May 31, 2018 at 11:16 am #28600
Is there an industry standard or a typical drum outage level that most sites shoot for? Specifically for maximizing amount of feed into the drum while maintaining a low risk of a foam over event. If so, where does this target level come from? Is it from Coker experience (like “well, with these specific drums, we have never foamed over when targeting under X feet”) or is there a source of empirical data and trends that allows industry to draw reasonable conclusions?
I expect outage levels will significantly vary with changing crude blends, drum conditions, antifoam make and use, etc… Besides just being relatively conservative with outage level targets, how do other sites account for these changes?
Thanks for your time,
June 3, 2018 at 12:07 am #28605
With the outage being measured after the coke drum has been quenched and the water drained, a reasonable target is 15 ft (4.5 meters) from the top tangent line of the coke drum. The goal is to prevent foam or coke fines from being entrained with the vapor leaving the coke drum and be carried over into the fractionator. For coke drums operating at the target drum outlet temperature of 825 F (440 C), the foam front on top of the coke bed is between 5 and 10 ft thick during active coking. During the little and big steam as well as the water quench, the coke bed is stripped of hydrocarbons and shrinks somewhat, as much as 5 ft. Some feeds foam more and some feeds foam less. Lower drum temperatures cause more foam. The drum should be switched before the top of the foam front reaches the top of the cylindrical portion of the drum. A 15 ft minimum outage from the top tangent line of the coke drum as a target should provide sufficient room to manage the foam front and minimize the amount of material that is carried over to the fractionator. This is assuming the maximum coke drum vapor velocities are followed. This limit is dependent on the pressure on the drum during coking.
With a uniform feed slate and with a non-foamy feed, a minimum outage target of 10 ft from the top tangent line of the coke drum has been used successfully. As the feed slate changes, the foaming tendencies change so a more generous outage target will avoid unintentional overfilling of the drum.
Most outage measurements are from the top of the nozzle for the top head. The height of the head plus the height of the nozzle has to be added to the minimum outage target from the top tangent line.
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