Lessons Learned from a Coke Drum Vent Incident

Presented By

Dale Wilborn - Chevron

Conference: Galveston 2016

The Delayed Coker experienced an incident that resulted in a reportable quantity of H2S emitted to the atmosphere. The Coker Structure Valve Sequencing (i.e. Interlocks) program makes sure one step is completed before allowing the new step in the sequence to proceed. At the end of the quench cycle, the blowdown valve is closed simultaneous with opening the vent valve. The Programmable Logic Computer (PLC) receives valve position from the Limitorque actuator as part of the permissive sequence.

In this instance, D drum Blowdown MOV registered CLOSED even though the valve was still about 30% open. When F drum was depressurized by opening the blowdown valve, the vapors had a “reverse flow” path through the D drum blowdown valve to the D drum vent to atmosphere. The false D drum MOV valve position was a single point vulnerability that had not been identified.

Because of the high risk from H2S or flammable hydrocarbons, it is essential that all MOV valve positions, either open or closed, be accurate. A back-up or redundant method to verify MOV valve position is being developed.

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