What are the reasons for the accumulation of carbon deposits in Purge air manifold?

 

It likely has to be liquid fuel that is somehow "dribbling" back into the purge air manifold.

If you have a purge air system, then you likely have a unit capable of dual-fuel operation (gas-distillate, and maybe distillate-heavy for the liquid fuel portion). And, if you have carbon deposits in the purge air system, then you likely shut the unit down, or it has tripped, while operating on liquid fuel.

When there is liquid fuel in the nozzle, and the turbine is hot after a trip or shutdown while operating on liquid fuel, that fuel will expand and can "dribble" into the AA and even into the purge air system. It can flow out of the nozzle tip and into the AA system, and it can flow through bad check valves into the purge air system.

The purpose of purge air is to purge the liquid fuel from the nozzle when transferring from liquid to gas so that the fuel doesn't carbonize in the liquid fuel nozzle with the high temperatures while operating on gas fuel.

But, because the purge air supply comes from the axial compressor discharge/AA compressor, when the unit is shut down on liquid fuel there isn't usually enough pressure/flow available to purge the liquid fuel nozzles. Sometimes, the sequencing will also prevent purging during a coast down from a trip.

So, if the unit is being shut down (normal, fired shutdown) while operating on liquid fuel or it has tripped while operating on liquid fuel that's when the likelihood of having liquid fuel get into the AA and/or purge air systems is greatest. But, if it's getting into the purge air system, then the cause is likely one or more failed purge check valves. And they're not easy to find, not without removing them and testing and inspecting them.

I've also seen one unit at a refinery that ingested hydrocarbon vapors into the axial compressor and they were condensed by the AA pre-cooler, but I don't recall seeing any deposits in the purge air manifold at that site.