Control.com - Nerds in Control

1. When we change the IGV to manual during part load on a combine cycle turbine, what will be the effect on the FSR, firing temperature, exhaust temperature (we keep the load constant )?

2. How to know on a certain IGV position (in manual mode) between 57 to 84 that the compressor won't go to a surge or stall condition?

This discussion does not apply to units with DLN combustors or to F-class units.

1. I believe the purpose of Manual IGV Control was to allow operators to use the IGVs to try to increase exhaust temperature manually during part load operation so as to enhance steam production when the gas turbine was exhausting into an HRSG (Heat Recovery Steam Generator, or "boiler") *when IGV Exhaust Temperature Control was either not provided or not enabled*.

It's been a very long time since I've seen a unit with Manual IGV control but if I recall correctly the IGVs can only be closed from the current operating position when Manual Control was enabled, and the effect would be to increase exhaust temperature. If I recall correctly, Manual IGV control feeds into a MINimum SELect block, and so one cannot open the IGVs above the value being determined by the "automatic" IGV control (as the Manual IGV position reference would be greater than the automatic control reference). I believe at the time Manual IGV control is initially enabled, there will be no change in any operating conditions as the CSP will use the current parameters as defaults to provide a "bumpless" transfer from Automatic to Manual control.

If the unit were operating at Part Load without Pre-Select Load Control enabled and Manual IGV Control was enabled and the IGV angle was reduced by the operator, FSR would not change and the decrease in air flow would result in a loss of load. If Pre-Select Load Control were enabled when Manual IGV Control was enable and the IGV angle was reduced by the operator I believe the Mark V would adjust fuel (FSR) to try to maintain the Pre-Select Load setpoint which would also add to an increase in exhaust temperature.

If the IGVs are in Manual Control and the IGV angle was manually reduced to increase the exhaust temperature, Manual IGV control could be used to increase the IGV angle up to the "automatic" IGV reference (remember, the Manual reference is one input to the MIN SEL block and as it increases and some other input is lower the MIN SEL block will use the lowest input value).

I would recommend that if you use Manual IGV Control and want to transfer back to automatic IGV control that you increase the Manual reference value until the IGVs will not open any further (meaning it's effectively back on automatic control via the MIN SEL block) and then select Automatic IGV Control to have a relatively stable transfer. (There should be ramp rates to prevent jumps in IGV position but sometimes they weren't programmed properly and usually weren't tested during commissioning.)

2. In general, when a GE-design heavy duty gas turbine is operating at rated speed, the control system will not allow the IGVs to be operated such that the compressor could be damaged, even in Manual IGV Control. Even when in Manual IGV Control, there's another input to the MIN SEL block that would prevent the IGVs from being closed and causing the exhaust temperature to increase above the maximum exhaust temperature limit for the current operating conditions (which doesn't have anything to do with surge or stall, but which would prevent closing the IGVs even further to a possible problematic condition). There's also the minimum operating IGV angle setpoint which is feeding into the MIN SEL block (or to at least one of the inputs) which would prevent the closing of the IGVs below that value.

You haven't told us if the unit has an HRSG and/or a bypass stack, or why you would want to use Manual IGV Control, particularly if the unit has neither. It would be wasteful to close the IGVs and restrict air flow through the unit just to raise the exhaust temperature if the unit were operating in simple cycle mode. Also, if the unit has IGV Exhaust Temperature control and is exhausting into an HRSG, it would likely be impossible to close the IGVs any more than they would be at part load because the exhaust temperature should be at or near the maximum allowable value for the current operating conditions.

As far as I can remember, Manual IGV Control cannot be used to increase IGV angle over the "automatic" reference calculation to reduce exhaust temperature. I could only think of one reason one might want to decrease exhaust temperature at Part Load and that would be to reduce the amount of heat going into the HRSG for some reason (whatever it might be) and still maintain a particular load.

If the unit has IGV Exhaust Temperature Control and it is enabled and one would like to lower exhaust temperature and still maintain a particular load at Part Load (assuming the operating conditions would permit), it should be possible to deselect IGV Temperature Control and in that case the IGVs might (depending on the load and ambient conditions) open slightly and reduce the exhaust temperature.

Remember, the purpose of IGV Exhaust Temperature Control is to maximize exhaust temperature at Part Load by keeping the IGVs closed as long as possible without exceeding the maximum allowable exhaust temperature for the given operating condition.

Thank you very much for the response. My point is actually in your last three paragraphs. Unit is with HRSG with part load and IGV is still 57. I would like to open more to reduce temperature and maintain load. I don't care about HRSG temperature. Ehy cannot I open more than 57? it will be safer for temperature will be lower, only CPD will increase. Please advise?? Thank you.

CSA,

Your excellent feedback on this is timely, as I was about to ask a related question.

At my plant, we are running 3 Frame 6 turbines, also on Mark V control. It is a Cogeneration plant, where the Gas Turbine is coupled to a HRSG and the steam is then used for our process.

Our operators have also used manual control as a means of controlling exhaust temperature, in our case, fully opening the IGVs to lower the exhaust temperature. This is because we do not require that much steam for our process, and want to minimise venting of steam.

My question is:

How does manually setting the IGV to a maximum opening of 84 degrees affect the turbine efficiency?

What happens to fuel gas consumption? Surely it should increase?

Since the IGV is opened at maximum, what happens during a surge?

Appreciate your feedback on this.

Thank you!

You haven't told us if the unit has IGV Exhaust Temperature Control or not. If it does, have you tried disabling it? That would start opening the IGVs sooner.

I don't understand. You say you want to "...reduce temperature and maintain load...." and then you say you "...don't care about HRSG temperature...." If the IGVs were to be opened more than the Speedtronic-determined value, the effects will be: reduced exhaust temperature and reduced load (more energy will be used to drive the axial compressor). So, to maintain the load it would be necessary to increase fuel which would then increase exhaust temperature. It's kind of a circular dilemma.

Opening the IGVs would decrease exhaust temperature which would reduce steam production and reduce load if the unit were operating on Part Load *without Pre-Selected Load Control enabled.* If Pre-Selected Load control were enabled, the Speedtronic would increase fuel to maintain load which would in turn increase exhaust temperature.

If the load were very low (say just above 0 MW), increasing the IGV angle would increase the air flow through the unit, which might afflect flame stability because the fuel flow is very low at that point.

But, I don't understand what you're saying at all. What is the "safety" concern?

And again, does the unit have IGV Exhaust Temperature Control and if it does have you tried disabling it?

I appreciate very much the responses and I focus on your statement (which I expected) that is by opening IGV then reduce temperature and to maintain same load must increase fuel which then will increase exh. temperature. I would like to know if the temperature will go back to the same degree or lower or higher. What I would like to know is which one has the larger effect on the exhaust temp.: the opening of the IGV or the increase of fuel (load constant = same load which is part load, the unit has temperature control but in this case I would like to open manually and increase the fuel manually also to get back the same load.)

- Yes, overall efficiency is absolutely effected.
- Fuel is increased when in pre-selected load.
- There are many factors should be observed before it goes to surge.

I think the auto reference will prevent the closing, not the opening, because closing will be more dangerous than opening. Victor has the experience opening it even to 84. Victor, please confirm that you can also open at a degree between 57 and 84 and HOLD it in that position, just say e.g. 70 degree.

Thank you for the information!!

Victor,

If you are using Manual IGV Control (not forcing logic or manipulating the IGV reference or changing Control Constants), then there should be no problem. I believe if you work backwards from CSRGV (which is the "main" IGV control reference) you will find a MIN SEL block that is monitoring the various IGV control references (such as TTRXGV and CSRGVPS) and should (normally) prevent putting the IGVs into a position that would be dangerous to the axial compressor.

I would expect that opening the IGVs to 84 degrees would adversely affect the turbine efficiency. You should be able to verify that by plotting fuel flow-rate, load, and IGV angle, then selecting Pre-Selected Load Control and setting some value of load, then selecting Manual IGV Control and opening the IGVs. If the fuel flow-rate increases for the same load as the IGVs are opened then I would think that the efficiency decreased.

Again, if you are using Manual IGV Control and not forcing logic or manipulating the IGV reference or haven't changed IGV Control Constants, and the Mark V is permitting the IGVs to be opened to 84 degrees there should be no issue with a surge when the unit is synchronized to the grid. I don't believe that GE would have programmed the Mark V to allow the IGVs to go to full open unless the compressor was adequately "protected" (staying within the margin of protection deemed safe to protect against surge).

(If you're not using Manual IGV Control to open the IGVs to 84 degrees during part load operation, please don't tell us and especially don't tell us how you're doing it. I believe that many older Frame 5s and Frame 6Bs didn't have "modulated" IGVs, and they were pretty strong and well-built machines, which were normally operated with their IGVs at "full open" when synchronized. I don't know if later Frame %s and Frame 6Bs equipped with modulated IGV control were capable of similar operation. In general, the limiting factor of most heavy duty gas turbines today is the ability of the axial compressor to flow as much air as could be used in the combustion of fuel. In other words, many heavy duty gas turbines, especially the larger machines, are being operated right at the limits of the axial compressor's ability to maintain sufficient load at all operating conditions.)

Has the unit experienced an axial compressor surge during such operation? A single shaft machine connected to an "infinite" grid should be pretty stable speed-wise. As I understand axial compressor surge, aren't there speed fluctuations involved? (I've only experienced axial compressor stalls, and they're *NOT* fun as the turbine/compressor drops a lot of speed very quickly. Both of my 'experiences' were during unsynchronized operation.)

The Wikipedia definition of surging also includes stalling so I'm not clear; I think there can be stalls at various places in the compressor under different circumstances. Also, the Wikipedia article is more geared towards jet engines, not single-shaft heavy duty gas turbines. Axial compressor operation seems to be very "theoretical" in nature and, kind of like a certain kind of speed control, a little mystifying to most people. I just haven't found someone who can really explain it so that the explanation can be related to others, and all the reference books or articles I've found are either very technical or very brief in their descriptions.