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from the control department...
loss of flame at 630 rpm
Power generation equipment control. topic
Posted by omer jumma on 6 January, 2013 - 1:07 pm
after greeting i have problem with starting the unite (frame 9e). when i start the unite and reach firing, after that warmup the unit go to trip by loss of flame. i check the atomizing system anything ok (6psi) and liquid fuel system ok. servo valve ok. flame scanner was cleaned and i cleaned the filter of servo valve. and check the IGV angle ok. and clutch of liquid pump ok. and FSR 12 and fqrout 2 at warmup. when to reach fqrout 3. 3sec tripping


Posted by CSA on 7 January, 2013 - 3:25 am
omer jumma,

WHEN DID THIS PROBLEM START?

After an emergency trip from load while running on liquid fuel?

After a maintenance outage when fuel nozzles were replaced?

After "re-calibration" of the LFBV (the liquid fuel control valve) LVDTs?

After replacement of the liquid fuel control valve servo?

How often is this unit started on liquid fuel?

When was the last successful start on liquid fuel?

What kind of control system is used on the unit?

Was the torque adjustor recently worked on or refurbished?

During warm-up, what are the three exhaust temperature spread values (TTXSP1, TTXSP2, and TTXSP3)? Do they increase during warm-up, or do they increase just after warm-up is complete and just before flame is lost?

When a GE-design heavy duty gas turbine is being started, the reference for firing and for warm-up is strictly a value. It has nothing to do with acceleration or flame detection or temperature control; nothing except a position, and in the case of liquid fuel the position is converted to a liquid fuel flow-rate which is compared to the scaled feedback from the liquid fuel flow divider.

The typical start-up sequence for a Frame 9E is for the unit to purge at a higher speed (something around 20% speed or so), then the torque converter is de-pressurized to decelerate to about 9.5% speed at which point the torque converter is reduced to minimum torque position, and when the speed decreases below 9.5% speed the torque converter is re-pressurized and the unit should start accelerating at a slow rate to 10% speed.

As the unit accelerates above 10% speed at the slow rate, fuel is admitted at Firing FSR and the ignitors are energized and shortly after flame is detected the fuel is reduced to Warm-up FSR (to reduce the thermal shock to the hot gas path parts).

At about the same time as flame is detected and fuel is cut back to Warm-up FSR the torque converter is supposed to be "jogged" from minimum torque setting to the maximum torque setting by pulsing the torque adjustor drive motor on and off periodically. Fuel is usually held at the lower Warm-up value for about 60 seconds. At the end of the Warm-up period, fuel is then "released" to assist with accelerating the unit.

If the unit has a Mark V or a Mark VI or a Mark VIe Speedtronic turbine control panel, when fuel is "released" from warm-up the reference for the fuel valve position becomes a function of the acceleration rate. There is an acceleration reference (THNAR) and the actual acceleration rate (TNHA) is compared to the acceleration reference and fuel is adjusted to keep the actual acceleration rate very close to the acceleration reference.

However, if the torque converter is pulsed too quickly to maximum torque setting, or the value of warm-up FSR is too high, either of these can cause the actual acceleration to be much greater than the acceleration reference during the warm-up period. So, when the warm-up time is complete if the actual acceleration is greater than the acceleration reference FSR will be reduced to Minimum to try to reduce the actual acceleration--and this can cause flame to be lost in one or more combustors. If FSRMIN is too low and the actual acceleration is greater than the acceleration reference then flame can be lost.

But, if the unit has been successfully started for some period of time and this problem has just started, then something has changed. So, what has changed? If we understood what has changed, we might be in a better position to tell you what to do to try to resolve the problem. But, you haven't told us enough about the problem and when it started for us to be of much help.

Sounds like from what you have told us, you have verified the presence of atomizing air for assist (though 6 psi sounds very high, actually--it's virtually impossible to measure atomizing air pressure during firing because the Atomizing Air Pressure Gauge is usually scaled for something like 0-300 psig, and 6 psi is very difficult to read when the maximum range is 300 psig). If the atomizing air pressure were too high during firing and warm-up, then that could be causing a problem with combustion.

Another thing you didn't tell us about your unit was whether or not it has conventional combustors or DLN combustors. If it has DLN combustors, then there is a LOT of air flowing into the head end of the combustor and that can make ignition and stable flame during low speed somewhat more difficult than on a unit with conventional combustors.

So, there's a lot we don't know. Provide answers to the questions above and we might be able to provide some assistance. Again, if the unit has been started successfully in the past on liquid fuel, then it's necessary to identify what has changed recently, just prior to the start of this problem. And, then we can try to help you with your problem.

It would also be very helpful to know every Process- and Diagnostic Alarm that's present prior to firing and during firing and warm-up and to a few seconds after the loss of flame trip.

Hope this helps!


Posted by omer jumma on 9 January, 2013 - 5:00 am
thank you for helping

the control system mk 6e and the unite is started with liquid fuel 700 hours before this problem, was normal stopped. now when we start again, i faced this problem. but yesterday i try to start the unit and when i reached the 630 rpm, i show the intensity of scanner start to decrease to low, and i increased the fqrout by forcing tell the reach 1000 rpm. and i remove the force the acceleration is normal and FSNL (the problem in fuel between 630 and 1000 rpm)

thanks for cooperation


Posted by CSA on 9 January, 2013 - 11:49 am
omer jumma,

Use the Trend Recorder the next time you start the machine and monitor the signals in the previous reply. I think you will find that the actual acceleration rate (TNHA) is exceeding the acceleration rate reference (TNHA) and FSR is being driven to FSRMIN when warm-up is complete and that is causing flame to be lost.

Someone knowledgeable about tuning should be reviewing the pulses applied to the Torque Adjustor Drive Motor after flame detection, the value of Warm-up FSR, and the values of the Control Constants which are used to calculate FSRMIN and make the necessary adjustment to any or all three to prevent the loss of flame trip you are experiencing.

And, I STRONGLY caution you not to force the fuel control valve reference. When doing this many people are quite nervous to begin with and if something unexpected happens they can very quickly forget to unforce the value and keep pumping fuel into the machine when it shouldn't be. Just because a value can be forced doesn't mean it should be forced--under any circumstances. Forcing fuel to get the machine started is VERY DANGEROUS and should not be done more than once. I find that many sites adopt this method of starting instead of resolving the root cause of the problem, and when the unit is opened for a maintenance inspection they find the hot gas path parts are fried (cooked; over-temperatured) and they wonder why. Worse, the unit develops high exhaust temperature spreads prematurely and they wonder why.

Please use forcing judiciously and prudently. Use Trend Recorder to prove (or disprove) the theory that the unit is accelerating too quickly (for some reason) and FSRMIN is too low (for some reason) and it will not be necessary to force the fuel valve reference.


Posted by CSA on 10 January, 2013 - 2:39 pm
omer jumma,

>I think you will find that the actual
>acceleration rate (TNHA) is exceeding
>the acceleration rate reference (TNHA)
>and FSR is being driven to FSRMIN when
>warm-up is complete and that is causing
>flame to be lost.

That sentence should have read:

I think you will find that the actual acceleration rate (TNHA) is exceeding the acceleration rate reference (TNHAR) and FSR is being driven to FSRMIN when warm-up is complete and that is causing flame to be lost.

Sorry for any confusion this might have caused!

But I still cannot stress enough for anyone reading this thread: Forcing fuel flow reference signals is dangerous.

It's sometimes helpful to be able to force outputs of the Mark VI (or Mark VIe) but it can also be very dangerous.


Posted by omer jumma on 11 January, 2013 - 6:42 am
Thanks All

i find the problem of loss of flame and solved the problem someone from moe ministry of electricity, change the value of fsr at warmup from 12 to 10. i return change to 12 and starting the unit any thing is OK without forcing the unit. now is working properly. thank you for all and i am happy to share information in the site.


Posted by CSA on 11 January, 2013 - 12:12 pm
ommer jumma,

Thanks very much for the feedback!

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