Hi

We have had problem with out gt's exhaust temperature spread since last summer HGPI. We replaced 2nd. and 3rd. buckets and 3rd. shrouds. Rest capital parts were refurbishment. During the test run GT tripped from high spread. We did analyze and change fuel nozzles positions. No help. We did more more inspections; air inlet condition, temperature sensors check, borescope inspection etc. No help. Then we replaced refurbishment fuel nozzles with totally new set. After that spread was not a problem anymore. This week we started GT again and same problem again. Now spread is ok between 0-25MW and after 38MW. Between that range it is living all the time, changes could be from 45 dec to 60 dec in 10 seconds!

Until now we have checked:
Exhaust temperature sensors and Mark IV chipsets OK
Bleed valve leakages, ok
Heat exchangers in air inlet, no leakages
Internal borescope inspection for the turbine, no findings

Does anyone has a good guess, what could be the problem?

Thanks for the tips!

 

Hi

We are facing same problem in our GTG F7E. Problem started after CI. We swapped the burners, no help. 1 st spread is always 100. We contacted GE, they recommended to see the sealing of Transit pieces, so we are shutting down Tuesday and restart on Thursday. I will let you know how this would affect? Contact me at [email protected], so I will remember you

Thanks
Sardar9

 

Hi JV and Sardar9,

As you write TCs were inspected.

You did not tell how the thermocouple readings are distributed: do you see for most thermocouples regular distribution and then the area of the reversed bell shape with lower readings or you see the single thermocouple with very high readings (or with very low readings) with big difference to adjacent to it thermocouples.

The first pattern directs to combustion problem. Combustion problem can be confirmed by observing that cold spot rotates with the load change. Using the swirl chart quite accurately it can be identified which combustion chamber has the problem. With one combustion chamber defective, 3...4 thermocouples show lower value than TTXM.

The second pattern directs to the defect closer to the diffuser area. In this case it is hardly possible to see rotation (swirl) of the cold or hot spot with load change.

That is how the defect of the second pattern can be explained.
Temperature of the hot exhaust gases leaving the third stage buckets is distributed in the way that the area with highest temperature is located more or less in the middle of the flow and temperature is lower in the areas closer to the walls. This distribution changes with the load and is additionally affected by total air flow - IGV position. If the passage of the hot gases between the turbine and exhaust thermocouples is smooth, all TCs read similar value. If, in the part of the flow, there is an obstacle that can change the hot gases flow, the temperature distribution (only distribution!) in the part of the flow will change and thermocouple located downstream of this obstacle will read different value than adjacent TCs. This reading can be lower or higher, depending on how the flow of hot escaping gases is diverted. Instead of the obstacle, hot gases flow can be affected by the ingress of the cooling air from 88TK fans (additionally from 88BN fans on Frame 7/9 machines).

For this kind of defect it can not be observed swirl, because obstacle that changes the hot gases flow is located too close to the thermocouples. It is possible that obstacle can divert the hot gases flow only after the total flow of escape gases exceeds some level: this could explain why spread is high only for higher loads.

Inspection of the diffuser area on frame 6 is difficult: accessible only during MI but on frame 7 it should be possible to enter and inspect visually this area. Take into consideration that inspection is done in cold state and temperature expansion during the turbine operation can change mutual location of components.

It can not be excluded that on the particular turbine both kinds of defect (pattern one and pattern two) are present.

Regards

 

Thank you ger7. We too learned something.

Take care
G.Rajesh