Several week ago somebody asked for the same problem I wrote this item
one of the recent accidents in recent years has been for this event of a loss of injection drum (plugged channel), the drum was ketp 5 days with out nothing (steam or water) the operators thought that the drum would be cold, after the accident was estimated that a drum of coke may take 15 to 27 days to cool a temperature below 100 C
when the maintenance staff opened the bottom flange was an explosion, two people died at the bottom of the drum
after this accident we suffer this plugging and based on this experience we apply the following procedure
we inject water from the top (alternate quench) until it had a water level above the bed of coke, as the water enters the water perchlorate and vaporizes
-Foam was injected (via a fire truck) through the top line
-we opened the flanges (we kept a team a group of firefighters around the bottom drum) and proceeded to cut the hole pilot ( speed was 2 to 3 feet per minute ) all the personal was protected (EPP)
Remember that when you open the bottom of the drum (depending on what type of unheading devide you have install) drum will have accumulated hot water , for that reason open partially the drum and let it drain
When for some reason we have been some problem with power, etc(blackout) our procedure if we dont loose the entry to the drum and depending how much hours this drum was in operation (less 6 hours) we inject flushing oil (sometime in the service header there is some conection (in all ours design we have 2″ line) and keep that injection for 5 or 6 hour after that we inject steam for 2 or 3 hour and then we sent to the coke condensate or blowdown system (this take between 1 and 2 day) because you have tarry (vacuumm and coke) and the filter in that system are going to plug and you have to keep cleaning
with this procedure (we did about four time with any safety issue) we dont send any tarry to the pad
best regards