Optimal Design of Coke Drum Skirt Slots and Analysis of Alternative Skirt Support Structures for Thermal-Mechanical Cyclic Loading
The skirt-to-shell attachment weld on coke drums is susceptible to low-cycle fatigue failure due to severe thermal-mechanical cyclic stresses. Therefore, various skirt attachment designs have been proposed and implemented to reduce stress and thus improve reliability. The most common skirt design is a cylindrical shell attached tangentially by a fillet weld to the coke drum vessel. One inexpensive method to decrease stress in the junction weld is to add vertical slots near the top of the skirt,
... e top of the skirt, thereby reducing the local stiffness close to the weld. The conventional skirt slot design is thin relative to its circumferential spacing. An alternative skirt design where the vessel is supported by a number of welded attachment plates and allowed to expand and contract freely through the use of lubricated horizontal sliding plates also exists. In this study, thermal-mechanical elastoplastic 3-D finite element models of coke drums are created to analyze the effect of different skirt designs on the stress/strain field near the shell-to-skirt junction weld, as well as any other critical stress locations in the overall skirt design. The results confirm that the inclusion of the conventional slot design effectively reduces stress in the junction weld. However, it has also been found that the critical stress location migrates from the shell-to-skirt junction weld to the slot ends. The results from an optimization study of the slot dimensions indicate that wider skirt slots improve the stress and strain response and thus increase fatigue life of the weld and slot area compared to the conventional slot design. An optimal slot design is presented. The sliding plate design is found to further improve the stress and strain response at the point of attachment. However, bending of the vessel due to the rising water level during the quench stage is found to cause severe plastic deformation in the sharp corners which are inherent to the design. Thus, a novel design which includes pinned connections at the point of attachment in addition to sliding plates is proposed. The pinned-sliding plate design is found to completely prevent plastic deformation from occurring at the point of attachment and significantly reduce critical stress. Accordingly, the pinned-sliding plate design is the most promising candidate from a reliability standpoint among the designs examined in this study. iii ACKNOWLEDGEMENTS I would like to express my utmost gratitude to my supervisor Dr. Zihui Xia, who has provided endless opportunities, guidance, and support throughout this endeavour.