We are equipped with GE 9FA+ turbine which uses MARK VI as control system.

It seems that we have a problem with IGV suddenly not following changes in setpoint.

So we've had first the alarm L86VGA_ALM which comes if for 5 sec command & feedback have a deviation of 7.5%. So what happens is that now IGV has a reference of 83.5% but it remains stuck at 53%. We've checked physical position and it is actually ~50% & movement is not obstructed somehow.

Also input filter DP was checked prior to hydraulic block & it is O.K. We've also stopped the plant & replaced the Moog valve & its filter, stroked it & worked O.K.

After plant startup it worked O.K. for 4 hours & then the same problem occurred.

Can somebody help us out with what else could be checked?
Do you think that if we force reference value for a while less than feedback & then remove force might overpass sticking of the valve?

 

Some IGV actuators develop wear in the hydraulic cylinder wall which causes the piston the hang up. I've seen a couple of these where it worked ok in calibration mode, but would hang when the actuator had the IGV aerodynamic load during unit operation. Only fix is to replace the actuator. There may be a revised part number with an improved hydraulic cylinder. GE should be able to tell you if there is a newer part number available for your unit.

 

Thanks for your reply..

It seems similar to the answer of GE since they suggested that we should schedule shutdown for today & replace the IGV actuator with a new one!

 

If the old actuator was made by Young & Franklin, and you have it rebuilt, they can tell you if it had the old-style hydraulic cylinder. (I believe their newer design has chromed cylinder wall instead of cast iron, and "glide rings" on the piston.)

 

Yes; trying to run a DLN 2.n combustor-equipped machine without a working IGV actuator, or worse--trying to run by forcing IGV references--is pretty much just begging for compressor and/or combustor problems.

The forces at work on the IGVs when the axial compressor is spinning at rated speed are very large.

The question which has not been asked or answered is: When did this problem start? After a maintenance outage? After a unit trip from load?

Also, what Diagnostic Alarms are being annunciated from the VSVO cards which provide the current to the IGV servo coils?

Have you used the Verify Current and Verify Position features of AutoCalibrate to try to see if they indicate any problem(s)?

Best not to try forcing IGV references on a 9FA machine; the possible consequences would not be pleasant.

 

Problem was noticed during normal operation, not after outage or any plant trip!

No diagnostic was active from VSVO..

Finally this weekend we've shutdown the plant & GE replaced IGV actuator with a new one!

In the old one it was found that IGV gears had a light rub & grease inside gears was quite dirty.

Also an inspection was made in the predischarge accumulator of hydraulic circuit & found defective (torn bladder).

All calibration action was made after replacement of IGV using verify current & verify position & everything was found within specifications.

Also slew time was constant in several strokes (5.1sec).

So at the moment we've just started up & let's see how it goes!
Do you have anything else to suggest?

 

mesi,

Thanks for the update, and we're looking forward to hearing the results!

Failed accumulators can cause a variety of problems, though it's not clear how it could have caused or contributed to this problem. Charging, and checking the charge of, hydraulic accumulators as a fairly simple task that is almost never done properly--mostly because of a failure to read instructions, which are sometimes posted on the top of the accumulator pressure vessel, and either painted over or missing. Failure to follow instructions can cause blown-out bladders as well as excessive use of nitrogen (the normal charging gas) as well as a complete loss of charge.

The other problem with accumulators is the valve positions. Everyone thinks all the valves should either be open or closed. One valve (or pair if there are two acumulators) is the "block", or isolation, valve, and this valve (or valves) must be OPEN for normal operation. The other valve (or pair of valves if there are two accumulators) is the "bleed" valve, which is useful during system maintenance or outages, and which must be CLOSED during normal operation. Usually, the block valve handles are slightly larger diameter than the bleed valves, but sometimes they have the same size handles. If the valves are not labeled or if there is any question, the manufacturer's instructions should be consulted.

New units don't usually ship with a lot of grease on the ring and pinion gears. And, it should be a normal part of maintenance outages (especially HGPIs and Majors) to check the rack and pinion gears, grease them if necessary, and keep them clean if necessary.

Again, thanks for the update and we are looking forward to hearing the results. Lastly, if there are no obvious indications from the Speedtronic (like no Diagnostic Alarms, etc.) then it's safe to presume the problem is outside the Speedtronic control panel even if the normal course of action is to assume it's the control panel and blame it for most problems (after blaming the servo-valve and LVDT calibration).

 

Dear Gas turbine user. The problem you subscribe is a common problem in GT. This problem is due to oxidation of the oil. Even if you change the oil and even if you change the valves this problem will come back quickly or within just a few months. Is takes a complete book to describe you this is but please contact me and we can discuss this. Yes, the problem is very easy to solve

Best regards,
George.
[email protected]

 

Could you post the values of the LVDT feedback voltages in the autocalibrate screen? If there is no physical obstruction and Hydraulic ckt is ok , you can try removing one of the LVDT's. put a manual command of 0 and 100 in the autocalibrate screen and check whether the IGV goes to full mechanical Open and closed positions. The same procedure can be repeated with only the other LVDT in line. If the IGC goes to the full open and closed positions, the problem lies with a defective LVDT feedback. Replacement and recalibration of the LVDT's would solve the problem.

 

The plot of both LVDTs can be seen in the AutoCalibrate screens, as well as by using 'Verify Current' which outputs the current required to keep the LVDT feedback changing at a constant rate (this is why it has to be done after an AutoCalibrate procedure). If the LVDT feedback was not changing at a constant rate then the servo current required would be non-linear to keep the rate of change constant--and that would be visible in the resulting graph. 'Verify Position' outputs a constant current to verify position feedback changes linearly, and that would also show a discontinuity if there was one.

But, as reported by the original poster, they have already replaced the actuator per GE's recommendation, and as so often happens with these kind of urgent issues: Once there is no more problem, there is no more feedback.

Sad, but true.

 

Don't worry CSA,

We hope, they may drop their feedback soon.

Take care
G.Rajesh

 

Believe it or not the GE recommendation is to not grease the rack and pinion gears. The reason we were given is this attracts dirt and leads to accelerated wear of the parts. The regulating ring however should be greased.

 

Hi CSA,

I appreciate a lot your help & I'm really sorry for not replying but you know probably best than all that if a Gas Turbine is under CSA then GE's engineering teams are deciding what's the best in matter of time & money so as to fulfill contractual obligations.

However our team believe that the problem is from IGV's body meaning that it was damaged in certain positions by internal springs,making the valve stucking.Furthermore we've performed a hardness test in internal springs & actuator body & found that springs are twice harder than body.That's of course a thing that was reported to GE as an unacceptable issue for Young & Franklin actuator design.

My background is Controls but I'm confident that our specific trouble had to do only with mechanical friction & not instrumentation or control problems.

Thanks a lot for the support.

 

mesi,

Thanks you very much for the feedback. But, I'm <b>VERY</b> confused. Even more than usual. I have <b>NOT</b> seen EVERY configuration of IGV (Inlet Guide Vane) actuator provided on every GE-design heavy duty gas turbine, but I have never seen an IGV actuator with internal springs. Every IGV actuator I have ever seen used a double-acting piston to position the IGV actuator ring, and there were <b>NO</b> springs--either internal to the actuator or external to the actuator.

Springs would normally be provided to return a device to some "fail-safe" position if the force used to position the device were lost. If, during off-line stroking of the IGVs, the hydraulic pressure went to zero when the IGVs were at some mid-stroke or full open position the IGVs would remain at that position. There were no springs to return the IGVs to either a closed or an open position.

You keep referring to "... the valve
stucking...." The IGVs are not, technically, a valve. And the IGVs don't technically have a "[valve] body". The IGV actuator is usually a double-acting piston, with the hydraulic flows to/from each side of the piston controlled by an electro-hydraulic servo-valve. The piston up-and-down movement is converted to a rotary movement to open/close the IGVs (which pivot on an axis) using a "rack-and-pinion" mechanism. This is not the description of a valve, and, again: There are usually no springs anywhere (except in the electro-hydraulic servo-valve) on the IGV actuator, rack-and-pinion mechanism, or the IGVs themselves.

And even if springs were used in a "valve body", it would be logical to presume that the springs would not normally be in contact with stationary components. So, I'm even more confused.

So, can you please provide more detail about this IGV "valve" with springs?

Thanks a lot for helping me to learn and understand!

(By the way, many people upload pictures or drawings to free image-hosting websites for others to download and view. tinypic.com is one; www.speedyshare.com is another. If you could provide the schematic representation of the IGV actuator from the IGV P&ID and/or the IGV actuator drawing (cutaway) it would be very helpful in understanding where the springs are located that caused the IGV control problems at your site. Thanks! You just need to upload the information to the site, and then copy the url ("link") to the information and paste it in a reply to this thread, and we can all benefit from the experience at your site. Again, thanks for helping me--and most likely others--to understand and learn!)

 

YES,

We too agree with CSA as our GE-9E gas turbine IGV also controlled by double acting piston not by spring force.

We are too keen to learn from Mesi.

take care
G.Rajesh

 

G.Rajesh,

I may be very wrong, because the "Belfort Bunch" of GE quite frequently do things very differently from other divisions of GE. Why? Because they can. So they do.

I've worked on several Frame 9FAs (early models that were built in the USA, though) and I'm pretty certain that the IGV actuators of those machines did NOT have any springs in the actuator body, or on the actuator, or on the IGV control ring.

Nowhere--except the null bias spring in the electro-hydraulic servo-valve.

I hope the original poster responds, but, since he believes his problem is solved and he believes his team has identified the root cause of the problem, it's not likely.

 

Dear CSA & G.Rajesh,

I can see that you are really interested in our case but together quite ironic with your comments.

My intention was not to confuse anyone but actually i did it because I wrote "spring" instead of "ring". Of course I mean the piston ring & that's what was compared in hardness against the cylinder itself.
CSA talking to you now, I've felt that you were helping GE users but actually you are a tutor that freely educates every one in basics of mechanics.

I appreciate it a lot that you've helped me in general understanding of valves & double acting pistons basics!

Of course no photos available for you & case is considered closed together with my presence in control.com

 

Thanks Mesi for your kind feedback.

All we are learning form each other.

keep it up and keep post your future issues.

Take care
G.Rajesh

 

There is a trip for GE machine that IGV not following the reference!
During this time IGV temperature control must be in off position otherwise GT will trip due to loss of flame.