Hi everyone

I'm working now on a GE frame9 gas turbine. when I was reviewing the atomizing air diagram in P&ID I noticed an odd thing (for me!). Output of atomizing air compressor goes straightly to atomizing air manifold(without any valve or checkvalve or ...)

Now I have a question: consider the unit is working with liquid fuel and we change the fuel to gas. VA18 will open, and a portion of air will recirculate through atomizing compressor. VA19 will open and a portion of air will go for purging the liquid fuel path, but what will happen for the air which enters the atomizing manifold or even is there any air flow to atomizing manifold? If yes would it cause some problem for firing in combustion chambers? if not how it is possible with considering there is no valve, checkvalve, ... in the path?

I'll be glad for any answer.

 

dontknowman,

The discharge pressure ratio of the Main Atomizing Air compressor when operating on liquid fuel and the bypass valve is closed is approximately 1.3 (absolute) (that's times the CPD, in absolute pressure). When operating on gas fuel with the bypass valve open, the Main Atomizing Air compressor pressure ratio drops to about 1.1 (absolute). So, there is ALWAYS flow through the AA manifold and nozzles into the combustor--even when operating on gas fuel. For there to be flow (anywhere, at any time) there has to be a positive pressure differential.

When operating on gas fuel, Atomizing Air isn't needed for helping to ensure the liquid fuel is burning as completely as possible with as little smoke as possible. So, the bypass valve around the Main AA compressor is opened to reduce the power taken from the turbine shaft to drive the compressor--improving the efficiency of the gas turbine slightly.

BUT, the AA manifold and nozzles need to be a positive pressure relative to the combustor pressure in order to prevent hot combustion gases from flowing backwards through the AA passages in the nozzles and into the AA manifold, which severely damages the nozzles and can cause damage to the manifold piping as well. (It can even set up flows between combustors through the nozzles/manifold--which is REALLY bad for the machine.)

The AA flowing through the AA manifold and nozzle passages is called "purge" air, and it's even cools the AA nozzle passages, so it serves even another purpose.

The dual fuel nozzles are designed such that the combustion of gas is, for all intents and purposes, unaffected by the slight additional flow of air from AA system (used, again, for purging and cooling, and at a lower pressure--and therefore flow-rate). In fact, I believe the combustion liners for dual fuel machines are designed to account for the additional air from AA system during gas fuel operation (to help with achieving the proper stoichiometric air/fuel ratios during operation).

Hope this helps!

 

Thanks a lot CSA for your reply.
I really appreciate your knowledge sharing.

 

Well explained by CSA

take care
g.rajesh