Gas Turbine IVG


Thread Starter


Is there a relationship between no of rows of VSV and the engine RPM? For example, gas turbine MS-5001 has 5089 rpm with only one row of IGV while PGT-10/2 has 10000 rpm with 5 row of VSV. Any one has a clarification?
Speed is basically a function of the design of the axial compressor. GE-design Frame 5 heavy duty gas turbines were designed decades ago, before VSVs were considered for use on heavy duty gas turbines. And, a GE-design Frame 5 is physically much larger than a PGT-10/2, and trying to control speed and vibration on even a Frame 5-size machine when it was being designed (probably mostly with slide rules) was difficult if not impossible.

If I'm not mistaken, a PGT-10/2 is based on a much more recent aircraft engine design and construction methods, with lighter materials.

Finally, it's my understanding that VSVs are primarily used to, one, be able to increase the mass flow-rate of air through the axial compressor, and, two, when possible, to make the axial compressor smaller (shorter) and decrease the size (physical mass) of the axial compressor rotor.


the reason why I am thinking of the speed is the following:

- for most heavy duty gas turbine (frame 5 /6/ 7 and 9H) which are used in power generation (single shaft-with fixed speed) the IGV is only one row.

- for small gas turbine (LM2500 / 6000 /PGT-10, which as you said aeroderivative designed machines) either single shaft or twin shafts the RPM is greater than heavy duty gas turbines (around 1000 rpm). Always there are VSV rows (not only one row of IGV)

I agree with what you have said. but I has a query. what is the benefits of having VSV (which is doing the same job of IGV)?

why in small gas turbines with higher rpm there are several rows of VSV? one time GE engineer told me that is because the flow in the first stages are supersonic.

I'm not a gas turbine designer (and I was hoping someone else would respond before I did), but as I wrote before VSVs (Variable Stator Vanes) serve two important purposes: they allow for higher air flows through the axial compressor (than would otherwise be possible WITHOUT the VSVs) while protecting the axial compressor at the higher air flow-rates (which I didn't explicitly write--but implied), and the axial compressor (for the same air flow-rate) can be smaller (shorter).

GE is now retrofitting some F-class turbines with multiple stages of VSVs, and the new HA-class heavy duty gas turbines are manufactured with VSVs standard when new. I believe some of the newest F-class turbines also have multiple stages of VSVs--it's the progression of axial compressor design for heavy duty gas turbines from a single stage of variable inlet guide vanes to variable inlet guide vanes PLUS VSVs.

One of the "bottlenecks" for heavy duty gas turbine power output is the air flow-rate through the axial compressor. Materials for the hot gas path sections (turbine nozzles and buckets (blades) to the point that if more air can be passed through the machines more power can be made--and the axial compressor has always been the limiting factor in air flow through the machine. One way to "maintain" current designs for turbine sections and footprint and auxiliaries is to change the axial compressor configuration/design to include VSVs to allow for higher air flow-rates which means more power can be produced by the gas turbine (since the hot gas path parts can withstand higher temperatures--which is also part and parcel of increasing power output). DLN combustion systems have also reduced the combustor gas exit temperature from the days of conventional combustors which is also part of the new designs being tested and produced, all of which are resulting in more power output.

VSVs are, again, used for improving the air flow through the axial compressor--and one of the ways that's done is to protect the axial compressor using the VSVs. And, because they can use VSVs it's possible to make the axial compressor shorter than would otherwise be possible--which helps to save costs on materials and package size, which means the units could be sold at a slightly lower cost to Customers. (What it really means it the profit margin is increased.)

Just to recap, aircraft engine technology is slowly making its way into heavy duty engines--and VSVs are one way. In the past you typically only saw VSVs on aircraft deriviative engines which used the gas generator sections of aircraft engines for power generation or pump/compressor applications. Rated speed is a function of the design of the compressor blades (stationary and rotating), the physical mass of the axial compressor rotor and the protection criteria established by the manufacturer for the axial compressor (remember, axial compressors are unique machines and have some very small operating regions which, if exceeded, can cause catastrophic damage to the axial compressor (stall and/or surge).

This is all I can add to this discussion. VSVs are coming soon to a heavy duty gas turbine near you. I wouldn't be surprised if GE, or another manufacturer, isn't already or will be soon offering enhanced axial compressor sections for Frame 5/6/& B/E class machines with VSVs. The units have long been known to be "overbuilt" (meaning they are robust machines which weren't producing all the power they were capable of--because of limitations with materials and the air flows through the axial compressors. And, now both of those limitations ("bottlenecks") are slowly dissolving, and with manufacturing costs coming down, too, these kinds of compressor upgrades are becoming more cost-effective and economical for Customers who want to keep their machines, and upgrade the power output--and longevity--of their existing machines, at the same time.

Hope this helps!


Good point of view. You know any designer engineer can answer my query?


>I'm not a gas turbine designer (and I was hoping someone
>else would respond before I did), but as I wrote before VSVs
>(Variable Stator Vanes) serve two important purposes: they
>allow for higher air flows through the axial compressor
>(than would otherwise be possible WITHOUT the VSVs) while
>protecting the axial compressor at the higher air flow-rates
>(which I didn't explicitly write--but implied), and the
>axial compressor (for the same air flow-rate) can be smaller

I failed to mention that the VSVs on GE-design heavy duty gas turbines I have seen have four stages.

I do not know any axial compressor design engineers who have not retired and left the industry.

There is, or was at one time, a fair amount of written information about VSVs for aeroderivative gas turbines--and that's what's being adapted to heavy duty gas turbines these days--on the World Wide Web.

Single-shaft GE-design heavy duty gas turbines rotated at 3600 RPM (for 60 Hz machines) and 3000 RPM (for 50 Hz machines) before the addition of VSVs, and they still operate at the same rated speeds.

Most two-shaft heavy duty gas turbines (heavy duty and aeroderivative) have gas generators (the "high-pressure" shaft of the units) that rotate in a range of speed, and the axial compressors are coupled to the HP shafts and the VSVs are used to control air flow and to protect the axial compressor at the high air flow from damage (stall/surge).

But, if we're talking about a relationship between VSVs and rated speed of units with VSVs and those without VSVs I'm at a loss for explaining what that relationship would be. For single-shaft heavy duty gas turbines of ANY manufacturer the rated speed of the turbine is directly proportional to the frequency of the generator. For multi-shaft gas turbine driving a generator, the drive turbine shaft (usually the LP, or low-pressure, shaft) is also a function of the generator frequency. The relationship between generator rotor speed and frequency is: F=(P*N)/120, where F=Frequency (in Hz); P=Number of poles of generator rotor; and N=Speed of generator rotor. You can solve for any variable (F or P or N). But, in order for the frequency of a generator to be constant the speed of the generator rotor has to be constant--and when a drive turbine is mechanically coupled to a generator rotor the drive turbine speed has to be constant.

For mechanical drive turbines (such as those driving compressor or pumps) the speed of the driven device (the compressor or pump) is usually not the critical parameter--it's either pressure or flow-rate or some combination of the two. And, so the drive turbine coupled to the driven device is more free to change speed (within a design range) in order to meet the operating setpoint (pressure and/or flow-rate). And, the gas generator (the HP shaft--which is not coupled to the driven device) is also free to rotate (within a design range) in order to supply energy to the drive turbine to spin the driven device.

And I don't think VSVs have anything to do with either the generator drive speed or the mechanical drive speed--they're just their to maximize air flow through the compressor (over and above what could flow through the compressor if it didn't have VSVs) and to help protect the compressor against damage. At least that's the way I've come to understand VSVs--but I've never actually given any thought to the relationship between rated speed of the axial compressor and the presence or absence of VSVs. Doesn't mean there isn't a relationship, and I would imagine if one were designing an axial compressor from scratch (not scaling one up from a smaller size to a larger size) that VSVs would figure into the design as well as the rated speed. Or, the VSVs would be designed to accommodate the desired rated speed.

I would suggest searching the World Wide Web for more information about VSVs using your preferred search engine. Look for technical papers presented to trade or industry organizations as they usually don't have as much high maths (which can be very confusing to me).

Best of luck in your search!

In my personal opinion, Jan Gorski's and Elias Tsoutsanis' definitions/comments/clarifications are the best ones. It seem the VSVs change the angle of attack/flow of the air flowing through the compressor to protect the compressor at off-speed conditions as well as (in my understanding) off-temperature conditions (extremely low or in some cases extremely high).

The above was found using the search term: "variable stator vanes compressor" (without the double quotation marks, though it found the same results with the double quotations). (I tend not to use Google for searching, and these were found using a search engine other than (not) Google.)

The above is probably going to bring up the question: What is stalling/surging?

I will say that IGVs (Inlet Guide Vanes) and VSVs (Variable Stator Vanes) are not the same. One controls the air flow entering the axial compressor (IGVs) and one controls the loading of the axial compressor (VSVs). From the above it seems like by changing the angle of attack of the air on the rotating blades (using the VSVs) the rotating blades can be protected during different speeds or air flows.

There are single-shaft GE-design Frame 5 heavy duty gas turbines, and two-shaft GE-design Frame 5 heavy duty gas turbines. In the latter, the HP shaft (to which the axial compressor is coupled) spins in a range of allowable speed--NOT at a fixed speed. The axial compressor is designed to operate safely (without stalling or surging) in a range of speed, not a single, fixed speed.

In a generator drive application, the speed of the shaft driving the generator rotor is fixed--because generator frequency is (should be!) constant, and because generator rotor speed and frequency are directly related if one wants a constant frequency one needs a constant generator rotor speed. On an AC grid, every generator synchronized to that grid is spinning at what's called its 'synchronous speed.' That is the speed that is proportional to the number of poles of the generator and the frequency of the grid. No single generator can spin faster than its synchronous speed (all two-pole generators must spin at 3000 RPM on a 50 Hz grid, and 3600 RPM on a 60 Hz grid). Frame 5s are coupled to generators through a reduction, or load, gear. This changes the Frame 5's speed (nominally approximately 5100 RPM) to 3000 or 3600 RPM so the generator can produce AC power (amperes and volts) at the desired frequency (50 or 60 Hz, respectively).

The GE-design Frame 6 heavy duty gas turbine axial compressor was designed based on Frame 5 axial compressors--and they operate at the same nominal speed. (By the way, some Frame 5s operate at 5130 RPM when connected to a generator; and others operate at 5094 RPM when connected to a generator; and others operate 5089 RPM when connected to a generator--it all depends on the construction of the reduction, or load, gear. How many teeth the bull and pinion gears have--and that is not always a fixed number. But because the Frame 5/6 axial compressors can operate safely in a small range of speed (from approximately 5089 RPM to approximately 5134 RPM) the reduction/load gear teeth number can vary--in order to maximize the strength of the gear based on the grain structure of the piece(s) of metal being used to make the gears.)

While aeroderivative gas turbines operate on the same principle as heavy duty gas turbines (they all suck, squeeze, burn and blow--meaning they all draw air into their axial compressors, they compress the air flowing through the axial compressors, they oxidize (burn) fuel using the air exiting the axial compressor, and the discharge their exhaust gases (sometimes at very low pressures, sometimes at very high pressures) through their exhaust) they are constructed very differently. Most of the high-pressure sections (or, the gas generator sections) of aeroderivative gas turbines are designed to be very light (because they are mounted on aircraft wings) and to operate at very differing air flows (based on elevation--which can vary GREATLY for an aircraft!). In order to produce more thrust (power output) often its advantageous for the HP shaft to spin at higher speed--and that often increases efficiency as well. And, because an aircraft engine isn't driving a generator its speed isn't limited to a single value--it can vary as the speed of the airplane needs to vary. And because the rotating mass of the axial compressor is lighter (than the rotating mass of a heavy duty gas turbine axial compressor) it can also spin at higher speeds without worry about vibration and balancing issues that heavy rotating masses have to worry about.

When the HP, or gas generator, section of an aircraft engine is used for a land- or marine-based application (such as for driving a natural gas compressor, or a pump) it is coupled to a different power turbine--one that produces torque instead of thrust. That torque provides the power for the compressor or pump. These types of engines, utilizing aircraft engine HP, or gas generator, sections are typically called aeroderivative engines--because they are based on, or derived from, aircraft engines--but they produce torque, not thrust.

The choice to use an aeroderivative engine or a heavy duty gas turbine for an application is based on many factors: size of the engine/turbine and available space for it; speed range; amount of required torque (which may be small or very large or somewhere in between); required speed of output shaft; etc. Some operators prefer aeroderivative engines for various reasons (if there's a catastrophic failure the engine can usually be swapped out for another in a relatively short few days, whereas if a heavy duty fails catastrophically it can take weeks or months to return to service); usually, aeroderivative engines can be started and loaded much faster than heavy duty gas turbines. And so on.

But, I think we've hit on your stumbling block: IGVs and VSVs are not the same and don't serve the same purpose. They might look similar and operate similarly, but they serve two distinct and different purposes. And, their inclusion on a particular style of gas turbine doesn't have much to do with the rated speed of the shaft the axial compressor is coupled to. I refer to the addition/modification GE is offering to some of their heavy duty gas turbines to add VSVs--the speed of the engine/compressor doesn't change. But the power being produced by the engine does, and in some cases the off-speed capability of the engine is enhanced (which can be important for places in the world where the electric grid is not very stable).

I don't think you're going to find very many axial compressor design engineers responding to questions on I am only trying to express, in layman's terms, what I've come to learn and understand. When I search things like this on the World Wide Web, I find a LOT of maths--which don't always explain things very well for me. Maths is great for predicting what will happen, or analyzing what will happen or what happened. But, maths doesn't always explain (to me) what is happening. Especially when higher maths are used to "explain" things. I'm good with multiplication and division and addition and subtraction and a little bit of trigonometry and an even smaller amount of calculus, but more than that and I'm still looking for a definition, in understandable words and terms--and that's what I mostly try to provide when responding to questions on And once I (personally) understand what's happening or supposed to happen, very often the maths reinforce my understanding--but rarely do they form my understanding.

I wish you luck in your search. I think you should first try to understand the difference between IGVs and VSVs--because I don't believe they are the same or interchangeable. At least not on heavy duty gas turbines. And you seem to be implying that aeroderivative gas turbines and heavy duty gas turbines are more alike than they really are. All of the heavy duty gas turbines I have seen that have VSVs also have IGVs; I can't say that's true for all the aeroderivative gas turbines I've seen.

Anyway, best of luck!

As per OEM Compressor Variable Design Practice:

Both IGV and VSV are referred as 'Variable Stator Vanes'.

Historically the variable stages in industrial gas turbine compressor from GE Power (E-Class and F-Class) have been limited to a single stage (IGV). On relatively new machine likes the H-Class and C-Class class now contains variable stator vanes on multiple stages.

On E-Class and F-Class machines, most of the application are Generator drive. On this application, machines always operating around 100% speed, except during start-up and shutdown.

At reduced speed (start-up/shutdown), operating first stage compressor at or close to STALL is necessary to sustain overall pressure ratio in order to keep final stage flows away from CHOKING. 53% speed is the ultimate limit with first stage STALLED and last stage CHOKED at the same time. IGV and Bleed valve serves the purpose of mitigating these effects.

Gas Turbines from GE Oil & Gas (aka Nuovo Pignone) likes PGT-5, PGT-10, MS5002, LM Series aims for wide operating speed 75-105% on compressor drive application.

Some machines likes LM series (Aeroderivative) or PGT series which adopted design from LM, pressure rotio is relatively much higher.

The compressor minimum speed with first/last stage STALL/CHOKE can be as high as 80%-90% speed. Maintaining requirement for STALL/CHOKE by IGV is not enough. Therefore, multi-stages variable stator vanes is necessary.

H-Class and C-Class's axial compressor obtains higher efficiencies and higher-pressure ratios by adopting new compressor geometry design from Aeroderivative counterpart.

This improvement has higher efficiencies and higher-pressure ratios requires more robust aerodynamics. The VSV system enables significant improvements in flexibility as well as robustness of aerodynamics for these machine classes.

Hope this information is contributing.

Great information, and I stand corrected on IGVs as variable stator vanes (though as I said, technically--they look like and operate like variable stator vanes--they serve a different purpose than variable stator vanes, as you have confirmed).

However, the original poster's question remains: Is there a relationship between the number of "variable stator vanes" and turbine speed?

Thanks again for the valuable information.

Back to the original poster's question: Is there a relationship between the number of "variable stator vanes" and turbine speed?

First, let's say we are discussing about a single shaft unit. Turbine speed equals Axial Compressor speed. As for turbine speed, it is a corrected speed with ambient condition. Aerodynamic of Axial compressor matching design speed. Higher pressure ratio on Axial compressor leads final stages to reach the CHOKING LIMIT.

Aerodynamic and design speed are main contributing factors for pressure ratio. The number of variable rows typically increase with the design pressure ratio.

Not sure that an explanation above answer to original poster's question.


Written like a TRUE design engineer.

Actually, the original poster's question is very nebulous. I find it very difficult to compare GE-design Frame 5-sized machine and a PGT-10/2. One is an industrial gas turbine and the other is an aeroderivative gas turbine. They are typically used for different applications (purposes), and they have a different design history.

I get questions like this a lot--as people try to look for similarities between "gas turbines." And usually they don't distinguish between the applications and the design histories. A gas turbine seems like a gas turbine.

But not even every Frame 5 is the same. Subtle differences in design ("bang-bang IGVs, versus modulated IGVs, for example) can make for trouble for people when trying to understand how a Frame 5 operates and why it operates that way. And why two supposedly identical units (they're both Frame 5s after all) aren't the same.

I've tried re-reading the original post several times, looking for some clue that can be used for explanation. I thought I had it--before you corrected me that IGVs are also VSVs. (I can understand that, but a single stage of VSVs at the inlet of the axial compressor serves a different purpose than one or even multiple stages of VSVs in the early or middle section of the axial compressor (if I understand your explanation).)

I'm not adding anything of value to this thread so I'm not responding any further, though I will continue to follow it just out of curiosity and interest.

Thanks for your help, cigreen_man!



Thanks for your participating , the reason why I'm comparing 2 different types are the following:

- I'm fully aware of the different between the 2 applications. PGT10/2 is a mechanical prime mover for a compressor which serve the change in load, (actually the LP module is controlling the application load more than the HP which has the VSV), while Frame 5 which is used in power generation is a single shaft with fixed speed. but I tried to give an example based on differentiation of speed.

- in my site, we have also solarturbines used for power generation with 15000 rpm, and siemens turbine with 15000 rpm. Both have VSV, so I tried to do the comparison based on speed.

- I know that new 9H frames has fixed speed but with VSV, not single IGV. So I agree with you, the question should be "why new GE frames has VSV instead of IGV specially if the speed is constant in the power generation application. I mean after loading and during full operation. as the main purpose of both VSV and IGV is limitation of axial compressor surge.

thanks for your time and I just liked to illustrate my point of view
I think the rated speed of rotation of the compressor has little to do with the need for VSV's. The purpose of VSV's and VIGV's is to prevent compressor stall (similar to surge on a centrifugal compressor). The rated speed of the gas turbine is a function of the diameter of the wheels. The air flow through the compressor is determined by the cross sectional area of the inlet stage to the compressor and by the speed of the compressor. For a given cross sectional area, the rated speed is limited by the tip speed of the compressor (and turbine) blades - you want to keep the tip speed below the speed of sound. So, the larger the diameter of the machine, the slower the rated speed.
Now, if the compressor always ran only at rated speed and load, you would not need variable guide vanes and stator vanes to prevent stall. However, you have to start the machine up from zero speed and gradually bring it up to the rated speed. And, for mechanical load applications (pumps, centrifugal compressors) you need to operate over a variable speed range. Aircraft applications put a much bigger demand for variable speed. The aircraft engines were where the VSV's were first used. Multi-spool engines are also used where different stages of the compressor run at different speeds to control stall. (I believe today's high performance military aircraft engines use both multi-spool and VSV's.)