IGV does not open during acceleration and trips

I have a frame 6B machine with 32 MW generator. Recently, The unit was tripped due to IGV trouble. I tried to stroke the valve later and found it not at all responding to the command. During the troubleshooting, it was noticed that the pressure in the accumulator was not released after the trip and oil started spilling all around while removing the servo valve. We replaced the servo valve suspecting its failure and stroked the IGV successfully. Unfortunately, the same problem occurred while starting the machine, and tripped due to the same reason. IGV stroking does not work unless the accumulator pressure is released before proceeding to start the hydraulic pump. Actuator was tested using a high-pressure hand pump and found no leak at all. looking forward for help from the members. Thanks.
 
“…it was noticed that the pressure in the accumulator was not released after the trip and oil started spilling all around while removing the servo valve….”

There is no automatic release of hydraulic oil pressure; it usually just bleeds down after a trip or shutdown.

If you need to lower hydraulic oil pressure manually do so by opening the bleed valve on the hydraulic accumulator. It’s one of the two small valve handles at the base of the accumulator assembly; but it’s never properly marked. One of the two small valves should always be open—that’s the “block” valve. The other valve should always be closed—that’s the “bleed” valve. (It’s a great idea to mark them some how for future …!)

As for why the IGVs aren’t moving, did you perform a servo-valve polarity test when you installed the new servo-valve?

It’s possible the IGV hydraulic dump valve isn’t working properly, or the IGV solenoid valve, 20TV-1 isn’t working properly. I have personally never seen the issue you are describing with the IGV servo failing to move the IGVs unless hydraulic accumulator pressure is bled off. That would mean during normal operation the IGVs would never move if required to. There is something else wrong with the system.

You failed to mention what turbine control system is in use on the machine. What other alarms—Process- AND Diagnostic—are active?
 
  • what is the hydraulic oil pressure?
  • after forcing relevant logic for calibration did trip oil pressure establish?
  • check Servo coil resistance
  • and check Servo voltage (if servo resistance and voltage are ok then the problem might be from the oil circuit side)
  • did IGV respond while calibration after replacing the servo? (at filed angle indicator? )
  • check 20TV operation (if there is an inspection window then observe the oil drain stop while forcing)
  • using P&ID and verifying the oil flow stage by stage (verify VH3-1 operation)
  • check if there are any oil leakages between the IGV module and the actuator ?
  • try to verify C1 and C2 pressure of HM3-1 (open and close supply line for HM3-1)

last time i faced a similar problem with Hitachi make frame6B and found the check valve was gone from the IGV oil circuit.
 
“…it was noticed that the pressure in the accumulator was not released after the trip and oil started spilling all around while removing the servo valve….”

There is no automatic release of hydraulic oil pressure; it usually just bleeds down after a trip or shutdown.

If you need to lower hydraulic oil pressure manually do so by opening the bleed valve on the hydraulic accumulator. It’s one of the two small valve handles at the base of the accumulator assembly; but it’s never properly marked. One of the two small valves should always be open—that’s the “block” valve. The other valve should always be closed—that’s the “bleed” valve. (It’s a great idea to mark them some how for future …!)

As for why the IGVs aren’t moving, did you perform a servo-valve polarity test when you installed the new servo-valve?

It’s possible the IGV hydraulic dump valve isn’t working properly, or the IGV solenoid valve, 20TV-1 isn’t working properly. I have personally never seen the issue you are describing with the IGV servo failing to move the IGVs unless hydraulic accumulator pressure is bled off. That would mean during normal operation the IGVs would never move if required to. There is something else wrong with the system.

You failed to mention what turbine control system is in use on the machine. What other alarms—Process- AND Diagnostic—are active?
Hi,
Thanks for the reply.
1: Polarity of the servo polarity test was done.
2: Control system is Mark VIeS
3: 20TV is working as the drain line is cool while it is energized and hot when de-energized, means not passing.
4: Alarm is "IGV control trouble"
5: Other alarm is SRG servo voting mismatch but trip is initiated by IGV deviation

We had replaced IGV servo a couple of times in another machines and not experienced such an oil splashing issue ever before. Will the oil pressure remain high, in servo-accumulator-dump valve loop, for a while after the hydraulic pump is turned off?
Whenever we replace the servo or depressurize the entire hydraulic loop the IGV works smoothly thereafter. However, when we go for start up, after IGV testing, it fails to operate during acceleration and unit trips. Its quite strange.


Thanks.
 
Yes; it's quite strange.

EXACTLY HOW was the servo-valve polarity test done? Do you follow the procedure in the Control Specification? If not, how do you verify servo current polarity?

The hydraulic supply system is basically a static system which only has flow when a hydraulic actuator is commanded to move a device (such as the SRV or GCV or IGVs, etc.). When the device (SRV, GCV or IGVs, etc.) are in the commanded position there is very little flow in the hydraulic system. (Don't confuse the Trip (or Control) Oil system with the Hydraulic oil system.) SO, when the hydraulic system is shut down it usually takes a few minutes for the hydraulic accumulator (which holds a good amount of hydraulic fluid) to bleed down--IF the block and bleed valves are in the correct position AND the bladder is properly charged. As was written, the block and bleed valves have the same size handles and no other identifying information. I found MANY valves closed when the only the bleed valve should be closed, AND I also found MANY accumulator bladders to be uncharged or charged to the wrong pressure (the pressure should be approximately one half of rated hydraulic system pressure).

The purpose of the hydraulic accumulator is to provide flow at pressure when one of the servo-operated devices (SRV, GCV, IGVs, etc.) calls for a large change in position. This serves to maintain hydraulic pressure (since--again--the hydraulic system is basically a static system under normal, steady-state operating conditions) so that other hydraulically-operated devices don't experience issues because the hydraulic system pressure is momentarily decreasing and then increasing. The next time a unit shuts down watch the Hydraulic System pressure gauge (or the transmitter input to the Mark* if present) and you will usually see that hydraulic system pressure takes a couple of minutes or so to bleed down to zero.

Have you looked at the IGV P&ID? And the hydraulic oil system P&ID? And the Trip Oil system P&ID? You need to logically work through each possible device in the oil circuits for the IGV system (solenoid-operated valve; hydraulic dump valve (if present)--some very early GE-design Frame 6B heavy duty gas turbines did not always have hydraulic dump valves in the IGV hydraulic oil circuit). Also, some very early Frame 6B units did not have an Aux. Hyd. Pump and the unit had to be CRANKed to have Hydraulic pressure/flow to manually position ("stroke") devices and perform LVDT calibrations. If there's a hydraulic dump valve (a simple, spring-loaded normally open valve which uses Trip Oil pressure (another essentially static oil system with very little flow) to control the flow of high-pressure hydraulic oil to the servo-valve humidity and oil vapors can cause the dump valve to stick and not operate properly thereby preventing hydraulic pressure from reaching the IGV servo-valve.

As vscontrol mentioned, there is usually a gravity-operated swing check valve with a small orifice hole drilled in the flapper which is used to quickly dump trip oil pressure when the solenoid-operated valve is de-energized to stop the flow of hydraulic oil to the servo-valve and actuator. I was at a couple of sites where the flapper had been installed backwards and it took a very long time for the trip oil pressure to bleed off through the orifice hole in the flapper.

If the machine is an older machine the IGV actuator is probably located under the IGV bellmouth in a very difficult position to access. And, installing temporary pressure gauges in the hydraulic supply lines to the actuator (if that's even possible to do--some of the hydraulic manifolds on those IGV actuators could not have temporary gauges installed) that would be a good thing to do to see if the servo-valve is actually passing high-pressure hydraulic oil to the IGV hydraulic actuator when commanded to. I have also seen the wrong servo-valves installed on IGV actuators which resulted in some very odd behaviors. The servo-valves used on those machines are all very similar from the outside and seemingly can be interchanged (some can be--many cannot be). I've even seen more than one servo-valve installed incorrectly on an actuator (180 degrees out of position!).

You say you have a Mark* VIeS system.... Is 20TV-1 controlled by the SIS portion of the control system?

Personally I don't think this is a controls-related issue; something seems to be preventing high-pressure hydraulic oil from getting to the IGV actuator, or the actuator/linkage is sticking, or ??? I don't know precisely what this suggested test would indicate but have you tried closing BOTH of the manual valves on the hydraulic accumulator (BOTH the block and the bleed valve) with zero pressure on the hydraulic system when the unit is at rest, and then STARTing the machine and seeing what happens? If the IGVs move, then it kind of says the problem is not the accumulator, and if the IGVs move then you can SLOWLY open the block valve to put the accumulator back in service. If the IGVs don't move then they are behaving just as before and it's still most likely it's something other than the hydraulic accumulator. The hydraulic accumulator is in service (presuming the block valve is open, the bleed valve is closed, and the bladder is properly charged) at all times--whether you are manually positioning ("stroking") a device or the unit is starting/running/shutting down. So, if it was the accumulator causing the problem then you should see it when you are manually positioning the device, also.

If the unit has an Aux. Hyd. pump, you can start the Aux. L.O. pump and then start the Aux. Hyd. pump and watch hydraulic system pressure with the unit at rest. Then after a couple of minutes or so, shut down the Aux. Hyd. pump and monitor hydraulic system pressure, it should decrease rather slowly down to zero (in a minute or two or three). That would indicate the hydraulic accumulator is properly valved-in and the bladder is properly charged (in most cases, anyway). (My personal experience is that it takes a couple of minutes or so for hydraulic pressure to bleed down when the hydraulic pump(s) are stopped--again, because there is not very much flow through the hydraulic system, ever--unless a large change in the position of one or more hydraulically-operated devices is being called for and that's NOT usually the case on shutdown or at rest.)

That's about all I have to offer at this time based on the information provided. I would be surprised if the problem you are describing is somehow related to the absence or presence of hydraulic pressure because of the operation of the hydraulic accumulator (unless there is a sticky hydraulic dump valve in the IGV oil circuits--VH3-1 or something like that). Hydraulic dump valves are simple to remove and reinstall, and very easy to disassemble and reassemble. I heard of a broken spring once on a hydraulic dump valve that was causing intermittent problems--but that was a very long time ago and I don't recall exactly what the problems were. It's pretty easy to visually observe the movement of the dump valve spool piece because the spring is usually exposed on the outside of the dump valve assembly, and it should move when the solenoid-operated valve (20TV-1, in this case) is stroked--if the unit has a hydraulic dump valve (again some units didn't have them on the UGV system).

Please write back to let us know what you learn and how you resolve the problem!
 
If something is in the oil circuits is preventing the hydraulic pressure in the accumulator from bleeding down that might explain that part of the issue. Again, there’s usually nothing in the GE design which automatically reduces hydraulic supply system pressure—it’s usually Trip Oil, acting through the hydraulic dump valve that shuts off the flow of high pressure hydraulic oil from getting to the actuator to open the IGVs. The hydraulic dump valve usually moves the IGV actuator (because air flow through the IGVs is trying to open them more than the setpoint)—and if the hydraulic dump valve is NOT working properly and keeping hydraulic supply pressure on the IGV actuator to keep them closed that is likely the problem in this instance.

Based on the information provided.
 
Hi,
Apologies for the late reply.
The problem got resolved. All the IGV hydraulic-related instruments were checked and found healthy and intact. The dump valve was disassembled and found nothing damaged or not even dirty. As I mentioned in the previous posts 2 new servo valves failed to function properly. We installed an OLD servo and the machine is running smoothly since then. It's quite strange that the new servos are not working. The oil is often tested in the lab and reported as good but I, now, suspect the quality of the oil as the servo is the only device malfunctioning in the loop. We have contacted GE and waiting for the comments.
I will update you if anything is confirmed.

Thanks for your time and suggestions.
 
Thanks for the feedback!

I have installed completely new, never-used electro hydraulic servo valves taking great care to connect the new servos with the same color-coding as found when removing the old servos. And during polarity testing discovered one, and on occasion two, of the coils were operating opposite of how they should. Reversing the servo leads always solved the problem. I have spoken with a couple knowledgeable colleagues who had also experienced the same issue with new servos from the manufacturer (MOOG).

A lot of sites choose to purchase refurbished servos to save money. (To my knowledge MOOG does not refurbish servos; refurbishment is done by third-party companies.) One former contributor to Control.com reported installing several (six, if I recall correctly) refurbished servos on one machine that would not work properly no matter was attempted. A new servo was finally installed and it worked just fine.

The problem was attributed to incorrectly adjusted null bias springs. The owner-operator of the site decided never to purchase refurbished servos again as the process took more than two days to get the unit returned to service.

Your “fix” doesn’t explain why the IGVs move when being manually stroked but don’t move when the unit is being started-up. It’s certainly very odd.

Anyway, glad to hear the unit is running again!
 
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