As you know, care must be taken when connecting fire water into process services. Backflow of the process into the fire water system has to be prevented. My preference is to not have a permanent fire water connection. A temporary connection on the discharge line of the quench water pump would route water to the drums without concern for backflow in a permanent connection. It also avoids the incremental head the quench pump would add if the connection were on the quench pump suction.
It is possible to over pressurize the coke drum inlet nozzle or the lower portion of the coke drum if the coke bed is plugged off if excessive pressure is used to try to introduce higher pressure steam or water in the case of a tarry drum. A maximum allowable pressure should be calculated and followed in the case where abnormal cooling is required.
The use of top water, meaning water that is introduced through the top of the coke drum rather than up through the coke bed from the feed inlet nozzle, can achieve bed cooling without the risk of over pressurizing the drum or inlet nozzle. Using top water in the case you describe is slow, but will work. Add water to cover the top of the coke bed and wait while the heat in the coke bed boils the water off. Add more and wait. Repeat until the water level does not change after adding water. Great care must be taken when trying to drain water and open the bottom head. The drum could have significant amounts of hot water held up above the plug. Frank Tracy of P66 gave a good presentation of an incident similar to the one you have described at the 2016 Galveston meeting. It would be worth reviewing it.
