I have 3 questions regarding gas turbine operation.

We have gas turbines running in Liq. as well as gas fuel.

When Gas turbine runs in Gas fuel, the liquid fuel nozzle is purged with the Atomising air continuously via the VA-19 pneumatic valve. At the same time the AA manifold also receives AA. In a way the AA line of the nozzle is also purged.

Why is continuous purging needed for nozzles? A maintenance person told me that in some system, the VA-19 valve is opened only for some seconds after fuel changeover to Gas from liquid fuel. Is that right?

What is the significance of Machine tripping in Low AA compressor differential pressure (actuated by pressure switch 63-AD across the AA compressor). Does that has anything to do with the purging of the nozzles?

Why should the liq fuel atomising air pressure be greater than CPD?

 

Seeker1988,

I'll answer the last question first. Excrement only flows downhill; unless a pump is used to get it to flow uphill. That's true for any situation where there needs to be flow--flow only occurs from an area of higher pressure to an area of lower pressure. Always. On planet earth, in this dimension, in this part of the universe.

If Atomizing Air pressure is less than CPD it won't/can't flow into the combustor.

Purging of the AA passages of the fuel nozzles is necessary to prevent the back-flow of combustion gases and CPD into the AA manifold. Back-flows of combustion gases and CPD into the AA manifold and between two or more combustors can lead to very serious failures and overheating of manifolds and pigtails and flexible hoses and fuel nozzle components.

Generally, when a dual fuel unit if running on gas fuel, the Main AA compressor is somewhat bypassed because less AA is required for purging than for combustion of liquid fuel. There should be a solenoid-operated valve to reduce the flow through the Main AA compressor, which reduces the AA pressure slightly, which also reduces the power consumed from the Accessory Gear, which means more power is available for the generator (a few kW, anyway, which over a period of years or decades does add up).

I would have to see the sequencing/logic/application code running in the turbine control system to comment on the exact sequence of events. Usually, liquid fuel purge air flows continuously to make sure the liquid fuel passages are free of fuel (which if present would carbonize (harden) and cause combustion problems)--and also to prevent combustion gases from flowing into the liquid fuel nozzle passages. I believe on some older (very old) machines the liquid fuel nozzles were only purged briefly, but it was determined that continuous purging was best to prevent carbonization of liquid fuel AND to make sure the liquid fuel check valves seat. Leaking liquid fuel check valves can allow hot combustion gases to get into the flow divider and can even, in some rare cases, cause the liquid fuel stop valve to be opened allowing even further penetration of combustion gases into the liquid fuel system, which can cause more carbonization issues and worse. So, a continuously functioning liquid fuel purge system is very beneficial to a dual fuel unit when running on gas fuel.

Without AA when running on liquid fuel combustion will be very poor and there will usually be lots of white smoke and the possiblity of explosion in the combustion wrapper and/or exhaust. When the unit is running on gas fuel, AA is required for proper purging and to prevent backflows of hot combustion gases into areas where they shouldn't be flowing which could lead to more serious problems. AA is definitely required for dual fuel machines (and for any machine running on liquid fuel).

Hope this helps!