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IGV Actuator Span in DGA too big
GE Frame 6B Actuator replacement. Gross stroke 38mm however IGV blades travel more than 60DGA.
1 out of 1 members thought this post was helpful...

Recently we had a Major Inspection for our GE Frame 6B GT and we decided to replace the IGV Actuator as part of the worklist. Before replacing, we took as found readings(old actuator measurements ie: gross stroke & cylinder fully retract length.) and made sure the new actuator has the same measurements(which it does with +/- 1mm accuracy).

For info, IGV blades were replaced as well since this is normal practice for us during MI.(not sure on other people's practice).

During IGV calibration, we faced a problem where our IGV actuator hit the mechanical stop which was set at 29DGA-90DGA. I believe the gross stroke of the actuator which is 38.1mm should reflect to approximately 56DGA(eg: 32DGA min- 88DGA max).

Now the problem we have is that we are not convinced to perform calibration since this is not the actuator stop, but the IGV ring mechanical stop.

I need help on whether i should just proceed with autocalib and set the min/max at 29DGA-90DGA? Or i should make any other adjustments? (to my actuator or mechanically adjust the mehanical stopper so that it goes approx 2 DGA beyond my actuator stop. (Eg my actuator min/max stroke is at 28-92 DGA then i should set mech stop at 26-94 DGA).

If you think this is a mechanical problem, kindly explain how this could happen. And if so, is there any other way that i could do when performing the IGV calibration to ensure this mechanical problem can be resolved with a good control. Everyone knows rectifying a mechanical problem would take days and everything has to be opened up again.

Some advice please as this is my first time performing this job. Thank you in advance.

Please contact me directly for fast response and further details. zulfaizzulhasni@gmail.com

2 out of 2 members thought this post was helpful...

Fram6BNewB,

Welcome to control.com. It would have really been helpful if you had told us what control system is being used on the GE-design Frame 6B at your site.

After replacing the actuator--and probably the LVDTs--it will be absolutely necessary to calibrate the LVDT feedback. And, if you're using a GE Mark*/Speedtronic turbine control system, you will probably be using AutoCalibration (which, by the way, ONLY calibrates LVDT feedback, and NOTHING else--no matter what anyone ever tells you).

You will need to set the zero stroke voltage at the minimum mechanical stop, and it should be set using a True AC RMS voltmeter, one that can measure voltages of less than 1.0 VAC RMS at a frequency of approximately 2.8-3.2 kHz--something that not all new voltmeters can do, even when they say they are AC RMS voltmeters. The zero-stroke voltage should be 0.700 VAC RMS, +/-0.020 VAC RMS, so anywhere from 0.68 VAC RMS to 0.72 VAC RMS is acceptable.

You should also be performing a servo current polarity check prior to doing any LVDT calibration. This topic has been covered MANY times before on control.com, and you can find them using the 'Search' feature of control.com.

I, personally, have NEVER measured the linear stroke of the IGV actuator and tried to equate that to IGV angle. It is really necessary to get in and measure the actual IGV angle using a machinist's protractor (see the website for Starrett or Mitutoyo for details on these), and it will be necessary to have a machinist using a milling machine to remove about half of the width of the ruler of the machinist's protactor to allow it to fit into the IGVs when they are "closed"). It seems you have someone measuring the IGV angle, so it seems they have a protractor or some other precision means of measuring the angle--which is important.

The minimum mechanical stop for the IGVs should be 1-2 DGA (DeGrees Angle) less than the minimum operating angle, which is usually 34 DGA (for the majority of GE-design Frame 6B heavy duty gas turbines). And the maximum mechanical stop should also be 1-2 DGA beyond the maximum operating angle. (The signal names for minimum- and maximum operating angles for a GE Mark*/Speedtronic turbine control system are usually CSKGVMIN and CSKGVMAX, respectively.) You need to know the minimum- and maximum mechanical angles (minimum- and maximum mechanical stops) to be able to tell AutoCalibrate what they are.

When using AutoCalibrate to calibrate (scale) LVDT feedback, when it is started it will put out positive servo current to drive the IGVs (in this thread) to the minimum mechanical stop and when the voltage feedback from the LVDTs has stopped changing for a few seconds it records the voltages. Then it puts out negative servo current to move the IGVs (in this thread) to the maximum mechanical stop and when the LVDT feedback voltage stops changing for a few seconds it records that voltage. Then it puts out maximum positive servo current to move the IGVs back to the minimum mechanical stop to check that it returns to approximately the same voltage feedback, and then it calculates the "offset" and "gain" for scaling the IGV LVDT feedback voltages.

BUT, before you start AutoCalibrate you MUST tell it what the minimum- and maximum IGV angles are because it doesn't know what they are unless it is told because it can't measure the stops by itself. That's why it's important to tell it what the stops are set at. And, as long as the stops are outside the normal range of travel (usually 34 DGA to 84 DGA, or sometimes 34 DGA to 86- or 87 DGA), then everything will be fine--as long as you properly tell AutoCalibrate what the stops are set at. As long as it knows what the stops are set at before executing an AutoCalibration, it will be fine.

And, how you tell AutoCalibrate what the minimum- and maximum mechanical stops are set for depends on the version of Mark*/Speedtronic in use on the turbine. (That's why it would have been of help to know what version you are working with; the Categories available on control.com only show Mark V/Mark VI, and they are not the same, and there is Mark VIe now, which is not the same as the other two versions.

The actuator stops are usually set close to the ranges defined above (1-2 DGA less than minimum operating angle, and 1-2 DGA more than maximum operating angle). And the IGV ring stops are usually set outside (beyond) those. It really depends on the vintage of the machine how the ring stops and actuator stops are set. When I'm referring to mechanical stops, I'm referring to whatever mechanically limits the IGVs at the "closed" and "open" position, and that's what AutoCalibrate needs to know about--what the physical position is when the IGVs are prevented from closing any further by means of the hydraulic actuator and what the physical position is when the IGVs are prevented from opening any further by means of the hydraulic actuator. Whether that's ring stops or actuator stops isn't really critical, as long as the range of travel of the actuator is slightly more than the operating range of travel (so if CSKGVMIN is 34 DGA and CSKGVMAX is 86 DGA, the operating range of travel would be 52 DGA, and the maximum mechanical travel should be approximately 32 DGA to 88 DGA). Usually, the ring stops are set outside (beyond) the actuator stops, so 29-90 DGA would be okay. But, you should use the measured actuator closed- and open readings for AutoCalibrate.

Hope this helps!

1 out of 1 members thought this post was helpful...

Fram6BNewB,

You are calibrating the actuator, stop to stop. The ring stops are just there in the event the actuator stops fail. And, neither the actuator stops nor the ring stops should be at the immediate ends of the desired actuator range of travel (CSKGVMIN to CSKGVMAX). You aren't calibrating the total possible IGV ring travel--only what the actuator can travel.

Again, AutoCalibrate only knows what it's told. And it should be told that the ends of travel are the actuator stops, not the ring stops. The actuator can't travel more than the actuator stops are set for. In other words, the actuator can't travel to the ring stops unless the actuator stop(s) fail. (At least that's the way it should be set up to work.)

By the way, I believe the ring stops are well outside the normal range. They should just be set to something a little larger than the actuator stops are set for. So, for actuator stops set to 32- and 88 DGA for an IGV operating range of 34- to 86 DGA, the ring stops should be set for something slightly less than 32 DGA and slightly more than 88 DGA, say 31- to 87 DGA.

1 out of 1 members thought this post was helpful...

Dear CSA,

Thank you very much for your thorough explanation. It helps A LOT. Sorry that I missed to mention that we are using MarkVie as our control system. According to our control spec, ring stop is to be set at 30DGA for min, 90DGA for max. Actuator stop is set at 32DGA for min, 88DGA for max(which I don't know how the proper procedure should be done) and lastly operating range from MarkVie at 34DGA min and 86DGA max.

Now back to the subject. It is very clear to me that during calibration, I have to tell Autocalibrate what is the minimum/maximum mechanical stop. However I have 2 issues now:

1) How do I ensure that at actuator minimum travel (minimum actuator stop), the IGV angle is at 32DGA as per my control spec?

-this is the reason why I equate the linear actuator travel to IGV angle. When the actuator was installed, I realized that the minimum IGV angle (measured at the IGV blades using a protractor) was at 27DGA. So i needed to make adjustments to the actuator minimum length by a certain mm (metrics) as per the the value i get from equating what is the IGV angle travel per mm (linear actuator travel). Please do share me what is the best practice for this, prior installation.

2) The actuator stops was way beyond what it should (ring stop is not in the picture yet) which is at more than 60DGA span (90-30DGA). This might not be a problem at all, as it is fine to have the actuator stop to be at (30-90DGA). So I could set the min ring stop at 28DGA (30-2DGA) and max ring stop at 92DGA (90+2DGA) as what you mentioned (the ring stop is to protect if the actuator fails). Please tell me this is not a problem (it is not necessary to follow exactly as per control spec which requires it to set at 32-88DGA). Or is it something wrong with the actuator?

Thank you in advance!

Frame6BNewB,

As long as the IGVs can travel between 34- and 86 DGA without hitting a stop (actuator stop or ring stop) it should be okay.

Unfortunately, we can't post pictures or drawings here on control.com, so it's difficult for us to know exactly which actuator you have and how to tell you to adjust the actuator stops. And, even more unfortunately, most (but not all) GE-design Frame 6B heavy duty gas turbine IGV actuators are located in a very difficult place to access, except through a small plate in the bottom of the inlet duct underneath the bellmouth, and even then there is limited space to work.

I don't know if your actuator extends to open or close the IGVs, but let's say that it extends to open the IGVs. That means it retracts to close the IGVs. You set the actuator stop for opening by changing the length of the turnbuckle. It's usually locked in place with either jam nuts at either end of the main adjusting nut, or with a lock tab tack-welded to something stationary to keep the main adjusting nut from turning. To set the minimum mechanical stop there is usually some kind of "horseshoe ring" which bolts around the actuator shaft which limits the retraction stroke. That's how most of the IGV actuators I've worked on are adjusted--but setting the length of the length of the turnbuckle (and being sure to "lock" it into place when finished), and by changing the thickness of the horseshoe ring (grinding it or milling it as necessary).

The IGV actuators do not usually come pre-adjusted; they have to be adjusted during installation. Again, because of the usual location of the actuator it's not an easy thing to do, but that's what millwrights get paid to do sometimes--get into difficult places and work their magic.

You don't want the IGVs to close to less than approximately 32 DGA, because on a trip when the IGV dump solenoid (20TV-1) is de-energized they will close to the stop, and if that stop results in a severely closed angle when the unit is coasting down from rated speed it can cause the IGVs to be sucked into the axial compressor (which is NOT a good thing to happen). At the minimum, the IGVs can be bent and that's not a good thing, either.

You also don't want the IGVs to go too far open (past 90 DGA) on some kind of very unusual failure because that can also cause mechanical damage to the IGVs and/or their bushings as the forces (very GREAT forces!) working on the IGVs when the compressor is at rated speed can damage things or even break them.

The ring stops, well, they should be relatively easy to adjust, as they are usually just long, thick bolts with jam nuts (usually--not always). And they are relatively critical, too, as if, for example, the horseshoe collar came off the actuator rod and the unit tripped and the IGVs went closed to 20-something DGA during coastdown bad things could happen.

Is it absolutely CRITICAL to have the setting exactly equal to the Control Spec values? NO. For most GE-design heavy duty gas turbines, the only really critical IGV angle is the "full open" maximum operating angle because that determines the amount of air flowing into the axial compressor and that affects the maximum power output (when operating at Base Load) of the unit. If the IGV LVDTs are not calibrated properly at the full open maximum operating angle and the Mark VIe thinks they are at 86 DGA when they are actually at 84.5 DGA then the unit will not be producing as much power as it could be. Conversely, if the IGV LVDTs are calibrated such that when they indicate 86 DGA they IGVs are actually at 87.5 DGA then too much air is flowing into the unit and that will result in higher-than-normal firing temperatures which will decrease the parts life of the hot gas path parts (liners; transition pieces; turbine nozzles and -buckets and even the exhaust diffuser).

I wish we could share photo's and drawings, but we can't, unfortunately. You need to look very closely at the mechanical drawings of the IGV actuator because they usually have, in the NOTES section, some information about how to make adjustments. And, because the actuator is usually in a very bad position it's not an easy thing to adjust--but it is usually necessary to make adjustments when installing a new actuator.

As for equating linear stroke to IGV angle, it should be a proportional thing. And, you may find, once the adjustments are properly made that the linear stroke is very close to or even equal to the measurement you expect, but it's very necessary to get at least one end of the actuator stop to be set correctly (I would try to set the minimum actuator stop as that's really important during a trip condition) and then adjust the turnbuckle length to get the proper maximum actuator stop position, and then measure the stroke length.

The stroke length can always be the same, but if the minimum actuator stop isn't physically set to approximately 32 DGA and the maximum actuator stop isn't set to approximately 88 DGA, then the desired angles aren't going to be achieved at the ends of travel.

I think it's important to mention that, in our example, since the turnbuckle length determines the maximum open angle that the actuator is fully extended and then the turnbuckle is adjusted to set the maximum actuator "stop"--which isn't really a stop in the true sense of the word. It's the length of the turnbuckle section when the actuator rod is fully extended--in our example (it may be the opposite at your site!)--that sets the maximum open angle.

The horseshoe collar thickness actually sets the minimum retraction stroke--in our example--and it is more like a true stop in the truest sense of the word/term.

Do the actuator stops have to be set to exactly 32 DGA and 88 DGA? NO. 31.8 DGA or even 32.8 DGA are just fine for the minimum position setting, and 87.1 DGA or even 88.5 DGA will be just fine for the maximum position setting. As long as it's not 34 DGA and 86 DGA it's fine. You don't want the actuator hitting the stops at either end of travel (open or closed).

And, the ring stops should be set just "outside" or "beyond" that. You don't even have to measure the ring stop settings. You should be able to see where the ring stop bolts are with respect to the ring fixture when the IGVs are closed (minimum actuator position) and open (maximum actuator position) and then adjust them so there's a few mm of gap between the end of the stop bolt and the ring fixture. And, that's that. Again, they're just there to prevent excessive over-travel in the event of some failure of the actuator (such as the mounting bolts of the actuator shear off or become excessively loose; etc.).

Finally, with respect to the settings for the IGV actuator and ring stops in the Control Specification, they are guidelines. Just know that the IGVs have to be free to travel just slightly beyond their normal operating range (34 DGA to 86 DGA per your reply) and not hit either stop at either end of travel. That's the key--not to be hitting the stops at either end of travel, without too much excessive travel, particularly at the closed end. 20-anything DGA is too much travel for an actuator stop at the closed end, and 90-anything DGA is too much travel for an actuator stop at the open end. That would be okay in a pinch for the ring stops--but not for the actuator stops.

I don't know if your actuator extends to open the IGVs, or closes to open the IGVs. You just have to use the example here and modify it if necessary to fit your application/actuator configuration.

Hope this helps! Please write back to let us know how you fare in this process. Some people have posted pictures and/or drawings to free web-hosting sites (tinypic.com is one; there are many). I don't want to exchange email addresses because I like to keep all posting available to all the people that read them on this site--they help a LOT of people. And, if I gave out my email address I would be deluged with requests for help and jobs, and I prefer to help LOTS of people via this public forum instead of just one person at a time via email, and I don't have any jobs nor do I hire people (but I do recommend people be fired once in a while).

CSA,

I don't know how I can thank you more to be able to reply me instantly and with very thorough explanation. I believe you have been in this field for so long that you understand every bit of it. In this reply I will quote what you said so it would be much clearer for you and all other readers.

"As long as the IGVs can travel between 34- and 86 DGA without hitting a stop (actuator stop or ring stop) it should be okay."

*- I'm relieved we have the same understanding. FYI, my turbine is currently running with no issues on the IGV. I made sure of this during the calibration process where we performed the stroke at 32,57, and 86 DGA and took 16 points from the IGV blades by measuring using a protractor. Each gave a very accurate reading of approximately +/-.5 DGA error.

"...we can't post pictures or drawings here on control.com, so it's difficult for us to know exactly which actuator you have and how to tell you to adjust the actuator stops. And, even more unfortunately, most (but not all) GE-design Frame 6B heavy duty gas turbine IGV actuators are located in a very difficult place to access, except through a small plate in the bottom of the inlet duct underneath the bellmouth, and even then there is limited space to work."

*- The actuator that we are using is as per details below:
Y&F Part Number: 7645C004 Series
GE Part Number: 187C8362 Series

I believe you can find this easily from Google. You could also have access to the drawing from the Y&F pdf file which is available for download. If I am not mistaken, the stroke (travel length) of the actuator can't be adjusted and that is 38.1mm or 1.5inch. The adjustment that we can do is just for the zeroing (minimum actuator length/at fully retract position) and any adjustment made will also affect the span (maximum actuator length/at fully extend position).

"I don't know if your actuator extends to open or close the IGVs..."
it extends to open.

"You set the actuator stop for opening by changing the length of the turnbuckle. It's usually locked in place with either jam nuts at either end of the main adjusting nut, or with a lock tab tack-welded to something stationary to keep the main adjusting nut from turning"

*- yes we did set the actuator stop by making adjustments to the turnbuckle (actuator stem) after the locking nut has been loosened. I believe this actuator has the lock tab you mentioned. (we call it the lock plate where we fold it to lock the nut and reinforce with tack weld to make sure it locks. However we believe the tack weld is unnecessary as long as the lock plate is neatly secured.)

"Again, because of the usual location of the actuator it's not an easy thing to do, but that's what millwrights get paid to do sometimes--get into difficult places and work their magic."

*- again you are definitely right. To make adjustments after the actuator has been installed requires a modified tool and a very skillful millwright. Might as well we remove the actuator, make adjustments and reinstall which we did. Whatever it takes as long as the results turn out to be the same.

"You don't want the IGVs to close to less than approximately 32 DGA, because on a trip when the IGV dump solenoid (20TV-1) is de-energized they will close to the stop, and if that stop results in a severely closed angle when the unit is coasting down from rated speed it can cause the IGVs to be sucked into the axial compressor (which is NOT a good thing to happen). At the minimum, the IGVs can be bent and that's not a good thing, either.
You also don't want the IGVs to go too far open (past 90 DGA) on some kind of very unusual failure because that can also cause mechanical damage to the IGVs and/or their bushings as the forces (very GREAT forces!) working on the IGVs when the compressor is at rated speed can damage things or even break them."

*- For this reason, we decided to put the Ring stop at 30DGA and 90 DGA.

"Is it absolutely CRITICAL to have the setting exactly equal to the Control Spec values? NO. For most GE-design heavy duty gas turbines, the only really critical IGV angle is the "full open" maximum operating angle because that determines the amount of air flowing into the axial compressor and that affects the maximum power output (when operating at Base Load) of the unit. If the IGV LVDTs are not calibrated properly at the full open maximum operating angle and the Mark VIe thinks they are at 86 DGA when they are actually at 84.5 DGA then the unit will not be producing as much power as it could be. Conversely, if the IGV LVDTs are calibrated such that when they indicate 86 DGA they IGVs are actually at 87.5 DGA then too much air is flowing into the unit and that will result in higher-than-normal firing temperatures which will decrease the parts life of the hot gas path parts (liners; transition pieces; turbine nozzles and -buckets and even the exhaust diffuser)."


"As for equating linear stroke to IGV angle, it should be a proportional thing. And, you may find, once the adjustments are properly made that the linear stroke is very close to or even equal to the measurement you expect, but it's very necessary to get at least one end of the actuator stop to be set correctly (I would try to set the minimum actuator stop as that's really important during a trip condition) and then adjust the turnbuckle length to get the proper maximum actuator stop position, and then measure the stroke length."

*- We did set the actuator stop correctly(the minimum actuator stop) but by doing this, I can never achieve the maximum actuator stop position as per what I mentioned above. ie: the stroke length (travel length CANNOT be adjusted).

"Do the actuator stops have to be set to exactly 32 DGA and 88 DGA? NO. 31.8 DGA or even 32.8 DGA are just fine for the minimum position setting, and 87.1 DGA or even 88.5 DGA will be just fine for the maximum position setting. As long as it's not 34 DGA and 86 DGA it's fine. You don't want the actuator hitting the stops at either end of travel (open or closed)."

*- I am sure this is true as if this is not met than the actuator might have a shorter life span. Even the servo might have a shorter life span as well. Am I right?

"Finally, with respect to the settings for the IGV actuator and ring stops in the Control Specification, they are guidelines. Just know that the IGVs have to be free to travel just slightly beyond their normal operating range (34 DGA to 86 DGA per your reply) and not hit either stop at either end of travel. That's the key--not to be hitting the stops at either end of travel, without too much excessive travel, particularly at the closed end. 20-anything DGA is too much travel for an actuator stop at the closed end, and 90-anything DGA is too much travel for an actuator stop at the open end. That would be okay in a pinch for the ring stops--but not for the actuator stops."

*- It is very clear to me now for how it should and should not work. May I know what would you do if you have an actuator which travels from 32DGA at minimum stop when given minimum travel command (0DGA manual command from MarkVIe)and 93DGA at maximum stop when given maximum travel command (100DGA manual command from MarkVIe)? How would you perform Autocalibrate (what are the values that you would tell to the Autocalibrate in MarkVie)? And where would you set your Ring Stop to be?

"Hope this helps! Please write back to let us know how you fare in this process. Some people have posted pictures and/or drawings to free web-hosting sites (tinypic.com is one; there are many). I don't want to exchange email addresses because I like to keep all posting available to all the people that read them on this site--they help a LOT of people. And, if I gave out my email address I would be deluged with requests for help and jobs, and I prefer to help LOTS of people via this public forum instead of just one person at a time via email, and I don't have any jobs nor do I hire people (but I do recommend people be fired once in a while)."

*- I understand your concerns and I myself would prefer it to be this way as well so there are many lessons to be learned for all. FYI, I have a total of 9 similar GE Frame 6B Gas Turbines and I believe I will be replacing a new actuator sooner or later. I may have more questions regarding the servo but I might be able to find the answers from other threads so leave that for now.

Thank you again in advance.