State-of-the-Art Improvements in Coke Drum Design and Life Extension Practices

Coke drums are vessels critical to the operation and profitability of today’s oil refinery and tar sands facilities. These vessels operate in batch cycles at relatively high temperatures, ∼800F+, and about once a day they are filled with vapor and hot oil and finally quenched with water. They have historically been designed according as ASME Section VIII Division 1 pressure vessels with little or no account for the cyclic operation or fatigue service in which they operate. In the past ten years, the frequency and severity of these cycles dramatically increased while cycle times steadily decreased from 24 to 12 hour cycles and, more recently, to as low as 10 hour cycles. As a result, coke drums with few or no previous problems began to crack through wall in the shell and/or skirt region. In addition, many new coke drums are cracking through wall in as few as four to five years because they were not designed for the more severe fatigue service in which they currently operate. These observations have been learned by extensive monitoring and evaluation: • drums experienced lower switch temperatures (inadequate pre-warm for the skirt), • drums were heated and cooled much faster than anticipated, • contraction of the drum walls compacted the coke, • feed stocks were now different and produced harder coke, • reducing cycle times by as little as two hours caused average fatigue damage per cycle to increase by more than 30%. All of these occurrences can increase cyclic stress on drums and significantly accelerate cracking.Copyright © 2007 by ASME