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Incomplete draining of the coke drum can result in very serious safety hazards. Burns from hot water out of both the top head and bottom head have caused fatal injuries from drums that were not adequately drained.
Good practices that help prevent a plugged feed line that will then prevent draining the drum properly include maintaining a forward flow in the feed line with steam as the feed is switched to the alternate drum. Performing the little steam (to the fractionator) and then big steam (to the blow down system) steps as recommended by the licensors will minimize the unconverted oil that is in the lower portion of the drum and could then plug the feed line or coke bed near the feed line. Keeping the drum hot during the coking phase is another practice that minimizes the unconverted oil in the drum at the time of the switch. Properly steam sweeping the section of line between the switch valve and the inlet isolation valve clears the resid out of that section of line so that it will not plug. Keeping the blocking steam pressure higher than the process pressure will prevent any resid from leaking past the block valves and plugging the line inadvertently.
If all these steps are done properly, the likelihood of plugging the feed line and being unable to adequately drain the drum of water will be low. Procedures to handle the rare case when the feed line is plugged should be in place to manage that hazard.