A Review of Optimising the Design of a New Coke Drum Skirt

Abstract

Coke drums are subjected to severe thermal cycling with the skirt to shell connection weld being vulnerable to fatigue cracking. It is essential this connection is well designed to ensure a long life before repairs are inevitably required. Much has been written on coke drum skirt design with the aim of reducing the thermal stresses and strains encountered at the skirt connection weld, some designs have removed the weld completely allowing the drum to sit in an “egg-in-cup” arrangement. This paper includes a short literature review discussing Coke drum skirt designs and explains skirt behaviour during the drum cycle that results in eventual skirt cracking. A case study is reviewed in detail for a new pair of coke drums, where the predicted fatigue life of the chosen welded connection is assessed using axisymmetric, quarter symmetry and half symmetry finite element analysis supported by thermocouple data. The optimised design focuses on a conventional tangential design where the effects of the essential variables such as skirt thickness, skirt connection location, skirt-to head-gap and slot design (length, location & spacing) have been modelled and optimised to obtain a skirt design that produces the longest fatigue life for the intended duty cycle. Coke drum skirts must be installed onto the shell to exacting tolerances during manufacture to ensure concentricity and minimal gap between the skirt and shell. A brief overview of how this is achieved will be presented.