Change the IGV Angle Curve on Mark V HMI

H

Thread Starter

hassanj

Hi All,

We have been advised to change the IGV angle curve on GE Frame 6B HMI. This change is to decrease the IGV maximum opening angle from 84 deg to 81 deg.

There is an idea that the change will improve the heat rates by decreasing the angle as per advice.

Just wondering if someone has relevant technical info and kindly share.

Thanks,
Hassan
 
HassanJ,

> There is an <b>idea</b> that the change will improve the heat rate<b>s</b>
> by decreasing the angle as per advice.

Well, if the GT exhausts into an HRSG, then increasing the exhaust temperature by closing the IGVs will certainly do that. In fact that's exactly what IGV Exhaust Temperature Control (also known as "combined cycle mode") does. So if you're operating the unit in IGV Exhaust Temperature Control mode <b>at part load</b> then you already know that when the IGVs are at 81 DGA the electrical output of the unit is less than it is when the IGVs are at 84 DGA and the unit is at Base Load.

So, you have a choice--more electrical output and a lower <b>overall plant heat rate</b> (which comes at the sacrifice of gas turbine efficiency (higher gas turbine heat rate)), or a more efficient gas turbine and a slightly higher plant heat rate with more electrical output.

Some plants do have a primary responsibility to produce steam and electricity is the byproduct--sounds like this may be the case at your site. If you want to test your hypothesis there are two very simple ways to to do so without making a permanent change until you've decided this is the preferred method of operation.

The first is to load the unit to Base Load and then use Manual IGV Control to reduce the IGV angle to 81 DGA. The unit will stay at 81 DGA for as long as you like, for the purposes of gathering data. (Find the IGV Control display to find the Manual IGV enable and set point buttons. DON'T WORRY--you won't trip the turbine by switching to Manual IGV Control and reducing the IGV angle. It's only possible to reduce the angle below the current reference: it's NOT possible to increase the angle above the current reference with Manual IGV Control--and neither will trip the turbine. Just remember when you're done with the test to return the IGV angle set point back to 84 DGA, and reselect Auto IGV Control--and be sure IGV Exhaust Temperature Control mode ("Combined Cycle Mode") is selected and in operation--which it most likely already is. )

The second is to load the unit using the RAISE- and LOWER SPEED/LOAD buttons until the IGV angle is 81 DGA, presuming the unit is being operated in IGV Exhaust Temperature Control mode. The load and IGV angle will change slightly during the course of the day as ambient conditions change, but you will still get the data you can use to evaluate whether or not you want to make the change permanent.

And by data, I mean the load as well as the heat rate. You can't have Base Load power output <b>AND</b> higher exhaust temperature/lower overall plant heat rate.

If you want to make the change permanent you would only have to change one Control Constant--<b>BUT</b> it has to be changed in two places, and the change has to be compiled and downloaded to the control processors.

It is suggested you use the first method above and see if you get the results you desire and write back if you want to make the change permanent or search the control.com archives for past threads about permanently changing Mark V Control Constants.

Hope this helps!
 
HassanJ,

I need to correct a statement....

>So, you have a choice--more electrical output and a lower
><b>overall plant heat rate</b> (which comes at the sacrifice
>of gas turbine efficiency (higher gas turbine heat rate)),
>or a more efficient gas turbine and a slightly higher plant
>heat rate with more electrical output.

It should have read:

"So, you have a choice--<b>LESS</b> electrical output and a lower <b>overall plant heat rate</b> (which comes at the sacrifice of gas turbine efficiency (higher gas turbine heat rate)), or a more efficient gas turbine with more electrical output and a slightly higher plant heat rate."

In other words, you can increase the exhaust temperature by decreasing the IGV angle, but the electrical power output of the gas turbine will decrease (less mass flow through the turbine, causing the turbine control system to also decrease the fuel flow-rate). You can't maintain Base Load electrical output <b>AND</b> increase the exhaust temperature by decreasing the IGV angle. Gas turbines are most efficient when the IGVs are at rated opening and the mass flow through the machine (air and fuel) is at rated; this means the heat rate for the gas turbine is lowest (best) when the IGVs are fully open.

The overall plant heat rate can be increased by maximizing turbine exhaust temperature when the gas turbine exhaust heat is used to make steam. <b>BUT,</b> to do this will result in a decrease in the gas turbine heat rate in order to increase the overall plant heat rate. Again, if the primary objective of the gas turbine installation is to make steam (and it is for many plants) then electrical output is secondary and can be reduced to maximize steam production.

But, the original poster said the <i><b>idea</b></i> was that the heat rate<b>s</b> (plural!) could be improved by decreasing the IGV angles, and the only two heat rates I know of in a combined cycle plant are the gas turbine heat rate and the overall plant heat rate--and they both can't be improved by decreasing the IGV angle. AND, the electrical output of the gas turbine will decrease if the IGV angle is decreased.

Hopefully, otised will chime in to help explain this further!
 
Thanks a lot CSA for your prompt reply. As you mentioned, closing the IGV angle will increase the exhaust temperature and respectively the efficiency in HRSG. But I think I had to mention that all of our 5 units in WA are open cycle. Is that making different in the result?

Please guide us by considering the open cycle condition.

Thanks,
Hassan
 
HassanJ,

Quite an important little piece of information that was neglected, wouldn't you say?

Why would it be necessary to increase the exhaust temperature when operating in simple cycle (open cycle) mode? Does Earth's atmosphere need additional heat?

And, the idea of changing the maximum IGV angle on the HMI? Yes, one needs a means of changing the maximum IGV angle <b>in the RAM of the Mark V</b> and that's usually done using a program on the GE Mark V HMI (Control Constant Adjust). And, to make the change permanent one would need to change the value of the Control Constant in the Control Constant source file and also in the downloadable Control Constant EEPROM compiled value file--both of which reside on the HMI. But, simply changing it on the Mark V HMI won't change it in the Mark V RAM--one has to change it directly using Control Constant Adjust in the Mark V RAM from the HMI, and this does NOT change it on the HMI, only in the Mark V RAM.

I have to say I don't agree with this way of thinking, and that there may be more to this story than we have been told. There may be the odd set of circumstances (site elevation; site atmospheric pressure; unusual exhaust duct back-pressure; unusual inlet duct pressure drop; some non-OEM hot gas path components (specifically turbine nozzles, buckets, shrouds, seals, etc.), compressor/IGV/bellmouth condition, fuel composition, etc.) that might result in a slightly lower heat rate at reduced IGV angle, but one would also want to be sure that the instrumentation is accurate beyond doubt and that any assumptions being made about fuel or air flow or exhaust flow or electrical power output are reasonable and within industry tolerance.

I would also think you should have told us how this thinking was arrived at, and what lead to this conclusion.

And remember this: Free advice is worth exactly what you paid for it.
 
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