GT tripped on FSNL with combustion trouble, exhaust temperature high spread" alarm.

Seeking opinions on a recurring issue with our 127MW GE Frame 9E gas turbine in a 1+1+1 combined cycle configuration.
We've experienced three consecutive turbine trips with the "combustion trouble, exhaust temperature high spread" alarm while the machine was on FSNL.

The most recent incident happened during start-up after outage for CI and rotor balancing activity. The turbine was on FSNL for 4 minutes, and the allowable spread limit constantly decreased, triggering the alarm and causing the trip. we also carried out BI of CI parts & no finding related to hardware was observed.

This issue also occurred consecutively during the last two shutdowns in August and September, with a possible connection to SRV/GCV hunting in August, but not in September. GCV & SRV calibration in the recent outage & no anomaly was observed.
In all cases, the trips occurred within 4 to 5 minutes of the machine being on FSNL.

Looking for insights and opinions on this
Welcome to

There is over 20 years of GE-design heavy duty gas turbine controls-related threads and topics which are searchable using the 'Search' feature at the top of every webpage. And combustion trouble trips (high-high exhaust temperature spreads) is one of the most common such thread on It's been asked and answered just about every way.


You didn't tell us what fuel the unit burns when it trips on high-high exhaust temperature spread. You haven't provided any actionable data--such as which exhaust T/Cs are really high and which are really low and where the highest and lowest T/Cs are located. (Graphs; recorded data; etc.)

Has the fuel source/supply changed recently?

You haven't told us which turbine control system is in use on the Frame 9E.

You haven't told us which combustion system is in use on the machine (DLN or conventional).

It's not uncommon for machines to have problems maintaining flame in all the combustors during starting and acceleration to FSNL. It just may be that the start-up and acceleration FSRs need a little tuning to make sure all the combustors are lit and stay lit during starting and acceleration. Remember there are usually only four flame detectors for the 14 combustors during starting and acceleration--so if flame is lost or flickering in combustors without a flame detector that's going to be reflected in exhaust temperature T/C differences.

It might also be that the ambient temperature has change recently--we don't know these things. You have to tell us for us to know these things.

But, you need to provide more data--actionable data (recorded data; graphs; screen captures) during starting and acceleration and once the machine reaches FSNL.

The combustion monitor function isn't active until the machine reaches FSNL. And then, the maximum allowable exhaust temperature difference usually ramps down to it's normal value (which can be affected by some sensors not working properly--that's why we ask about Diagnostic Alarms and Process Alarms!!!). And, if flame was lost in one or more combustors--with or without flame detectors, because it could even be lost in a combustor with a flame detector, but there WOULD BE Process Alarm(s) to indicate that!!!--and it didn't get re-established before the IGVs opened and the compressor bleed valves closed it might be difficult to re-establish it at FSNL (for some combustion systems--and we don't know what combustion system the unit has--because you haven't told us).

EVERY FRAME 9E IS NOT LIKE EVERY OTHER FRAME 9E. Full stop. Period. There are subtle differences between almost all of them--they've been produced for more than 30 years, and packaged by several different packagers--not just GE.

Just because you have a Frame 9E and you can tell us a tiny bit about the problem(s) you are experiencing doesn't mean we can tell you precisely what the problem is and exactly how to fix it. Thank you for attempting to describe what you've done (I presume BI means Borescope Inspection and CI is Combustion Inspection) and what the results were.

I presume that you are able to get the machine to start and make it to FSNL and synchronize on the next attempt, when the machine is "warm", and I presume the machine is "cold" when it has this problem of tripping on high-high exhaust temperature spread when it reaches FSNL. If so, again, it sounds like some tuning of start-up and acceleration FSRs needs to be done. But we can't tell you how to do it or what to do because we don't know what kind of combustion system the machine has, or what fuel(s) it burns.

Help us to help you. Give us more information. You will probably be pleased with the responses. But, you haven't given us enough information to work with.
Hi sir,

What's your turbine usual spread value?
Should be around 20°C or so

are all the TTXD healthy and within the same value with each other?
On my turbine TTXD drifting at low load happen, because :
TTXD is not correctly mounted and fully inserted, and reduced exhaust flow make it difficult for them to catch the metrage

Also, do you have other system that monitor combustion? Do they catch some anomalies at the same time?

do you get combustion tunning done before those incidents appear?
If FSNL spread reaches more that could be due to one or more liner does not have flame. if possible increase the load, the machine may crossfire after the little loading
before going to touch the hardware kindly make sure all instruments(TTXD, GCV, GCV2(IF), and SRV) are okay.
  1. what about the flame intensity?
  2. Check the integrity of (TTXD)TC first (measure the mv, physical inspection inside the exhaust plenum for tip damage or not correctly inserted )
  3. from firing to the FSNL monitor, the lowest and highest temperature readings are changing or standing at the same TC
  4. if its TC(TTXD) values are changing while firing to FSNL that means you have a real combustion issue and if not changing TC issue
  5. fuel nozzles chocking
  6. liners or TP damage
  7. check the hot air leaks inside the Turbine compartment
The machine usually runs on partial load as per grid requirement on average at 100MW with two fuels availability i.e natural gas and high speed diesel. The unit primarily burns natural gas with conventional combustion system and Mark VI control.
The usual spread values during partial load are on average around 10°C.
For first two trip events the machine was in "warm" condition while for oct-21st event the turbine had cold startup after combustion inspection outage.
I am attaching the alarms summary and radar graph of T/C temperatures right before trip, along with the graph showing comparative values of allowable spread limit and spread values.
No chocking of fuel nozzles was observed during the Combustion inspection. There was minor Liner and TP damage tho.
The preventive maintenance of exhaust thermocouples was also performed during the recent combustion inspection outage. No anomalies were observed.


So, thank you for the answers to some of the questions.

I see multiple alarms for P2 pressure high and low--it seems like something is causing the P2 pressure to oscillate, and that's usually either the gas control valve OR the supply pressure is fluctuating; either will cause the SRV to hunt to try to maintain P2 pressure either because the gas control valve is hunting or the gas fuel supply pressure is hunting, respectively. I could conceivably see a situation where when one problem started (say the gas fuel supply pressure started swinging) that would also start the other system swinging (the gas control valve when trying to maintain speed at FSNL).

I'm presuming the machine is being started on gas fuel--but it's not really clear, since it can run on either fuel (gas or liquid/distillate).

I ALSO see several instances of flame detectors losing flame--which is what I described in my earlier response. If the P2 pressure is unstable then the GCV is probably also going to be unstable and that's going to cause speed and fuel flow-rate to be unstable, and if the machine's start-up parameters are iffy to begin with, then this is a problem.

Were the SRV and GCV "calibrated" during the CI? If so, were there any significant differences noted between the as-found and as-left LVDT feedback calibration values?

I have NEVER heard of exhaust thermocouple preventive maintenance. In almost 40 years of working on GE-design heavy duty gas turbines, I've never heard of that. Can you explain what exactly exhaust thermocouple preventive maintenance is, please? Does it involve determinating the exhaust thermocouple wires and reterminating them? Does it involve removing the exhaust T/Cs from the radiation shield and re-inserting them? I presume the preventive maintenance involved someone going into the exhaust plenum and inspecting the thermocouple tips to make sure they are correctly installed, not touching metal, and centrally positioned in the radiation shield. And, I would expect this would be done AFTER any exhaust thermocouple preventive maintenance, especially if the thermocouplese were removed and re-installed. And, if they weren't removed I would still expect the exhaust thermocouple preventive maintenance procedure to include a visual inspection of the tips in position in the radiation shield in the exhaust plenum.

Photos need to be clear to be of real use. If you have or can obtain the Alarm History from the web browser for the trips that would be very helpful.

Based on the information provided, it appears the gas fuel flow to the fuel nozzles is unstable and flame is not stable in all the combustors at FSNL, and it would seem another indicator of that would be turbine speed would also be unstable at FSNL. This is actually a very dangerous situation, as unburnt fuel can amass in the exhaust plenum, and if the turbine exhausts into a boiler (HRSG) it can also collect there and it can suddenly ignite which would lead to serious problems, injury or even death.

It's not clear why this only happens occasionally (as you say you are able to keep STARTing the machine and it will eventually lead to a successful synch and loading), but if the gas fuel supply is not stable, or unstable at certain times (say when multiple machines are running, or not running, as the case may be) this could explain why the problem is intermittent. Cracked combustion liners can crack more with each successive trip--even if it's just from FSNL, the sudden thermal differential can worsen cracking (and when the unit trips, the compressor discharge pressure decreases quickly, too).

But, based on the information provided, I don't have anything else to offer. If you have Alarm History files from the trips that would be very helpful, more helpful than unclear photos with little explanation. I can't tell if the unit tripped before or after synchronization (I was presuming it tripped before synchronization, but if it tripped after synch that could possibly worsen the loss of flame in individual combustor(s) problem because immediately after synchronization the Mark* VI will be increasing fuel flow-rate to reach Spinning Reserve (usually about 5 MW, which is not that much fuel flow for a GE-design heavy duty Frame 9E.... but if the Mark* VI is trying to increase fuel flow in a machine which doesn't have flame in all the combustors that's not good, either).

Again, the problem may be tuning. The problem may have been made worse by "calibrating" the fuel control valves (which IS NOT what AutoCalibrate does!!!) improperly and without comparing the as-found and as-left calibration values. AND, if the unit(s) get their fuel from LNG and the source of the LNG has changed recently, that could also contribute to the inability to establish and maintain flame in all the combustors during starting and initial loading.

FINALLY, there is often a misconception that the Combustion Monitor (which looks at exhaust temperature spreads and calculates an allowable spread value) is ALWAYS active--meaning during STARTing and acceleration. And, that is simply NOT true. Fuel flow-rates are changing during STARTing and acceleration and they should be fairly stable at FSNL, so the Combustion Monitor IS NOT ENABLED until the machine reaches FSNL when the fuel flow-rate is--or, rather, should be--more stable.

Please let us know how you resolve the problem. If you require further assistance, we require better data (often called "actionable" data). Trends are available from the Trip History function of the Mark* VI, and they would also very helpful.

Best of luck, fatima.
thanks for the report

what looks strange to me, is that the spread value doesn't swing
and looks stable and with good value when the limit rise

but those chart doesn't tell about live value

Is this common to you to have exciter trip and 52G breaker trip on start up ?

The Trip History display of the Mark* VI WILL have individual exhaust thermocouple values clearly recorded and visible.

The allowable exhaust temperature spread (signal name TTXSPL) has a maximum value and when the machine reaches rated speed (FSNL) and the Combustion Monitor function is enabled the allowable exhaust temperature spread (TTXSPL) will start ramping down from the maximum value to the calculated value which is determined from several operating (real-time) values, including but not limited to axial compressor discharge temperature thermocouple readings. The ramp time from maximum to calculated allowable exhaust temperature spread is usually around 30-90 seconds. If any of the values that feed into the allowable exhaust temperature spread calculation are amiss that will affect the calculated value of allowable exhaust temperature spread.

Combustion trouble alarms and trips (there should be a COMBUSTION TROUBLE Process Alarm approximately nine (9) seconds BEFORE a COMBUSTION TROUBLE TRIP (yes; you're right, nine (9) seconds is not enough time to take corrective action but that's the standard). If the condition isn't corrected (or something isn't done to correct the problem with the allowable exhaust temperature spread problem) the unit will trip.

A Combustion Trouble trip is NOT just a problem with one or more of the actual exhaust temperature spreads (TTXSP1, TTXSP2 and TTXSP3) exceeding the calculated allowable exhaust temperature spread. For enhanced reliability and redundancy the Combustion Monitor function checks for the adjacency of the highest and lowest exhaust thermocouple temperature values (the Combustion Monitor function sorts all of the exhaust thermocouple values from highest to lowest, AND then it sorts them by location--all in the same scan of the application code. And it uses those two arrays created from the sorting functions to check for magnitude of temperature differentials AND adjacency. (It's normal to see a combustion problem manifesting itself in the exhaust temperature readings by having a really cold temperature next to the next lowest temperature and possible even to the third lowest temperature.)

It's also a common misconception that the Mark* throws out the highest and lowest exhaust thermocouple temperatures when calculating spreads and adjecency--which is false. It does that ONLY for calculating the average exhaust temperature reading, NOT for the Combustion Monitor function.

USUALLY when a turbine trips for some reason like Combustion Trouble there IS NOT a Process Alarm for exciter trip or 52G breaker opening--because that's expected to happen during a turbine trip (emergency shutdown). THERE IS, however, an SOE (Sequence of Event) for these conditions if that function is enabled for monitoring unit operations (and, yes--it seems to be enabled for this machine). Those AREN'T Process Alarms (SOEs and EVENTs); they are just messages to let a conscious and properly trained operator and Operations Supervisor to things changing state when the machine is operating--even if those things are NORMAL and to be expected. They're not alarms--they're just like a sequence of event recorder recording changes of state during operation (SOE means Sequence Of Events).

FSNL means Full Speed-No Load. IS THE MACHINE TRIPPING WHEN LOADED (after synchronization when 52G is closed) OR WHEN IT'S NOT LOADED (when 52G is open)? Because, that's a BIG difference--unloaded operation versus unloaded operation. If you mean it's tripping after synchronization (when it's loaded), that NOT the same as FSNL (which is unloaded operation).

You might think I'm being difficult--I'm trying to help you understand you need to be precise when you are asking for help about a technical problem. It's important, especially in today's world when so many things are so technical--and GE-design heavy duty gas turbines are very technical. Please be clear when asking for help; it avoids misunderstandings.

AND, remember: If you could have solved the problem without any assistance you wouldn't have posted on an anonymous World Wide Web forum for help. So, if we ask questions it's because we're NOT sitting or standing beside you and we don't know what you know and we haven't seen what you have seen. That's why I asked for the Alarm History and the Trip History--because those things can answer a lot of questions without you having to write all of the answers to my--and others'--questions. We are trying to help you--help us to help you.

Exhaust spread trips are not the easiest things to troubleshoot. It's even MORE difficult when the exhaust spread trips are intermittent and not repeatable. In my personal opinion, this problem started AFTER a CI (Combustion Inspection)--when exhaust thermocouple preventative maintenance was performed. And we/I still don't know what that is or what it entails or how it's performed. BUT, now, mysteriously there are intermittent issues with exhaust thermocouple readings (and maybe even axial compressor discharge temperature readings). There are flickering flame detectors (L28FDC and L28FDD, if I recall correctly)--which indicates that flame IS NOT STABLE during acceleration and possibly not at FSNL (or initial loaded operation...).

One thing I can tell you with a very high degree of certainty: The problem IS NOT the Mark* VI. The problem is either something amiss with one or more of the combustors or transition pieces or side seals or cross-fire tube assembly/installation, OR it's a problem with exhaust thermocouple wiring or installation, or the axial compressor discharge thermocouple installation or wiring (signal names CTDA1, CTDA1 and CTDA--which would mean there are Diagnostic Alarms which aren't being investigated and resolved...!). The Mark* VI is as good as it's inputs--and that includes LVDT inputs and thermocouple inputs (ALL thermocouple inputs).

Without more actionable data, there's nothing more we can do. Again, we haven't seen what you've seen; we don't know what you know; we don't have access to the data you have. If you need some help accessing the data, we can try. MOST--but not ALL--GE Mark* VI HMIs store alarm history and operating data and history. But many of the files are overwritten after a few days or weeks. That's why it's important to access that data and history immediately after a trip. And store it in a safe place. For future use.

And learn how to use Trend Recorder in Toolbox. It's not difficult; it's the best troubleshooting tool there is--it's free, and it's powerful. And it won't trip the turbine. Ever.
as a trained operator, i've seen alot of trip and runback by combustion trouble
which were initiated by unpoperly mounted TTXD or dying one

that's why my guess was faulty TTXD at first
and i'm curious about seeing the TTXD trend when trip happend

Sequence of event, will be stated as class SOE in the alarm viewer, which is not, that's why i'm asking why breaker trip is alarmed prior to combustion trouble
because brutal trip can cause combustion trouble especialy when bleeds valves / IBH trips open

Also, i agree that P2 pressure alarms swinging between HH and LL are not goods omens,
and probably show a trouble with VGM or VGC to regulate smoothly

But why spread doesn't show us this swing if combustion is so bad ?
from the trend Fatima provided, it doesn't look bad
Actually, it could be the Mark* VI. And, yet again, I have failed to ask what Diagnostic Alarms are present during these start-ups and acceleration to FSNL and possibly initial loaded operation.

I UNDERSTAND--a single Diagnostic Alarm will not trip a turbine. BUT, there are combinations of Diagnostic Alarms that will trip a turbine. NO; there is no list of which combinations of Diagnostic Alarms will trip a turbine--there are literally thousands of possible Diagnostic Alarms, so that's out of the question for a comprehensive list of combinations of Diagnostic Alarms that will result in a turbine trip.

Diagnostic Alarms are trying to alert a conscious and diligent operator to problems with the Mark* hardware which could lead to operational problems. Yes; the Diagnostic Alarm messages are cryptic and difficult to understand. BUT--the Mark* VI has Diagnostic Alarm troubleshooting help in the Mark* VI System Guide, GEH-6421. So, that is a lot better than people have it when working with Mark* IV or Mark* V! A LOT better.

Diagnostic Alarms left unresolved are just problems begging for more serious problems. (Just like 125 VDC Battery Grounds--left unattended, there can be multiple grounds which makes finding them more and more difficult.)

Do the right thing. Troubleshoot and resolve Diagnostic Alarms (and 125 VDC Battery Ground alarms) as soon as possible. If you need help diagnosing Diagnostic Alarms--ask here at Just be prepared to do a little investigation and answer some questions.

Contrary to extremely popular belief and urban myth posting a couple of sentences about an issue with a GE-design heavy duty gas turbine and expecting detailed and concise answers and instructions for solving problems is unreasonable and very, very infrequently results in quick, concise recommendations; it does occasionally happen, but it's rare. The World Wide Web has only changed one thing about troubleshooting Mark* turbine control issues: A wider variety of people can read and respond to your queries and requests. But, without actionable data--and answers to questions asked in trying to understand the circumstances and situations--we, even us experts, are really just making a guess (or guesses) as to what might be wrong.

I, and others, recognize that sometimes the problem can be overwhelming AND that sometimes posters don't speak English well which makes asking questions and posting answers a little more difficult. But, there is more than twenty (20) years of GE Mark* turbine control-related posts here on searchable using the magnifying glass at the top of almost every webpage. And, MANY people who ask for help or quidance or recommendations write back to let others who read these posts if the information provided was helpful or not. And THAT is what makes one of the most helpful World Wide Web forums around--because people who read these threads can see and understand what works and what doesn't and can learn from the discussion and troubleshooting. Really, there is a WEALTH of Mark*-related information here on; some would say it's an embarrassment of riches (meaning there's so much information in one place it's kind of embarrassing to have all that information so easily available).

Sure, you may not find the exact answer to the question you have--but then maybe that's because you aren't seeing the problem from a different perspective. MANY people write in with a preconceived notion of what the problem is or may be and they frame the questions to get validation of the answer they want, when, in reality the problem is not what they think it is or is caused by. So, when you're searching for information on you don't always come across it quickly or in exactly the same question as you have. (We have repeated the help suggestions for many questions in many different ways over the two-plus decades of questions and answers here on Reading through past threads will often be very helpful and trigger other thoughts and questions. There SERIOUSLY is a lot of information on And, if you need clarification, you can always ask.

So, to finish, it COULD BE the Mark* VI--but if it were there are likely several, perhaps many, Diagnostic Alarms trying to alert someone to the problems with components/card of the Mark* VI. But, again--the Mark* VI is only as good as its inputs (and outputs), and that includes proper LVDT calibrations. And sometimes Diagnostic Alarms will also warn of improper LVDT calibrations, as well.
Huh? I thought the original poster had responded that the problem had been solved (though with zero details...).

Hope he decides to post again with some details. A past contributor to always said, "Feedback is the most important contribution to!" (Or something very similar to that, anyway.) Again, it's the feedback that lets other people reading the thread--now and in the future--what the problem was and how it was solved. THAT'S the real beauty of these World Wide Web forums--not just getting quick answers to questions, but the journey of solving problems and getting to a solution.


Just for closure, this was what was originally posted:

"This was a recurring issue but has been rectified in the recent outage and we have been able to have a smooth startup after that."

Happy to hear the problem was rectified.

Sad not to hear the details.

It's the life!