Unit runs on Gas with Primary and Secondary Fuel Nozzles. Exhaust Spread was 27C after MI in March 2009. Problem started in July 2010. Chronology of the problem as follows:

Mar 2009: Turbine Major Inspection performed
Apr 2009: Exhaust Spread was 27C
01/30/10: 3 Exhaust Trips Signals reached limits
02/01/10: Exh T/C H & K looped. Spread normal.
07/02/10: Plant tripped on high exhaust spread
07/11/10: Sludge noticed in Fuel Gas Pig Tails
07/11/10: FG lines cleaned & FN Tips replaced
08/17/10: Unit started. Spread still abnormal.
July/Aug’10: Low & High Reading T/C looped
08/31/10: Spread 58C with 2L & 2H T/C rejected.
09/4-18/10: Spread rise from <80C to 97C
09/19-29/10: Spread rise from 100C to 105C
10/1-6/10: Spread remains high at 98C to 100C

Exhaust T/C Readings in "C" on 10/19/2010 as follows:

T/C 21TE450A:522C
T/C 21TE450B:513
T/C 21TE450C:502
T/C 21TE450D:499
T/C 21TE450E:485
T/C 21TE450F:484
T/C 21TE450G:491
T/C 21TE450H:492
T/C 21TE450I:450
T/C 21TE450J:429
T/C 21TE450K:476
T/C 21TE450L:463
T/C 21TE450M:508
TTXM:488
TTXSPL:63
TTXSP1:93
Low Limit:449
High Limit:521

Gas Turbine drives Syn Gas Compressor in Ammonia Plant that is critical for Plant running. The Controls team have confirmed that all the 13 exhaust T/C are normal and readings are real. It is quite obvious that it is not only a real but an excessive combustion trouble. Plant operation is vital and hence all probable causes needs to be identified for giving a clear action plan to shutdown the unit. Some of them have tried to list down.

Fuel Nozzle
-Clogging of nozzle
-Effective area flow not within limits
-Nozzle not calibrated to the specified limits
-Incorrect nozzle internals

Combustion Air
-Hot air leak from Cross Fire Tubes
-Excess air leaking into WI piping
-Fouled compressor

Fuel Gas
-Gas leak from Pig Tails
-Contamination of gas upstream

Combustion
-Huls spring seal fit improper

Transition Piece Assembly
-Missing TP end seals

Request:

1) Please advise additional probable causes.

2) Is HGPI advisable? to inspect the Stage Nozzles, Buckets & Shrouds, for the reason that unit had been running on high exhaust spread since July'10 until now.

Please let me know any further information needed.

 

> 1) Please advise additional probable causes. <

That's a pretty comprehensive list. The only thing I didn't see was mention of inspecting the exhaust T/Cs and their radiation shields to ensure the T/C tips are properly located in the shields and there is nothing blocking air flow through the shields.

> 2) Is HGPI advisable? to inspect the Stage Nozzles, Buckets & Shrouds, for the reason that unit had been running on high exhaust spread since July'10 until now. <

Since you're confident the problem is not with the exhaust T/C inputs to the turbine control panel, and since the cause of most exhaust temperature spreads is "mechanical", then some kind of inspection seems warranted. We don't know if the exhaust T/Cs and radiation shields have been inspected (see above), so that would be a quick check that could be done relatively quickly without a lot of disassembly.

A borescope can be used to inspect some of these things, but not all of them.

If there was a problem with cross-fire tubes, they would likely be "glowing" or at the least very, very hot and that could be easily detected with a non-contact infrared temperature sensor.

You mentioned finding sludge/deposits in the pigtails. That would be a sure sign to me that something undesirable is entrained in the gas, and that the gas temperature is low enough to cause it to "congeal/solidify" in the pigtails. So, it's probably also doing the same in the nozzles, where there is another pressure drop/temperature drop.

Hope this helps.

 

Hello over there...

Excessive combustion trouble is usually related to operating your GT relatively outside the FG specifications. Maybe it’s good to find or at least understand first the root cause of the problem. What is your FG specification and do you comply with these specs? Is the FG scrubber near the GT or do you get the FG "conditioned' from a LNG supplier or process plant or are you conditioning it at your plant? Any maintenance activities performed to the FG scrubber? Think about the dew point, hydrates and heater problems etc.

It appears that the GT had operated properly prior the Major Inspection and sometime post MI. To me it look like that you have some mechanical integrity issues which might be caused post the inspection. Who did the inspection the OEM or third party? Where all the hot gas path parts replaced with new parts or refurbished parts (maybe aftermarket parts)? Is this GT combustion system using the standard with LHE or extender liners? Bear in mind that the FG nozzles / tips are designed for particular GT type and based on the FG specs as given by you during the design phase of the GT. Did you also check the calibration of the nozzles with the "GO" and "NOT GO" method? In the meantime I realized that this topic jumped over to mechanical discussion. Maybe we should discuss this topic somewhere else? Maybe for the benefit of the group, you can give some feedback.
YES for borescope inspection by qualified service provider.

Good Luck,
A. Oztas

 

Thanks for your advise.

1) Will inspect the T/C radiation shields.

2) Planning for IR Temp Survey. Will confirm the fuel gas temperature w.r.t. dew point.

Ash

 

the list you have given is quite comprehensive , but try to do a load test before the machine is shutdown. The swril angle for frame 5 machine is about 110-

120 deg for no load and about 5-10 deg for base load of the machine , the exhaust gas thermocouple reading taken at full load , 75% load , 50% load , 25% load

and at no load will give you a idea about were the combustion trouble is taking place. I have given a actual test reading taken in a machine . it will give

you a example of how to proceed about doing it. Ge has thirteen thermocouples and 10 combustion chambers , thus any problem in a combustion chamber is

reflected in two thermocouples. it can be analyzed by<pre>

n/l 25% 50% 75% 100%

TT-XD-1 285 345 411 465 496
TT-XD-2 280 337 401 463 505
TT-XD-3 277 328 394 470 513
TT-XD-4 275 332 395 485 524
TT-XD-5 273 343 414 470 516
TT-XD-6 270 336 404 468 496
TT-XD-7 283 340 406 475 513
TT-XD-8 282 344 408 470 515
TT-XD-9 286 347 411 471 518
TT-XD-10 285 349 413 493 516
TT-XD-11 288 354 420 488 511
TT-XD-12 285 354 418 475 512
TT-XD-13 286 349 416 471 510
TTXM 282 343 407 473 511
TTXSPL
TTXSP1 16 22 26 33 29.8</pre>

if you will see the low spot temperature in the fsnl is in thermocouple region 5,6 , then at 25 % load it shifts to 3,4 , then at 50% loading it shifts to

2,3 then at 75% loading and at base load it is at 1,2 region. this is the effect of the swril. from this we can see that the combustion trouble (though there

is none appreciable here ) is at combustion chambers 10 and 1. in a machine where there is a huge spread problem like yours , you can see this quite clearly. this will help you in pinpointing the the combustion chambers where the trouble actually is.another region you need to look into during the shutdown is the thermocouple shields , the thermocouple should be seated in between them , not too far off nor too fan in front. this analysis will also help you in determining weather the thermocouple is a problem or not . if a thermocouple is showing consecutive low value or consecutive high value for all the load ranges (fsnl to base load) ,then that particular thermocouple is salty / susceptibility faulty.

and yes hot gas path inspection is necessary for this case . though this is a routine one , do check combustion liners too as they are one of the common cause for high spread.

 

Thanks for your advise. We cannot bring down the load as plant is running @ 100% load. We suspect most probably that the primary FN has some issue.

Could I please know the reason to justify as to why a HGPI is required?

Ash

 

Additional Feedback on the problem:

We have Primary & Secondary Fuel Nozzles as informed. Primary tips had passed the calibration test at GE, Meelsa. 5 out of 12 Secondary tips have failed in the test. To eliminate Secondary contributing to the problem, we isolated 20GN fuel gas valve. Typical of this unit. Appears probably due to increased velocity thru Primary exhaust spread has come down from 96 to 62C.

JFYI.