Coke Drum Skirt Life-Cycle Management

Abstract Delayed coking is a form of thermal cracking used for processing “bottom barrel” residuum in a refinery delayed coking unit (DCU). Coke drums, which are key components of the process, are subject to thermal cyclic and aggressive operating conditions and have a finite life. A safe and reliable operation before replacement requires special considerations to manage the key aspects defining the coke drum life-cycle. The skirt to vessel connection has been identified as one of the critical locations limiting the coke drum life; the skirt-to-vessel weld is vulnerable to fatigue cracking, while the skirt itself is vulnerable to bulging and key holes cracking. In this paper, key aspects of coke drum life-cycle management will be discussed, covering the useful life of coke drum skirts from “cradle” to “grave.” Important aspects such as supplemental requirements for skirt design and fabrication, specialized in-service inspection, on-line monitoring, engineering assessment, repairs and replacements will be covered in detail. This paper includes a short literature review of common skirt designs and damage mechanisms of greatest concern with consideration given to fabrication of skirts for both new coke drums and skirt retrofitting projects. Five case studies are reviewed in detail including: the analysis of the skirt configuration for a new pair of coke drums; two coke drum set skirt retrofitting projects; two skirt repair events in two separate coke drum sets. The paper will include closing remarks and recommendations to help improve the management of skirts for these complex and critical pressure vessels.