Testing of Shot Peening for Coke Drum Application

Due to batch cyclic operation and water quenching process, coke drums in Delayed Coking process units are exposed to severe damage caused by low cycle fatigue. Improving resistance to this damage mechanism is one of the major considerations when designing new coke drums. In a major project replacing totally eight coke drums, shot peening was considered and tested on specimens made of C-0.5Mo backing plate with alloy 625 cladding. This was based on the proven effect of this technique for enhancing fatigue performance of welds in other industries, such as bridges and aviation. After shot peening, surface stress profile of the specimen was measured. Cantilever bending fatigue testing was conducted to determine if fatigue resistance had been improved. Coke drum operation involves thermal cycles which has the potential to negate the effect of shot peening. To quantify the influence of such thermal cycles and detect whether there is a practical residual benefit, simulating heat treatment cycles were designed and performed on the fatigue coupons after the shot peening. This paper summarizes the shot peening process, surface stress measurement and fatigue testing results. It can be used for evaluating the application of shot peening on coke drums for both new drum fabrication and repair/maintenance of operating drums.