Abstract
Coke drums are large pressure equipment critical to the Delayed Coking Unit (DCU) of refineries, which are used for the carbonization reaction involving dehydrogenation, rearrangement and polymerization. Due to the severe fatigue loading, high temperature and corrosive environment, cracks, bulging and local wall-thinning are frequently occurred at the root of the top-nozzles, shell and along the bead direction of the fillet weld in skirt-to-conical head joints. A typical case is given in this paper on the inspection process of a newly improved coke drum, and 18 longitudinal cracks have been found in the fillet weld of the skirt-to-cone joint when it is in service. According to the stress state, operation conditions and structural strength of the structure with these longitudinal cracks, cracking mechanics for provoking these longitudinal cracks are given. Innovative repair procedures are proposed to prevent the same type of cracking. These repair measures were proved to be effective by successful applications in repairment of similar cracking in another coke drum for continual two years. Some improved suggestions for the local structure of the fillet weld in skirt-to-conical head joints are put forward. Suggested inspection strategy for detecting the longitudinal cracks in the fillet weld of the skirt-to-cone joint and some issues to which high attention should be paid during inspections are given as well.
