Ebi/abdi,

According to Mambo's and mhwest's explanations, the problem could very well be the ignitor position. What they're describing is that the tip of one or both of the ignitors--if inserted too far into the combustion liner--gets VERY hot during diffusion flame operation in Primary and Lean-Lean combustion modes, hot enough to ignite fuel in the primary combustion zone once the air/fuel mixture gets above minimum combustion mixture (richens up enough to be capable of diffusion flame).

Once more, during a Lean-Lean to Premix transfer, the fuel to the primary combustion zone is completely shut off (it's all directed to the secondary combustion zone). Then, once flame is NOT detected for a few seconds, the gas control valve that directs fuel to the primary combustion zone starts opening to re-admit fuel to the primary combustion (as the fuel to the secondary combustion zone is reduced at the same rate--this to prevent load swings). IF there is some ignition source--the extremely hot tip of an ignitor that is inserted too far into the combustion line, for example--then the fuel that is being re-admitted to the primary combustion zone can be re-ignited. So, just as you describe some time after fuel starts being re-admitted into the primary combustion zone if there is something hot enough to ignite the fuel into a diffusion flame then, voila! Extended Lean-Lean combustion mode is switched to. (In fact, if flame is detected in the primary combustion zone during a Lean-Lean to Premix transfer or during Premix mode, the ignitors are ignited for a short period--just to ensure that ALL the primary combustion zones are ignited (through the cross-fire tubes) to prevent high exhaust temperature spreads from damaging the hot gas path turbine buckets.

When in Premix combustion mode, there is no diffusion flame in the primary combustion zone--even though approximately 80% of the fuel is being introduced into the primary combustion zone. To get the low temperatures required to reduce NOx emissions it's necessary to extinguish the flame in the primary combustion zone that exists during Primary and Lean-Lean operation-and the only way to do that is to remove all the fuel from the primary combustion zone.

Once the diffusion flame is out (because there is no fuel flowing into the primary combustion zone for a few seconds) then fuel starts being re-admitted to the primary combustion. Under normal circumstances there is no ignition source in the primary combustion zone and the air/fuel mixture is so lean that diffusion flame does not exist in the primary combustion zone. The premixing of the gas fuel in the primary combustion zone is sufficient to produce the hot gases required for the turbine to produce torque--but not so hot that excessive NOx is formed.

The most common source of primary combustion zone re-ignition in GE-design heavy duty gas turbine DLN-I combustion systems is the presence of hydrocarbon-based solids which are so hot they can ignite the natural gas fuel--once there is enough fuel to be ignited. Such solids come from liquid contaminants in the gas fuel that "coke" and form solids on the fuel nozzles (usually) that get VERY hot, hot enough to ignite gas fuel--once the air/fuel mixture reaches the minimum mixture--and cause the unit to transfer to Extended Lean-Lean mode. Such contaminants include lubricating oil (from the natural gas compressors), seal oil (from the natural gas compressors), diesel and gasoline (yes--both do mysteriously get into natural gas pipelines; gas analysis reports have confirmed this on many occasions), and other hydrocarbon-based liquids entrained in natural gas pipelines.

So, try to envision what's going on during a Lean-Lean to Premix transfer when fuel is shut off to the primary combustion zone then re-admitted, and recognize that any ignition source (such as a hot or glowing ember, or the over-heated tip of an improperly inserted ignitor) can cause the fuel to ignite into a diffusion flame when the air/fuel mixture gets high enough as fuel is being re-admitted to the primary combustion zone as the Lean-Lean to Premix transfer is being completed.

Were the ignitors replaced during the HGPI? Was the insertion depth set when the new ignitors were installed? (This usually requires a unique tool to measure the distance from the ignitor mounting flange to the inside edge of the combustion liner.) If the ignitors, or even just one of the two ignitors, were/was replaced and the insertion depth was not set correctly then this could indeed by the "sparking gun" in your problem.

Please write back to keep us informed of the progress and resolution.

 

Dear CSA,

This happened on Fr6 DLN-1 GE machine. When GT start it is on Primary mode of firing, after it gets transfers to LL mode when flame is present in Both Zones. Transferring from LL to Premix mode happens around 18-23MW. At this time flame has to be present only in secondary zone side. Due to the spark plug insertion length more into the chamber it was red hot and didn't allow the flame to extinguish and reignite primary zone. the spark plug when removed on shutdown was found burnt and in damaged condition (steel cover over porcelain found worn out). The spark plug replaced with new and adjusted as per spec. just few mm near to combustion linear.

 

Mambo--and mhwest,

Thanks for the explanations and clarifications!

I continue to learn from the experience and knowledge of others here at control.com--which is what makes this site so useful to so many people.

Thanks, again!

 

Thanks to you also CSA. I have found your frequent and detailed tutorials very educational. I don't believe I've ever met you, hope to someday - your discussion style is unlike anyone I recall working with over the years.

 

Dear CSA and the team

Thanks for your appreciation, this what encourage everyone to share their experience.

Cheers
Mambo

 

hi,

WE TEST THE SPARK PLUG IGNITION THERE IS not any problem about the ignitor.

but I think THERE IS A PROBLEM FOR TRANSFERRING NOZZLE of combustion chamber # 1, because at any load just during the transferring this combustion have the minimum temperature.

THE MINIMUM TEMPERATURE WAS 424 deg_c with the load of gas turbine was 115 MW.

THE second low temperature was 443 deg_c for combustion chamber # 2, the third low temperature for combustion chamber # 10 was 470 deg_c.
the high temperature was 588 deg_c for combustion chamber # 6.
AM I true? there is problem for transferring nozzle of combustion chamber # 1. but I don't know what problem exactly exit?

THE three of gas detection in down of combustion chamber was activated during the high load.

thanks a lot.

 

Hi,

We test the spark plug ignition there is not any problem about the igniter. But I think there is a problem for transferring nozzle of combustion chamber # 1, because at any load just during the transferring this combustion have the minimum temperature.

The minimum temperature was 424 deg c with the load of gas turbine was 115 MW. The second low temperature was 443 deg c for combustion chamber # 2, the third low temperature for combustion chamber # 10 was 470 deg c. The high temperature was 588 deg c for combustion chamber # 6.

Am I true? There is problem for transferring nozzle of combustion chamber # 1, but I don't know what problem exactly is?

The three of gas detection in down of combustion chamber was activated during the high load.

After Fix the gas leak from the gas inlet to three manifold of primary, secondary, transfer for can # 7 and start the unit in 10 mw we have 500 ppm CO in turbine and combustion chamber cabin. After increase the load to 45 mw concentration of co reach to zero, why?

 

hi,

you said The presumption is that if the four primary flame detectors all indicate the absence of flame then there is no flame in any of the other ten combustors. But, the high spreads--especially if they are in Celsius--indicate that flame may still be present in one or more combustors when it's not present in the four with the flame detectors.

Or, if the fuel flows to all the secondary nozzles are not uniform then uneven pressures can exist in the combustors which could cause primary zone re-ignitions which prevent attainment of Premix mode.

I have a question we can change the flame scanner from the one can to another can? because in document shows the flame detector must be in can# 4, 5, 10, 11 but in really there is in my unit at can # 1, 2, 3, 4.

thank a lot for reply me.

 

ABDI,

Primary flame detectors (I presume you are talking about primary combustion zone flame detectors--you weren't specific...) can only be mounted on combustion cans with mounts and that have combustion liners and flow sleeves with the appropriate sight "tube" holes through which the primary zone flame detector can see diffusion flame. Most turbines only have four combustion cans with primary flame detector mounts, and four combustion liners (and flow sleeves) with sight tube holes. It might be possible to shut the turbine down and shuffle cans/flow sleeves/liners around to change the positions of the primary zone flame detectors.

Please write back to let us know how this progresses.

Thanks!

 

Hi,

We check the combustion hardware but we didn't see any problem.

We can't go to premix mode. every try go to premix mode during transferring the flame at primary was detected and we have spread just during transferring. then we don't know what thing must be check.

during unsuccessful premix mode we have just extended lean lean mode high emission alarm that means the unsuccessful premix mode.

We check the trend of 24 thermocouple of exhaust just during transferring. we have spread around 100 deg C and when the GCV3 (GAS CONTROL VALVE FOR TRANSFERRING) HAD maximum opening the load (MW) REDUCED around 4 MW, BUT I don't know why?

Please help me because we can't increase the load of gas turbine and we don't know which load is better for this unit.

THANKS A LOT FOR REPLY ME.

 

ABDI,

The fundamentals of transferring from Lean-Lean to Premix are as follows:

1) Reduce the fuel going to the Primary combustion zone to extinguish the flame in the Primary combustion zone; this means all the fuel flowing to the Primary combustion zone has to be stopped. To do this <b>and</b> maintain load it's necessary to shift all of the fuel to the Secondary combustion zone.

2) Once the Primary combustion zone flame has been lost for a few seconds, then start increasing the fuel flow back to the Primary combustion zone while decreasing the fuel flowing to the Secondary combustion zone. (Load is a function of fuel flow--to maintain stable load, the total fuel flowing to, and burning in, the combustors must remain stable.)

If at any time after Primary flame was lost and fuel is being re-admitted to the Primary combustion zone if flame is again detected then the ignitors are energized and fuel flow to the two combustion zones is driven back to approximately 50-50 (50% to the Primary and 50% to the Secondary).

As has been explained, there can be many reasons for Primary zone re-ignition: an ignitor or ignitors inserted too deep into the primary combustion zone; something entrained in the gas fuel flow that ignites or leaves a residue behind that is glowing hot enough to ignite a diffusion flame in the Primary combustion zone; problems with the combustion liner design and construction (particularly if they were not built by a reputable company).

There is a venturi in the combustion liner that separates the Primary and Secondary combustion zones. The purpose of the venturi is to increase the velocity of the air/gas flowing from the Primary combustion zone to the Secondary combustion zone to be so high that diffusion flame from the Secondary combustion zone cannot travel in a reverse direction back into the Primary combustion zone and re-ignite the fuel into a diffusion flame. So, if the combustion liner is not built properly then this could also be a cause of the problems you are experiencing.

But, there must be some ignition source in the Primary combustion zone (not necessarily the ignitors being energized by the control system <b>PRIOR</b> to the detection of diffusion flame by the Primary flame detectors). It's either oil or gasoline or natural gas liquids that are being ignited by the diffusion flame in the Secondary combustion zone and working their way back to the Primary combustion zone, or some other source (carbon build-ups that are hot enough to ignite the gas fuel being re-introduced to the Primary combustion zone during the latter stages of the Premix transfer process) that is igniting the fuel in the Primary combustion zone into a diffusion flame.

You say you have high spreads during transfer attempts, and that load drops during transfer attempts. That's an indication that the fuel flows to the various combustors are not equal--equal fuel flows would mean the exhaust temperatures were equal. (It could also mean that air flows to the combustors were not equal, but that's hard to imagine when the combustors have no moving parts and should all be virtually identical--again, here's where a reputable supplier is important.)

You have also indicated that the same thermocouples always indicate a spread--and you have indicated that you believe the hot gases flow straight through the turbine to the exhaust. Unfortunately, the hot gases do NOT flow straight through the turbine to the exhaust; there is a "swirl" effect, particularly at low load. So, while exhaust T/Cs 1 and -2 are directly downstream of combustor #1, they are not always directly indicative of what's going on in combustor #1 at all loads.

When you say you have checked the combustion hardware, that's not very descriptive, unfortunately.

Is the natural gas flowing to the turbine pressurized by a compressor nearby? Or, is the natural gas supply pressure stable during the transfer? Natural gas supply pressure fluctuations during Premix transfers can cause sufficient problems with the fuel flows to allow diffusion flame from the Secondary to travel back through the venturi into the Primary combustion zone. Also, if the IGVs were not stable during the transfer this could cause a problem.

You have never said if the unit has IBH (Inlet Bleed Heating) and if it is active/enabled during these Premix transfers.

Lastly, if the fuel flows to the secondary combustion zones are not all equal then it's possible that the spreads and other problems could be the result of plugged secondary fuel nozzle orifices, and if the secondary fuel nozzles have "transfer" gas passages (which is sounds like they do) then if some of them were plugged that could be a part of the problem as well.

But, my money is on something in the gas fuel flow or something in the Primary combustion zones of the combustion liners igniting the gas fuel flowing into the Primary combustion zone when it gets high enough to sustain diffusion flame (i.e., when the fuel-air mixture gets to the point that it can sustain diffusion flame).

If my memory serves me correctly, there is one other condition which can cause a Premix transfer to be aborted or a transfer to Extended Lean-Lean: high exhaust temperature spreads. The theory here is that there Primary flame detectors are NOT installed in every combustion can. This means that if diffusion flame were to be ignited in the Primary combustion zone of a combustion can without a Primary flame detector that the flame detection system would not be able to detect the diffusion flame. HOWEVER, diffusion flame burns much hotter than Premix combustion and this results in a high exhaust temperature spread (the hot gases from this combustor with diffusion flame would be much hotter than the combustion gases from the combustors on either side that were in Premix steady-state combustion mode)--so therefore a high exhaust temperature spread in Premix Steady State (or while transferring to Premix Steady State) could be an indication of diffusion flame in a combustion can without a Primary flame detector. So, it also triggers a transfer to Extended Lean-Lean, or aborts a Premix transfer procedure resulting in Extended Lean-Lean operation.

You must use the troubleshooting tools available on the Speedtronic turbine control system to determine which is occurring: Primary zone flame detection <b>OR</b> high exhaust temperature spread that is resulting in Extended Lean-Lean operation, and an inability to transfer into or remain in Premix combustion mode.

If it's high exhaust temperature spreads, then it's most likely the result of uneven fuel flows into the combustion chambers of one or more combustors WITHOUT Primary flame detectors--which helps narrow down the number of combustors somewhat. The fact that you say load is dropping when the exhaust temperature spreads are higher is also indicative of the possibility of loss of secondary flame in one or more combustion cans (because if the same amount of fuel is flowing into all cans and load is dropping that means the fuel is one or more Secondary combustion zones is not burning--which also results in a high exhaust temperature spread (though it means the hot gases from that can/cans is cooler than the gases from the cans on either side).

I <b>strongly</b> suggest you get someone knowledgeable to site to help with the problem. You seem entirely pressured to just solve the problem--and that's next to impossible if you can't take the time to understand what the possible causes might be (we've explained them several times over here) and then logically eliminate the possible causes until you arrive at the real cause. An understanding of how DLN combustion works in the combustor is key to this troubleshooting.

And an understanding of how the Speedtronic works is also key. The Speedtronic does NOT control the amount of fuel flowing into individual combustors. It has valves which control the amount of fuel into manifolds which surround the axial compressor casing and the sizes of the orifices of the fuel nozzles controls the amount of fuel entering the combustor by virtue of the pressure differential across the orifice (gas pressure upstream; combustor pressure downstream). The Speedtronic cannot increase or decrease the gas fuel flowing into any single combustor with respect to any other combustor. It's all done using manifolds to which all fuel nozzles are connected and by having open and free-flowing nozzle orifices--the Speedtronic only controls the total amount of fuel flowing into the manifolds. The amount of fuel flowing into each combustor is a function of the size and number of orifices--and whether or not the orifices are clean and clear. Plugged fuel nozzle orifices can cause imbalances in fuel flow-rates into individual combustors, resulting in high exhaust temperature spreads.

<b>PLEASE</b> have a knowledgeable person come to site to help with the problems--and then write back to let us know what was found to be the problem.

Troubleshooting is a logical process of elimination--when you don't know what the problem is, you have to eliminate the possible causes.

If the unit has "knock-out drums" (cyclone separators; coalescing filters; etc.) have you verified that no liquid is being collected, or that the amount of liquid is not excessive? Fuel gas used for GE-design heavy duty gas turbines <b>MUST</b> not have a temperature that is close to the dewpoint temperature of the gas. Because as gas flows through valves there is a temperature drop across each valve <b>AND</b> a temperature drop across each fuel nozzle orifice, and this can cause gas fuel liquid vapors to condense and cause primary zone re-ignitions. GE recommends a the gas fuel temperature be at least 50 deg F above the dewpoint to prevent the possibility of condensation across valves and orifices--and condensate has definitely been known to cause Primary combustion zone re-ignitions, with PERFECT combustion hardware.

Lastly, you need to remember: The unit was working fine <i><b>before</i></b> the outage. So, think long and hard about what was changed and what was worked on (mechanically--and on the control system: valves, Control Constants, etc.). And eliminate all the things which were "touched" or changed during the outage as possible causes of the problem.

A logical troubleshooting process--derived from an understanding of DLN principles and system configurations (P&IDs)--is the only way to solve a perplexing problem.

As for operating the turbine in Extended Lean-Lean combustion mode--yes, the Speedtronic will allow that. <b>HOWEVER, <i>YOU</i> MUST</b> multiply every hour of Extended Lean-Lean operation by a factor 10 because that's how hard on the combustion liners it is. DLN-I turbines were NOT designed to be operated for extended periods in anything other than Premix Steady State mode. Not even Lean-Lean mode--which is just a transition mode to get to Premix Steady State, and has it's own multiplier for equivalent hours of operation.

Whoever did the maintenance outage, installing new combustion parts and reassembling the unit, should be held responsible for achieving Premix Steady State operation after the outage--especially if the unit was operating properly before the outage. The ONLY way they cannot be held responsible is if they didn't provide the combustion parts installed during the outage and the combustion parts came from someone other than the OEM or one of its packagers or from a reputable third-party supplier (and there are really only two such suppliers other than the OEM; yes, lots of salespeople all say, "It's just like GE!" but it's really not).

It's not very likely that this is a controls-related problem. It sounds very much like the Speedtronic is doing the right thing. Again, please, Please, PLEASE, <b>PLEASE</b> have someone knowledgeable come to site to help solve this problem. If you need a recommendation, tell us where the turbine is located and perhaps someone here can provide the name of a knowledgeable person who can travel to the site to assist.

And--PLEASE write back to let us know how the problem is being troubleshot and resolved!!!!!!!

 

Dear Abdi,

I can only repeat what CSA said as there is no way I can explain things the way that he did.

1) Get someone knowledgeable to site to assist you in diagnosis.

2) Your exhaust splits are too high for a properly operating system. Prior to a transfer I would expect a spread of less than 10 degc.

3) This issue almost has to be related to hardware tolerances or installation issues. Unless your staff did something really wrong with replacement or adjustment of gas valves or IGV's LVDT's then the control system is trying to do what it needs to.

You need to be clear on what you did to check the hardware. Who did the hardware come from? What pieces were replaced? If this truly was a HGP then normally fuel nozzles, liners, transition pieces at minimum. As far as turbine area this can be 1st, 2nd and 3rd stage nozzles and buckets, shroud blocks etc.

If you truly found loose fuel hoses causing a gas leak in earlier post this point to sloppy work.

Diagnosing this type of issue is very difficult. But with the large exhaust spread I would hope that a knowledgeable person could analyze the data and narrow down the possible components. Once this is done they should have some idea as to what they would hope to find if they have to disassemble the machine again.

This problem has been going on for a while based on your past posts. Running in lean-lean mode is very bad for the machine and you really should get someone there to help and repair the problem before more damage if caused.

 

hi everyone.
i have this problem on my turbine too.
during premix operation, as 1st flame distinguish at primary zone. at this time we have high vibration (2mm/s extend to 10mm/s) on the No. 2 bearing. the flame detectors recognize the flame at primary zone then energize the spark plugs.
evidence:
our operator say that they have this problem at low ambient temperature( Tamb=18 c)

 

ahmad baktash,

I don't think you have the same problem that was being described previously. Are you saying that while running in Premix mode for some period of time the Mark* detects flame in the primary combustion zone and energizes the high-energy ignitors to go to Extended Lean-Lean or Lean-Lean? Again, the most common cause of this is some kind of ignition source which is reigniting the fuel in the primary combustion zone into diffusion flame--something like a hot ignitor tip or some kind of hot fuel nozzle tip with hot deposits (carbon-type deposits). Also, unstable fuel supply pressures can cause this kind of problem. High #2 bearing vibrations (on machines with three bearings--such as Frame 9Es or Frame 7Es/EAs) can be traced to one or more combustors without flame detectors which have re-ignited to diffusion flame and are causing excessive flame instability (along with high exhaust temperature spreads--which can also lead to the Mark* energizing the ignitors). But, you haven't told us what you've done to try to troubleshoot the problem, or what the results of your troubleshooting were. It's not likely based on the information provided that we can tell you precisely what the problem is caused by and how to resolve it--there's just too much you haven't told us.

What Frame size machine are you working on?

What DLN combustion system is in use on the machine you are working on?

Does the machine have IBH (Inlet Bleed Heat) and is it working properly?

What is the TTRF1 when the unit is trying to transition from Lean-Lean to Premix?

Is the unit being started "cold" when this problem occurs (that is, has the unit been on Cooldown for several hours since the last shutdown)?

Are you certain all of the fuel has been shut off to the primary combustion zone when this occurs?

What is the load when this failure to transition occurs?

What are the exhaust temperature spreads (TTXSP1, TTXSP2 & TTXSP3) when this failure to transition is occurring?

What alarms are active and annunciated when this failure to transition is occurring?

What are the primary- and secondary flame detector intensities when the failure to transition is occurring?

When was the last time the unit was tuned for emissions ("DLN tuning")?

GE and its packagers have extensive charts and troubleshooting procedures for problems like this. Have you used the Gannt charts and tables to investigate the problem? If so, what were the results?

What is the gas fuel supply pressure when this failure to transition is occurring?

 

amahd baktash,
We have six simple cycle 7EA's gas only units and we had this same problem on a few of our units in cold ambient temperatures. The problem was corrected by lowering the constant FXKTL2 by 10 degF - 15 degF. We change these every late Spring and late Autumn. We found if we leave them at the lower setting in hot ambients, it creates problems and trips the unit during shutdown when transferring from Premix to Lean-Lean. I believe we also had to change the constant FXKTL3 one time. I believe FXKTL2 is the constant that determines at what TTRF1 is considered Premix and FXKTL3 determines Extended Lean-Lean. It's been a while so I will have to confirm.
PLEASE NOTE--These changes were made after a THOROUGH Engineering Review on OUR particular units. Please perform an Engineering Review on YOUR units to determine if this would be an acceptable change and if any other constants may need changed in addition.

 

Hi

Turbine is Ge_frame9 DLN1 AND ESTABILISHED at sea level(300m) and hot climate (10 centigrade at January).

HGPI:

We performed the HGPI successfully and filled every related forms. During this inspection, we saw the highly deformation on all combustion liners at dilution hole area then we change them.

Operation:

Engine was started normally and turbine give the load between 70-80 MW at lean-lean mode. The exhaust spreads between 18-25 Centigrade at this mode. We set the reference temperature (TTRF1) =1087 C for premix operation. During this operation and extinguishing the of the flame in the primary zone, the vibration at the No. 2 bearing is increasing from 2mm/s to 10mm/s (5 time higher). Flame extinguish at primary zone successfully and as we estimate load is decrease. During the increase of load (as result of primary zone fuel injection), after second the flame detectors detect the flame at primary zone. Because of it, the spark plug energizes then we have flame at primary zone and we couldn’t have the stable premix operation.

We calibrate the spark plug and flame detectors. As I said the temperature spread is normal (20degree centigrade).

IBH (inlet blead heat) is disable.

Unit is not cold. Turbine work at lean- lean mode and we try to go premix mode and it fail.

U asked me about fuel shut off. The flame is extinguishing completely at primary zone. Then I conclude the fuel is shut off.

U asked about the load when the failure occurs. As temp=15 C and h=200m the load is between 80-85 MW.

About the active alarms: relays alarm, resignation alarm, high emission

We never tune the turbine for emission.

 

RichT1
How can we change the FXKTL2 and FXKTL3? They are fix and unchangeable.

 

ahmed baktash,

I only offer this information based on the information you provided.

You mentioned finding combustion liner damage in the area of the dilution holes--which are usually located in the primary combustion zone. For there to be damage in that area it's pretty likely that the unit was either operated for long periods of time in Primary combustion mode, Lean-Lean combustion mode or Extended Lean-Lean combustion mode,or the unit desperately needs DLN tuning (which you say has never been done). And, as is the case with many issues like this, it is very likely that it's a combination of these two and maybe some other issues as well.

GE-design heavy duty gas turbines with DLN-1 combustion systems are designed to be operated, continuously, in Premix Steady-state combustion mode. Primary- and Lean-Lean combustion modes are transitory combustion modes--necessary to get the unit into Premix Steady-State combustion mode. Sure, the Mark* doesn't prevent hours and hours of operation in either Primary- or Lean-Lean combustion mode--but that doesn't mean it's good to do so. The OEM does publish guidelines (in GER-3620) about the "cost" of long-term operation in either Primary- or Lean-Lean combustion mode--there are factors above 1.0 which are to be used on the Fired Hours in those modes of operation when determining when to schedule maintenance outages (Combustion Inspections; Hot Gas Path Inspections; Major Inspections).

THEN there's the matter of Extended Lean-Lean combustion mode--which is meant to be a temporary combustion mode when it's necessary to run the unit to either gather information (data) or to keep the unit running in order to transfer back into Premix Steady-state combustion mode. Again, the Mark* doesn't prevent long-term operation in Extended Lean-Lean combustion mode, but the factor for Fired Hours in Extended Lean-Lean combustion is 10. That means every hour of operation in Extended Lean-Lean combustion mode counts as 10 hours of operation in Premix Steady-state combustion mode.

Why? Because when the unit is operating in Primary-, Lean-Lean or Extended Lean-Lean combustion modes the combustion is occurring primarily in diffusion flame--and that flame is much hotter than premix flame. And at higher loads (such as in Lean-Lean and Extended Lean-Lean combustion modes) the amount of fuel flowing into the combustor and being burned in diffusion flame can--and does often--impinge on the walls of the combustion liner causing deformation and loss of thermal barrier coating, even cracking. There have been units with DLN-I combustors which have had several trips while operating in Extended Lean-Lean combustion mode for extended periods of time. These units could not be operated because of HIGH exhaust temperature spreads. When the liners were removed, many had collapsed.

And, yet, the operators and the plant management said, "But the Mark* didn't prevent us from operating the unit for extended periods of time in Lean-Lean- or Extended Lean-Lean combustion modes!!!" (It's always the control system's fault.) No; it didn't. It's expected that reasonable people know and understand the limits and consequences of keeping the unit running when it's not running in Premix Steady-state combustion mode. (This is where training and familiarization is very important.) If the control system was shutting the unit down or tripping it when it was being operated improperly it would be deemed unreliable. If there was an emergency and the unit needed to run but it couldn't run in Premix Steady-state combustion mode it wouldn't be a good thing if the control system shut the unit down or tripped it.

One of the main reasons for using Inlet Bleed Heat is to allow the unit to transfer from Primary- to Lean-Lean combustion modes and then from Lean-Lean to Premix combustion modes sooner that would otherwise be possible (in other words, without IBH). And, one of the good side effects of IBH is that it allows the transfers to occur at lower fuel flow-rates than would occur without IBH--and that means the temperatures inside the combustors won't reach the same levels as they reach when the unit is being operated without IBH.

If the above paragraphs and information don't actually describe how the unit was operated at your site, ahmed baktash, it's still applicable for others reading this thread now and in the future. Based on the information provided, it's difficult to come to another conclusion.

DLN tuning is important--for many reasons. If the fuel source changes over time (since commissioning) it's even more important.

As for the high vibrations, there are units which were supplied without gas transfer control valves (called "transferless") and they require IBH to be operating when transitioning from Lean-Lean- to Premix Steady-state combustion mode. And even when they do make that transition with IBH enabled and operating (at lower loads than would otherwise be possible without IBH running) the vibration around the combuston section usually increases slightly. People standing close to transferless units can often hear a high-pitched noise (akin to a screeching sound made by little girls) during the Lean-Lean-to-Premix transfer process (for a few seconds). If this is attempted on a transferless unit being operated without IBH then the noises are much louder--and so are the vibrations (higher). This is a result of higher-than-normal fuel flow-rates through the secondary fuel nozzle (because there is no transfer fuel system/nozzles).

There's a lot we don't know about your unit and how it's configured and how it's operated. Again, I offer this information based on the information provided and my personal experiences. It may or may not be directly applicable, but it does fit the conditions indicated (liner issues; high vibrations during Lean-Lean-to-Premix combustion mode transfers) which might be helpful to others reading this thread now and in the future.

Lastly, the OEM has multiple DLN-I troubleshooting charts which can be very helpful when all the facts and conditions are known.

I have heard that some DLN operating parameters (Control Constants ???) have been hard-coded into the turbine control system. (This usually happens when these parameters are modified without consulting the OEM and units have had damage.)

One more time: The above information was provided based on the information provided and my personal similar experiences. Something happened and is happening. I suggest working with the OEM or knowledgeable services to get to the root cause of the problem(s).

We would really appreciate any information you can share with us as you work through the issues. Best of luck!

 

ahmad baktash,
We simply went to the Control Constants Display on the HMI and made the changes for each MARK V.

What do you mean when you state " We set the reference temperature (TTRF1) =1087 C for premix operation." Are you setting a Control Constant?

What we were seeing on our units was TTRF1 would reach the transfer point to Premix, but when the Primary Zone was extinguished, TTRF1 would fall below FXKTL2 setpoint (1990 degF or 1087.78 degC) and relight the Primary Zone. After an engineering review, we change the constant to 1975 degF (1079.44 degC) in cold ambient temperatures. Even though we have six identical 7EA units, FXKTL2 is not the same for each unit. The numbers stated above are for only one unit.

 

ahmed baktash,

I only offer this information based on the information you provided.

You mentioned finding combustion liner damage in the area of the dilution holes--which are usually located in the primary combustion zone. For there to be damage in that area it's pretty likely that the unit was either operated for long periods of time in Primary combustion mode, Lean-Lean combustion mode or Extended Lean-Lean combustion mode,or the unit desperately needs DLN tuning (which you say has never been done). And, as is the case with many issues like this, it is very likely that it's a combination of these two and maybe some other issues as well.

GE-design heavy duty gas turbines with DLN-1 combustion systems are designed to be operated, continuously, in Premix Steady-state combustion mode. Primary- and Lean-Lean combustion modes are transitory combustion modes--necessary to get the unit into Premix Steady-State combustion mode. Sure, the Mark* doesn't prevent hours and hours of operation in either Primary- or Lean-Lean combustion mode--but that doesn't mean it's good to do so. The OEM does publish guidelines (in GER-3620) about the "cost" of long-term operation in either Primary- or Lean-Lean combustion mode--there are factors above 1.0 which are to be used on the Fired Hours in those modes of operation when determining when to schedule maintenance outages (Combustion Inspections; Hot Gas Path Inspections; Major Inspections).

THEN there's the matter of Extended Lean-Lean combustion mode--which is meant to be a temporary combustion mode when it's necessary to run the unit to either gather information (data) or to keep the unit running in order to transfer back into Premix Steady-state combustion mode. Again, the Mark* doesn't prevent long-term operation in Extended Lean-Lean combustion mode, but the factor for Fired Hours in Extended Lean-Lean combustion is 10. That means every hour of operation in Extended Lean-Lean combustion mode counts as 10 hours of operation in Premix Steady-state combustion mode.

Why? Because when the unit is operating in Primary-, Lean-Lean or Extended Lean-Lean combustion modes the combustion is occurring primarily in diffusion flame--and that flame is much hotter than premix flame. And at higher loads (such as in Lean-Lean and Extended Lean-Lean combustion modes) the amount of fuel flowing into the combustor and being burned in diffusion flame can--and does often--impinge on the walls of the combustion liner causing deformation and loss of thermal barrier coating, even cracking. There have been units with DLN-I combustors which have had several trips while operating in Extended Lean-Lean combustion mode for extended periods of time. These units could not be operated because of HIGH exhaust temperature spreads. When the liners were removed, many had collapsed.

And, yet, the operators and the plant management said, "But the Mark* didn't prevent us from operating the unit for extended periods of time in Lean-Lean- or Extended Lean-Lean combustion modes!!!" (It's always the control system's fault.) No; it didn't. It's expected that reasonable people know and understand the limits and consequences of keeping the unit running when it's not running in Premix Steady-state combustion mode. (This is where training and familiarization is very important.) If the control system was shutting the unit down or tripping it when it was being operated improperly it would be deemed unreliable. If there was an emergency and the unit needed to run but it couldn't run in Premix Steady-state combustion mode it wouldn't be a good thing if the control system shut the unit down or tripped it.

One of the main reasons for using Inlet Bleed Heat is to allow the unit to transfer from Primary- to Lean-Lean combustion modes and then from Lean-Lean to Premix combustion modes sooner that would otherwise be possible (in other words, without IBH). And, one of the good side effects of IBH is that it allows the transfers to occur at lower fuel flow-rates than would occur without IBH--and that means the temperatures inside the combustors won't reach the same levels as they reach when the unit is being operated without IBH.

If the above paragraphs and information don't actually describe how the unit was operated at your site, ahmed baktash, it's still applicable for others reading this thread now and in the future. Based on the information provided, it's difficult to come to another conclusion.

DLN tuning is important--for many reasons. If the fuel source changes over time (since commissioning) it's even more important.

As for the high vibrations, there are units which were supplied without gas transfer control valves (called "transferless") and they require IBH to be operating when transitioning from Lean-Lean- to Premix Steady-state combustion mode. And even when they do make that transition with IBH enabled and operating (at lower loads than would otherwise be possible without IBH running) the vibration around the combuston section usually increases slightly. People standing close to transferless units can often hear a high-pitched noise (akin to a screeching sound made by little girls) during the Lean-Lean-to-Premix transfer process (for a few seconds). If this is attempted on a transferless unit being operated without IBH then the noises are much louder--and so are the vibrations (higher). This is a result of higher-than-normal fuel flow-rates through the secondary fuel nozzle (because there is no transfer fuel system/nozzles).

There's a lot we don't know about your unit and how it's configured and how it's operated. Again, I offer this information based on the information provided and my personal experiences. It may or may not be directly applicable, but it does fit the conditions indicated (liner issues; high vibrations during Lean-Lean-to-Premix combustion mode transfers) which might be helpful to others reading this thread now and in the future.

Lastly, the OEM has multiple DLN-I troubleshooting charts which can be very helpful when all the facts and conditions are known.

I have heard that some DLN operating parameters (Control Constants ???) have been hard-coded into the turbine control system. (This usually happens when these parameters are modified without consulting the OEM and units have had damage.)

One more time: The above information was provided based on the information provided and my personal similar experiences. Something happened and is happening. I suggest working with the OEM or knowledgeable services to get to the root cause of the problem(s).

We would really appreciate any information you can share with us as you work through the issues. Best of luck!

CSA,
Good explanation on DLN operation /tuning for a GE HDGT, I Just would replace the word "vibration" by "pulsation" ...Thats why you can see some Alstom TA's ( now GE TA's engineer called themselves "Emissions & Pulsations monitoring/tuning " for Ex Alstom HDGT for example... And GE TA's called themselves "DLN/Combustion Tuning engineer" thats my own opinion/feeling after I did some DLN tuning with support by remote center in USA on GE 9FA units...

BTW thanks for sharing your experience and feedback on such thema !

Happy new year ! all my best wishes !Stay safe &healthy!

James