In one of our units, GE Gas Turbine frame 9E, Controlled by Mark IV+, the SRV start hunting and P2 pressure is flactuating as well from firing till FSNL. After synchronization directly, SRV is not hunting and performing very normal. there are no other alarms or diagnostics.
We have replaced the moog servo-valve, stroked the valve from 0 to 100% and every thing is normal. Mechanical team replaced the hydraulic cylinder also but still the issue is not yet solved.
could you please advise in this regard.

 

it looks like that the dead band/time integral of the controller function block is too low so that any fuel gas demand can cause very fast controller response.

 

I’ve never heard of a Mark* VI+.

One of the most beautiful things about GE-design heavy duty gas turbines was that the “deadband/time integral” of hydraulically-operated valves and IGVs was calculated by the factory and stated very clearly in the Control Specifications document so that as long as the correct value was put into the servo regulator it operated perfectly. No guessing; no tuning; no problems. None of the “increase the gain until the process is unstable and then decrease it a little” crap. Just enter the right values into the regulator and move on to the next task. It was genius.

That was possible because the same actuators and valves were used for decades—and the same servo-valves, also.

How much is the P2 pressure fluctuating? +/-0.5 bar? +/-1.0 bar? Are the pressure fluctuations affecting synchronizing (causing longer then normal synchronization times)?

But, over the last decade or so many things have changed, including valves and actuators. We have no way of knowing what valve and actuator is used on the units at your site. We don’t know what the fuel gas supply system looks like—is there a pressure regulator upstream of this particular unit that may be experiencing problems when the flow-rate through it is low (such as during firing and acceleration)? Are there any fuel filters upstream of the SRV that might be contributing to the problem? Does the unit have DLN combustors? Is the SRV a rotary ball valve or the older combined vertical SRV/GCV assembly? Are the P2 pressure transmitter valves in the correct positions?

What Diagnostic Alarms are being annunciated prior to STARTing?

Are the individual servo currents similar or is one much different than the others during STARTing and after synchronization?

What is the null bias current value for the SRV?

How was the servo polarity verified when the new servo was installed? Using the procedure in the Control Specification or by testing under the control of individual processor servo outputs?

How is the GCV acting—during STARTing and after synchronization? Is it always stable under both conditions? (I am presuming the unit does not have DLN combustors and multiple gas valves.)

It would be odd for the GCV to be stable at all times while the SRV was unstable because during acceleration after warm-up the Mark* is trying to control the acceleration rate and after synchronization if the P2 pressure was unstable the load would be unstable but you say the SRV stabilizes after synchronization.?.?.?

If the SRV is “old” and has never been refurbished or replaced it’s possible that the internals (seat; plug; etc.) are worn or have some kind of build-up/contamination from contaminants in the natural gas. You’ve replaced the actuator and the servo and the problem is still existing, so it’s either the valve assembly or internals, or the supply system or the P2 pressure transmitter valves or the shaft speed pick-ups (and that’s pretty unlikely).

That’s a fairly comprehensive list of possibilities based on the information provided. Please write back to let us know what you discover and how you resolve the issue.

 

Our 6B unit (MARKV) was doing the same thing. We did all the things you mentioned, same issue. GE on site and found that the GCV was the valve "hunting" slightly during a valve calibration. They recommended changing out its servo. We changed it out with a rebuilt one (not GE's recommendation). Same issue. We finally talk our boss into changing it out with a new servo. Issue gone.

 

I've had the same experience with refurbished ("rebuilt") servo-valves--and others have reported the same problems, also. And, as others have written before on Control.com about servo-valves, I, too, have pulled brand new servo-valves out of the box and installed them with exactly the same color code connections--only to find that one or two of the servo coils are reversed, from the manufacturer. That's why it is SO IMPORTANT to perform a proper servo polarity current test when installing a new servo valve. It's NEVER necessary to "recalibrate" a valve of the IGVs when installing a new servo (if that's all that's being replaced!) as changing the servo-valve does nothing to the LVDT feedback--and calibration ONLY affects LVDT feedback scaling--NOTHING ELSE.

I think CSA wrote that at one site they pulled six or seven rebuilt servos out of the warehouse (at a warehouse that served multiple turbines) and found only one that was usable. And, mostly that was the result of improper null bias spring tensions--that CANNOT be properly adjusted in the field.

AND, the procedure usually written in the Control Specification for performing servo polarity checks (the one that say, basically, manually stroke the device and if the action is jerky replace the servo) is complete BS. A proper servo current polarity check involves using a single processor's servo output to hold a device in some midstroke position and checking all processor outputs individually. CSA, a contributor, wrote many procedures on Control.com for this critical test and verification. AND, when replacing servo-valves, it's always a good idea to check the null bias current value for the device/regulator being worked on and if it's NOT set to 2.67 (the default value for TMR control panels) then it should be reset to 2.67 before testing servo current polarity--something I think CSA assumed was not necessary to check/change, and so didn't include that little important tidbit of information when replacing servo-valves and testing servo polarity).

 

I've had the same experience with refurbished ("rebuilt") servo-valves--and others have reported the same problems, also. And, as others have written before on Control.com about servo-valves, I, too, have pulled brand new servo-valves out of the box and installed them with exactly the same color code connections--only to find that one or two of the servo coils are reversed, from the manufacturer. That's why it is SO IMPORTANT to perform a proper servo polarity current test when installing a new servo valve. It's NEVER necessary to "recalibrate" a valve of the IGVs when installing a new servo (if that's all that's being replaced!) as changing the servo-valve does nothing to the LVDT feedback--and calibration ONLY affects LVDT feedback scaling--NOTHING ELSE.

I think CSA wrote that at one site they pulled six or seven rebuilt servos out of the warehouse (at a warehouse that served multiple turbines) and found only one that was usable. And, mostly that was the result of improper null bias spring tensions--that CANNOT be properly adjusted in the field.

AND, the procedure usually written in the Control Specification for performing servo polarity checks (the one that say, basically, manually stroke the device and if the action is jerky replace the servo) is complete BS. A proper servo current polarity check involves using a single processor's servo output to hold a device in some midstroke position and checking all processor outputs individually. CSA, a contributor, wrote many procedures on Control.com for this critical test and verification. AND, when replacing servo-valves, it's always a good idea to check the null bias current value for the device/regulator being worked on and if it's NOT set to 2.67 (the default value for TMR control panels) then it should be reset to 2.67 before testing servo current polarity--something I think CSA assumed was not necessary to check/change, and so didn't include that little important tidbit of information when replacing servo-valves and testing servo polarity).

Dear WTF

Thanks for trying to help in troubleshooting and for sharing your experience,

I will try to answer your questions\ remarks mentioned in both replies in the below points:

- We have replaced the servo twice and in both times it was brand new servo. We didn't do polarity check actually but we stroked the valve many times from 0 to 100% and it was smoothly operating. As I mentioned on the title, Once unit is synchronized, the SRV is behaving very normal without any hunting.

- P2 pressure fluctuation is around +\- 3 to 4 bars. It is not affecting the synchronization at all.

_ Are there any fuel filters upstream ?
Yes filters are there but we have a Diff. pressure switches across them and no alarms are there. Moreover, It is being serviced during each overhauls (please note that this hunting issue in SRV is happening since many years).

- GCV is hunting during start up, acting normally.

_ the unit is having having single GCV (No DLN)

- I will try to note down the null bias current and servo current when I go again to the plant and share with you.

Soon I will come back with more details. I just wanted to highlight the point of SRV hunting during startup only and ask if anyone experienced same phenomena before.

Thanks to you and other members for participation.

Regards,

 

It would have been VERY HELPFUL to know that this problem has been existing for many years.

Smooth stroking under manual control (while in AutoCal) DOES NOT mean the servo polarity is correct. (Neither does jerky movement during manual stroking (and what is jerky to one many is not to all others, and vice versa.)

Since you said you have other units which don't experience this problem, I suggest that either the regulator gain for the SRV is not correct on the unit having the problem, or the null bias current is not correct. Early Mark* VI turbine control systems used P2 pressure as feedback to the SRV regulator (something that drives most people very crazy). Later versions of application did not do that. If the unit having the problem was not installed at the same time as the others, it's possible that there is something amiss with the configuration of the Mark*. (Again, I'm unfamiliar with a Mark* VI+; if you could provide more details, it would be helpful.)

If the SRV is a rotary ball valve, historically they have not been the most stable valves, especially during start-up and acceleration. But, +/-3-4 bars (45-60 psi) is a LOT of oscillation. And it would also explain why the GCV is not stable. (Of course, an unstable GCV can cause the SRV to be unstable, so, your mission, James, if you accept it, is to determine if the dog is wagging the tail or the tail is wagging the dog.)

It's also possible the P2 pressure Control Constants for this unit are incorrect.

AND, if the P2 pressure is swinging around as much as you say it is, then it's pretty certain the gas fuel supply pressure is also swinging too. And, interaction with whatever should be controlling P2 supply pressure can be aggravating a small problem and making it worse.

Anyway, that's all I got. Best of luck. Hopefully you will write back to let others know what you discover.

Tchau!

 

It would have been VERY HELPFUL to know that this problem has been existing for many years.

Smooth stroking under manual control (while in AutoCal) DOES NOT mean the servo polarity is correct. (Neither does jerky movement during manual stroking (and what is jerky to one many is not to all others, and vice versa.)

Since you said you have other units which don't experience this problem, I suggest that either the regulator gain for the SRV is not correct on the unit having the problem, or the null bias current is not correct. Early Mark* VI turbine control systems used P2 pressure as feedback to the SRV regulator (something that drives most people very crazy). Later versions of application did not do that. If the unit having the problem was not installed at the same time as the others, it's possible that there is something amiss with the configuration of the Mark*. (Again, I'm unfamiliar with a Mark* VI+; if you could provide more details, it would be helpful.)

If the SRV is a rotary ball valve, historically they have not been the most stable valves, especially during start-up and acceleration. But, +/-3-4 bars (45-60 psi) is a LOT of oscillation. And it would also explain why the GCV is not stable. (Of course, an unstable GCV can cause the SRV to be unstable, so, your mission, James, if you accept it, is to determine if the dog is wagging the tail or the tail is wagging the dog.)

It's also possible the P2 pressure Control Constants for this unit are incorrect.

AND, if the P2 pressure is swinging around as much as you say it is, then it's pretty certain the gas fuel supply pressure is also swinging too. And, interaction with whatever should be controlling P2 supply pressure can be aggravating a small problem and making it worse.

Anyway, that's all I got. Best of luck. Hopefully you will write back to let others know what you discover.

Tchau!

WTF he has a Mark 4 plus it sounds like to me. We had one. It is 1990 vintage Controls with a B/W monitor. SRV does swing a little with that P2 pressure.
The real question I have is he hitting FSRT on startup and it is swinging by gaining speed as the PCD increases.
If memory serves me correct that SRV is like wide open and you are on the Control valve at that point. 2 Valves in the same body. Since like you stated limited info given. FSRT limit been messed with? PCD Transmitter calibrated or Constants messed with? I'll follow along in the thread just to see if they report anything.
BTW our MKIV plus was on a frame 7EA 1990 vintage..Good Day.

 

Yeah, a lot of people have the mistaken idea that when the unit reaches 100% speed that the SRV should go to 100% stroke. NOT. Never. Not ever. Not for any version of Mark*.

I cut my teeth on Mark* IV, including Mark* IV Plus (there were, I think, actually six (6) versions of Mark* IV, a few with battery-backed RAM and some with EEPROM, some with Dynamic Rung Display, some with the ability to name inputs (most without that ability)), etc.

I have a REALLY DIFFICULT time believing that a typical Mark* wouldn't trip the unit (presuming nothing is forced!) with swings of +/- 3 to 4 bar--that's just wild pressure swings and the GCV of any Mark* newer than any Mark IV controls fuel after warm-up to maintain a set acceleration rate. And, as you say, there's always FSRT and exhaust over-temperature alarm and -trip logic. I just can't really understand how +/- 3-4 bar swings could be happening and the unit make it to FSNL without something jiggered or frigged or forced (actually SEVERAL things--not just one, but I wasn't gonna bring that up and I'm glad you did!).

Yeah, I'm gonna follow this but there's just too much we don't know and will probably never know. MANY people will not admit to forcing logic so it's not worth asking. They're just looking for something to try or somewhere to look for a problem they haven't thought about. Many people are just looking for validation of their idea of what the problem is (and they won't really ever say what they think the problem is...).

And they never provide feedback--which really irks me. This is a free forum--we get paid the same amount as anyone who posts here for help: Nothing. Zero. Zilch. Nada. Zippo. Niente. Diddly squat. And some of us spend some serious time trying to help.

Anyway, I know what to expect--and I'm pleasantly surprised when there is feedback. And it's useless to ask people to provide feedback.

Thanks; I hadn't thought about Mark* IV Plus in a long time. Brings back some mostly good memories. It was SOOOOO difficult to troubleshoot most problems with Mark* IVs because there was no real configurable high-speed data capture function. It make one have to think and study and understand systems and field devices to do serious troubleshooting. And, I really wouldn't want to go back to do those days.