IGV Opening at Base Load.

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Thread Starter

Amr1589

An upgrade has been performed to 9FA unit recently. this upgrade included combustion system from DLN+2.0 to DLN 2.6+, advanced hot gas path & mark 6 to mark 6e.

Recently noticed that during base load operation IGV goes below its  max operating angle (88.1). the message "Base load" in HMI and FSR in temperature control mode. Does it relates with ambient conditions? because this happens only during high ambient temperatures. How could unit be at base load and IGV still can open further more?
 
Thank you in advance :)
 
Amr1589,

What are the Process Alarms being annunciated when this "problem" is occurring?

What are the values of CSGV and CSRGV when this "problem" is occurring?

Also, what are the values of TTRXP, TTRXS and TTXM when this problem is occurring?

If the IGV reference, CSRGV, and the IGV position feedback, CSGV, differ by more than 0.1 or 0.2 DGA, then either something is amiss with the calibration procedure or the servo current polarity or the null bias current being applied. Or possibly all three.

It's also possible, though not very likely, one or both of the IGV LVDTs is worn at the maximum open position and the output is not stable.

The IGV reference, CSRGV, is the output of a Minimum Select function. So, use ToolboxST to find where CSRGV is written to and monitor all the inputs to see what's happening.

And then ask the supplier of the upgrade to explain why the HMI is indicating Exhaust Temperature Control when the IGVs are not at CSKGVMAX. I suspect the 'Base Load' indication is the active Load selection, because the usual proper indication of Base Load is the Status message "Exhaust Temperature Control." This same question usually comes up when operators are using a Pre-Selected Load Control setpoint that results in a value of TTXM that is nearly equal to TTRX--and the control "bounces" in and out of Droop speed control and Exhaust Temperature Control. This is YET ANOTHER reason NOT to use Pre-Selected Load Control for anything other than load changes (and there are many more reasons NOT to use Pre-Selected Load Control for anything other than load changes).

And, further there may be some issue with either IGV LVDT calibration (including the value of null bias current specified) and/or the polarity of the servo current being applied to one of the IGV servo coils is not correct.

But, please re-read your post and the Summary and clarify the question: Are you asking why the IGV angle can be less than CSKGVMAX when operating on CPR-biased exhaust temperature control, or how the IGVs can open more than CSKGVMAX, or???

Please write back with clarification and answers.
 
Dear CSA,

Sorry I forgot to clarify that the upgrade has been performed to two gas turbines and that happened to both of them and at the same time. so it can't an issue with LVDT of IGV nor the servo.

There is no alarms being annunciated prior to that action. That's why i asked if it relates to ambient temperature because while weather gets hotter during noon and afternoon. ambient temp becomes higher than 35 c. IGV lowers from 88 to 82. CSGV is following CSRGV normally and no problem with that.

we already asked GE representative. he checked the toolbox for a block where CSRGV is an output. and what he said was "it is not a problem, it is an extra control that limits IGV opening to achieve maximum performance of the unit" which I can't understand how could it be maximum performance while IGV is not fully opened! I mean more air mass flow rate can be achieved!

-Yes my question was (how the IGV angle can be less than CSKGVMAX when operating on temperature control and IGV control message in HMI is (max firing temp)?

and if you can give us more explanation about this NEW control?
 
Is this unit simple cycle or combined cycle? If it is combined cycle, "performance" will refer to the combined cycle output, not just the gas turbine output, and it could be that with the IGV's slightly less than full open the exhaust gas temperature is higher and the steam side of the system is performing better.

I'll admit I'm still scratching my head on this and I think GE should provide you with a better explanation as to why the performance is higher. (At the time I retired from GE, I believe the full open IGV angle on a 9FA was 87 degrees. However, at that time there were programs in place to improve performance and to reduce startup times on the combined cycle units.)
 
Amr1589,

Can't comment on the NEW control code without being able to see it. I suspect what the field service person meant to say was that a new algorithm was included to protect the axial compressor against surge/stall conditions which can occur under certain ambient conditions (which should have been outlined) and that when active the algorithm will temporarily close the IGVs to protect the compressor. The intent of the new algorithm is to maximize power when possible, but because the axial compressor is the weakest link in many heavy duty gas turbines as far as being able to flow maximum amounts of air under typical conditions compressor protection algorithms are necessary to protect the compressor when conditions approach a limit--and that limit should have been detailed/outlined at the time of explanation.

That's just my SWAG (Scientific Wild-Arsed Guess) based on past experience, and I can tell you that GE-design F-class heavy duty gas turbines (including FA machines!) are built with extremely little engineering margin, and as mentioned, the axial compressors are the weakest part of the system.

I also failed to ask if, during these IGV "problems." the grid frequency was stable or not. Because off-frequency operation of these machines can contribute to axial compressor issues, or, more correctly, put them closer to the limits of safe operation. A LOT of 9F and -FA machines seem to be operating in parts of the world where the grid frequency is very unstable, and they weren't really designed for that. And, many owners and operators and operations supervisors mistakenly believe that the unit should continue to produce rated power--if not more power--when the grid frequency is unstable, which is just not possible.

The OEM has designed the new HA-class turbines--and tested them extensively--to operate in off-frequency conditions so as to be more capable of responding to grid events to support grid stability. But, I don't expect that new axial compressor designs for F- (and FA-) class machines will be forthcoming soon, or if they are, that they will be "economical." In the meantime, with increased IGV openings compressor protection becomes even more important. Many 9F- and 9FA-class gas turbines have had various compressor modification over the years--we don't know what's been done at your site, if anything. But, again--the ability to make more power with current F- and FA-class heavy duty gas turbines is limited by the axial compressor designs (and materials), and while attempting to produce as much power as possible as often as possible it's often necessary at times to limit air flow to protect the compressor.

Axial compressors are unique machines, and have unique characteristics from other types of compressors and many believe they are very robust--as robust as the gas turbine section of the unit, which is not always true. F- and FA-class axial compressors are, for the most part, being operated at the very limits of the design (and materials), and as such, require monitoring and protection which doesn't always operate "as expected."

But, without being able to see exactly what's running in your Mark VIe, it's not possible to say anything more than the above based on the information provided and past experience.

Hope this helps!
 
The algorithm is called Model Base Control [MBC] which limits IGV opening to keep compressor discharge temperature CTD within allowable operating limits.

* CTD highly increases in case of high ambient temperatures (high CTIM) Throttling IGV will decrease CPD and therfore CTD. That's what i could get till now as the units still under reliability period after the upgrade.

Why this happened after the upgrade? I mean neither IGV nor compressor section has been touched in that upgrade so it is supposed that no change in compressor protection algorithms be done.

But i should mention that a correction has been done in bleed lines from the compressor in a way to decrease the amount of air being extracted for cooling the turbine as the new advanced hot gas bath parts (made of better alloys to withstand higher temperatures) don't need much cooling like previous ones. Does it have a relation to our topic?
 
Amr1589,

Another victim of the OEM Sales group.... This particular OEM, when selling gas turbine control upgrades, believes they are both entitled, and required (by safety regulations and standards--many their own), to provide every turbine they retrofit with the same control and protection algorithms provided with similar new turbines. Instead of just duplicating what was in the previous control system, and then applying safety updates and asking the Customer if they want this or that enhancement, they start the engineering process from zero as if it were a new turbine and then have their engineers go back and make modifications to try to get the control system to work with the auxiliaries in place on the unit (which may or may not be the same as on a new unit).

If you're getting a new programmable control system on an existing unit wouldn't you naturally think and presume the supplier is going to duplicate what was done in the old control system first, and then make enhancements and upgrades? But, NOOOO--this division of the OEM doesn't do it that way. And therein lies a lot of the problems with turbine control system upgrades and their implementation and commissioning.

This causes a LOT of unnecessary effort and re-work, and leads to a LOT of commissioning problems. Worse, when pressed to explain what changes were made--even for safety purposes--the OEM can't detail them or provide any information except on a one-by-one basis as problems become apparent. And, for most Customers like yourself--who just expect that the turbine is going to operate like before the upgrade, but with newer and better control systems hardware and software methods--are <b>extremely frustrated and unhappy</b> to learn of the changes to their machine. Changes which,in some cases, require changes to their standard operating procedures--which they should have been made aware of prior to awarding the turbine controls upgrade contract, or at least before commissioning!

MBC--Model-Based Control--is touted as "the next best thing since sliced bread" (which means it's a GREAT modern invention--if you like sliced bread, that is!) for GE-design heavy duty gas turbine control and performance. It has great potential, but really only when connected to the OEM's IIoT (Industrial Internet of Things) which provides them with free and unlimited access to operating data. Field service people can't look very deep into the algorithms of MBC, get very little training in how it works, and as such rely almost exclusively on remote support for troubleshooting and problem resolution. It's a wonderful thing, the promise of MBC--but it's most wonderful for the OEM because now you are tied to them for service and support since most everything is locked down and password-protected. You get the explanations and information they want you to have, while they get access to your operations and data to enhance their "big data" database.

The OEM has done and continues to do a masterful job of spreading fear amongst owners of older heavy duty turbine control systems by issuing end-of-life letters and doing a poor job of supporting older control systems they manufactured and produced. They are only interested in selling new control systems, and in fact, the division that produces the control systems doesn't even provide the installation, commissioning or after-market service and support--it's all done by different divisions of the same Company, each with its own profit targets and objectives.

And, while it's not necessarily the operators and technicians who fear lack of OEM support, it's the middle- and upper-level managers of companies that own and operate the turbines that fear lack of support. They don't want to tell their management and shareholders they didn't upgrade the control system and that's why its on an extended forced outage (which isn't usually the case, that control systems cause forced outages--its the lack of spares and proper maintenance and neglect of alarms and housekeeping that cause control system failures and outages). So, when they decide to buy a turbine control system upgrade, it's mostly to protect their position/job--and they buy from the OEM, because, well, they can be trusted, right? It's their turbine and auxiliaries, right (the OEM's)? And if something goes wrong, well, the OEM has deep pockets to fix the problem.

So, your management has fallen prey to sales and marketing fears propagated to sell new control systems which also increase your dependency on the OEM for service and support--and for explanations when these new-fangled controls seem to not work as expected, or as they worked in the past.

Again, F- (and FA-) class machines have extremely little margin in materials and design, and many performance upgrades involve new, subtle tweaks to existing control--and even cooling--schemes and practices. These performance enhancements are usually only fractions of a percent, compared to the increases which can be achieved on many non-F- (and FA-) class machines when upgrading to improve output. The system you are describing is one of these fractional-percentage increases only possible with control and cooling system changes. (Now, a fraction of a percent increase in performance for a 200+ MW machine over 10- or 15 years can be a lot of money, but it just speaks to how little margin was built into these turbines when they were designed that the only performance enhancements which can be made are fractions of a percent. The machines are quite literally running at the very limits of the materials and designs and have been since they were new.)

<i>Based on the information provided</i>, I would say that CTD is being limited so as to limit the temperature of the air being used for cooling because the materials or the clearances can't withstand higher temperatures--which you probably experience regularly in your part of the world. Another unexplained "benefit" of the performance enhancement. Or maybe it was explained to upper management and justified with some spreadsheet or estimate of long-term revenue enhancement. "You'll lose a little bit of output (money!) on those really hot days, but in the long run you'll make more money than if you didn't purchase this performance enhancement." That explanation was not pushed down to operations and technicians, as these things are generally not made known to operations and technicians until after the fact--the fact that someone is complaining that, "It's never done THAT before you put that Mark VIe in!!!".

The OEM should be held accountable for explaining every "enhancement" and safety improvement "included" with a turbine control system upgrade in some kind of operations document provided with the turbine control system upgrade. The OEM believe they are entitled to make these changes without notifying the operators and owners. It's "their" (the OEM's) machine, after all, right? At least that's their argument. And locking owners and operators into long-term service agreements while being able to access vast amounts of operating data for free is hugely profitable to the OEM.

Middle- and upper-level managers of the OEM have been heard to say things like, "The automakers don't give you access to the code in the computers in the cars they sell--why should we give access to the code in the control systems we sell with OUR turbines?!?!" Autos are commodities, turbines are not--and autos don't have the auxiliaries turbines do, and aren't (generally) as complex as turbines and turbine auxiliaries--all of which require some understanding and familiarization and an ability to look into the computer code to troubleshoot, or even to understand (since operation isn't documented anywhere else!).

You're now going to be more beholden to the OEM for service and support. And, how did that work out with the previous control system? Were they helpful and supportive with the previous control system--even before they made it obsolete and decided not to support it? What makes anyone think the Mark VIe is going to be any different?

Oh--and by the way, the Mark VIe is into it's tenth year of production.... How does management feel now about their decision to buy the latest and greatest heavy duty turbine control system?

As more and more units fall under the MBC lockdown, it's going to be more and more difficult to find information from other sources and third-party consultants and individuals. There's already a dearth of good information out there, and it's only going to get worse.

Wish the news were better, my friend. Actually, it's pretty good for the OEM and its shareholders (and, in the interest of full disclosure, I own a boatload of shares of the OEM--though I disagree with their practices).
 
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