On the Effect of Hot-Box Size on Coke Drum Skirt Fatigue Life

Due to the severe cyclic thermal and structural loading that coke drums experience during operation, thermo-mechanical low-cycle fatigue failure has become a prominent consideration in the design of coke drums. Fatigue failure is commonly observed at the skirt-to-shell attachment weld, due to thermal gradients that develop during heating and cooling cycles. To reduce temperature differences between the coke drum shell and skirt near the attachment weld, “hot-boxes” are often implemented in skirt designs. Hot-boxes are enclosed regions of space near the attachment weld that maintain the skirt at temperatures similar to the nearby shell through natural convection and radiation. The present study investigates the effect of hot-box emissivity and height on the fatigue life of the attachment weld for skirt designs with and without keyholes. The results presented herein indicate that, for the skirt height investigated, fatigue life has negligible sensitivity to the height of the hot-box when no keyholes are present on the skirt. In contrast, varying fatigue performance is observed as a function of hot-box height when keyholes are included as part of the skirt design. Although the functional relationship between hot-box height and fatigue life for the skirt design with keyholes is non-monotonic, the results provide directional insight into the optimization of hot-box size.