The general rule is no more than about 1-1.5 degrees of variation is acceptable.
The problem usually lies in the measurement method and the patience of the person/people taking the measurements. There is some hysteresis ("slop") in the rack and pinion gear teeth which causes any and every blade to move about 1-2 DGA just by virtue of grabbing the blade witn one's hand/fingers and rocking/wiggling it (under normal circumstances (good bearings (not worn or damaged); normal wear on the gear teeth). This is normal and to be expected. And it must be eliminated to the extent possible when measuring IGV angles manually with a machinist's protractor or some other means or else the readings will be skewed and the IGV mechanism may appear to be excessively worn when it really isn't.
So to negate this from the measurement at any position, one needs to always hold the blade being measured in the same manner. Air moving through the IGVs (of MOST machines--but not ALL machines!) tries to open the IGVs (blades). I, therefore, chose to hold the blade of the IGV in the position as if air were trying to open it--for every blade I measured. This effectively removed any normal hysteresis ("slop").
Measuring IGV angles is NOT for people who don't have patience and who aren't given the time to take the measurements properly. Many people are--understandably--nervous about taking IGV angle measurements because they are worried about them suddenly closing (by the Mark*). AND, since most people don't take IGV angle measurements on any kind of regular basis they are also unfamiliar with the necessary requirements to negate/mitigate the normal hysteresis ("slop") in the gear teeth. And, so they rush and fumble to get the measurements, and then reading the protractor is also difficult, as it's usually not very well lit inside the inlet/bellmouth area. (It should be noted that with most machinist's protractors made by Starrett and Mitutoyo and other machine tool manufacturers, the width of the "pointer" of the machinist's protractor is one-quarter of the distance between each degree of the scale, allowing the ability to read the angle to one-quarter of a degree (0.25 degrees). Again, with poor lighting and a time crunch and unfamiliarity the readings can be very jumbled and make it appear as if the gear teeth/rack and pinion mechanism is excessively worn.
There are ways to mitigate the dangers of the IGVs moving unexpectedly and possibly causing finger damage or IGV damage because the protractor is still in the space between two blades. One of them is to move the IGVs to some position (fully closed; mid-stroke; fully open; almost fully open) with the Mark* and then shutting down the Aux. Hyd. Pump, and the L.O. pump (with the Mark* still calling for some position/angle), and then going to the hydraulic accumulator and using the bleed (drain) valve to relieve the hydraulic system pressure and leaving the bleed (drain) valve open while the measurements are being taken. This is the safest method in my opinion. The problem with this method is that the bleed (drain) valve (AND the block (supply) valve) are almost NEVER properly identified--AND they are usually physically the same size and located next to each other. There's an easy way to solve this problem once the Aux. Hyd. Pump is shut down, and that's to begin opening one of the two valves whilst watching hydraulic system pressure. If the pressure drops as the valve is opened that is the bleed (drain) valve--AND IT SHOULD BE APPROPRIATELY MARKED AS THE BLEED (DRAIN) VALVE!!! If the chosen valve is already open and can't be opened any further but can be closed a turn or two that is the block (supply) valve and it should remain open--AND IT SHOULD BE APPROPRIATELY MARKED AS THE BLOCK (SUPPLY) VALVE!!! (If both valves are open or closed, then the hydraulic accumulator HAS NOT BEEN properly put in service, and it's likely that the hydraulic system pressure will drop very quickly when the Aux. Hyd. Pump is shut down.)
This requires two or three people, sometimes more, because it's usually very difficult to see the hydraulic system pressure from the location where the hydraulic accumulator block and bleed valves are located, and it's usually a VERY good idea to have two people measuring IGV angles in the inlet/bellmouth (one holding the light and providing support and using a radio to relay angle measurements) to someone sitting in front of the HMI and stroking the IGVs and recording the measurements.
Time and familiarity and experience are the best attributes for someone taking IGV angle measurements. And knowledge of how the equipment works and how air flow affects the IGV actuator/positioning system. (In most cases, the Mark* and the IGV actuator/positioning system are trying to keep the IGVs from fully opening when the unit is running; most people think they are trying to open the IGVs against air flow, but they're usually not. (There are some IGV designs which have a slightly different angle to them which causes air to try to close them, but those are usually only found on very large GE-design heavy duty gas turbines. But they STILL have the normal hysteresis ("slop") of approximately 1-2 degrees which must be negated/mitigated when taking measurements to obtain consistent readings.)
And, it must always be remembered that the only IGV angle which is really important is the one at the maximum operating angle--because that is where the IGVs will be when making rated, optimal power. Any other position/angle is not as critical because in those positions the IGVs are NOT being used to control air flow, they are being used to control exhaust temperature at Part Load and will be moved to whatever angle is required to achieve the desired exhaust temperature.
