possible under the "wrong" conditions for both to go to 100%.

If the gas fuel being supplied to the unit changed significantly since the unit was commissioned, in particular, if the BTU content dropped significantly, then it's also possible that even if the pressure/flow was correct that there just isn't enough BTU content to develop the torque required to maintain the turbine speed/load setpoint, and the GCV would go to 100% while the SRV maintained P2 pressure because the supply pressure/flow was sufficient.

The SRV is NOT a position control loop--it is a pressure control loop. Pure and simple--just look at the formula at the top of this thread. Yes; the SRV has LVDTs for position indication. But, it's NOT a position control loop. The reference for the SRV is P2 pressure (FPRG), not position. The GCV is a position control loop--the reference (FSR) is the position the GCV should go to and maintain. If FSR changes, then the GCV position should change to maintain GCV position to be nearly equal to FSR. (Some people call the GCV the FSR Valve.) But, if I mis-typed somewhere in one of my responses that the SRV was a position loop--I was mistaken (I'm not a good proof-reader of my writing; sorry). The SRV is a pressure loop--not a position loop. The turbine control system will put the SRV at whatever position it needs to be at in order to make the actual P2 pressure equal to the P2 pressure reference. If that means the SRV has to be at 46.7% or 59.3% to maintain actual P2 pressure equal to the P2 pressure reference the turbine control system will move the SRV to make whatever position it needs to in order to maintain actual P2 pressure equal to the P2 pressure reference.

As gas fuel supply pressure/flow drops, this would tend to make the P2 pressure drop (presuming the GCV was stable). BUT, the turbine control system would open the SRV to keep P2 equal to the P2 pressure reference--until the SRV hit 100%, and then it can't do anything to increase P2 pressure. At this point, the GCV would then probably start opening to maintain a load setpoint (presuming, that as most turbines are, it is running in Pre-Selected Load Control, or with an external Load Reference). If the gas fuel supply pressure/flow continued to drop then it's possible the GCV could also go to 100%. But, a conscious operator should be aware of what's happening and be taking appropriate action (trying to restore gas fuel supply pressure/flow, or unloading the unit until gas fuel supply pressure/flow is restored). If the GCV and SRV were both at 100% and the gas fuel supply pressure/flow continued to decrease, then the load will start to drop and eventually the unit will lose flame--just because there's isn't enough gas fuel to maintain flame. A "Loss of Flame" trip would be annunciated.

If the unit is a generator drive (and I'm presuming it is) AND the grid frequency is stable, then the turbine-generator speed will be stable, and the P2 pressure reference when the generator breaker is closed will be stable, which should make the actual P2 pressure also stable.

And, again, if the gas fuel make-up has changed significantly AND the gas fuel supply pressure/flow is not correct, this could also cause problems.

But, it's just not normal for the GCV to ever go to 100%, if operating conditions are normal. Unless someone didn't calibrate the GCV LVDTs properly (which is another possibility).

Hope this helps! If you're having a specific problem, please describe it and we can try to help.

CSA,

Thank you so much for explanation.

After reading your post two questions arise in my mind:

#1- As you mentioned that SRV has pressure control loop, and it's concern is only with P-2, but sir what about its LVDT? TO whom it gives feed back? IS this feedback signal is only used to indicate SRV opening or it is used in any controlling function too?

#2- If gas turbine is unde normal operation, gas supply pressure is normal but GCV both LVDTs get fail then what will happen?

1 out of 1 members thought this post was helpful...

MSF,

#1- The LVDT is used as an "outer loop" to the pressure loop. When the actual P2 pressure is equal to the P2 pressure reference, the output of the regulator summing junction is zero (because the two are subtracted from each other). If the actual P2 pressure deviates from the P2 pressure reference, then the output of the regulator summing junction will be positive or negative to change the position of the SRV until the actual P2 pressure equals the P2 pressure reference. The LVDT feedback is used for stability purposes, nothing else. The primary control loop for the SRV is P2 pressure.

In all actuality, the calibration of the SRV LVDTs is NOT critical--not at all. Because the valve is going to go to whatever position it needs to--whether or not the LVDT feedback is correct or not--to make the actual P2 pressure equal to the P2 pressure reference. So, if the LVDT feedback is telling the Mark* turbine control system the SRV is at 45.7% but it's really at 53.9%, the Mark* doesn't care. It's still going to move the SRV to whatever position is necessary to make the actual P2 pressure equal to the P2 pressure reference. If that's at an LVDT feedback of 45.7% when the SRV is really at 53.9% then the SRV will be at 53.9% (actual, physical stroke) with an (incorrect) indicated position of 45.7%--and the unit will run just fine. Pretty cool, huh?

#2- Depends on the turbine control system. The predominant failure mode is for the LVDT feedback to go to zero (negative, actually). And, the regulator in a Mark* turbine control system chooses the higher of the two LVDT feedback signals, so the control will shift to the remaining good signal if one LVDT feedback fails low (negative). Digital Mark* turbine control panels will also alarm (Diagnostic Alarm) on the loss of one LVDT signal. If a second LVDT signal is lost, most digital Mark* turbine control systems will "suicide" the servo-valve output, meaning that there will be zero current flowing to the servo-valve and the fail-safe spring in the servo will cause the hydraulic fluid to be removed from the servo and the closing spring of the GCV will slam the valve shut. Also, the design of the GCV is such that P2 pressure will assist in closing the GCV and keeping it closed (against CPD). When the GCV closes fuel flow will stop and the unit will lose flame and be tripped.

I believe some of the newer digital Mark* turbine control systems will sense the suicide of the servo output and initiate a trip as a result of that condition.

But, when you say "...both GCV LVDT fail ..." you really need to specify how they fail--high or low. The description above only applies to both LVDT inputs failing low--which is the predominant failure mode. They can fail high, but don't usually.

3 out of 3 members thought this post was helpful...

Isulamu,

The SRV (Speed Ratio Valve) controls P2 as a function of speed. The purpose is to keep P2 pressure a constant amount higher than compressor discharge pressure so as to maintain a more or less constant differential pressure across the GCV (Gas Control Valve). The SRV also acts as a stop valve to shut off fuel flow on a shutdown or trip.