Hello Guys,

I have a question about flame detector uneven counts. In Mk-V Idos, the flame indicates:
Flame 2 = 513
Flame 3 = 739
Flame 7 = 374
Flame 8 = 214
(not a DLN Machine)

If all flame indicates logic-1, we continue running the turbine, but if we loose 3 out of 4 flame detectors, we trip. Can anyone explain what it indicates if the flame count shows very high and what it indicates the flame is low in count as in above mentioned readings? Does it indicate over firing, under firing, quality of the gas burnt into turbine, and what is the impact of that on the turbine? We are using Honeywell flame detectors and are brand new installed after a major overhaul on turbine. Spread 1 = 44 DegC and Spread 2 =35 DegC, FSR= 49 @32MW Base, IGV =86Dga, CTIM= 22DegC, TTXM=560, CPD = 9.39Bars

Best Regards,

Mambo

 

Mambo,

First of all, let me congratulate you on the magnitude of the flame detector intensities at your site. There are many sites around the world which routinely have one or more of the Geiger-Mueller (Honeywell) flame detectors which indicate less than 100 counts all the time! So you have a better than average unit, even if it's not as good as it was before. You gave some very good data, except: What were the intensities before the flame sensors were replaced? Good analysis of a question like this really requires data over time to compare against.

Flame intensity indications can be an indication of combustion problems, but from the data you provided, it doesn't look like there is anything to be concerned about. It certainly wouldn't be an indication of over- or under-firing; that could really only be measured with lots of instrumentation not normally installed on a turbine.

Usually uneven flame indications have more to do with "aiming" issues than anything, or with dirt or moisture on the lenses. Contrary to popular belief, moisture can condense on the lenses of flame detectors and impact sensitivity. The distance of the flame detector from the flame; the "straightness" of the piping/connection to which the flame detector is connected to the combustor; the alignment of the holes in the combustion liner with the flame detector piping/connection; all of these can greatly impact the sensitivity. Also, the amount of fuel being admitted through the nozzles will affect the flame intensity, but any great discrepancies between combustor fuel flows and flame intensities will usually show up first as high exhaust temperature spreads, or at least higher than normal spreads.

Since a maintenance outage was performed, the flame detectors were removed, and you even say new ones were installed. It's likely an installation issue or something similar. One would even wonder about the cleanliness of the tubes which the flame detectors are mounted on and which mount on the can. Some units have manual shut-off valves (gate valves!) mounted between the detector and the can which can be not fully open or stuck (they rarely get operated and it is a very high temperature application they are being used in).

There is a huge move afoot to eliminate flame sensors; many feel that a combination of CPD and exhaust temperature is as good an indicator of the presence of flame, which is all the flame detectors are really there for anyway. Just a safety instrument, and, like mechanical overspeed bolts, they will probably soon go by the wayside. The only reason they haven't disappeared before now is the resistance of many insurance companies to their elimination. But, they will probably not be provided on new units in the very near future.

This issue, intensity indication, has provided a lot of fodder for discussion over the years. Me, I miss the old days, when flame indication was just a logic signal, a simple "on" or "off". Too many people try to attribute all manner of perceived problems to flame detector intensity, or vice versa.

 

Dear CSA
I would like to add further that as we continued to run machine to base we came across high wheelspace temperatures, the first stage aft. and second stage forward temperatures showed very high crossing the limits and then we received high vibration alarms for turbine bearings 1, 2.

We decided to stop turbine and inspect, findings were first stage nozzle and bucket coked and a bit deformed and decolourized, they were really hot when unit was on load.

I would like to further share my experience about these flame detectors if the yellow flame is presence flame count increases..and means that bad quality of gas or improper mixture of air and gas. If more of blue flame is presence then flame count decreases with a good combustion.

Earlier flame counts were within 300 for all the flames as indicated by operational personals.During installation all the 4 detectors were cleaned properly, block valves were fully opened.
I will share more informations once everything is assembled back and restarted.

Best Regards,
Mambo