Reliability and life assessment of coke drums through boat samplebased testing

Life assessment of coke drums is vital to prevent circumferential through-wall cracking and to ensure it's safe and serviceable. Cracking in the shell/skirt-shell junction/welds, permanent distortion, and bulging (due to the ratcheting effect) are common damage mechanisms due to low cycle thermal fatigue. The difficulties in performing an insightful inspection and life assessment are limited by the wall thickness, location heterogeneity due to service conditions, and test methods. In this study, a boat sample-based testing procedure is developed for the life assessment of coke drums. The boat samples are extracted from four different locations named crack, bulge, notch, and benchmark from a 40-year serviced coke drum. The boat samples are machined for sub-size smooth tensile, notched tensile, low-cycle fatigue, and Charpy toughness specimens. Smooth tensile, notched tensile and low-cycle fatigue tests are performed at three test temperatures (room temperature, 250 degrees C, and 480 degrees C). The results of sub-size specimens are calibrated to full-size and compared with the knuckle plate (least temperature in the coke drum). The fracture toughness of boat samples is predicted by the Haggag method using the smooth tensile and metallographic results. The boat samples have gained strength but have a reduction in ductility and toughness compared with knuckle plate material. The boat samples have shown a change in the microstructure and hardness. Carbide precipitation is observed on the grain boundaries and within the ferrite grains. This precipitation might have reduced the ductility and toughness.