On-Line Steam Spalling
This method is used for spalling one pass (at a time) of a delayed coker heater without removing the heater from service. Hydrocarbon flow is removed from the pass being spalled, the flow of steam to that pass is increased, and temperature is cycled to break coke loose. (The steam used is injected either as steam or as steam condensate, not boiler feed water or other water containing dissolved solids; if steam condensate is used, the heater firing supplies the needed heat of vaporization.) The technique requires a heater configuration in which temperatures of individual passes can be controlled at least semi-independently.
The coke particles are swept into the coke drum. (There are occasional reports of this technique being used on crude unit heaters; it is not recommended because of the chance that large coke fragments could damage or plug the fractionator column which follows the heater in those plants.)
The advantage is that decoking occurs with the plant and heater continuing in normal operation with only a reduction of feedrate. The main disadvantage is that decoking is incomplete and the extent of decoking cannot be accurately determined. Operators expect that, after several on-line spallings, the heater will have to be shut down for complete decoking. The chance of tube plugging, which would require an immediate plant shutdown, is higher than that associated with other decoking methods. Certain steps must be carried out quickly to avoid tube overheating on the one hand or production of wet, off-specification coke on the other. The high and variable volume of steam used may cause premature coke drum foamover.