Coke Drum Keyhole Optimization With Alloy Weld Overlay

Abstract

Coke drums are subjected to severe cyclic thermal and structural loading, and have therefore become canonical examples of thermo-mechanical fatigue failure. One of the most common locations of fatigue failure is at the skirt-to-shell attachment weld. To increase the fatigue life of the attachment weld, vertical slots and keyholes are often machined in a circumferential pattern near the top of the skirt to increase local flexibility near the weld. While this methodology provides a cost-effective means to improve the fatigue life of the attachment weld, stress concentrations that form at the keyholes result in crack initiation and propagation. As such, frequent repairs are necessary to prevent crack growth at the keyhole locations. In the present investigation, a new keyhole design consisting of N06625 overlay at the keyhole locations as a means of delaying crack initiation has been studied using FEA simulations. To further optimize the new design, a sensitivity study involving keyholes with and without fillets was also performed to determine the effect of fillet size on fatigue life. The results presented herein indicate that increasing the keyhole fillet radii mitigates the effect of stress concentrations on the corners of the keyholes, which delays crack initiation. Furthermore, it has been demonstrated that adding N06625 overlay at the keyhole protects the base metal from excessive plastic strain. In particular, the proposed design has been shown to improve the fatigue life of the keyhole by at least an order of magnitude when compared to analogous keyhole designs with no overlay, which has the potential to improve the overall cost-effectiveness of keyhole implementation by a significant margin.