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IGV Operational Principle
Question on how IGV's work and the principle of operation that makes them work.

This question is not intended to solve an issue but in the hopes of expanding my general understanding. I am an aux. operator working towards Control Room Operator at a simple cycle GE7fa plant and was wondering how the VIGV system works? I'm not interested in the mechanics as that is fairly well understood, but rather in what the operating principle is that makes it function. For example in relation to exhaust temp. control do the IGV's adjust for increased/decreased exhaust/turbine cooling or does it affect air flow or air pressure used in combustion and therefore combustion temp. or is there some other concept that makes VIGV's work. If it helps I work at a 7fa plant with dln 2.6 combustors on gas only.

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The answer to your simple question is not so simple. So, please, be patient. I've tried to provide a little history (which I hope a knowledgeable contributor, otised, will correct, if necessary) to understand how the technology got to the point it's at today. It would help if it were possible to post a picture of a GE-design combustor and fuel nozzle, but it's not (possible). Just realize that the air flowing into each combustor cannot be controlled individually; the air flow is a function of various fixed openings: slots, holes and orifices in the fuel nozzle(s), combustion liners, and flow sleeves (if the turbine uses them). So, while the fuel flowing into the combustors can be varied over a wide range, the air flow into each combustor cannot--except by controlling the total amount of air entering the inlet of the gas turbine's axial compressor through the Inlet Guide Vanes, or IGVs.

Essentially, the IGVs of a GE-design F-class heavy duty gas turbine with DLN combustors are being used to control air flow into the turbine, while attempting to minimize emissions by keeping flame temperature low, keeping the air-fuel mixture in a stable combustion range (without knowing what the air-fuel mixture actually is!), all while protecting the gas turbine, axial compressor and exhaust.

The IGVs on an F-class turbine really have no operator-selectable or -controllable capability; the Speedtronic does everything. For F-class gas turbines with DLN combustors, IGV control is complicated and important--with no operator control capability.

Originally IGVs were either "closed" or "open." They were two-position, and sometimes affectionately referred to as "bang-bang" IGVs--banged up against the full closed stop or the full open stop. There was no "modulating" or control of the opening. Limit switches indicated full closed or full open. I believe one of the original purposes was to help protect against axial compressor surge/stall during start-up and shutdown (and to reduce the torque required to accelerate the turbine shaft). Pretty simple.

VIGVs (Variable Inlet Guide Vanes), or modulated IGVs, which are commonly referred to as just "IGVs", were a way to increase the operating range of GE-design heavy duty gas turbines by controlling the total air flow into all the combustors to prevent from leaning out the air-fuel mixture too much during low load operation. They are, essentially, like the butterfly throttle valves of carburetors on reciprocating engines, controlling the air flow into the engine but not to each individual cylinder.

Variable IGVs are modulated (opened and closed) as load is increased and decreased. The control scheme for this early type of Variable IGV control was relatively simple. The IGVs were opened from the closed position (approximately 34 DGA, DeGrees Angle) to the Minimum Modulating Position (approximately 57 DGA) as the unit was accelerating, and then when the unit was loaded and the exhaust temperature reached a certain value (700 deg F or 900 deg F; I can't recall exactly which) as fuel/load increased the exhaust temperature would tend to increase. But, the IGVs were opened to maintain the temperature as fuel was increased, until they were fully open (usually approximately 84 DGA), and then as fuel increased the exhaust temperature would also increase.

However, it was soon found that the exhaust temperature of the gas turbine could be increased at part load operation by further reducing the air flow into the gas turbine so that when the gas turbine is exhausting into an HRSG steam temperature can be increased (when the turbine is operating at part load and exhaust temperature would otherwise be low). This was termed 'IGV Exhaust Temperature Control,' or 'Combined Cycle Control.' It was usually an operator selectable mode of control, because it's not always necessary to maximize exhaust temperature, and some gas turbines even had diverter dampers which can direct exhaust gas flow to an HRSG or to atmosphere--and it would be wasteful to maximize exhaust heat when exhausting directly to atmosphere (also called Simple Cycle operation, when IGV Exhaust Temperature Control is selected OFF).

Now, enter GE-design heavy duty gas turbines with DLN (Dry Low NOx) combustors. DLN combustors employ an extremely lean air-fuel mixture to reduce internal hot gas temperatures thereby reducing the formation of NOx. The air-fuel mixture is so lean it literally borders on the stability of combustion, so control of air into the combustor is very important. To run at lower loads with DLN combustors and maintain low emissions it is necessary to reduce the air flow as fuel is reduced and that's one of the primary functions of the IGVs on a GE-design heavy duty gas turbine with DLN combustors: controlling air flow to be able to operate at lower loads while maintaining low emissions.

While it has been a great surprise to many combined cycle power plant managers and operations supervisors (and some technicians), the running air-fuel mixture of most gas turbines is not monitored nor controlled in real time (or at any time!!!) as the turbine is operating. (That's right--some automobiles with computer-controlled fuel injection are more sophisticated than GE-design heavy duty gas turbines--which just completely infuriates some combined cycle power plant managers when they are having trouble making emissions, and they see the Speedtronic as this extremely complicated and all-powerful computer-based control system which can't be counted on to automatically adjust all turbine operating parameters to reduce emissions a half ppm.) All of the parameters affecting the air-fuel mixture are set by the configurations of the hot gas path parts and the fuel nozzle orifices and control valves. All of this is taken into account when selecting the various components and setting control parameters (Control Constants).

So, controlling air flow in DLN combustors is very important to maintaining flame stability and keeping emissions at a minimum. The IGVs are kept closed during loading while fuel flow is lower (than at higher loads) to maintain flame stability and not enrich the air-fuel mixture too much to keep emissions low. This also increases exhaust temperature (just as when IGV Exhaust Temperature Control is ON). Exhaust temperature is really the defining limit for protecting gas turbine hot gas path components, so as fuel is increased and exhaust temperature would tend to increase if the IGVs remained closed, they are modulated open to prevent the exhaust temperature from exceeding the exhaust temperature control reference (limit).

Also, as fuel is increased it's also necessary to increase the air flow to prevent leaning-out the air fuel mixture too much and tripping the turbine on loss of flame.

It's all a balancing act, but unfortunately the programming and parameters for controlling air to minimize emissions are all kind of unknown, since GE considers all of this DLN stuff proprietary--and they are EXTREMELY protective of their intellectual property and control schemes these days. Basically, GE knows what the air flow through machine is when the IGVs are openings that correspond to certain exhaust temperatures and this is all factored into the control scheme and component selection. Again, it's all considered proprietary and very little is known publicly--but, amazingly enough, when everything is as it should be it works incredibly well. (It's when people put parts into the machines with poor quality control of dimensions, and IGV LVDT calibrations are done willy-nilly without actually measuring IGV angle versus LVDT feedback, that problems--incredibly frustrating problems--begin.)

There are also other factors and control schemes at work in the overall IGV control scheme, including protecting the compressor from surge/stall conditions at certain operating points. The IGV control reference, signal name CSRGV, is actually the result of a minimum select function in the Speedtronic control panel, which chooses the least of several references in order to protect the turbine while minimizing emissions.

This is one reason why IGV LVDT calibration is so important, especially on turbines with DLN combustors--because the "hidden" control parameters and programming rely so much on the estimations of air flow at certain IGV angles. Truly, small errors in IGV calibration can cause emissions limits to be exceeded by small, but costly, margins.

Hope this helps. It was a good question, and difficult to answer. Sorry for the length of the answer. Please write back with any questions or for any clarification.


Thank you very much for your response and for the historical background. That really helps to "build" the concept. I have been visiting this forum almost daily for a few months now and although my local training has been very good at explaining the specific tasks and functions related to my job, this forum and others have been invaluable in creating a broader understanding of the machinery and concepts with which I am working.

With that said, I would like to take this opportunity to thank you and all of the other regular contributors to this forum.

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You are most welcome for any help I may have provided. I hope you have been using the 'Search' feature of to look up various topics in your visits to the site. There is more than a decades'-worth of information here, and the best threads are those with feedback from the people asking questions, letting others know if the information was helpful or not.

I think now that you have the history and some of the other nuances of IGVs, you need to know that during normal operation of a GE-design F-class heavy duty gas turbine with DLN combustors the IGVs are positioned pretty much based on what the exhaust temperature is. The IGVs are usually held "closed" (at the minimum modulating position) until the exhaust temperature reaches somewhere near the CPR-biased exhaust temperature control reference (TTRX) and then as fuel is increased and the exhaust temperature would tend to increase the IGVs are opened to maintain the CPR-biased exhaust temperature control reference. At about the time the IGVs are fully opened (84 or 86 DGA, sometimes more depending on the configuration of the turbine and axial compressor), you will see that the unit is at or very near Base Load.

Again, the air flows through the machine are all known by GE for the various exhaust temperatures and IGV angles and this information along with the fuel nozzle sizing and combustion liner openings and flow sleeve dimensions and fuel control valve positions are all used to achieve a desired emissions value. All of that is "hidden" to the operator (and technician), and there is really no operator control of any IGV function or position.

So, for your unit, under normal loading and unloading, it's safe to say the IGVs are opened and closed based on exhaust temperature.

To be a good operator, one really needs to understand the equipment being operated in order to properly respond to unusual situations. It seems you are off to a good start.

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The following statement is incorrect:

Also, as fuel is increased it's also necessary to increase the air flow to prevent leaning-out the air fuel mixture too much and tripping the turbine on loss of flame.

It should have read:

Also, as fuel is increased it's necessary to increase the air flow to prevent over-enriching the air-fuel mixture too much and increasing emissions.

Sorry for any confusion this may have caused.

Very well said as usual from Mr. CSA. I hit like button.

Take care

By ifada ejiro on 12 April, 2017 - 12:00 pm

Thanks a lot you just saved a soul. But i did not get the moog servo valve part of it. Because in my plant, GE frame 9 GT, the IGV system has limit switch, LVDT, moog servo valve and an actuator.

Please kindly talk about the moog servo valve.

irfada ejiro,

The subject of servo-valves as used on GE-design heavy duty gas turbines has been covered many times on Fortunately, past threads are archived, searchable and accessible using the 'Search' function of

If you can be more specific, we can probably provide more information, but it is strongly recommended that you read a few of the past threads (some more than 12 years old!) to develop your understanding of and form your questions.

In general, IGV limit switches have been phased out of use on ge-design heavy duty gas turbines for many years, so that suggests your unit is older. Servo-valves are used to control the flow of high-pressure hydraulic fluid to the IGV actuator to open/close them based on an electrical signal from the turbine control system. The LVDT(s) provide position feedback to the turbine control system, sometimes just for indication (on older machines), and sometimes for position feedback for a position reference.

Without knowing much more about the age of the unit at your site and the turbine control system in use on the unit it's very difficult to say much more. While GE-design Frame 9E heavy duty gas turbines have been manufactured and sold for decades, all GE-design Frame 9E heavy duty gas turbines are NOT alike. The auxiliaries and control systems have changed a lot over the product life.

Search and read and learn--and ask clarification questions here. We like to help, but the more information about the machine(s) at your site you can provide the better answer and information we can provide. The Operation and Service Manuals provided with the units are also an excellent source of information. And if you're trying to learn and understand the unit(s) at your site the very best information you can get are the P&IDs in the Manuals. Make large size copies for yourself, and make notes on them as you learn and increase your understanding and knowledge.

Hope this helps! There is a great deal of information available on; it's a great resource.


How does the combustor pressure drop relate to the following two parameters mathematically:

1. IGV angle

2. Load on the Gas turbine. Suppose my turbine is working on part load.


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Sorry, I have no mathematical formula(s) for such calculations.

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

Mrinal... the following patent may be what you seek:

Phil Corso

By Chiranjeevi on 13 June, 2017 - 8:08 am


What is the variable name which will be minimum value that will go to CSRGV OUT, when GT is running with part load?


It could be one of several, depending on which type of combustion system, if the unit is operating in Simple Cycle or Combined Cycle mode, etc.

Work "backwards" from CSRGVOUT to find the MIN VALUE selector function in the IGV control scheme--it shouldn't be very "far" from the CSRGV/CSRGVOUT signal. And, you will need to monitor all the inputs, and the output should be the minimum value of all of the inputs.

Please write back with more details about the unit you are working on, AND what you find.