GE 7 transfers from premix to extended lean firing mode

L

Thread Starter

LT

Looking for reasons why a GE 7EA DLN1 will go into extended lean lean firing mode, at or near base load.

Unit would do this every 3-4 months for the past couple years (more so in winter months). Since a major overhaul this past summer (installed refurbished HGP parts, refurbished combustion hardware, new fuel gas valves, new IGV actuator and calibration of all valves/IGV/transmitters) the lean lean events are more frequent, every few weeks or less.

Tuning shows higher than usual NOX (high 30’s minimum) and low CO (single digit ppm). Moving fuel spit any where’s from 81 to 78 doesn’t resolve issue. Exhaust thermocouples and low spreads do not pin point any specific burner issue.

Points being trended in high speed view 2 files do not pin point any probable causes.

Unit runs on pipeline spec natural gas and base loaded sister unit in plant does not have similar problem.

Looking for suggestions as to what we could try or look at next.
 
Dear LT,

you would most likely be a rich man if you could answer this question for all the Frame 7 machines out there. As you most likely know the typical reason for a unit to fall out of premix and go into extended lean-lean mode is a re-ignition of flame in the primary zone. This can be caused by many issues. You sound like you have covered many of them. I doubt if monitoring points will help as the event is usually so quick you can't catch it, or all you see is a primary flame sensor picking up and the unit refires plugs to relight the primary zones if that hasn't already occurred.

You say this was an issue before the major but is now more pronounced. This can be due to differences in tolerances between the old hardware and the new. Tuning the splits is one way to try and remedy the situation. A full tuning of the part load and base load curves is the other. It sounds like this may be the next best step in your case. You mention this seems to occur more in the winter months which is also somewhat typical.

I don't know the specifics of your site so there are some things that can contribute to primary re-ignition events. If you have unheated fuel you may be getting some liquid hydrocarbons causing issue. Not sure if you have inlet bleed heat, this should not cause a problem at base load, unless the valve is doing something erratic.

As usual more information can help others on the site come up with more ideas (guesses on my part), but this type of event is difficult to diagnose as you have already found. Please write back and I am sure myself and others will try to contribute, or at least learn from your experiences.
 
Thanks, I appreciate your review and feedback MIKEVI,

So one of the points being trended is the spark plug firing and we are not seeing a call for re-ignition, which leads us to believe it’s a flashback and poor combustion dynamics. Any thoughts on going through the exercise of installing a combustion tuning kit analyzer on the unit (was one of the recent recommendations brought forward)?

A far as I know we have only tuned at full load and not part load (is it normal to tune at part load), but that brings up another question around tuning of the combustion liners. There were modifications 9-10 years ago around row 17 compressor blade issues (including staged dilution modification of liners and TP’s) as well as upgrades for heat rate gains 5-6 years ago. I read something in these old reports and upgrade documents that the liners can or should be sent off site for factory tuning (not sure if that was ever done but probably not). I’m wondering if anyone is familiar with this factory tuning of the liners and the specs for staged dilution of the liners/TP’s. There are also questions raised recently around firing rate control curves and 2035 or 2055 TTRF. Although since none of this was changed recently (last 5-6 years), it does not explain why we would only now (with in the last few years) be faced with the firing issues.

As for the hardware overall, the question has been raised as to the continued refurbishment of CI components (how often can this be realistically done) and the QAQC of such refurbishment’s. I guess there is always two sides to any story. Then there is the quality control during the overhaul and the possibility of foreign objects getting into the fuel stream during the work. I understand it may be hard to detect secondary burner issues (contaminants lodged in the internal burner orifices) when looking at flame spreads or exhaust thermocouples, is that correct?

As for the fuel, we preheat the natural gas to around 65 C and have not noticed any contamination’s or liquids in the fuel gas strainers at the GT or in the main gas filters/knockout drums common to both GT’s.

We do have bleed heat valves and have these trended as well, there were a number of lean lean events where the bleed heat valves were not open at all.

Looking forward to any further suggestions or recommendations.
 
Dear LT,

good to hear back from you. For your comments I offer the following as I know it.

I have never seen a combustion dynamics monitoring (CDM) system installed on a Frame 7EA DLN1 machine. My understanding of the 7ea DLN1 was that dynamics were not an issue, as long as fuel splits were not moved beyond the normal 78-82% range, and emissions at the lower loads were in spec. The only time I have heard a TA express concern is when NOX numbers are extremely low at low loads, where dynamics and lean blow out could be an issue. I have heard of CDM's being placed on the newer DLN1+ machines where dynamics can be an issue due to the leaner mixture and the enhanced combustion nozzles.

Most tuning that I am aware of is done throughout the load range, again to minimize NOX and CO at all ranges and maintain proper pressure drops in the combustion liner and fuel nozzle head end by controlling compressor mass flow. In the units I help to maintain we rotate combustion hardware between several machines. The dilution orifices are supposed to be sized to maintain the same flow rates for each can liner and TP. But as we rotate hardware at each CI we can see minor changes in NOX and CO numbers that theoretically should not be changing.

I can only guess as you can that since the CI there has been a slight change in flow patterns that has destabilized the combustion in at least one can. This change in flow is enough to disturb the pressure drop in the liner where the flame is able to make its way back into the primary zone.

This is such a balancing act as you know to keep te flame at the proper area of the liner. Any small change to dilution flow either through changes to the TP hole sizing, tuning pins, or otherwise can lead to primary re-ignition events.

As you said I would have to focus on what has been recently done. If you are able to trend and monitor the instrumentation on the unit and feel good about its operation then I have to go back to hardware.(I do this partly since I can use my I+C title as an excuse!)

Let me know if I can help further, or if you have more comments or suggestions for me.
 
Do you have historical data of CTD and with ambient temp? Could a major leak on a IBH valve cause this much of a shift in NOx? Should be relatively easy to check
 
Thanks again for the comments and advice.

We also move the CI hardware between the two machines and did not have these extended lean lean issues with this hardware when it was in the other unit.

But I would tend to agree with you, it is probably hardware related.
 
Yes we have both trended Mike, CTD was running around 622F at base load when the ambient was 19F (during one of the events). CTD only moved around a couple of degrees before and after the lean lean event (when looking at the entire data capture time frame for the event). I haven't looked at all the data myself as we have a couple different specialists looking at the trends and high speed data reports.

We had new trim installed in the IBH valve during the major overhaul last fall. I'm quite sure it isn't passing, but I'll double check just in case.

There are still questions around the staged dilution hole sizing on the liners/ TP's and the firing curves.

Thanks
 
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