Combustion Trouble GE Frame 6B GT.

S

Thread Starter

saber

We have 4 nos GE 6B GT. 2 of them are having "COMBUSTION TROUBLE" for last 6 months. All are having DLN system and all are 10 yrs old.

When the machine deloads from base (say 33 to 35 MW) at 30 to 29.5 MW, the machine jumps to 22 to 24 MW generating the following alarm "COMBUSTION TROUBLE". Then it comes back to 30 to 29 MW immediately and finally it goes to required preselected load.

We checked all exhaust thermocouples and found okay. We calibrated all gas valves (SRV, GCV and GSV) and found "Incorrect null bias servo current" message for GSV only!! But valve movement is fine and physical position is same with the required value.

What else should be checked to solve the above problem?
 
When was the last maintenance outage for each of the two units experiencing the problems?

Do the units have IBH (Inlet Bleed Heat), and is IBH active when the problem occurs?

Do the units have Gas Fuel Transfer Valves (GTVs)? Or are they "transferless"?

What is the TTRF at the higher load (before the Pre-select Load Setpoint is lowered)?

What is the TTRF at the lower Pre-select Load Setpoint?

The precipitous drop in load suggests a loss of diffusion flame--or failure to establish diffusion flame--in the Primary Combustion Zone of more than one combustor.

What are the IGVs doing prior to, during, and after the load swings?

Have you inspected the tips of the high-energy ignitors (fancy name for the "spark plugs" used in DLN combustor-equipped machines)? They do require replacement, and the insertion depth is critical to proper operation and optimum life. (Look at the combustor drawings in the Service Manual to find the insertion depth.)

As for the GSV 'incorrect servo null bias' Diagnostic Alarm, they are common for Mark V-equipped units with DLN combustors. As long as the actual measured position is nearly equal to the calibrated LVDT feedback, it's not a problem.
 
> When was the last maintenance outage for each of the two units experiencing the problems?

---- snip ----

When was CI (Combustion Inspection) done? What are the wheelspace temperatures, are there any cold spots in exhaust thermocouple ring?
 
Dear experts,

sorry for being out of touch.

I have VIEW2 file during combustion trouble. It is better to send you via email. I need your email id.

<b>Moderator's note:</b> most of the "experts" on this forum prefer to not have their email addresses public. I suggest you find one of the free upload sites, upload the VIEW2 file there, and post the url here on Control.com.
 
Sear saber,

If you have been running the unit with this problem since you originally posted the question (and, likely for some time before the question was posted) you are damaging the hot gas path components. No matter what the cause of the problem is the combustion liners, fuel nozzles, and transition pieces, not to mention the turbine nozzles and buckets, don't like the thermal stresses they are being subjected to when sudden load swings are experienced. If, as suspected, diffusion flame is being lost in one or more combustors and then being re-established in those combustors it's like a miniature explosion each time that happens. Raw unburned fuel is flowing into each combustor when flame has been lost and then, probably through the cross-fire tubes, flame is being re-established. And it's probably not happening smoothly, which means the combustors and hardware are being subjected to pressure waves as well as the thermal stress of the loss of flame and then the sudden re-establishment of flame.

You have answered NONE of the questions asked originally. Yes; you've provided some data, and, as you know, it's very difficult to analyze the data without converting it to a spreadsheet format and that takes some time (unless one has a macro or app to do that, and I don't have one at this writing).

We also can't translate many of the values (like DLN_MODE_GAS and FSR_CONTROL) without access to some of the files in the UNIT1 directory to convert the values to text.

Looking at the actual turbine speeds in both files, while they are relatively stable they are WAY high! Approximately 102% of rated speed, which translates into as much as 100 RPM over rated speed--for a DLN machine! YYIIIIIKKEESS!!! Either something is WAY wrong with the speed feedback configuration or the frequency is 102% of normal, too, which is pretty darned high.

It's also pretty clear that the unit is dropping out of Premix Steady State and into Lean-Lean or Extended Lean-Lean.

But, it is highly recommended that you get someone to site to help resolve this problem as quickly as possible if you want to avoid catastrophic damage to the turbine, which means it will be be shut down for a LONG time while it's being repaired--which will cost a LOT more than having a knowledgeable person come to site and resolve this issue.

Please, don't continue to beat this turbine by subjecting it to the abuse it seems to be enduring at this time.

And, please answer at least one question: Is the grid frequency really at approximately 50.7 to 51.0 Hz? Continually?
 
Dear CSA,

Following your previous thread dated on 20 September, 2012 - 9:59 pm. My answers are as follows:

1. No IBH system.

2. No GTVs.

3. 1114 Deg C at Base Load. (you'll get all data sheet on the link below)

4. 1065 Deg C at 23 MW Load (Preseleted Load). (you'll get all data sheet on the link below)

http://www.mediafire.com/file/bj79sf4v9owevov/Attachments.zip

Frequency: Grid frequency is not stable. Frequency varies from 48.9 to 51.4!!!!! This normal in our Grid.
 
In Attachments.zip on the above link you will get CSP.txt. To read this file a font (Aahhh.ttf) is attached.
 
Dear saber,

When grid frequency is as unstable as you say it's very difficult to operate DLN-equipped machines reliably. VERY difficult. When the grid frequency is higher (or lower) than nominal, turbine speed--and, hence, axial compressor speed--is higher (or lower) than rated. DLN combustion systems, in Premix Steady State, run in a very lean condition to begin with and when the air flow entering the primary combustion zone (where the air/fuel mixture is very lean to begin with--in fact there's not supposed to be any diffusion flame in the Primary combustion zone when the unit is operating in Premix Steady State), maintaining stable Premix operation is extremely difficult.

The fuel split for DLN operation is based on a presumed air flow at rated speed. When axial compressor speed is higher (or lower) than rated, and can change very quickly because of grid frequency disturbances, it's extremely difficult for DLN combustion systems to remain in Premix Steady State mode (the low emissions mode).

It doesn't make any difference what the salesperson said when these units were being ordered: DLN combustion systems DO NOT operate reliably when the grid frequency is unstable, and varies by as much, or more, than 2%. Full stop. Period. End of discussion. It's just not going to be reliable in Premix Steady State. And running the unit in Extended Lean-Lean at high loads has been deemed harmful to the combustion system as well.

And neglecting to tell us of this condition (abnormal grid frequency) when posting for help is being less than honest with us.

I know you can't fix your grid frequency problem, but if that's how the grid in your part of the world "operates" then that's the conditions of operation for you. But, there's nothing that can be done by the Speedtronic or the OEM which can make DLN operation more stable when the air flow through the axial compressor/turbine is unstable.

DLN combustion is, by its nature, operating on the limits of stability. DLN combustion is often described as a four-sided "box" that the air/fuel mixture must remain inside of for stable operation. And this is all theoretical, because the sensors required to actually monitor all of the conditions don't exist on the turbine; they would be prohibitively expensive.

So, presumptions are made about operating conditions, and none is more important than the speed of the axial compressor and the air flow through the turbine. Which is a function of grid frequency. And, when grid frequency is unstable, then the presumptions are incorrect.

Alternating Current (AC) electrical systems--most importantly the consumers of power transmitted by AC electrical systems (grids)--are predicated on a relatively constant frequency. The speeds of motors (induction as well as synchronous) are directly related to the frequency of the AC applied to them. The speed, and torque produced by the motors, is therefore a function of the frequency. And when the frequency is not stable, the power produced by the motors is not stable. And when that happens, the grid becomes very unstable. It's a vicious circle that is difficult to "open" or stop.

It's impossible for us to help you with your situation.
 
Dear saber,

Another very important thing to note is that when not using OEM combustion hardware (fuel nozzles; combustion liners; transition pieces) the ability to troubleshoot DLN combustion problems is that much harder. Many companies claim to produce compatible or similar combustion hardware--for a cheaper price than the OEM--but when there are DLN reliability issues they are nowhere to be found supporting the troubleshooting. They blame it on the Speedtronic, and if the unit was running fine before the problem(s) started with OEM or different combustion hardware then it's not likely a problem with the Speedtronic.

When was the last time the DLN system was tuned at your site? Did the problems start after the tuning?

Are you certain the make-up of the gas fuel you are receiving hasn't changed recently (higher or lower BTU content, or slightly different Wobbe index)? These things can require different tuning and even, in some cases, different combustion hardware.

So, you may have a lot of factors contributing to your problem: unstable air flow because of unstable grid frequency (and unstable in this usage includes stable operation at frequencies much higher or lower than nominal, meaning that even if the frequency is stable for an hour or longer at 51 Hz, that's still outside the desirable range of operation, deeming it unstable at 50 Hz). While we would always like to have or identify a single cause, with problems like this it's not always possible.

And FINALLY (I mean it this time--this is the LAST possibility I can think of!), if you are trying to operate the turbine(s) at TTRFs that are just slightly above or below the combustion transfer temperatures then operation is NOT going to be stable if that operation is around the Lean-Lean-to-Premix transfer, or the Premix-to-Lean-Lean transfer. That is an OPERATIONS problem--the operators need to know what those transfer temperatures are, and they need to NOT choose a Pre-Selected Load Setpoint that is near to those transfer temperatures, because with an unstable grid frequency/turbine-compressor speed, it's pretty likely upsets will occur. (One would think the operators at your site, after 10 years, would understand this, though many times new supervision comes into a plant and has to be educated because they just don't understand "We've always done it that way!" and believe they can change things without really understanding the nature of the "beast"--in this case DLN combustion--and no one on site really can explain it better than, "We've always done it this way!" so there's a difficult learning curve which sometimes last an interminably long time.)

Best of luck! Report back, please, so we can all learn from your experience.
 
Hi.

You say about wobbe index what is it?

In my site LHV gas change frome 45311 to 45340 and MW gas change from 17 to 18. temperature gas is around 24 to 32 deg c.

Is there any logic in speedtronic about the wobbe index?

My plain frame9 markv DLN1 don't go to premix because the flame loss in primary about 16 second after that the flame detect in primary and during transfer we have high spread around 165degc. I want to know about the role of wobbe index in the stable of combustion and what must we do to resolve the problem?
 
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