All this while that i've been working with instruments on frame vi GE gas turbine system, I notice that IGV angle start from 33degree and not zero degree and end at 86 degree and not 100degree. Please I need detail information about this from the whole house.
Please take time to read and re-read (and re-re-read) this response if necessary, because it's not intuitive and it doesn't make many people happy when it's first presented. It WILL begin to make sense if you consider it, and re-consider it, and mull it over in your mind.
The positions (not the angular measurements--the positions) of the IGVs of GE-design heavy duty gas turbines are a function of the air flow through the machine at various operating conditions (starting and acceleration; rated speed, zero load; part load operation; full ("Base") load operation; shutdown).
The designers could very easily have chosen 0- and 100 for the minimum and maximum positions--but the choice of engineering units (degrees; percent; etc.) was foremost in their minds as designers--and as practical people (yes; practicality did play a part in many decisions, though it's not always evident or clear). The designers chose degrees angle, based on the everyday measurement of angles--where 360 degrees is a full circle, and 90 degrees is a quarter circle.
By doing so, it enabled mechanical workmen (factory workers and field workers) and instrument technicians to measure the IGVs using a standard machinists' protractor, something which is (usually) readily available in any self-respecting machinist's or mechanics tool container. And, it just so happened that the minimum operating angle for GE-design B/E-class heavy duty gas turbines occurs at around 34 degrees angle, as measured with a machinists' protractor, and the maximum operating angle, as measured with a machinists' protractor, occurs at around 86 degrees.
Now, they could have chosen 0- and 100 "degrees", or 0- and 100 "percent"--but then there would have had to be a special tool that would have to be used to measure against some specified and unmovable reference point to use to confirm 0- and 100 "degrees", or 0- and 100 "percent"--which would have increased the cost of the machine to produce and supply, and which would have made measuring and setting angles impossible without. And would have increased the complexity of the operation, also.
Instead, the designers chose to use a readily available tool which could be obtained from any machine shop supply store, and to use readily available reference points for that tool (the adjacent IGV blades).
It's no more complicated or simple than that. The minimum air flow during starting, acceleration (and shutdown) occurs when the IGVs, as measured with a machinists' protractor against adjacent IGVs are at an angle of approximately 34 degrees. If you want to think of that as "zero" that's entirely your prerogative. And the minimum air flow at rated speed with zero load occurs at approximately 57 degrees when measured with a machinists' protractor against adjacent IGVs, and the maximum air flow at rated speed and full load occurs at approximately 86 degrees when measured with a machinists' protractor against adjacent IGVs. And, if you want to consider that as "100" that also is entirely your prerogative.
The designers could have chosen 0- and 100 "degrees" or "percent"--but then some special tool, or a conversion chart could probably have been made to convert degrees as measured with a machinists' protractor, would have to be used to measure (and convert) IGV positions. Instead, they used a readily available tool with readily available reference points and standard measurement units: degrees.
If you, as a technician, had to use a special tool to confirm or measure IGV positions or use a machinists' protractor and a conversion chart every time there was a question about IGV positions/LVDT calibrations, that would probably greatly increase the time required as well as the complexity--and you would likely have to be continually explaining the measurement and/or conversion every time there was a question. AND, we all know how well GE documents things. NOT very well, as it turns out.
So, given the above description and possibilities for describing IGV positions and angles, what would you choose? (I know what I would choose!)
Yes; the designers did provide a crude pointer/indicator on the side of axial compressor casing for IGV angle measurement--but they also recognized that that would have to be calibrated and verified in some manner and that mechanics and pipe fitters and electricians would ALL use the indicator as a foot-hold when crawling around the unit and would ONLY be as good as the day-to-day "care" it received. Using a readily available machinists' protractor to check actual IGV positions (angles) against adjacent IGVs or even to verify or (something I've RARELY seen done!) adjust the pointer/indicator on the side of the axial compressor casing is a simple, and elegant, choice given the alternatives.
AND, given that GE uses percent speed (TNH) to describe shaft speed--and EVERYONE (especially at first!!!) complains loudly and to everyone else within earshot that GE should have used RPM instead, using standard degrees for IGV angle measurement and description is just plain simple (and elegant).
Hope this helps! If it's not immediately clear on the first read, re-read (and re-re-read) the above; it will make perfect sense after some time has passed.
Now, for the answer to the NEXT questions....
Why didn't the designers make the minimum angle 0 degrees and the maximum angle 100 degrees, and let the turbine control system control to whatever positions (34, 57, 84 or 86) were required?
Well, they didn't want the IGVs to close below approximately 33 DGA, because if the IGVs ever went to less than 34 DGA when the axial compressor was at rated speed, well, it's possible the IGVs would be sucked into the axial compressor, or (has happened on occasion) a sudden closure of the IGVs (to even 33 DGA) while at rated speed could cause a collapse of the combustion liners as well as damage to the axial compressor (surge/stall).
And, same above 84 or 86 DGA--too much air flow could also damage the axial compressor, especially when ambient temperatures are below design. And, excessively open IGVs could lead to excessive over-firing of the unit when operating at Base Load, which decreases hot gas path parts life.
SOOO, the designers chose to mechanically limit the IGV travel at both ends of travel (closed and open) to protect the IGVs, axial compressor and combustors and hot gas path parts life.
Finally, how does one measure 0 DGA with a machinists' protractor? (NOT VERY EASILY!) So, the chosen minimum- and maximum mechanical stops also serve another purpose--to make it easy to measure using a machinists' protractor and adjacent IGV blades.