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IGV Modulation
IGV modulation as a function of CTIM

Please, is IGV Modulation ever a function of CTIM (Compressor inlet temperature)?
Detailed explanation as usual will be highly appreciated.


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>Please, is IGV Modulation ever a function of CTIM
>(Compressor inlet temperature)?

For a GE-design Frame 6B heavy duty gas turbine--not usually. There are some machines which can modulate IGVs based on very low ambient temperatures (which will make CTIM be very low), but usually when that happens there are alarms (at least one), something to the effect of 'Exit Guide Vane Protection Active' (if I recall correctly).

For most non-F-class GE-design heavy duty gas turbines, IGV modulation is either a function of exhaust temperature or exhaust temperature. That sounded redundant, but, that's the way it works. When in "simply cycle" mode (not exhausting into a waster heat recovery steam generator (boiler)) the IGVs would be held closed until the exhaust temperature reached something like 900 deg F (I don't recall the exact value; it could have been 700 deg F--it's been a LONG time). And, then as load was increased and exhaust temperature started to increase, the IGVs were modulated open to hold the setpoint. Once the IGVs reached maximum operating angle (CSKGVMAX) as the unit was loaded they could not be opened any further and the exhaust temperature continued to rise until it hit CPD-biased exhaust temperature control reference/limit ("Base Load").

In combined cycle mode where it's usually desired to maximize exhaust temperature at part load (less than Base Load) in order to maximize steam production the IGVs were just held closed until the exhaust temperature reached very close to CPD-biased exhaust temperature control reference (TTRX). At that point if load were increased exhaust temperature (TTXM) would tend to increase above TTRX--but the IGVs would be modulated open to keep TTXM equal to TTRX. And, once the IGVs were at maximum operating angle then the unit would be at or very near Base Load.

There are some unusual applications where some unique things were done with the IGVs to control exhaust air flow-rate, and other such odd things. And, as mentioned, there were some units which required protection for the axial compressor exit guide vanes at low ambient temperatures where the IGVs would be automatically modulated to reduce air flow-rate to protect the exit guide vanes--but there was usually an alarm to let the operator know why the power, and IGV angle, was being limited.

CSRGV, the Control Stroke Reference-Guide Vane, is the IGV position reference, it's usually the output of a selector block (sorry; I don't have access to any turbine information at this writing so I don't remember if it's a High or a Low Select block). You can work "backwards" in the control logic/code from CSRGV and find all of the inputs to the block and then what drives those inputs to see what might be causing the IGVs to modulate on CTIM.

If you can describe the problem you are experiencing we might be able to offer some ideas on how to gather more data or troubleshoot or even possible resolve the problem. But, you just keep asking, "Please, is IGV Modulation ever a function of CTIM (Compressor inlet temperature)?" and we don't know exactly how to respond. I inferred from your other post that the unit in question is a GE-design Frame 6B heavy duty gas turbine; that would be another really great piece of information to know.

I've been getting a lot of calls lately about strange operations--and when I ask I find out that the grid frequency has been low. What's the grid frequency doing at your site?

Hope this helps! Without being able to see the actual sequencing or application code running in the turbine control system all I can offer is typical operating conditions.

I would feel grateful if you allow me to contribute on this thread!

First of all i can confirm that IGV modulates as a function of the CTIM (compressor temperature inlet)

Explanation: as Mr CSA used to explain on most of his replies about the compressor and IGV functions, the IGV during startup and shutdow of the machine are always fully closed to avoid compressor surge during low speed (lets take an example of 42 is fully closed and 84 is fully open). so during startup the IGV or the IGV reference CSRGV is 42. Okay so once the HP speed (TNH) is high enough the IGVs would tend to open (CSRGVPS) part speed reference to 56. so this is the first opening (which is a function of the CTIM), and the second openning is a function of the exhaust temperature (either in simple cycle or combined cycle) just like Mr CSA explained above.

Now lets go back to the first openning of the IGVs which like we said is a function of the CTIM. actually what i said above that once the HP speed (TNH) is high enough the IGVs will open to let more air through the machine, but in reality is the TNHCOR which is envolved. TNHCOR= CNCF x TNH, Where TNHCOR is the the corrected hp speed, and CNCF is the temperature correction factor, and it is calculated in function of the CTIM.

Lets take an example: during ISO conditions (CTIM = 15C , 60F)

CNCF=1 and the TNHCOR = TNH always

the IGV would tend to open at 82% of TNHCOR which is 82% of the HP speed (it depends on the project those examples are taken from a 5002C GT algorithm).

now if the CTIM = 0C, 32F the CNCF = 1,027, and this time when the IGVs will open at TNHCOR=82%, and TNH would be =79,84%.

On the other hand if CTIM is 45C, 113F the CNCF = 0,9516 and this time when the IGVs will open at TNHCOR=82%, this time TNH=86%.

I think it has a relation with the mass flow of air during hot and cold weather, and how it affects the compressor and the turbine performance.

hope this helps, waiting for your feedback.

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


The original post is unclear. You are correct; CTIM may change the turbine speed at which the IGVs begin opening, but it doesn't change the angle they open to. And this only occurs during acceleration when starting. Since turbine speed can't be controlled below ~95% this doesn't seem to be applicable.

We should ask rotimi88 to clarify his question. I took it to mean above 14HS (95%) and I did ask him to tell about any problems he may be experiencing (which is basically asking for clarification).

I find very often when people ask a vague question like this they usually have an answer they are hoping to receive to support a hypothesis they have formed and most likely have promoted to others in their attempt to explain a problem, real or perceived. And, generally, if the answer they receive doesn't fit their hypothesis they don't want to continue the discussion.

Well see how this goes....

Dear CSA,

I found this thread very interesting and informative. With this in mind i have consciously being following it for some periods now. To reply CSA sir, just as I have told you in one of my posts, the refinery power plant i am working on is still at the detailed design and engineering stage in which I am fully involved and we are hoping it will be commissioned by 2019. I used this thread to learn from experienced people like you in which you cannot know how much I look forward to your replies and I hope to be like you in few years to come (smiles). So please pardon me if you feel sometimes newbie like us ask vague questions.

So concerning my question, it was someone on another group of gas turbine that asked whether IGV Modulation is a function of CTIM (Compressor inlet temperature) or not. According to him, he said he has experienced an issue that IGV at baseload is modulating from 86 to 84deg as CTIM increases to 30degC (GT is running at combined cycle mode). So for better understanding, I brought it to this thread in which I believe you and others will have detailed explanation for this.

Thank you sir

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


I'm glad you find this thread useful and informative--you started it!

However, there is still not enough information to be useful to begin to try to understand why the IGVs might be moving when ambient temperature is "high" (personally, I don't consider 30 deg C to be that high; 40 deg C, yes). We don't know what Frame size machine, we don't know what kind of combustors and/or fuel, we don't know a LOT about the machine and its configuration.

And, if it's a GE-design Frame 6B or 9E heavy duty gas turbine built and supplied by GE-Belfort, well, all bets are off because they have just thrown long-standing GE standards and conventions out the window and invented their own. They will also specially configure just about any machine for just about any Customer requirement and in so doing will make changes to long-standing conventions and philosophies. So, without really understanding a LOT more about the configuration, the packager, and the situation (especially, again--if the grid frequency was stable at the time of the occurence), there's just NOT enough information to be of any further information. Unless one of the readers has first-hand knowledge of the situation or has a similarly configured unit and can provide specifics--and without knowing more about the unit configuration, how can anyone say their machine is similar or not?

There are some things which can be explained without too much information because there are some control philosophies and practices which have been in force and convention for decades--literally. And which GE Belfort are free--and willing--to throw aside and do something completely out of the ordinary, for just about any reason, including "just because we can."

And, I believe we have tried to address some of the more conventional configurations and control philosophies--but without more details about the specifics of the machine and its particular configuration and operating conditions there's really nothing more to add, at least from my perspective. Personally, I would have to know a LOT more about the situation, and since you can't provide that information I can't add any more to this thread--unless someone can prove something other than has already been described and discussed.

Thanks for the feedback on the context and origin of the question. Again, I most often find that when posters are unwilling to provide necessary details that in all to frequent cases it is again because of a "hidden" agenda in which they are looking for validation of a particular theory or hypothesis and when they don't get that they just stop replying or supplying requested information. I'm sure if you read past threads, even a small percentage of them, that you will find that we ask for a LOT more information than is ever provided in an attempt to be more concise and specific. We often go to great lengths to cover several possibilities because of the lack of information provided in the original or even subsequent posts trying to cover several possible contingencies in the hopes that it will spur more, better information in response. Sometimes it does; sometimes it doesn't. But we always try to get more detailed information to provide a more concise and detailed reply.

At least I hope that's the way it comes across. Many people, for whatever reason, firmly believe their problem has one and only one possible cause and that when they supply their anecdotal information that one or more people will instinctively understand their problem and now precisely what the cause is and how to rectify the root cause. Unfortunately, some problems can manifest themselves in many different ways and some people can falsely attribute symptoms to causes and vice versa--still believing there is only possible reason for their problem and that someone in the world can tell them exactly what is causing their problem and precisely what to do to quickly and finally resolve their problem. And the truth is that rarely happens when there is little in the way of actionable data and only anecdotal data, and NO alarm information provided to help troubleshoot and understand the problem. Because so many people do not pay attention to any alarms, and when the unit trips can't understand what caused the trip or the alarms preceding the trip which would have alerted a conscious operator to an impending trip, well, they just don't every pay any attention to alarms. And, yet when a proper review of alarms is conducted it quickly becomes VERY obvious and clear that the condition leading up to situation was alarmed for some time, and no one was paying attention to alarms. That's why you will very often find I ask for alarms, and very inoften get the alarm information I seek. Alarms (both Process AND Diagnostic!) are meant to be helpful and informative and instructive. And without them, just as in this case you have brought to us, without them there's often little we can provide in the way of help. We still try, but sometimes there's just nothing more we can add.