Frame 6B GT Spread Issue

J

Thread Starter

JJss

Frame 6 GT with Mark V (TMR) control.

Recently we replaced Fuel nozzle because we faced combustion trouble alarm at 25 MW. Swapped the nozzle between hot and cold area still spread was found high SP1=56,SP2=46,SP3=44 deg C.

OEM suggested replace all the fuel nozzle. Now we are facing high exhaust spread issue at FSNL, 70, 69,68 Deg C SP1, SP2 and SP3 respectively on gas fuel and Unit tripping on High Exhaust spread alarm. We observed that 20% to 97 % TNH flame was OK in all four cans but after 97% flame lost in the can No. 3 . ( compressor bleed valve closing at 95% Speed)

But the second attempt, immediately after reaching zero speed unit started on gas and everything found normal. FSNL spread was less than 15 Deg C. Unit loaded all the load up to base load everything found OK.

This unit is normally used for the peaking purpose.

OEM suggested to check SRV/GCV calibration, Inlet pressure, Gas composition, filters etc.

All are found Normal.

Last four startup we observed the following :
1. first attempt 20 % to 97 % TNH flame OK > 97% TNH flame out in Can 3 and high exhaust spread alarm observed.

2.Second attempt every thing found normal, unit synchronized and running normal.

Any body can help? Any suggestion?
 
JJss,

We presume the unit does not have DLN-I combustors.

When did the spread problem begin--after a maintenance outage? After a trip from load? Did it gradually get worse?

You seem to purposely mention that the unit was re-started on gas fuel. Does that mean it was being started/run on liquid fuel when the problem started? It's very important to know which fuel is experiencing the spread problem--or if the spreads aren't really affected by which fuel is being burned.

Were the gas valve LVDTs recently re-calibrated (before the spread problems started)?

Were any servo-valves recently replaced?

When you say fuel nozzles were moved, was that done using a swirl angle chart/calculator?

Do you have the flow test data for the "new" fuel nozzles? Are they within at least 10%?of each other? Did you make sure NOT to put the nozzles with the highest and lowest flow ratings in adjacent combustors?

When you moved a nozzle from a cold spot to a different location was that nozzle examined for any signs of problems?

If the spread problem is present on liquid fuel as well, has anyone been recording liquid fuel pressures using the manual selector valve at the liquid fuel flow divider? If so, what were they (all 10 pressures) when the spreads were high?

When you replaced the fuel nozzles with new/rebuilt nozzles were the nozzles that were removed from the unit examine for signs of problems? If so, what was found?

While MOST high exhaust temperature spreads are caused by fuel nozzle issues, not all of them are. If the spread is unaffected by fuel then it's likely caused by something else--a cracked liner or transition piece, or incorrectly installed cross-fire tubes; etc.

I have recently run across two machines which were experiencing loss of flame at 95-98% speed. After looking at start-up data for about 10 seconds it was clear the units were accelerating VERY quickly to 95% speed and when speed control took over the fuel was being cut back so much that the flame was being lost in one or more combustion cans.

Usually the unit will try to cross-fire (through the cross-fire tubes) but it was also discovered that both units (in different parts of the world) had incorrectly installed cross-fire tubes during a recent maintenance outage.

Fixing the cross-fire tubes helped the loss of flame issue on one machine, but not so much on the other machine. Neither site would hear of reducing the acceleration rate--they were very proud of how quickly their machines accelerated. One machine has recently discovered three combustion liners have developed serious cracks and has had to have all combustion liners and transition pieces replaced--but still won't consider reducing the acceleration rate. (It also continues to have intermittent spread issues at 95-98% speed....)

So, there's a lot we don't know about the way the machine is operated at your site. It's a pretty safe bet that SRV/GCV LVDT calibration isn't the problem, but without data from verification checks it's difficult to say for sure.

Again, most spread problems are fuel nozzle related--and most can be spotted on visual inspection. Some are the result of improper control system setup/tuning which is intentional or the result of some other problem. There's just a lot we don't know.

Please write back to let us know what you find!
 
I am adding some more details

This unit was in service and running quite long time after the MI at normal condition.

After Combustion trouble alarm reported we were run the unit at different loads to analyse the spread issue, based on the temperature pattern, we swapped the fuel nozzle, but it would not improved the spread . OEM suggested to replace all the fuel nozzles. We replaced all fuel nozzles.

After replacement we are facing this typical problem:-

First attempt above 95 % speed can 3 flame off and high spread at FSNL.

Second attempt every thing is normal. Last four start up this sequence was same. We are starting the unit on gas at FSNL we are stopping, start the unit after reaching zero speed. Second attempt everything normal.

appreciate your valued suggestions.
 
B
After reading this post, I am curious about the acceleration rate. How to determine whether it is optimized or not?

Which parameters are responsible for change of this in mark-V/mark-VIe?

can you elaborate please.
 
JJss,

When the LVDT calibrations were "checked"--what was done? Were the LVDTs "re-calibrated"?

Were the servo-valve(s) replaced?

Another thing which happens on occasion is that the fuel nozzles which are installed are not direct matches for those being removed. This can cause flow differentials.

Combine this with unintended changes in LVDT "calibration" and these two can combine to cause intermittent problems.

As far as acceleration rate, the Acceleration Rate reference is called TNHAR. It is usually fairly constant, except for one period, during acceleration when it speeds up a little bit. I don't have access to a Control Specification at this writing, but if you would provide the value of TNHAR during acceleration (say, at 30% TNH; 55% TNH; 70% TNH; and 85% TNH), as well as the time the unit takes to accelerate from firing/establishment of flame to FSNL we could comment on the acceleration rate.

A lot of people mistakenly believe they can increase the starting/acceleration rate with little or no effect on the turbine--and that's not true. And, when they increase the acceleration rate and nothing untoward happens immediately they have a false sense of security. And, they increase it more--and more. Until, some day, something untoward happens. And then they complain LOUDLY that the Mark V shouldn't have let them increase the acceleration rate as much as they did! (Yes; it's the Mark V's fault--isn't everything??!?!!)
 
Thanks lot

Dear CSA

1.Unit does not have DLN combustion.

2.Spread issues was gradually get worse.

3.Unit was not started/run on liquid fuel. Liquid fuel is used as emergency fuel , ie. Any gas interruption and base load fuel switching test in every two months as per PPA. But after this spread problem we did not test/run the unit on liquid fuel.

4.After facing these problem we never tried liquid fuel startup.

5.Servo valves are not replaced before and after the spread problem.

6.Calibration checks done after the spread problem, It was found OK.

7.Based on swirl angle chart/calculator we swapped the fuel nozzles between cold and hot area, and no improvement noticed, nothing found abnormal in the removed nozzles and based on OEM recommendation replaced all fuel nozzles. Then we start facing this typical spread issue.

8.New fuel nozzles flow was within the range, can numbers are marked on each nozzle and installed in the respective CAN.

9.Before the last startup following constant changed as per the OEM recommendation but no improvement noticed.

FSKRN1- FSNL FSR constant from 16.2% to 16.6%

FSKRN2- FSR droop correction factor 18.3 to 18.15%

I forget note one thing, every time unit coasting down at 70% speed flame out occurring in all cans.

********

Dear barindra75

FSKSU_IA -Satrtup FSR Acceleration Rate 0.05%
FSKSU_IM -FSR Ramp Rate to Maximum 5%<pre>
TAKL1 0,11 %/sec Normal Acceleration Rate Limit #1
TAKL2 0,11 %/sec Normal Acceleration Rate Limit #2
TAKL3 0,31 %/sec Normal Acceleration Rate Limit #3
TAKL4 0,31 %/sec Normal Acceleration Rate Limit #4
TAKL5 0,1 %/sec Normal Acceleration Rate Limit #5
TAKN1 40 % Acceleration Rate #1 Breakpoint
TAKN2 50 % Acceleration Rate #2 Breakpoint
TAKN3 75 % Acceleration Rate #3 Breakpoint
TAKN4 95 % Acceleration Rate #4 Breakpoint
TAKN5 100 % Acceleration Rate #5 Breakpoint</pre>
once again I will explain

1. First attempt: Unit starting on gas, firing speed to 95 % speed all the flames are Ok, >96% TNH can 3 flame getting off and spread is high at FSNL. Then we are stopping the unit.

2. Second attempt: Immediately after reaching zero speed, unit started on gas, everything OK, startup and running parameters all are OK.

3. Shutting down: while coasting down at 70% TNH 'Chamber flame out ' and tripping the unit.

This was repeated last five startup.

The BIG Question is why second attempt every thing is normal?
 
JJss,

How long since the last maintenances outage? What was the most recent maintenance outage? Was it planned or unplanned (before the spread problem)?

Again, when you removed the #3 fuel nozzle, was there any investigation of the nozzle to check for blockage?

What is the range of flow-rate differentials of the newly-installed set of nozzles?

What is the flow-rate of the nozzle installed in the #3 combustor?

How long do you wait on the first start attempt for the unit to try to cross-fire before shutting down and starting again? What are the spreads when #3 combustor loses flame at/near rated speed? What are the spreads right before #3 loses flame? (At FSNL it should cross-fire (through either or both of the cross-fire tubes of the combustor). So this is telling if it doesn't.)

Have you considered exchanging flame detectors to see if, perhaps there's a weak or faulty flame detector?

What is the flame detector intensity of #3 before it loses flame? Do the other flame detectors lose intensity (or gain intensity) when #3 loses flame? Particularly #4 flame detector (it's presumed the flame detectors are mounted on combustors 3, 4, 7 & 8.)

The acceleration Constants appear to be the typical values. Your new information about changing the FSKRN1 value is consistent with an attempt to put a little more fuel into the unit when Speed Control takes over. Again if the actual acceleration rate is high the Mark V will try to reduce fuel--perhaps by just a little too much to maintain flame in the #3 combustor.

Why was FSKRN2 changed? That's not a Constant that is normally changed as it affects Droop Speed Control and loading/unloading rates. So, it's odd that it was changed as it doesn't affect FSNL operation.

By the way, what is TNH at FSNL at FSNL--during the first/ AND second start attempts?

Your other revelation indicates either the Minimum FSR Constants need adjusting--which are also related to acceleration Constants in a twisted GE way--or there is still something amiss with the gas fuel system. Has the unit always lost flame when shutting down at approximately 70% speed, or is this a recent phenomenon?

I'm still of the opinion, based on the information provided, that there is something amiss with the gas fuel system--most likely a restriction of some sort, OR there are worse problems with the combustion hardware. The y-strainer may be plugged, or filters upstream. If this is the case, the SRV will be open more than previously during firing and FSNL operation to maintain the same P2 pressure.

Also if you're exchanging fuel nozzles with no change in spreads, then it's time to start looking at things like cross-fire tubes and liners. Because if it's nozzle-related then exchanging nozzles should have some affect. (Unless the wrong nozzles are being exchanged.) AND, if the spread condition/location remains unchanged after the new nozzles were installed then it's even more likely the problem is combustion hardware related and not controls-related.

Most sites don't check LVDT calibration--they just perform an AutoCalibration of the LVDTs (without measuring stoke--either during the verification, or during the check if a check of as-found LVDT calibration is performed). So, without knowing what has been done--and how--it's difficult to say if LVDT calibration is correct or has changed by some small amount which might affect starting. Did they record as-found calibration values and measurements that could be shared?

Many intermittent problems like this can be the result of Control Constant settings which are borderline and when the machine is cold/cool might cause intermittent problems.

It would also be helpful to know what Process- and Diagnostic Alarms are active during acceleration.

It's very likely based on the information provided that a combustion hardware issue is making slightly borderline Control Constants cause intermittent problems. Many times, difficulty is the result of more than one issue, and smoldering controls issues can be magnified and appear to be the root cause when they're really not. The more I consider and write about this the more convinced I become that the root cause is combustion hardware-related and any controls issues are secondary--especially if the unit had been running fine before the spreads started increasing.

Finally, what are the values of the FSKMIN and FSKMIND arrays?

Lots of questions, but it's very likely there is more than one problem. And since this is primarily a controls site we tend to focus on controls, but most spreads are NOT controls-related. Especially if the location and magnitude of the spreads isn't changing with new fuel nozzles and exchanging fuel nozzles.

AND, if there are combustion hardware-related issues the unit will "struggle" when switching from acceleration- to Droop Speed control near FSNL--at the same time the compressor bleed valves are closing and more air is moving through the machine. This sounds more and more like a real combustion hardware-related issue at the root of what may be more than one issue. But, there should be sufficient fuel and a low air flow at FSNL for any combustor to reignite if flame is lost--AND there should be no flame instability--or high spreads--at FSNL which would require cross-fire tube flow for reignition. Yet more reasons to suspect combustion hardware problems.

Also, many thanks for the improved information. It is most helpful and appreciated.

PLEASE write back with more details as you investigate the problem and--hopefully resolve it. A LOT of people read these posts and good information and good feedback on problem resolution helps MANY people
 
Oh! Thanks a lot!

How long since the last maintenance outage? What was the most recent maintenance outage? Was it planned or unplanned (before the spread problem)?

Ans:-Last maintenance was Combustion Inspection on Aug 2012 .
Spread high reported last December 2015
1st week of January 2016-Fuel Nozzle removed for inspection.
We did not find anything abnormal.

Swapped cold spot and hot spot Can 2 & 9
No improvement noticed in the spread.

Last week of January 2016 Complete set of fuel nozzles replaced (refurbished)
After replacement of complete set of fuel nozzles, we are facing this typical issue
First attempt on gas startup flame lost at 99.5% TNH<pre>
TNH | 85% | 92% | 98% | 99.5%
CSGV | 34 | 57 | 57 | 57
FD1 | 207 | 233 | 244 | 233 Can # 2
FD2 | 242 | 253 | 280 | 0 Can # 3
FD3 | 209 | 215 | 211 | 233 Can # 7
FD4 | 254 | 260 | 259 | 266 Can # 8</pre>
FSNL spread 79,77,76 Deg. C spread 1, 2, 3 respectively

Second attempt, immediately after zero speed, everything went normal.
FSNL spread 9.8, 9.5, 9.5 Deg. C spread 1, 2, 3 respectively

This was repeated the same manner in last 5 startup/run.

Following inspections/checks done so far :-

SRV /GCV calibration (certainly, stroke length verification) auto-calibration never used, Gas inlet filter, gas condensate drain trap, exhaust thermo-couple, compressor bleed valve. Nothing found abnormal.

Fuel Nozzle refurbishment report says "The final flow test results indicated that all fuel nozzles were flowing within the drawing specified effective area."

Flow rate is not given/missed.

We kept the unit more than 2 minutes at FSNL to try re-establish the fire. Starting the unit again immediately after zero speed.

We are not suspecting any flame scanner, because all the scanners are OK up to FSNL and second attempts all are OK , more over the first attempts flame failed in can#3 and real problem was reflecting in the exhaust spread.

Why was FSKRN2 changed?

Ans:-I am also feel the same, OEM may compared old records and new value, did very minor change, and it will not affect the droop and this issue.

By the way, what is TNH at FSNL at FSNL--during the first/ AND second start attempts?

Ans:-First attempt TNH reached 100% then come down to 97%

Costing down flame out is recent phenomena, noticed after replacing the FN set.

First attempt FPG2 was 16.13 bar and FSG 28.72% at FSNL
Second attempt FPG2 was 16.5 bar and FSG 22.14% at FSNL<pre>
FSKMIND1 6,6 % FSRMIN FSR Corner 1-Shutdown Curve
FSKMIND2 8,2 % FSRMIN FSR Corner 2-Shutdown Curve
FSKMIND3 9,8 % FSRMIN FSR Corner 3-Shutdown Curve
FSKMIND4 11,3 % FSRMIN FSR Corner 4-Shutdown Curve
FSKMINN1 5 % FSKMIN Spd Corner 1
FSKMINN2 40 % FSKMIN Spd Corner 2
FSKMINN3 82 % FSKMIN Spd Corner 3-IGV Closed Speed
FSKMINN4 85 % FSKMIN Spd Corner 4-IGV Opened Speed
FSKMINU2 11,6 % FSKMIN FSR Corner 2-S.U.(=FSRWU)</pre>

We changed FSKMIND4 from 11.3 to 11.7 after that unit not started.

Day before yesterday we did fuel switching from Gas to liquid and back and BL test done successfully

Gas Base Load Spread was 39,26,25 Deg C respectively
LF Base Load spread was 41,31,30 Deg C respectively

JJss
 
Dear All,

I am repeating the same startup issues are still there.

1. first attempt 20 % to 97 % TNH flame OK > 97% TNH flame out in Can 3 and high exhaust spread alarm observed.

2. Second attempt every thing found normal, unit synchronized and running normal, Base Load Gas and LF OK , switching fuel OK.

3. While stopping about 65-70% speed Chamber flame out trip.

Kindly give more suggestion, if any.

JJss
 
Dear JJss,

I am repeating: It's extremely unusual for a GE-design Frame 6B heavy duty gas turbine NOT to cross-fire at FSNL when running on gas fuel. Extremely unusual. The IGVs are at minimum operating angle for rated speed, and this should not prevent flow through the cross-fire tubes on either side of the #3 combustor from re-igniting the flame in #3 combustor.

When there is no combustion in one combustor, the pressure in that combustor is lower than the pressure on either side. This pressure differential is what causes hot gases to flow into the combustor with low or no flame and ignite or re-ignite flame. (This is exactly how flame propagates around the unit during firing from the two combustors where ignitors ("spark plugs") are located.)

At high loads with the IGVs at or near full maximum opening there can be situations where the air flow past the ends of the cross-fire tubes is so high that it can prevent hot combustion gases from flowing from a combustor with higher pressure into a combustor with lower pressure--but that only occurs at high loads and IGV angles.

From the Control Constants provided, it would <b>seem</b> there is no problem. But, without being able to see a trend or data capture of a start-up to see how fast the unit is accelerating when it reaches FSNL on the first attempt versus how fast it reaches FSNL on the second attempt, and what is happening to FSR when it reaches 95-97% speed on the two attempts, it's virtually impossible to say anything other than: It's extremely uncharacteristic for flame to be lost in one combustors at or very near rated speed (with the generator breaker open) and for the unit not to cross-fire and re-ignite the fuel in the combustor where flame was lost.

I could envision a scenario where if the fuel was cut back very sharply from acceleration to speed control at the same time the compressor bleed valves were closing that flame could be extinguished in a combustor--but I would also think there was a flow mis-match or something similar with that combustor's fuel nozzle, too. And I would still think that all things being normal the combustor would re-ignite from one cross-fire tube or the other--or both.

Chambers (Combustors) flaming out during shutdown could definitely be the result of a Control Constant tuning issue. That happens all the time. And, it's kind of difficult to tune out, some times. But, on a machine running gas fuel with the expected fuel characteristics for the gas valves (SRV and GCV) AND the fuel nozzles installed in the machine it should definitely be possible to get to about 20% speed before flame is lost or the SRV is closed to extinguish the flame.

I might recommend forcing the compressor bleed valves to remain closed until the unit reaches 100% speed to see if that helps--but it's really not a long-term solution, and the only thing it would really indicate is that there is still something amiss with #3 combustor and/or its fuel nozzle. Because, whether the bleed valves close at 95% speed or 100% speed or even at 5 MW--all combustors should still be able to maintain flame whenever the bleed valves close. So, this isn't really a valid test.

There's something here we're missing--and when you're told to "get more data" that usually means that the information provided hasn't been enough to be of help and that more information is required.

And, in this case--the fact the unit won't cross-fire at FSNL is very, Very, VERY unusual. We would need to see high-speed data of SRV position (FSGR), P2 pressure (FPG2), speed (TNH), GCV position (FSG), FSR, acceleration (TNHA), acceleration rate reference (TNHAR), and speed control FSR (FSRN). We would also need the Control Constants of FSR during start-up (FSKSU_FI, FSKSU_WU, FSKWU_IA, for example--all values of FSKSU_xx). It would be helpful if you could post a Trend Recorder file of these values--during an unsuccessful first start, and then during a subsequent successful second firing attempt--to a free web-hosting site (such as www.tinypic.com, or one of many similar sites). If not a Trend Recorder file, then the exported values in .csv format from a Trend Recorder file. Then you can post the URL to the location and we can download the file(s) and analyze them.
 
Dear CSA,

Thanks a lot

These are the constants:-<pre>
FSKSU_AR 30,6 % Startup FSR Acceleration Limit
FSKSU_FI 17,5 % Startup FSR Firing
FSKSU_IA 0,05 %/sec Startup FSR Acceleration Ramp Rate
FSKSU_IM 5 %/sec FSR Ramp Rate to Maximum
FSKSU_TC 1 sec Startup FSR Time Const During lower
FSKSU_WU 13 % Warmup FSR</pre>
20CB forced FSNL test on gas fuel:-

https://dl.dropboxusercontent.com/u/91779409/12mar16_20cb_forced_1st_attempt.csv

20CB unforced FSNL test on gas fuel:-

https://dl.dropboxusercontent.com/u/91779409/12mar16_20cb_unforced_2nd_attempt.csv

This is 24th Feb startup:-

https://dl.dropboxusercontent.com/u/91779409/24feb16_1st_start.csv

https://dl.dropboxusercontent.com/u/91779409/24feb16_2nd_start_fsnl_ok.csv

I will update if anything change/found/solved

Thank you for your help

JJss
 
FINALLY WE RESOLVED THE ISSUE

VA18 atomizing bypass valve was not opening or not opening fully. However second attempt it was opening fully and bypassing atomizing compressor while operating on gas. This was repeating all the time. Replaced VA18 with new one, unit is operating perfectly.

Thanks for all your helps and advises.
 
Top