Hello, we have a gas turbine model 5002D that we cannot start after major repairs. The problem is that during the start and when the flame is formed, only 3 of the 4 flames are detected and the exhaust temperature difference is very high, but at around 1650 rpm, all 4 flames feel the flame and the exhaust temperature difference is normal. It returns, but the average exhaust temperature rises and finally it trips at 2600 rpm with a high exhaust temperature alarm that reaches 565 degrees Celsius.

 

Based on the information provided, it would seem that there is some issue with fuel control--or possibly IGV angle (though that would be somewhat unusual).

It's not uncommon for one or more flame detectors (sensors) to lose flame during starting, warm-up and acceleration. And, remember--there aren't flame detectors in every combustor; there are four flame detectors on your machine per your original post) and it could mean there are more combustors without flame.

There's a lot of information you haven't provided--such as what turbine control system is in use for the machine (it's likely a GE Mark* turbine control system, but what version (V, VI, VIe?).

What kind of combustion system is being used on the machine--conventional or DLN (Dry Low NOx)?

Also, what fuel is being burned during starting and acceleration?

Were the fuel control valve LVDT feedbacks calibrated during the outage? Were they removed, refurbished and/or replaced?

Does the machine have variable second-stage nozzles (between the HP and LP shafts)? If so, are they stable during starting and acceleration and operating normally?

What Process Alarms are being annunciated during starting and acceleration? (This is often very important to understanding the situation and should not be overlooked.)

What Diagnostic Alarms are being annunciated during starting and acceleration? (This is also often very important to understanding if the control system is working correctly--most people just consider Diagnostic Alarms to be nuisances, but they really aren't. Yes; the alarm text messages can be cryptic, but they can also be very helpful in understanding what is or might be happening.)

What has been done to try to troubleshoot the problem--and MOST importantly, what were the results of the effort(s) (besides the problem still persists)?

The combustion monitor function of a GE-design heavy duty gas turbine usually is not enabled (made active to protect the machine) until it reaches normal operating speed (for the HP shaft). It still makes all the calculations, it's just that the protection (Combustion Trouble alarms and trips) aren't enabled until start-up and acceleration are complete (for the simple reason that it's not uncommon for some machines to have problems establishing and/or maintaining flame is all the combustors during starting).

If this is something new to this machine, then you need to go back to what was done to the machine during the major "repairs." Such as what was done (if anything) to the fuel control valves and/or fuel control valve LVDTs; what was done in the gas turbine exhaust (were exhaust thermocouples removed and replaced, for example); things like this. Because, again, if this problem just started--and it was after a maintenance/repair outage then something occurred or didn't occur or wasn't done properly or wasn't reassembled property, or the wrong parts (like fuel nozzles, fuel check valves, etc.) were used in the reassembly.

It's also very common for the gas turbine exhaust temperature to reach the maximum allowable exhaust temperature during acceleration because of the open compressor bleed valves, and the "closed" IGVs (Inlet Guide Vanes), and the low speed of the axial compressor. BUT, it's NOT normal for the turbine control system to annunciate high exhaust temperature alarms because it should be capable of limiting the fuel flow so as not to exceed the maximum allowable exhaust temperature. (The maximum allowable exhaust temperature is calculated at all times, even when the machine is not running, including during starting and acceleration). That alarm, during starting and acceleration, is usually caused by problems with regulating fuel flow (fuel supply pressure/flow issues; fuel control valve instability; etc.).

A LOT of information and questions, I know--but there are a lot of variables. AGAIN, if this problem for this machine started after the outage and everything was working fine before that then people need to sit down and make a list of what was done and look at this, prioritize it, and start working though the possibilities. VERY often troubleshooting is a process of elimination--especially when there are a lot of variables. Every GE-design heavy duty gas turbine IS NOT like every other GE-design heavy duty gas turbine, even for the same frame size machines. They're similar (they all draw into the compressor, compress the air, add some fuel to it and pass the resultant hot gases through one--or in your case--two compressor sections to produce torque (in your case torque to drive the axial compressor (HP shaft) and torque to drive the load connected to the LP shaft (centrifugal compressor; generator; etc.).

You have a pretty comprehensive list of possible causes for this situation. I'm sure you wanted a few sentences to tell you precisely what to do to solve the problem quickly--but, unfortunately, we just don't have enough information to be of help. I see the site has multiple machines, and if they are similar to this machine it might be helpful to look at some start-up data for similar machines compared to the start-up data for this machine to spot some differences that might be useful in resolving the problem.

Please write back if you need further assistance--but, PLEASE provide answers to most, if not all, of the questions asked above. Help us to help you.

And if you manage to solve the problem please write back to let us know how that was done; a LOT of people read these posts now and in the future and knowing what the ultimate root cause was and how it was resolved can be helpful to many other prople.

Best of luck!