Dear CSA and all,

This is my first post in this wonderful website, as I was only a reader since a long time ago.

In my plant we have 4 GE 5002D Gas Turbine which is a 2 shaft mechanical drive machine in a combined cycle application. originally the combustion system was a DLN system but 3 years back we did a retrofit to LHE combustion system. We experienced different flame counts on different machines.

2 months ago we had a flame out trip on machine (T4). we were suspecting the flame detectors to be the problem and we did replace 3 out 4 scanners and restart the machine. the situation improved since most of the readings went above 100 while they were around 55 to 80. Then, 2 weeks after the forth scanners reading dropped down to 50. We are investigating why we have low flame detectors reading on the fourth machine (T4)? That is reading lower counts than other machines, here are the current readings:

Can#11 is currently very low 50
Can#10 is bouncing around 130
Can#3 is around 140
Can#2 is fluctuating "50 to 80"

Other machines have higher readings in T2 for example
Can#11 is around 250
Can#10 is around 190
Can#3 is around 260
Can#2 is around 330

Looking at 3 years history data of the flame detector and we found out that T4 readings were not exceeding 180 except can#3 detector was around 220 and they start to drop gradually over time. On other trains looking 3 years back I can see most detectors were reading above 300. yes they have also dropped gradually over time but still considered acceptable an average of 200 to 250. What I am trying to say here that it is not really a new problem that just appeared, except can#11 which is showing 50.

Checks that we already did:

- Alignment of the flame detectors pipe and flanges with the holes in the liners. As per our mechanical group it was found ok?

- Wiring checks are done and found ok

- We confirmed that the right Honeywell scanner part number for LHE system is being used (GE oil & gas part# RRO58985, Manufacturer P/N LG1093AA36, GE P/N 261A1812P014)

- We swapped T4 detectors with T1 detectors and we noticed that those scanners which were reading low in T4 are showing a good reading when installed in T1 and vice versa

- Wheel space after inner thermocouples have a difference of 200 deg F, A = 660 while B = 460, most probably this is a TC failure and it is there since few months unless you have any other advice.

- No dust found on the flame detectors lenses; actually we still have a low reading even after replacement?

- IGV is going wide open @ noon time and it around 56 in night time this is happening in all trains with the change of ambient temperature, however in T4 two months ago this was not happening and can#11 detector was a little higher (100)

- Exhaust readings from TC#1 to TC#13 are (1025, 1021, 992, 1003, 1003, 1001, 1024, 1010, 1003,1001, 1002, 1010)

- Exhaust TC allowable speared TTXSPL (249 F)

- Exhaust TC actual speared TTXSP1 (29 F)

- TNH 4720 rpm sometimes @ noon time increases to 4870

- TNL 4280 to 4314 rpm

- CPD (88 to 95 psi) night to day

- Compressor inlet temp (85 to 112 F) night to day

- Compressor discharge temp (130 to 149 F) night to day

- Ambient air pressure 13.6 PSIA other units are 14.3 PSIA, any effect?

- Exhaust duct pressure 0.2 psi other units are 0.04, any effect?

- P2 pressure 239 psi

- Differential pressure across intake filters 4.2” h2o other unit around 3” h2o

- SRV 29%

- GCV 43%

- IGV (56 to 78 DEG) night to day and in some days it goes wide open when TNH exceed 94%

- Fuel gas temp (130 to 149 F) night to day same as others

- Turbine compartment temp reaches (150 F) which is almost (15 F) higher than other trains

Concerns that I want know:
- Is there any way to confirm fuel to air ratio here to ensure proper combustion? If yes, how? As of my knowledge fuel ratio is maintained by controlling P2 at set point which is calculated by the formula "FPRG = (TNH * FPKGNG) + FPKGNO" which is "FPRG= (TNH * 2.9) - 29" in our case, but what about air ratio? Is IGV adjusting to maximize exhaust temperature only or it will adjust itself in a trail to optimize the combustion and maintain a certain air to fuel ratio?

- I just figure out that FDG the fuel gas flow transmitter is faulty and showing 0, does it have any effect?

- Is there any recommendation to replace that Honeywell scanner with a conventional 4-20 ma one? Do you think this would improve the readings?

- Is there any specific clearance between the tip of the scanner pipe and the liner wall?

- In the inner part of the scanner pipe we have a small hole, what is the purpose of that hole, is it there to prevent condensation on the lens?

- Could there be an issue with the fuel nozzle itself especially the one in can#11? Is it possible to adjust it to control fuel quantity and direction in to the linear? Do you think it is wise to try swapping fuel nozzles?

Please advice? I am ready to provide any additional data required to analyze this problem. FYI, I have already read this article http://www.control.com/thread/1226432295#1226779774.

Thanks in advance and looking for your valuable feedback

 

I did not think I would ever say this, but there's <i>almost</i> too much information here.

But, I am not familiar with the acronym LHE. I'm going to guess (because I like guessing some times) that it's some kind of low hydrogen fuel that requires conventional combustors and that when the conversion to LHE was performed the combustion system was converted back to either single nozzle per combustor or multi-nozzle per combustor system.

Would you please explain about LHE and tell us what type of combustor is being used now. Please tell us about the decision to switch from DLN combustors to another combustion system.

Next, please tell us how the flame detectors are oriented on the sides of the combustion cans. Are they mounted perpendicular to the outside of the can, or are they angled such that they are pointed towards the fuel nozzle?

Also, do these machines start on LHE or are they started on natural gas fuel or distillate fuel and then transferred to LHE above some load?

Next, do these machines use water- or steam injection for NOx reduction or power augmentation? If so, what are the flame intensities when there is no injection?

 

Also, are the flame detectors cooled?

 

- LHE means "Lean Head End" combustion system where there is only a single fuel nozzle per combustor while DLN has 2 fuel nozzles (primary & secondary). It's different from DLN "Dry Low NOx" in the way that DLN have more than one operating or fuel mixing modes "Lean to Lean, Premix ...etc" and DLN

- The decision to switch from DLN to LHE was because the DLN system was not working as expected. It was not doing what it is supposed to do, instead of reducing NOx we had a high Nox going out of our turbines exhaust stacks "environmental issue". GE explained that this was happening because we have a bigger than required turbine driving a small load compressor, so when the compressor is near to its 100 % capacity the turbine is only around 75% which causes the DLN system not to work properly.

- The flame detectors are angled and are pointed toward the fuel nozzles, same in all trains.

- The turbine uses "natural gas fuel" only at all times, before and after the LHE retrofit.

- NO water or steam injection is used for NOx reduction.

- On DLN system we used to have a flame detectors cooling system, but this cooling system was demolished and removed from CSP logic during the LHE retrofit by GE.

- Currently we are in a very bad condition where 2 flame detectors are around 45 "calling for a trip" and the other 2 detectors are around 140. The 2 with low reading are in the upper half of the turbine (can 2, 11) while the other 2 are in the lower part of the turbine (can 3, 10), any comment?

 

Yes; I contacted a former colleague who's still with O&G and he filled me in on LHE.

Your comment about the turbine being much larger than the compressor is interesting. With so much air flowing through the head end of the combustor the flame intensity is probably going to be lower than with either DLN- or conventional combustors. And you have mentioned some pretty big swings in IGV angle for what would seem to be "small" load changes given the relative sizes of the turbine and load compressor which might also be contributing to the low flame intensity.

Also, this being a relatively new combustor one has to wonder about the angle of the flame detector sight tubes, meaning one has to wonder whether if the flame ball (which may already be of low intensity) doesn't move back and forth from the expected location more than "usual."

I couldn't ask about this sight tube hole you alluded to so I don't know how that may or may not play into this problem, if at all.

I wish I could be more help, but this new to me. Please write back and let us know how this progresses.

 

You asked a couple of questions I didn't address.

GE uses Reuter-Stokes Flame Trakkers which are usually much better at detecting flame. I say usually because there have been a couple of occasions when replacing Honeywells with them hasn't improved the intensity, and in fact has made it worse. One of those occasions was when the flame ball wasn't in the expected area. They are 4-20 mA devices.

Something which isn't clear to me is when this problem started. If it's fairly recent, then swapping fuel nozzles mIght be worth trying. But the exhaust temp spread seems low so nozzle problems don't seem to be the likely cause.

If this is something which started after the switch to LHE combustors then I would "lean" towards either a flame ball location or sight tube angle problem, or a Control Constant setting problem.

But, I don't have any experience with LHE liners, and I don't understand the hole in the sight tube you have mentioned.

 

>Also, are the flame detectors cooled?

Hi, thanks for your reply
- I mentioned before that we don't use flame detectors cooling system any more, it was removed when we change the combustion system from DLN to LHE.

- What do you mean by "if the flame ball doesn't move back and forth from the expected location more than "usual." I really thought about this, I suspect something like this" maybe flame is there but the detector cannot see it although it is properly aligned with the hole in the linear wall because flame is shifted forward. How can we confirm this, what to check, what to do to improve the situation?

What control constant setting exactly would you suspect? But why this is happening only in Train4 but not in the other trains.
Since we are currently in a bad situation (2 scanners showing good reading 140 and 2 scanners showing low reading 40) ,what about lowering the drop out setting for at least one scanner of those showing low reading temporarily? The settings are in I/O cfg and are expressed in 1/16 so if we put 3 it means 48 CNT.

What about these 2 items:
-Ambient air pressure 13.6 PSIA other units are 14.3 PSIA, any effect?

- Exhaust duct pressure 0.2 psi other units are 0.04, any effect?

looking forward to hear from you as we are planning to shutdown the machine to do more checks, but I don't know really what good checks we can do more

 

since you have sighted the flame scanner on the flame ball convincingly, the issue is pulsations not visible to the naked eye.

can be resolved in part by the filter options that come with some scanners and is affected by the waveband being detected (UV versus IR) and the use made of the flame pulsation signal.

flame instability can increase at low loads and associated with pulsating flows within the combustion chamber akin to incipient surge conditions.

 

I haven't ever seen or heard of using filters on GE-design heavy duty gas turbines. Boilers, yes; gas turbines, no.

The fact that this is only occurring on one out of four units is very odd. The fact that the exhaust temperature spread is not very high is <b>MOST</b> confusing. The fact that two out of four flame detectors are indicating low intensity--and that the two are adjacent--is also confusing.

If it was something mechanical, such as problems with combustion liners or cross-fire tubes I would expect that if there were problems with cross-fire tubes or liners the exhaust temp spreads would be higher.

So, just about everything here is confusing.

I think the only thing we haven't really looked at is wiring and possible electrical noise problems--which would be a controls problem. It's likely the wiring for the two upper detectors is run through the same conduit and/or junction box.

Not knowing much about the LHE combustion system, it's possible that there is some "tuning" which could be done to try to resolve the issue, though again it would seem that would affect the exhaust temp spreads.

So, I'm down to wiring issues at this point. Check for high resistance in the flame detector wiring between the Speedtronic and the flame detectors. Check for properly grounded shield drain wires. Megger the wiring from the Speedtronic to the flame detectors at 500 volts, or 1,000 volts with the flame detectors disconnected.

Swap the flame detector wiring at the Speedtronic to see if the problem follows the flame detector/wiring/combustor or stays with the Speedtronic input.

With the unit shut down and using a suitable high intensity light source (the same source for all testing) it should be possible to determine if it's the wiring or an actual problem with the flame intensity. If by using the same intensity light source it's determined that all the flame detectors are indicating properly--or will indicate properly with a light source of the same intensity--then it's likely back to something mechanical.

Use an oscilloscope to monitor the feedback during the offline testing to look for differences which shouldn't exist using a light source with the same intensity for all the testing if the wiring and inputs are good.

Lastly, swapping fuel nozzles is also an option, but I don't think that should be done until the wiring and inputs are all eliminated as possible problems.

Do write back to let us know what the results of your efforts are.

 

Dear air005,

Now you are in the air...loud and clear...

Interesting discussion, it appears that your Fr5002C was upgraded to FR5002D. This upgrade basically consists of, replacing the 16 stage HP rotor to FR6001 HP rotor and some other mechanical and control parameters changes. This upgrade is most likely done by the OEM. Is this unit used as LNG train or as midstream booster compression application? Based on your information, it appears that the temp. control is set to combined cycle control. The HP speed is operating near the min. operating point of 92%. You might need to review the surge control line, the load control from the master controllers and the interface to the turbine control program.

Now back to reality about your flame detectors low counts. LHE liners are very common in one shaft GT’s and known to be operating with minor issues (simple cycle mode). OEM's experience with two shaft LHE liner is probably not that much (combined cycle mode). Due to the fact that the LHE liners have a leaner reaction zone, the flame zone will also change accordingly. The standard used flame detectors have known problems detecting the flame properly. Since the HP rotor is operating around the min. speed point, this will also affect the flame property.

Did you contact the OEM? Are the existing flame detectors upgraded to different type? I don’t know what the TCP is MKV or MKVI. I guess it’s a MKV panel, if this is the case check the <P> core and the associated cards such as TCEA etc and ensure that the flame detectors supply voltage is about 330Vdc.

Solution: Check with the OEM whether they have issued TIL's or NIC's which tackles these kind of problems.
You might consider switching the current operating mode to simple cycle mode from your TCP. Most likely the flame detector counts will increase.

Good Luck...
A. Oztas