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Coke Drum Foam-Overs: Causes and Cures

By Norm Lieberman

There are two types of coke drum foam-overs – bad and very bad:

Results of a drum carry-over are quite variable, depending on how much coke has been carried into the fractionator, and now the fractionator is designed to deal with carry-overs. On most newer cokers, a circulating fractionator bottoms pump, and an external filter permit modest amounts of coke (a few tons) to be extracted from the fractionator in a day or two. A typical 12′ diameter fractionator can tolerate a single carry-over of 20 or 30 tons of coke. Carry-over amounts greater than these amounts will:

Carry-Over After Switching

I’ve had a wonderful opportunity to study this problem in great detail last month. My client has four coke drum density level indicators, drum top pressure indicators and the combined drum outlet temperature located on the switch deck. This allowed me to observe the response of the coke drums, as the operators manipulated the drain, steam, switch and vapor valves. This coker has a tendency to carry-over on almost every switch. My observations: Made after watching half a dozen switches indicated.

  1. Coke drum outage. A small outage being 18′ to 22′
  2. Low coke drum top temperature (i.e. less than 815°F) promotes unstable foam fronts, which requires a more positive ascending pressure profile to suppress.
  3. Lighter coker feeds, slop in feed, low coke drum pressure, high recycle rates, excess steam in the heater passes, and other factors that increase drum vapor velocities, require a larger “x” valve.
  4. Many of my clients use a tremendous amount of steam in the coke drum structure, most of which goes into the coke drums, which increases velocity.

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