Stay tuned, prevent blowout trips of 7FAs

1. Immediate aftermath of a flame blowoff event is shown here. The flame is at the bottom of the photo, some distance from the aircraft

An informal telephone survey by the editors of CCJ ONsite seems to indicate that more operators of GE 7FAs with DLN2.6 combustion systems may have experienced blowout trips than not. Such intermittent trips are virtually instantaneous and typically referred to by the acronym LBO (for lean blowout). Survey respondents suggested that blowout trips are common for at least some DLN2.6 combustors not equipped with an appropriate advance-warning system—such as a combustion dynamics monitoring system (CDMS). Negatives of LBO include accelerated degradation of parts and the obvious loss in kilowatt-hour production.

The editors called Dr Timothy Lieuwen, PE, associate professor, School of Aerospace Engineering, Georgia Institute of Technology, and asked the expert to explain in plain English exactly what a blowout is. Lieuwen, well known in the electric-power community, chose a picture over words. Fig 1, from a paper published by Lieuwen and two colleagues in Elsevier Ltd’s Progress in Energy and Combustion Science, was snapped immediately after a flame blowoff event. It shows the flame has blown out of the engine and is behind the aircraft.

Next, the editors called Mitch Cohen, senior systems engineer for Orlando-based Turbine Technology Services Corp, to learn more about lean blowout and how to protect against it. Cohen, respected on the deck plates for his engine tuning expertise, presented on the topic at the last 7FA Users Group meeting.

He said that LBO trips, which typically are annunciated by the control system as either “High Exhaust Spread” or “Loss of Flame” trips, do not always have a well-defined cause or clear-cut corrective action. Cohen identified these potential sources of DLN2.6 blowout:

• Combustor tuned to a fuel/air ratio that is too lean.

• Low-frequency dynamics.

• Worn combustion-system hardware.

• Instrumentation failure or shift in calibration.

• Shift in control-valve calibration.

• Continuous emissions monitoring system (CEMS) out of calibration.

• Note that the first two causes of LBO are tuning-related; remainder are impacted by hardware condition.

Continuing, he identified the following as LBO influence factors:

• Rich- or lean-PM1 operation.

• Ambient temperature.

• Unit configuration—that is, simple- or combined-cycle operation.

• IGV part-load temperature control curve.

Cohen said there are practical steps operators can take during maintenance outages, combustion tuning, and normal operation to reduce the likelihood of an LBO trip. He recommended guidelines for allowable emissions and dynamics levels to avoid blowouts and explained how those values are influenced by split schedule and control curve adjustments.

The ABCs

2. DLN combustion systems operate very close to the lean flammability limit and have a very narrow equivalence-ratio operating range

The DLN2.6 combustor, which is designed for 9 ppm NOx, demands very tight control of the fuel/air ratio over the entire load range. Just how close this combustion system must operate to “the edge” to assure that emissions and performance goals are achieved is illustrated in Fig 2.

Note how near the lean flammability limit (LFL)—the point at which the fuel/air ratio is too lean to support combustion and LBO occurs—is to the desired operating range. Such close proximity suggests that if any one of a 7FA’s 14 combustors operates to the left of the dashed line the turbine probably would trip.

Equivalence ratio is another important term to remember. It is the actual fuel/air ratio divided by the stoichiometric fuel/air ratio. Recall that the defining characteristic of a diffusion flame is that fuel and air are introduced into the combustor separately and the mixture burns at an equivalence ratio of 1.0, where flame temperature is highest and NOx production typically is highest as well.

Above 1.0, there is fuel in the mixture that can’t burn because the amount of air available is insufficient to support complete combustion. That’s why high CO levels are a characteristic of fuel-rich flames. Below an equivalence ratio of 1.0, oxygen remains after complete combustion of the fuel and it serves to suppress flame temperature. A ratio of 0.5 means there’s twice as much air as needed for complete combustion.

High CO is not just a characteristic of fuel-rich flames. It also may be found in lean flames because they are relatively cool and the conversion of CO to CO2 slows as flame temperature decreases. CO ultimately limits engine turndown; the low combustion temperatures associated with low-power operation cause a rapid increase in CO level.

3, 4. DLN2.6 fuel-nozzle arrangement is shown normal to the combustor axis at left, through the combustor axis at right. Premix burners of the same color operate together

In the premixed combustion process used for DLE-equipped turbines, air and fuel are mixed upstream of the combustionchamber, allowing tight control of mixture stoichiometry and, therefore, flame temperature. Figs 3 and 4 show the arrangement of fuel nozzles for a typical DLN2.6 combustor. Note that the staging of multiple fuel streams through the six fuel nozzles is required to achieve turndown flexibility while maintaining flame stability, controlling NOx emissions within requirements, and minimizing destructive dynamics.

Premix (PM) burners of the same color in the diagram are internally manifolded together within the end cover. The quaternary, or fourth, fuel manifold (a ring around the outside of the combustor casing and upstream of the premix burners) serves the series of radial pegs shown in Figs 3 and 4. Quaternary is used on some units to reduce the amplitude of specific dynamic pressure frequencies responsible for wear and distress of the combustion hardware.

Mode 6 split schedules for rich- and lean-PM1 combustors in Fig 5 show how fuel flow is distributed among the various nozzles/manifolds as load changes. The term “Mode 6” means that all six fuel nozzles are receiving fuel. The horizontal scale, combustion reference temperature, varies with gas-turbine load: The higher the temperature, the higher the load. At base load, TTRF1 typically is in the range of 2380F to 2420F.

5. Mode 6 split schedules for rich and lean PM1 combustors show how fuel flow is distributed among the various nozzles/manifolds as load changes. Typical PM1 split in Mode 6 is at left, PM3 at right

Cohen told the editors that lean PM1 was the method of varying load on the gas turbine with the original DLN2.6 combustion system. In the left-hand chart of Fig 5, PM1 was at its highest split percentage at base load (extreme right) and dropped as the unit was unloaded (moving to left). Load turndown to about 50% of rated capacity was possible using this approach. Rich PM1 was developed after lean PM1 for the purpose of achieving greater turndown. It typically enables the combustion system to achieve a turndown of 40% or less while maintaining CO emissions within the desired range.

The dashed line at 15% of the PM1 split schedule represents the optimal split for NOx in a DLN2.6 combustor—that is, the split at which minimum NOx emissions can be achieved. The terms “lean PM1” and “rich PM1” are used relative to this split value. At a fixed firing temperature, NOx increases as the split is modulated from this value in either direction—on the lean-PM1 side because the PM3 nozzles are getting richer and hotter; on the rich-PM1 side because the PM1 nozzle is getting richer and hotter.

6. All fuel nozzles operate at fuel/air ratios above the lean flammability limit at base load. In lean-PM1 operation at base load (left), PM1 (light yellow tint) runs slightly leaner than PM2 and 3 (orangey-yellow tint); in rich PM1, PM2 is slightly leaner than PM1 and 3 (right)

This is illustrated in Figs 6 and 7 for base-load and part-load operation, respectively. In Fig 6, all nozzles operate with fuel/air ratios above the lean flammability limit. For the lean-PM1 case at the left, PM1 is slightly leaner than PM2/PM3 (orangey-yellow tint); for the rich-PM1 combustor at the right, PM2 is running slightly leaner than PM1 and PM3.

At part load during lean-PM1 operation, illustrated at the left in Fig 7, PM1 is the leanest nozzle and the PM1 and PM2 nozzles are operating below the LFL; the PM3 nozzles are the richest nozzles and anchor the flame. LBO is more likely to occur with this operating scheme than with the rich-PM1 scenario at the right where the single PM1 combustor anchors the flame. Reason is that it’s much easier to keep one nozzle above the LFL than it is to keep three above the LFL.

7. At part load during lean-PM1 operation (left), PM1 is the leanest nozzle, the PM3s are the richest nozzles and “anchor” the flame. The PM1 and 2 nozzles operate below the lean flammability limit. At right, PM1 is the richest nozzle (flame anchor) and the PM2 nozzles are the leanest

LBO with lean PM1

Cohen offered a common LBO signature for lean PM1 combustor operation in Fig 8, pointing out the characteristic step change in exhaust spread (from 25 to 209 deg F in this case) with no other anomaly identified in the trip log or in historical PI data. In this case LBO occurred at 75 MW, just as the engine was transferring out of Mode 6. The trip was not anticipated and occurred so quickly (2 seconds), there was no action operators could have taken to prevent it. 

8. Common LBO signature for lean PM1 combustor operation is characterized by a step change in exhaust temperature spread; no other anomaly was identified

At base load on the lean-PM1 split schedule, Cohen continued, all nozzles operate at close to the same fuel/air ratio and all nozzles, individually, are above the LFL. As the unit unloads, PM1 and PM2 are leaned out—eventually dropping below the lean flammability limit. PM3 must operate at a sufficiently rich fuel/air ratio to prevent LBO.

An indicator of insufficient LBO margin, Cohen said, is very low NOx values. To provide adequate margin, he suggested adjusting split schedules to maintain NOx values above 6.5 ppm during part-load operation. Cohen stressed that LBO can occur at any load within Mode 6 if the unit is not well-tuned.

 

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