Steam-Air Tube Decoking
This is the method most often used. It consists of the spalling step and the cokeburning step.
In the spalling step, the tubes are heated, steam is passed through them, and as much of the coke as possible is broken loose and blown out of the tubes. Steam velocity, the impact of other coke particles, and cycling of temperature (thermal expansion and contraction of the tubes) break the coke loose.
In the burning step, air is admitted to the tubes in low concentration along with the steam. The remaining coke burns to oxides of carbon and sulfur.
In most cases, the effluent generated in both of the above steps is quenched by injection of water into the effluent line. All or a portion of the steam is condensed. The mixture is separated in a special decoking knockout drum. Noncondensible gases and remaining uncondensed steam are ducted either directly to the atmosphere or to the heater stack, where the flow of flue gas helps to disperse them. Quench water containing coke particles goes via the sewer to treating and disposal.
In rare cases, the effluent, without quenching, is ducted directly to the stack. This is feasible only in locations where a dark plume is tolerated.
The heater is fired at low rates as part of the decoking procedure. The usual safety procedures related to firing must be taken. In addition, all coils in the heater must be protected from over-heating. This involves putting steam through the process coils and any steam generating or superheating coils and putting boiler feed water through any BFW heating coil.