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Reply To: Cutting coke drums under single isolation from overhead vapor line

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#24882

Mike Kimbrell
Participant

I think every Coker has had problems with the overhead vapor line valves at some point during the life of the unit. My experience has been to schedule repair or replacement of the offending valve as soon as practical when problems occur. The number of drum cycles should be kept to a minimum prior to the repair or replacement. Additional precautions should be utilized to protect any personnel on the structure should the remaining valve not seal completely.
Routine coke removal requires positive isolation from potential hydrocarbon sources. Two valves in series with steam pressure higher than the process (blocking steam) is the traditional method of proving the drum is isolated so that the drum can be opened. A single slide valve like DeltaValve or Z&J could possibly be used for this purpose as the steam pressure on the body of the valve is higher than the process. We use slide valves for top and bottom heads during the coking phase so they should be adequate for drum isolation. There is limited experience with using a single slide valve as drum overhead isolation.
A single conventional valve has a significant risk of leaking hydrocarbons back into an open drum. Several significant incidents have occurred by allowing hydrocarbons into an open drum and some of those incidents have resulted in fatal injuries. The source of those hydrocarbons is not always understood or confirmed; however, utilizing two valves in series with blocking steam pressure higher than the process pressure is a way to ensure proper isolation and protection from backing process into an open drum.

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