IGV Angle andd AHOP Problem


Thread Starter


we have Fr. 9E GE machine in which Aux. Hydraulic oil pump does not pick-up in auto as IGV is below minimum opening angle (31 deg) when machine is in stand-still condition.

Is it advisable to provide mechanical stopper to IGV Link (by shimming the gap) to restrick it at 33 deg instead of 31 deg as minimum position when AHOP is not operating ?

If done, what are the long run after effects ?

Is their any alternative solution to the problem ?
The probably reason the Aux. Hyd. Pump won't start is because the IGV angle is "tripping" the unit. It's not "normal" for Aux. Hyd. Pump operation to be "controlled" by IGV angle, so it's more likely that an IGV trip is causing the problem.

But, you haven't told us when the problem began, or how long it has been a problem.

The reason the IGV angle is tripping the unit is probably because a recent maintenance outage was performed and the IGV LVDTs were or were not calibrated, or were not properly calibrated. It could also be that after a maintenance outage that the mechanical stop for the IGV actuator or the IGV rack was not properly installed.

Have you physically measured the IGVs to determine the actual position they are in?

I don't recall if the IGV actuator rises to open or rises to close. If it rises to open, then there is supposed to be a "shim" (called a "horseshoe shim" because it sort of resembles a horeshoe in shape) that acts as the stop for the actuator in the down (retracted) position. It may also be that the shim is worn, damaged, or needs to be changed. But, again, I don't recall if the actuator rises to open or rises to close. You need to look at the IGV actuator drawing in the Service Manual and at the IGV actuator itself to be sure.

Adjusting the IGV actuator linkage turnbuckle is another method of adjusting travel.

If they are physically at 31 DGA and it's not possible to fix the cause (broken or incorrect mechanical stop; incorrectly adjusted IGV actuator linkage; etc.), then you could change the Control Constant that defines the minimum position (which is really just sensing for an LVDT failure). You will need to look at the sequencing or application code for the unit at your site (work "backwards" from the Alarm Drop Number through the CSP or Toolbox file--you never told us if you have a Mark V or a Mark VI) to find the Control Constant that defines the limit.

This is only recommended if:

1) you have confirmed the IGVs are indeed at 31 DGA (not just the LVDT feedback that's indicating 31 DGA)--which would indicate that either the mechanical stop is misadjusted or broken, or, more likely that the IGV actuator turnbuckly linkage is misadjusted)

2) it's impractical to shut the unit down at this time to fix the cause (misadjusted or broken stop; misadjusted IGV turnbuckle linkage).

There would be no long-term effects of operating with the IGVs at 31 DGA, as that's just the angle the IGVs go to when 20TV-1 is not energized and the unit is not running at that time. When 20TV-1 is energized during starting (and there is Hyd. Pressure!) then the IGVs should go to the minimum operating angle, which is usually 34 DGA, but can vary from site to site. So, when the unit is cranking or starting or running or even shutting down, the IGVs should go to an angle of 34 DGA or greater, depending on the speed and mode and ambient conditions.


We have MARK VI and the start permissive is such that IGV should be 32 DGA for AHOP start-up (why such logic?). The LVDT was calibrated in shutdown recently with actual blade angle and after that only this problem started. We have Actuator rise to IGV close, but the air gap with 34 DGA is appox. 2.8mm between the stopper, and when AHOP is stopped IGV closes to 31 DGA and rest at mechanical stopper.

To increase the angle by 31 DGA to 32-33 DGA we have to shim the stopper by 3.0mm (32.5 DGA) for AHOP startup without any logic forcing.

But why such logic for start permissive, kindly share.
LK3IGVFLT1 is described as the Control Constant which sets the limit for:


The logic is trying to check to make sure the IGV LVDTs are working before allowing a unit START.

When the IGV LVDT feedback is less than LK3IGVFLT1 for more than LK3IGVF4 time then L3IGVF1 goes to a logic "1". When LK3IGVF1 is a logic "1" then L3IGVFLT goes to a logic "1", which will prevent L3STCK0 from going to a logic "1".

L3STCK0 is one of the start-check permissives, which will prevent a START command from being initiated, which will prevent ultimately prevent the Aux. Hyd. Pump from starting, but, the IGV position isn't just controlling the Aux. Hyd. Pump; it's preventing a unit start, which would prevent the Aux. Hyd. Pump from picking up.
IGV going below a preset value (LK3IGVF1)or over a value (LK3IGVF2) results in L3IGVFLT. L3 IGVFLT is derived from L3IGVF1 & L3IGVF2 which are for loss of feedback and for vanes open. The normal values for the above constants LK3IGVF1 & LK3IGVF2 are 32 and 35 dga and the machine permissive to start (L3STCK0) will include L3IGVFLT. Since the IGV angle is below 32 dga, it is not gettiing permissive to start.

One thing can be done for start up is to force the logic L3IGVFLT to zero so that startup can be done.
Forcing is always the "answer" to everything, isn't it?

Why not fix the issue, instead of having to force the logic?

Forcing is for emergency situations, maintenance or troubleshooting, but is <b>ab</b>used by many people as part of normal operations, which contributes to lots of operational problems.

Why not educate people about the issue and what can be done to solve the root cause instead of advising to use forcing?

Give a man a fish, feed him for a day.

Teach a man to fish, feed him for life.

Forcing gets more people into more trouble than just about any other feature.
I agree with CSA on this one. Specially in this situation.

Normally, you don't want to have the AHOP running unless you are in start-up sequence. Stand-still has no need for it. If you force that signal related to the LVDT health, then what? You start the AHOP, start the unit, and find out you just damaged something because the IGV was not opening properly.

Forcing was always one of my big issues when related to the customers/operators. I am very happy when, after I am forced to force some signals at the end they force me to show them no forcings are still active :) (couldn't help it). And I am always very concerned when I hear operators or maintenance crew speaking too easily about forcing signals.

On the situation at hand, I would recommend checking again the calibration settings, measuring again the real angle to confirm the LVDT calibration was right (take at least 8 measurements in 8 different points). If everything looks good, you should only then consider mechanically readjusting the IGV stopper.
Hi all,

We have just experienced a very similar issue (angle < start inhibit point), which came about post replacement of some IGV segments during a planned shutdown.

All of the points raised on this forum were considered, however, for us, an adjustment of the turnbuckle and a subsequent calibration was required and performed.

I would strongly recommend going after the actual issue opposed forcing logic points. Logic forcing must only be done as a last resort. We have experienced too many occasions when persons have either left forces active during run up or have forced to the wrong condition and have actually tripped a unit.

Further to this, we have been considering for a while now of making L30FORCED a pre start check because of the issues we have experienced!

Teaching a man to fish - is always a great thing but how about fishing in troubled waters........not that practical.

I fully agree to views that rather than being circumspect, it is always advisable to go to the root cause of the problem and solving the problem once for all but when you have reasonably low time at at hand a you need to go for a start-up, you have to opt for forcing. Moreover in this particular case, it is not at all perilous since it is tested and proven at one of the known places.
Troubled waters present a challenge to any fisherman. If those are the only waters available, then one needs to understand how to fish in them, or perish. Unnecessary risks can also lead to fatal consequences of one sort or another.

Forcing is for maintenance activities, troubleshooting and emergencies. This author seemed to be asking what the reasoning was behind the logic in an attempt to understand what the options might be; hopefully one of the options was not logic forcing, but we'll likely never know.
It is okay to disagree without being disagreeable - anonymous. I think the matter is being dragged too far. I understand that the forum is open to express individual views and it would be better to stay technically focussed so that we gain something out of it.