Partially filled coke drums especially those of a few hours or less will contain only partially coked contents even though heater outlet temperatures are above 935aF. This is due to the lack of sufficient coke mass in the drum which normally provides a thermal source for the continuation of the coking reactions after charge has been discontinued. Such drums need to be steamed stripped for longer periods of time, up to four to five hours, versus 1-1/2 hours for a normal drum in order to help solidify the contents. However, partially charged drums of several hours, generally four hours or less, that also have residuum in them from emergency steaming of the heater will have a tendency to have soft asphalt like properties due to partial polymerization, and will have a plastic like deformable nature. This material, although allowing steam and water flow into the drum, can prevent water draining by plugging drum channels much the same as a check valve prevents backflow in a line.
you can found some point about that in coke drum /tarry drum
espana2005