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Reply To: Drum vent opening


Mike Kimbrell

New Delayed Cokers have interlocks to prevent the inadvertent valve misalignment that would allow hydrocarbons to be released to the atmosphere. Opening a vent valve on a coking drum is only one of several different potential valve misalignments that could result in a release of hydrocarbons. Interlocks require actuators on the valves that can prevent the valve from opening without the proper permissives. Typically these actuators are electric motor driven and the permissives prevent the operation of the motor. The handwheels have to be controlled as well as the manual operation of the actuator.

Typically, the vent line is routed to a safe location which provides a layer of protection for this hazard. A Layers of Protection Analysis (LOPA) can be used to determine if your safeguards are adequate to reduce the probability of the hazard causing injury. A key piece of this analysis is the initiating cause frequency. The CCPS (Center for Chemical Process Safety) has a large library of books dedicated to process safety including LOPA. In the book on LOPA there is a reference to the frequency of operator errors on a routine procedure for personnel that were well trained, unstressed and not fatigued to be at 10-2 (one in one hundred) opportunities. A 2-drum Coker on a 16-hr fill cycle has 548 venting, draining and deheading opportunities per year. The CCPS value would then indicate that there are 5.5 incidents per year where the procedure was not properly followed. This seems high to me and I think not all incidents would result in an injury. If 1 in 10 of these incidents resulted in an injury, there would be 0.55 incidents per year as an initiating event frequency for a 2-drum Coker, which seems reasonable. A 4-drum Coker would have twice the incidents and a 6 drum Coker would have 3 times the incidents.

Using the initiating event frequency above and using a targeted event frequency of 10-4 or 10-5 injuries per year (This is the risk tolerance your company has for large process safety events) it is not possible to achieve this frequency without an instrumented system. A SIL 1 system would be my minimum recommendation and depending on the risk tolerance the SIS might need to be a SIL 2 system.

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