It is common to have difficulty draining water out of a coke drum if there is unconverted oil in the drum. Raising the drum temperatures during the coking phase will help finish the conversion of hydrocarbons in the drum to coke and lighter hydrocarbons.
The initial steaming of the drum to the fractionator is also important. This phase of steaming is used to push the feed up through the coke bed up to the liquid layer at the top of the coke bed so that no partially converted feed is contained within the coke bed. This steaming is also used to transfer heat from the coke bed to the reaction mass that is still converting and still needs energy. Too much steam at this step will slow the reactions and allow pitch or tar to run back down into the coke bed. Not enough steam will fail to push all the feed out of the coke bed and into the liquid reaction layer.
If the initial steaming is not done properly, when the coke bed is steamed to the blow down system much of the partially converted feed can be stripped out of the drum if the steam rate is high enough for long enough. As much steam as possible should be used in the phase of the off-line cycle without causing carry over of coke fines into the blow down system. So, higher drum temperatures while coking, adequate steaming to the fractionator and to the blow down system should help with draining water out of the coke drum once the coke bed has been quenched.