Prevent coking fuel oil nozzle


Thread Starter


Flame 5 machine, dual fuel nozzle, spring loaded check valves. Found nozzles some particles like coking material in the fuel oil nozzle dip.

Normally fire with gas, and switch over to fuel oil two week once. Experiencing adjacent TC hit TTHT (low exhaust temp.limit). Request for recommendation on how to prevent fuel nozzle from coking during fuel oil firing.

I am unfamiliar with TTHT, so can't help with that.

The liquid fuel nozzles should be purged with air whenever the unit is running on gas fuel.

Having said that, there have been documented cases of liquid fuel check valves leaking backwards while running on gas fuel, allowing hot combustion gases at CPD to get into the liquid fuel flow divider and cause liquid fuel to dribble into other fuel nozzles and cause coking.

If the unit is shut down while burning liquid fuel, there is no liquid fuel purge and if the fuel nozzle is hot enough some coking can occur.

Liquid fuel purge air does not flow on a gas fuel start until the unit reaches 14HS, so if there were any liquid fuel in the nozzle after a stop or trip while running on liquid fuel and the unit was re-started on liquid fuel there could be some coking until 14HS.

Lastly, if liquid fuel purge air were NOT flowing when it should be that could cause coking, too. If I recall correctly, the liquid fuel purge air flow is controlled by VA19-1--but check your P&IDs to be sure!

That covers all but the oddest possibilities and is a good place to start troubleshooting.

Please write back to let us know how you care!

Normally this will be happen in Dual fuel machine cocking is major problem So for further prevent from this situation you need to Recirculate your fuel up to fuel nozzle while operation on Gas. Follow Machine vendor recommended Procedure about Fuel recirculation During Gas Firing.

Second need to Clean your Spring Loaded check valve. if fuel oil filtered good and recirculate from fuel nozzle then need to clean every year During Turbine Combustion Inspection.

Purging set 35sec after switch firing gas to oil. Is it timing sufficient? How to change the timing in MarkV logic. example a time constant as plan to increase the timing to 60sec.

How the circulation work for fuel oil when turbine on gas firing. Can send me the hook up layout for reference.

what other practice in the market to prevent coking on nozzle.

Do you use water and air purge? or just Air purge?

Have you checked all sequences working properly? I mean all respective valves operating properly in the field? NRV Ok? Enough air/water pressure?
You don't say what type of fuel oil.

If it is a heavy fuel oil and is heated perhaps it is the heater control that is causing problems? Poor heater control could lead to fuel being carbonised on the heat exchanger surfaces and then being carried through and deposited in the nozzles.
It seems they use #2 distillate/fuel oil (diesel).

A small amount of carbonization is unavoidable. The problem is the original poster hasn't said how much (volume per nozzle) was found, how many nozzles it was found in, and how long the unit ran before it was found.

If the unit is being transferred to liquid fuel and then shut down on liquid fuel, and not re-started for a few hours, then it's entirely possible for a small amount of carbonization to occur. If the liquid fuel purge check valves are leaking while the unit is running on gas fuel, that could cause fuel oil to dribble into the liquid fuel nozzles when it shouldn't be.

But, we haven't heard back from the original poster about the information we've asked for.