C-½Mo Vs 1Cr-½Mo: A Comparative Study of Coke Drum Shell Materials

Abstract To mitigate the impact of severe thermal loads during operation, two different design methodologies are typically adopted when selecting base metal materials for coke drum fabrication. The first approach favors the use of certain chromoly steels (i.e. 1¼Cr and higher), prioritizing strength characteristics to improve resistance to bulging. The second approach, which favors carbon steel and C-½Mo, acknowledges the inevitability of bulging and targets materials with lower strength and finer grain size to optimize fracture ductility, allowing larger bulges to develop before cracking occurs. The trade-off between optimizing bulge resistance and cracking resistance is a critical consideration, particularly when other factors, such as the superior reparability of C-½Mo, may influence material selection. This study examines eight coke drums from the same upgrader unit that were designed using “lower strength” base metal materials; six were fabricated using a C-½Mo alloy, two were fabricated using a 1Cr-½Mo alloy. Despite differences in material composition, both sets of drums share many design and operating characteristics, providing an ideal framework for directly comparing the two base metal materials. An extensive experimental testing program was conducted on boat samples extracted from all eight coke drums, along with a review of historical inspection data. Based on the performance metrics outlined in this study, the C-½Mo drums generally outperform their 1Cr-½Mo counterparts.