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We are facing typical problem with regards to IGV control. About a month back GT ( Frame VI) Tripped on IGV Control Trouble trip. It was observed that IGV was opening smooth at 57 degree at part load. All of sudden it started hunting and within few seconds machine tripped. We cleaned the pensil filter for moog valve and started the m/c again. M/c continue to run for one month and last week the m/c tripped again on the same pattern.

We changed the filter and checked the IGV opening. IGV opening 2 degree less than the set point. It was thought that the null bias of moog valve got disturbed and started the m/c. But now the difference varies between 1 degree to 5 degree. Like m/c continue to run with 1 degree diff and all of sudden diff increases to 4 degree then the m/c continue to run with this difference for a day or two then again the difference reduces to 1 degree. Can anyone suggest the probable cause and remedy of this problem.

Please send me more details of the Gas Turbine(solar or GE) and the control system etc and I hope I will be of help to you.

I have solved a lot of problems on Gas turbines, and most of the time it was due to air locks in the hydraulic system, noise in the signals, oil/moisture carry-over in the gas, or clogged filters, dirt in the hydraulic system or dirty earthing system.
Regards.

Looking forward to your reply.

What's typical about an IGV Control Trouble trip on a Frame 6?

Which control system does the unit have? TMR or SIMPLEX?

Okay; you've cleaned the servo-valve "pencil filter" at least once, and changed it. What did the old filter look like after you put in the new one? Was it dirty again? If so, it's an oil maintenance problem--one which is made worse by a change in oil manufacturing practices in recent years. Do you regularly have your L.O. (which is also the fluid used for the Hydraulic system on most GE-design heavy duty gas turbines) tested for particulates and viscosity? When was the last time the L.O. was replaced? Servo-valves get a lot of blame for problems related to poor L.O. or poor L.O. maintenance practices.

Having said that, if the L.O. is contaminated the servo could also be contaminated. Filters don't catch everything; so, if the "pencil filters" are dirty when you clean them or replace them it's entirely possible the servo internals are also dirty. Have you considered replacing the servo-valve, "pencil filter" and all?

Have you looked at the servo-currents when the differentials are highest and lowest? What are they? Are they balanced (meaning, are the currents from <R>, <S>, and <T> (presuming the control system is TMR) all about the same value or magnitude)?

You mentioned the null bias of the servo, but it's not clear what you meant or what was done (if anything) to the servo or the control system as a potential remedy to a perceived null bias issue. Can you be more specific about that? What is the null bias current value of each processor?

Dear Rishi,
We have observed the similar situation and solved it out for the Frame-IV.

I think You must observed the following:

* Heavy hunting in the Hydraulic Oil Pressure and so in the IGV angale. Which resulted in tripping of M/c.

You must check whether the Accumulator near the Servo is in line? i.e. inlet valve to the Accumulator is Open. If Yes, replace the Accumulator and Check the same.

Another thing, cleaning the moog servo or replacing the filter,spare of moog servo requires the extream extream exteram(03 Times)care to be taken for the cleanliness of the tool tackels, oils, Hands of personals and envoirement at which the servicing is being carried out. So if possible try to replce the servo, then serviceing the servo.

If the problem is solved at your end, kindly share with us.

With regards.

Mehul Jesalpura brings up a good possibility: Have you checked to see the accumulator is properly "valved in?"

If you have a GE-design heavy-duty gas turbine, there's not usually an accumulator in the L.O. system. On a GE-design turbine the hydraulic accumulator will have a "block" valve and a "bleed" valve, sometimes two of each if there are two bladders. If the block valve is closed, the accumulator won't function to prevent hydraulic system pressure fluctuations as hydraulically-operated devices are commanded to move. If the block and bleed valves are both open, usually the hydraulic pressure (not the L.O. pressure) will not be at rated.

The block valve should be open during normal operation, and the bleed valve should be closed during normal operation. Unfortunately, the valves are not usually marked (sad, but true). However, usually, the handle of the block valve is smaller than the bleed valve handle, but that's *not* always the case.

Dear Mehul,

Our observation is that the hunting in the hydraullic pressure is both ways, i.e. whenever there is hunting in the hydraulic pressure, hunting may be there in IGV but the reverse is also true: whenever there is hunting in IGV, hydraulic pressure will hunt to the tune of around 10 Kg/cm2. So it is very difficult to judge whether the problem is with hydraulic oil system or with IGV. But one thing is very clear... This turbine control system is very, very complicated as compared to the other control system. In my short span of 2 years of turbine maintenance I have never seen the same problem repeating again except for the Warren Pump L.O. temperature high trip. What you guys have done in this regard... have you ever thought of 2-O-O-3 logic for warrent L.O. temperature very high or still running the m/c with single temp. switch? As we are people from the chemical industry, so reliability and constant running is the main issue...

Dear Friends,

Ultimately a reason has ben found for the tripping but the m/c is still under observation. One of the LVDTs was showing fluctuating values. The strange thing is that everything was found normal during the check up like no grounding, feedback voltage was perfect. Only when the turbine is in running mode then only the fluctuation increases. Maybe high vibration in the turbine compartment is one of the reasons. Feedback for further discussion is highly valued as through discussion only we can clear our fundamentals.