In frame 6 model MS6001B the inlet gas pressure is 19+ in bar. there is a SRV where we generally found a very little difference between inlet & outlet of this valve. Then GCV is regulating according to load. so why we are using SRV? Is GCV not enough to regulate the line pressure?

 

iii,

The 'S' in SRV stands for STOP. The primary function of the SRV is to be the gas fuel stop valve.

Next, another purpose of the SRV is to limit the pressure upstream of the GCV so that flow through the GCV is proportional to the opening ("stroke") of the valve. This is important when the turbine is at rated speed.

Finally, the SRV serves to reduce the pressure upstream of the GCV during firing and acceleration. If, for example, the gas fuel supply pressure is 18 barg, and the pressure at the fuel nozzles only needs to be 0.2-0.3 barg, if the entire pressure drop were to be taken across a single valve (the GCV) the valve would not be able to control flow very well; it would be very nearly closed and regulation at that position is very poor. So, the SRV drops the pressure to something around 1.5-2.0 barg (approximately) upstream of the GCV during firing, and then as the turbine accelerates the SRV opens as a function of speed to increase the pressure up to the running P2 pressure value at rated speed.

If the SRV inlet and outlet pressures are very nearly equal, then one of three things is wrong. Either the gas fuel supply pressure is very low (not much higher than the P2 pressure reference) which is making the SRV have to be fully open. Or there is a problem with the control loop for the SRV; either the P2 pressure transmitter isn't working properly or there's some problem with the hydraulic actuator or the valve itself. I've also seen improperly adjusted P2 pressure reference constants cause the SRV to be full open at 100% speed--which is not how the valve is supposed to operate.

Or, there is usually a strainer and/or filter upstream of the SRV, and if that strainer/filter is plugged then the flow through the device is restricted which usually causes a high pressure drop across the device which reduces the pressure upstream of the SRV.

So, the GCV is NOT enough to provide the proper pressure drop for ALL operating conditions, and the SRV should not be full open, or even at 97-99% of full open, under normal operating conditions. Something is amiss with the gas fuel supply pressure (it's too low, or there is some restriction upstream of the SRV), the valve and or the actuator is not working properly, or there's some problem with the turbine control system (not very likely).

Please write back to let us know how you solve the problem.

 

You are only looking at the pressure when the turbine is at 100% speed. The SRV is set to maintain intervalve pressure as a function of turbine (compressor) speed. Monitor the pressure during startup or a fired shutdown.

 

Hi all,

I agree CSA about the use of SRV.

My question is simple.

What is the difference and what will happens if we use SRV + GCV on the gas turbine? (example GT GE frame 3)

and we use a metering valve for the another same Gas turbine? (GT GE frame 3)

I need your advices about this

Sorry for my english

 

DAMENE AMAR,

Long time no hear from you; welcome back.

Basically, to accomplish what you want to accomplish requires something like an equal percentage trim valve, or something very similar. I guess another name for an equal percentage trim valve might be "metering valve". Back when GE was first designing heavy duty gas turbines, such valves were VERY expensive and difficult to service and maintain (read, more expense). So, GE designed their own combined gas valve assembly to accomplish the required tasks of stop and control, and pressure control upstream of the control valve. One of the advantages of this is that flow through the GCV is basically proportional to stroke when the turbine is at rated speed and the P2 pressure is constant, which makes programming the control system a little easier (and reduces control system complexity and cost).

GE has offered some electrically-operated control valves which are capable of handling the pressure drop and controlling flow with a single valve, but there still has to be a stop valve (or a double block-and-bleed valve assembly) downstream of the control valve (it's usually downstream of the control valve). The problem with these valves is that they don't like dirty gas--not at all. And the gas in a lot of places is not very clean (some is downright filthy!). So, they require more frequency maintenance, and sometimes stick before the planned maintenance outage--which can have not-so-good results (as I'm sure you can imagine). That's another nice thing about the SRV/GCV combo--with good quality internals they usually last a good long time, even with poor gas quality (unless there is sand and small rocks in the gas, which there is at some sites). Rebuilding them is not that complicated, either. And, the hydraulic actuators are really pretty compact and simple (single-acting).

There are probably less costly hydraulically-operated equal percentage trim control valves available today than were available back "before dirt", but I have yet to encounter one.

Does a metering valve require a means of measuring fuel flow-rate (because the SRV/GCV combination does not, which is yet another advantage of this combination). Which would require more complexity still in programming and configuring the control system (and maybe even more inputs). Which is just trading one set of problems for another.

If you're having problems with "servo-valves" the problems are not likely the servo-valves, but the oil--both the quality (maintenance) and type (formulation) of the oil. That's what most people complain about--servo "problems" when it's not really the servo at all. There's nothing like a good hydraulically-operated valve--nothing. But, that means the hydraulic "fluid" has to be good, too.

I'm sure some turbine sites have tried this, and probably with varying degrees of success. I do know of one site that tried to use a pneumatically-operated equal percentage trim control valve with pretty bad results.

Hope this helps!

 

Hi,

I think we should remember that one of the major duty of SRV is to create and maintain sonic flow through GCV(s) by keeping the upstream pressure constant entire load range of gas turbine. When flow speed reaches sonic levels at lower pressure ratios (critical pressure ratio), the flow becomes choked and independent of downstream pressure changes. In this case, flow can be altered by changing the flow area only. So the mass flow is controlled by proportioning the flow area.
Also remember that the SRV valve shape is always a ventury shape due to above reasons.

Regards
TOral

 

Hi,

Just a correction on my last sentence. It must be GCV instead of SRV. So most of the GCVs trim are venturi shaped to create sonic flow.

I recall GE uses Woodward made "SonicFlo" gas control valves on 9FBs.

Regards
TOral

 

I think we need to remember the original poster has a much larger problem on his hands: Why is the pressure drop across the SRV so small at rated speed?

I doubt the original poster will replace the SRV, but he should definitely be very concerned about the low pressure drop and be working to correct that situation to the extent possible. A fully open SRV presents a potential for tripping on exhaust overtemperature should the supply pressure suddenly increase and the SRV not be capable of responding quickly enough to the pressure change.

The signal name typically used for the P2 pressure reference is FPRG, and the signal name typically used for the actual P2 pressure is FPG2. If at steady-state operation/speed FPG2 is equal to or less than FPRG then there is some problem with the gas fuel supply pressure/flow (plugged ("choked") y-strainer and/or filters upstream of the SRV; low gas fuel supply pressure; etc.). If at steady-state operation/speed FPG2 is greater than FPRG then something is definitely wrong with the control loop or the valve/actuator is sticking open. FPG2 should be equal to FPRG under normal operating circumstances, since that's the SRV's secondary mission: to make actual P2 pressure equal to the P2 pressure reference. But, it should be able to do so at SRV openings of less than 99-100%.

FPRG should always be slightly less than gas fuel supply pressure, or gas fuel supply pressure should always be greater than FPRG--by at least 0.5 barg or more. There is a slight pressure drop across the SRV during normal operation but since gas fuel supply pressure is usually several barg greater than P2 pressure reference it's never noticed and doesn't affect normal SRV operation at all--again, since gas fuel supply pressure is supposed to be several barg greater than P2 pressure. As the SRV approaches 98-99% open the pressure drop across the SRV will become more apparent, and when the SRV is at 100% the pressure drop across the SRV will be very obvious (again, only about 0.5 barg or less).

Sonic, sub-sonic, super-sonic--that's all important, yes--but only if the SRV/GCV combo is being replaced. It is good to know this if one is going to be changing gas fuel control valves, and the engineer(s) involved in that effort should be cognizant of all of the requirements through careful analysis of the existing valves and flow conditions.

Hopefully the original poster will write back with details of what was found and how the problem was resolved. I would also like the original poster to post:

1) the gas fuel supply pressure upstream of the SRV, and,

2) the two following Control Constant values from the turbine control system at his site: FPKGNG and FPKGNO. We can use this to help analyze the situation.

 

From last 2 yrs i am getting this type of value in SRV. For example last hour reading is Inlet-18.9 Outlet- 18.3 & GCV-10.3. Our supply pressure always above 19 Bar. And we have not faced any problem regarding this issue.

 

iii,

Okay. I'm confused. Why can't you just tell us what the gas fuel supply pressure is (specifically--not just that it's always above 19 bar). The pressure drop you cited across the SRV is very typical of a working SRV, but I don't know how you're measuring it with the typical gauges/transmitters provided with GE-design heavy duty gas turbines. Because, the pressure upstream of the SRV is usually gas fuel supply pressure, which is usually greater than 19 barg.

What is the physical position of the SRV? (It varies with load in order to try to maintain P2 pressure, but it also varies with gas fuel supply pressure.) But, in general, at some load (please tell us what load, in percentage of rated) you are taking the SRV position reading at? The signal name is usually FSGR.

And, I've never seen a gauge for pressure upstream of the SRV that read a different pressure than gas fuel supply pressure--unless the y-strainer/filter upstream of the pressure gauge was plugged.

But, I think your question was answered, since you haven't provided the requested information. The SRV is primarily a gas fuel stop valve (though many new turbines have "auxiliary" stop valves installed upstream of the SRV these days). Secondarily, the SRV (according to the "R", or "Ratio" part of the valve's name) is to control P2 pressure upstream of the GCV in proportion to turbine speed.

 

Has the fuel gas supply pressure decreased or has the fuel gas composition changed significantly from the time the gas turbine was commissioned? If the LHV has decreased, you will need a higher gas flow to achieve the rated output power, which will result in both the SRV and the GCV being more fully open. You may need to contact the turbine supplier with a current gas analysis to see if any changes to hardware or control constants are required.

 

Dear CSA,

Yes it was time.

The reason why I ask you about the difference using a SRV/GCV and a metering valve, Is that we had a bad experience during project.

I want, if possible, to get a contact with a specialist to explain him the problem.

In one GT GE Frame 3 with GCV/SRV and Mark 6 it was GOOD no problems. In another GT GE Frame 3 with a new metering valve and Mark 6 we got a big problem in combustion chamber. I can not discuss it here more. Sorry for that

See you

 

DAMENE AMAR,

Lots of simple questions....

Was the same metering valve used on all three turbines? Or, at least the same type of metering valve (not necessarily the same size) from the same manufacturer used on all three turbines?

Do these have electric actuators, or hydraulic actuators, or pneumatic actuators?

Was the gas fuel clean and free of dirt/sand/hydrocarbon-based heavy liquids?

What was different in the application on the unit which experienced the problem?

You say "...combustor..." It was only one combustor? That would seem very unusual, if the fuel flows to all six combustors through a common manifold, and not necessarily related to the metering valve.

How are you intending to locate/contact a specialist or consultant?

I understand you can't say too much, but, you're really piqued my interest. I'm very curious. I know of some Mark VI panels which were supplied with electically-operated/actuated metering valves, but because of the poor quality of the fuel the valves were continually sticking or failing to control properly. Wasn't a fault of the valve, per se, but of the cleanliness of the fuel supply. The valve manufacturer said the valve was never intended for such a dirty application, but I'm not sure anyone understood that before the valves were applied. And, of course, the turbine owner/operator insisted they never had problems with the hydraulically-operated SRV/GCV--which was a crock because they demanded an electric valve because of all the problems they were having with the hydraulically-operated valves, including, but not limited to, excessive servo replacement (traced to dirty lube oil), and valve seat/plug/shaft/seal problems (traced to dirty natural gas fuel supply).

But, anyway, any information you <b>could</b> provide would be much appreciated.