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Why Does Liquid Fuel Get into the Combustion Wrapper
Why LF can get into the Combustion Wrapper during false start?

Greetings,

The lower half of the combustion wrapper has a manhole with a pipe which drains either Water Wash or Liquid Fuel. For the Water Wash it is clear for me how the water gets into the combustion wrapper. But I don't see any way of liquid to get into the combustion wrapper. The liquid if failed to ignite will be in the combustion liner and transition piece. The lower cans will have a false start drain tubes which is on the forward casing of the Can. The liquid can't get into the combustion wrapper unless the hula seal is only air tight and not liquid tight. Any explanation please.

Thanks

2 out of 2 members thought this post was helpful...

Aptx4869,

What do you think happens to the liquid fuel that is not ignited when the unit fails to establish flame when starting on liquid fuel? The atomized fuel condenses on the inside of the combustion liners and transition pieces, the nozzles and buckets, and in the exhaust. None of the joints--including the hula seals of the combustion liners--are water-tight (or even liquid fuel tight). Some of the liquid fuel that doesn't burn in a failed liquid fuel start does make it out of the stack in a vapor form, but if the turbine exhausts into an HRSG then if the HRSG is cold it will even condense there before making it out of the HRSG stack.

If liquid fuel were to dribble into the combustion liners during fired operation that would mean flame was lost in one or more combustors, there would be high exhaust temperature spreads and white smoke coming out of the stack. Any unburned liquid fuel would combust incompletely, causing the white smoke.

So, you're right--it would be extremely difficult for liquid fuel to puddle and collect in the combustion wrapper during fired operation on liquid fuel--but it would certainly collect in any low point in the unit during a failed start attempt on liquid fuel and would, if not allowed a path to drain from any low point, cause a serious explosion if the unit did fire on a successive start attempt--whether on liquid fuel or gas fuel.

The hula seal isn't even air-tight--it works as a seal because of the pressure differential on the outside of the seal versus the inside of the seal, and because of the high flows when the unit is running. There is s small amount of leakage through the seal during normal running operation. If you don't believe that, look at the inside of the transition pieces the next time they are removed from the unit during a maintenance outage. And the amount of leakage is calculated and even required for proper operation (cooling and diffusion of the combustion gases, as well as taken into account for fuel-air mixtures for combustion purposes.

Finally, combustion liners are not liquid-tight. They have slots and holes (some more than others) and unburned, puddling liquid fuel would find a path to a lower point in the machine. There could be small puddles in some liners, but the purpose of purging is to try to help the fuel evaporate and make its way out through the exhaust.

Hope this helps!

CSA,

Yes this helps. also you brought a good point about the holes in the liner. I believe the liner has two types of holes. One for cooling and the other hole is involved in the combustion process. If possible, can you mention some of the points involved for introducing air into the combustor while burning diffusion gas (at lower loads)? is air still introduced through the flow sleeve while on diffusion gas (lower loads)?

many thanks