Limiting the Gas Turbine Load and Operating at Base Load


Thread Starter


At one of our facilities, the GE Frame 6 machine has been operated at "Pre select" Mode. At Present, GT is operated at 70% of base load (roughly). Due to limitation, the GT should not required to be operated at 100% load. Operating the GT at "Pre Select" mode causes the GT to be operated at lower GT Exhaust Temperature (and also lower firing temperature). This has an impact on GT heatrate/efficiency. Is it feasible to alter the baseload IGV opening setting to a lower value and operate the Gas Turbine at Baseload Mode? Will this improve the GT heat rate and also help us to maintain lower GT Power output?


>No to all questions.

Thanks CSA....Why does the Exhaust Temperature with "Pre Select" model be lower compared to Exhaust Temp Vs CPD Profile of typical part load curve?

WE have another machine where the machine capacity is limited to 90% of baseload capacity to maintain spinning reserve. i.e., at baseload the IGV opening is only 78%. However, this machine has been intentionally set in this way from its commissioning to maintain some spinning reserve. Please share your thoughts.
Your post is not clear.

It's not known if the unit in question is operating in Simple- or Combined Cycle mode, or if IGV Exhaust Temperature Control is enabled and active.

<i><b>Regardless,</i></b> there is no way to improve heat rate and/or gas turbine efficiency when operating at Part Load versus Base Load. Not through normal means. Full stop. Period. It's just not possible. The physics of the design of the gas turbine won't allow it.

If the unit must be operated at Part Load, the efficiency and heat rate will be what they will be. And short of redesigning the axial compressor and combustors there is virtually nothing to be done other than keeping the inlet guide vanes and axial compressor as clean as possible.


Thanks CSA....

What would be the shape of the IGV opening profile across different GT load (typical) at part load operation with and without enabling Preselect mode for a Frame 6 machine operating in combined cycle?

Preselect Load Control is a lazy man's way of changing load on a GE-design heavy duty gas turbine. It is not usually configured properly and results in load swings of +/- 1-2 MW around the intended load setpoint.

It has nothing whatsoever to do with heat rate and/or efficiency.

If a GE-design heavy duty gas turbine is being operated in combined cycle mode--which is another way of saying it's being operated in IGV Exhaust Temperature Control mode--then the IGVs are being held closed during part load operation in order to maximize exhaust temperature and steam temperature/production in the HRSG ("boiler"). This has the effect of <b>decreasing</b> the gas turbine heat rate and efficiency but increasing the heat rate and efficiency of the overall plant cycle (GT and HRSG and ST (Steam Turbine)).

In Combined Cycle mode (IGV Exhaust Temperature Control mode) the IGVs are held closed much longer during part load operation than they would otherwise be--again to maximize gas turbine exhaust temperature. Gas turbines are mass flow machines--meaning, the more mass (air and fuel) that flows through them the more power they produce. So, by reducing the air flow the efficiency and heat rate of the gas turbine are decreased.

If this is the way the turbine has to be operated in order to meet the plant needs then the gas turbine efficiency and heat rate are going to be what they are when operating under these conditions. If plant personnel are "challenged" to try to improve heat rate and efficiency (and incentivized to do so through bonuses) then the best things that can be done are to ensure that the contract is written such that the operating conditions are taken into account (part load) and a suitable baseline heat rate and efficiency condition is established (something other than Base Load heat rate/efficiency, since the unit cannot ever hope to achieve Base Load heat rate/efficiency when being operated at part load). And then, the operators and maintenance crews have to work diligently to keep all equipment and parts of the unit and its auxiliaries in the best and cleanest conditions in order to operate as efficiently as possible for the way the plant has to be operated.

The turbine control system, while a powerful thing, cannot be used to compensate or make up for the design of the gas turbine when it's being operated at part load to try to make the turbine heat rate and efficiency be as close to Base Load conditions as possible. It just can't be done. You have to look elsewhere for performance and heat rate and efficiency improvements--and I can tell you from (painful) experience that HRSGs are extremely notorious for being very poor on heat rate and efficiency, at any load/firing rate. There are likely many more opportunities to improve the heat rate and efficiency of the HRSG than there are for the GT.


Thanks CSA for your genuine replies...I really enjoy reading your replies...

Your explanation has really coincided with my line of thought which is improvement of Combined Cycle Efficiency with higher GT exhaust Temperature...I had wrongly mentioned in my previous post by referring to the GT heat rate improvement. Our intention is more to improve the Combined Cycle Efficiency (in our case, it is Cogeneration and not the combined cycle. We use the HRSG for generating steam for the refinery process). I could see that the GT exhaust temperature at 70% load is extremely low with preselect mode as the IGV is in full open position. If we can have of the lower opening of IGV, then it might increase the GT Exhaust temperature and further enhance the HRSG performance. The increase in GT exhaust temperature would also result in lowering in the HRSG stack temperature thereby increase in the overall efficiency. Am I correct in this point? Appreciate your patience.

Most GE-design heavy duty gas turbines have two "buttons" on the HMI display to allow selection of "SIMPLE CYCLE" or "COMBINED CYCLE" modes. Sometimes they're labeled "SIMPLE CYCLE" and "IGV EXH TEMP" control, or something similar.

In "simple cycle" mode, the IGVs are left open until the exhaust temperature reaches approximately 700 or 900 deg F (I can't recall which at this writing), and then as the unit is loaded the IGVs open to maintain this temperature until they are fully open. This means that the exhaust temperature will rise as the unit is loaded until it reaches the defined temperature (700 or 900 deg F) and then will maintain that temperature as the unit continues loading until the IGVs are fully open. Continuing to load the GT will then cause the exhaust temperature to increase until the unit finally reaches CPD-biased exhaust temperature control--Base Load. (This modulation of the IGVs at a predetermined temperature is done to prevent dynamic pressure pulsations in the combustors to minimize combustor hot gas path part wear and damage. The HMI screen will usually indicate "IGV Exhaust Temp Control" when the IGVs are opening while trying to maintain the defined exhaust temperature, and that field will then go blank when the IGVs are fully open.)

Combined cycle and cogeneration--as far as the GT is concerned--are the same thing. In "combined cycle" or "IGV exhaust temperature control" mode, the IGVs are held closed as the unit is loaded until the exhaust temperature reaches the calculated exhaust temperature reference "limit"--signal name TTRX. This is the maximum allowable exhaust temperature for the machine, which is a calculated value which is never higher than the Control Constant TTK0_I, the "isothermal" exhaust temperature limit (usually 1100 deg F, or 593 deg C). And, then as fuel is added to the unit to increase load the IGVs are opened to maintain exhaust temperature equal to TTRX, which, as CPD increases will begin to decrease from the isothermal value, until the IGVs are fully open and the unit is on CPD-biased exhaust temperature control--Base Load. The HMI display will indicate "IGV Exhaust Temp Control" at all times during loading, until the unit reaches Base Load (CPD-biased exhaust temperature control).

Units designed for simple cycle, or peaking, application which have no HRSG to produce steam from the GT exhaust may not have these two buttons as operating in "combined cycle" or "IGV exhaust temperature control" mode is inefficient. Why heat the GT exhaust when it's just being directed to atmosphere and not being used to produce steam--there's no reason to do so, especially when it reduces unit heat rate and efficiency at Part Load.

Turbines which have a diverter damper to send the GT exhaust to either atmosphere or an HRSG, or which exhaust directly into an HRSG, will usually have these two buttons to allow the operations personnel to maximize exhaust temperature to produce more/hotter steam at lower loads, or, if steam production is not so important or the exhaust is being directed to atmosphere through a diverter damper to minimize exhaust temperature to the extent possible.

So, you should be looking for the "COMBINED CYCLE" or "IGV EXH TEMP" buttons and making sure they are selected if you want to maximize exhaust temperature during Part Load operation. However, you should be aware that the IGVs will <b>NOT</b> close below 57 DGA (DeGrees Angle) even if "COMBINED CYCLE"/"IGV EXH TEMP" is selected--they cannot close below 57 DGA when the unit is at rated speed. So, the exhaust temperature will be what it will at light loads--though it will be hotter sooner than if "SIMPLE CYCLE" was selected, and it will be hotter longer than if "SIMPLE CYCLE" was selected.

If the "Main" ("Startup") HMI display does not have these two buttons, then the IGV HMI display may have them. If you can't find the buttons on any display, well, then, you will need to get someone to site to help with ensuring the Speedtronic is properly configured to allow IGV exhaust temperature control, and then enable or add the buttons to a display. I have been to some sites that demanded IGV exhaust temperature control be disabled during start-up--even though the unit exhausted into an HRSG. (Yes; there are lots of start-up and operations managers who don't properly understand this function and its usefulness and like to "flex their muscles" by forcing the OEM start-up engineer to disable the function in front of their minions to show how powerful they are....)

Also, sometimes the HRSG is not capable of the maximum allowable GT exhaust temperature (1100 deg F, 593 deg C) and so IGV exhaust temperature control is disabled for that reason.

So, you should be doing some homework to ensure the HRSG at your site is capable of full GT exhaust temperature, and that everyone is aware of what you are planning to do--before enabling IGV exhaust temperature control if it's not enabled, or if you have to have someone come to site to assist with enabling and putting the appropriate buttons on the display.

There is one more possible test you can try--presuming the HRSG is capable of maximum GT exhaust temperature. Usually, on the IGV HMI display, there is a button for "MANUAL IGV CONTROL." This and other related buttons can be used to close the IGVs but only until the exhaust temperature reaches the calculated exhaust temperature reference "limit" (TTRX). It can't be used to open the IGVs more than the calculated IGV reference, only to close the IGVs to some position less than the calculated IGV reference and only until the GT exhaust temperature reaches the exhaust temperature reference "limit" (TTRX). You could see if this would help maximize exhaust temperature--again, presuming the HRSG is capable of the maximum allowable exhaust temperature (usually 1100 deg F, or 593 deg C). Just don't forget to disable Manual IGV control when finished with your testing.

Finally, Preselect Load Control Mode has <b><i>absolutely nothing</b></i> to do with IGV position or GT exhaust temperature. Nothing whatsoever. It's just a lazy man's way of changing load and operating the GT by using load setpoints when changing load.

Hope this helps! (By the way, if you like the posts--that is, if you find them helpful--there is a 'thumbs-up' button you can use to indicate your like, or a thumbs-down button to indicate your dislike.)


Thanks CSA once again...

yes, as you had mentioned, the GTs are not operated to its full exhaust temperature range. We need to explore the IGV exhaust temperature control setting more. Yes, now i understand that the pre-select load operation alone has nothing to do with lower GT exhaust temperature.


Myself and the operator tried to search for the "arm" which would help us to change the IGV Exhaust temperature control mode to "ON". But, we could not find it in the MARK VI Screen (under Control/IGV Control). One of the Mark VI trained operator says that he would be able to change it though it is not displayed in the MARK VI Screen. Is this the case for all the Gas Turbines?

>yes, as you had mentioned, the GTs are not operated to its
>full exhaust temperature range. We need to explore the IGV
>exhaust temperature control setting more. Yes, now i
>understand that the pre-select load operation alone has
>nothing to do with lower GT exhaust temperature.

I have been <b>extremely</b> remiss for not asking: How many (and which) of the units at your site that we are discussing have DLN-I combustors?

Because, if any of the units we have been discussing have DLN-I combustors then 'Manual IGV Exhaust Temperature Control' <b>WILL NOT</b> likely be available on the IGV Control Display. Nor will 'IGV Exhaust Temperature Control' be an operator-selectable function on the HMI. Most GE-design heavy duty gas turbines with DLN-I combustors have IGV Exhaust Temperature Control enabled at all times--again, to help with maintaining the proper air/fuel mixtures in the combustors at low loads (particularly in Premix Steady-State Combustion Mode).

If any of the units do NOT have DLN-I combustors then I have to believe--and strongly suggest--that IGV control functions have been purposely removed/disabled as appropriate--for some specific reason or reasons. Reasons might include an inability of exhaust components (HRSG superheater; stack components; exhaust transition duct components; static emissions reduction components; etc.) to withstand the maximum possible exhaust temperature which might be reaced with IGVs at relatively low positions at part load. I've even had the "pleasure" of working at sites where some representative of the owner/operator or member of the commissioning crew demanded, ..., er, ..., uh, ..., requested that some function--such as IGV Exhaust Temperature Control--be permanently disabled. And, then learned that the site required that function and was questioned about why it had been disabled. You might find some mention of this in the Start-up Report prepared by the OEM/packager gas turbine commissioning engineer.

At this point, without knowing which--if any--of the units we are discussing, and without knowing a LOT more about the exhaust components of the GTs we are discussing I am <b>VERY STRONGLY</b> recommending that you have a knowledgeable GE-design heavy duty gas turbine controls engineer visit the site and spend a few days getting familiar with the site, how it's operated, what the exhaust components are, and then make some recommendation about what is or isn't possible. This is the best way to ensure that no harm is being done to the GT or the exhaust components by trying to enable some IGV function which was likely disabled for some specific reason. Please, have someone come to site to assist with this effort.

And, please write back to let us know how you fare with this effort.


Thanks CSA.

The machine is neither provided with DLN-I combustor nor operating with enabled IGV exhaust temperature mode. IGV at part load is always at its maximum open position (84 Deg). Also, when the machine is operated at base load the GT exhaust temperature is maintained above 1000 Deg F which confirms that there is no apprehension with high GT Exhaust temperature. As you had mentioned, we don't know why the machine is not provided the IGV exhaust temperature enabling option be an operator-selectable function on the HMI. Probably the machine is very old (1980s make).

We are exploring the possibility of undertaking site trial to operate the machine at Manual IGV control to observe the performance at different IGV angle say 79 deg, 75 Deg and 70 deg. This might probably help in ascertaining the best lower operating angle as a temporary measure to improve the efficiency. In the mean time, the subject would be taken up with GE, who in general takes some time to answer.
You dont expect your IGV to be fully opened at 84 degrees when you are on spinning reserve. Your IGV will open according to your load preselected value and your load demand. The gas valves will tend o open so as to compensate for the variations in the grid frequency while the IGV opens correspondingly though not as spontaneous as the servo.

If these machines are old(er) they might not have modulated IGV control. That was very common for early Frame 6s, especially ones that were only originally expected to be operated in peaking application (simple cycle mode) OR which were originally planned to be operated at Base Load instead of Part Load.

That may explain why you don't have the SIMPLE- or COMBINED CYCLE buttons, and why there's no Manual IGV control function.

What you may need is to have the proper hydraulic actuator installed, and the modulated IGV function enabled in the turbine control system.

I apologize profusely for not asking more questions about the turbines; it could have saved a lot of time. In the future, when you post it would be very helpful if you could tell us about the turbines at your site; while you work with them day in and day out we don't. And, contrary to popular belief every Frame 6 is not like every other Frame 6. Sometimes there are very subtle differences; other times, the differences can be very great. The more information you can provide in your original post the better and more concise your responses will likely be.


Thanks CSA...

I could see the option in HMI for "Manual IGV". You had mentioned in your post that "no Manual IGV control function".



with the preselect control and with disable IGV exhaust temperature control, the machine is running at 84 deg even at partload.

> You don't expect your IGV to be fully opened at 84 degrees when you are on spinning reserve. Your IGV will open
> according to your load preselected value and your load demand. The gas valves will tend o open so as to compensate
> for the variations in the grid frequency while the IGV opens correspondingly though not as spontaneous as the servo.
Your explanation on GE's heavy duty GTs operating modes are so good.
Can you pls help me in following matter;
1. Can we use CTIM corrections curve of base load to correct part load performance (heat rate) also.? If no than any corilation to correct part load heat rate at different CTIM.

As per my knowledge GEs provided base load performance correction curve is applicable for base load only. At diff CTIM air flow will vary at constant volume compression (84 deg IGV). But at fixed part load i.e. 100 MW airflow will remain same with difference CTIM.

I'm not, and have never been, one to use a lot of maths to analyze turbine performance; that's the purview of people with degrees in thermodynamics and access to lots of formulae and spreadsheets and MANY assumptions (because, as you know, there are lots of assumptions made with thermodynamic analysis). Me, I'm just a glorified technician, trying to explain things to operators and other technicians--many of whom have never had any "advanced" maths, and if they did, like me--it's long forgotten because of lack of use. I'm very good with "straight-line" maths (y=mx+b, or f(x)=mx+b), simple algebra, but after that's it's all Greek (literally--because of the use of so many Greek letters!).

1) I don't really understand the question. If you are referring to using CTIM (a median-selected values of axial compressor inlet temperature) for some kind of analysis, then I would say yes. I've seen performance engineers (the people with the thermodynamics degrees) use it all the time.

2) It's my understanding that most OEMs ONLY guarantee performance (and heat rate) at Base Load--when the IGVs are at maximum operating angle and the unit is at rated speed (critical for MANY parts of the world!). It's much too difficult, with typical instrumentation. to guarantee--or even accurately calculate--air flow and performance at anything less than Base Load. AND, because guarantees are made at the machines nameplate rating, which is specified at specific ambient temperature and pressure and often humidity, GE provides performance correction curves for things such as ambient temperature (which I believe is axial compressor inlet temperature, CTIM) and possibility ambient pressure and I've only once seen a correction curve for humidity.

That's about all I can say with any degree of certainty. GE has many publications about unit performance and calculating performance; you should look through the manuals provided with the turbine & auxiliaries and search the World Wide Web for them. (It's pretty amazing all of the pub's they have, not just for the Mark*--but also for basic unit operation and maintenance and performance.)

Hope this helps!