Hot-box Study

Abstract – Coke drums are subjected to severe cyclic thermal and structural loading, and have therefore become a canonical example of thermo-mechanical fatigue failure. One of the most common locations of fatigue failure is at the shell-to-skirt juncture. To reduce temperature differences between coke drum shell/skirt near the juncture, “hot-boxes” are often implemented in coke drum designs. Hot-boxes are enclosed regions of space near the shell-to-skirt juncture that maintain the skirt at temperatures similar to the shell through natural convection and radiation. Reducing temperature differences between the shell and skirt mitigate thermal-gradient stresses, thereby increasing the fatigue life at the juncture. The present study employs FEA to investigate the effect of hot-box emissivity and height on shell-to-skirt juncture fatigue life and skirt resistance to buckling. Hot-box height ratios – defined as hot-box height divided by overall skirt height – between 0.25 and 0.5 were investigated. The simulations presented herein indicate that lower emissivity values within the hot-box retard heat transfer across the shell-to-skirt juncture, thereby increasing temperature differences across the shell and skirt and reducing fatigue life. In contrast, no significant impact on fatigue life was observed at the juncture when the height of the hot-box is reduced. Finally, a noticeable reduction in buckling resistance is observed as hot-box height is lowered, due to the movement of the hot-box “thermal hinge” closer to the structurally weaker keyhole positions.

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