Coke Drum Failure Mechanism And Life Assessment Research

Delayed coking technology develops quickly, and it is one of the most widely used petroleum coking technology. Coke drum is the core equipment of delayed coking unit, the reliability and integrity of which impacts the whole delayed coking system.In this paper, the work process throughout the complete cycle of the coke drum is simulated, based on the results of finite element analysis, mechanism of weld cracking and drum bulging is analyzed. The transient analysis is considering the different convective heat transfer coefficient of inner boundary in the process, the changing temperature boundary conditions with time and height, and the dynamic generation process of coke by life and death element method.Transient thermal analysis and thermal-structural coupling analysis shows that:The temperature and temperature gradient of drum wall nodes is changing complexly, and the overall level of TG is higher in water cooling stage; at the same time, the average equivalent stress of the lower drum covered by coke is much larger than material yield strength, which caused extensive yield. In conclusion, the water cooling stage is the key stage of stress and strain field development. And the key points of stress concentration and plastic yield mainly occur on the first cylinder weld, skirt weld, the skirt itself, conical head and the lower part of the cylinder area.The failure mechanism analysis based on numerical simulation results shows that:In water cooling stage, the coke hinders the contraction of the lower drum area covered by coke, resulting in a sharp rise of stress level in this area, and because of the material discontinuity and geometric discontinuity, the weld heat-affected zone happens stress concentration, so the cracks of lower drum weld expand most rapidly under high cyclic stress amplitude. In oil filling stage, the linear thermal expansion of drum is not uniform along axial, and the coke generated is uneven, which will inevitably lead to nonuniform plastic strain of lower drum after water cooling contraction, with many years of work cycle, the nonuniform plastic strain along axial accumulates gradually, eventually drum body happens irrecoverable bulging deformation.The life assessment of20g is based on thermo-mechanical fatigue life test and finite element analysis of cylindrical notched specimen. The specimen material is from the service coke drum, the dangerous stress and strain of cylindrical notched specimen is analyzed by FEM. And then according to Manson-Coffin formula, using equivalent plastic strain range method, the life evaluation equation of the damaged material is fitted. Finally, combining with the maximum equivalent plastic strain range of coke drum, the remaining life of the coke drum is predicted.