Safety Assessment And Remaining Life Estimation Of Coke Drums

Coke drum is the key device of delayed coking in petrochemical industry, and due to its long exposure to the severe service environment the temperature of which changes form room temperature to 480℃cyclically and the effect of cyclic loading, it is easy to raise some kinds of failure relating to structural strength, which may lead to unexpected grave accident, so it is necessary to do a rounded structural analysis and estimate the remaining life of coke drum.For the accuracy of the remaining life estimation, the structural analysis of coke drum in service conditions has been carried out, and the stages of coke generating and water cooling were emphasized on, in which the coke drum experiences the largest temperature change. Based on the FEM theory, the transient heat transfer between the drum wall and fluid medium rising up with steady rate was simulated numerically first, then the time history of the stress and strain fields were acquired with indirect coupling method. The results indicate that the outboard of drum wall falls in plastic yielding in the water cooling stage and the welded seam is under multiaxial stress state.Thermal mechanical fatigue (TMF) under multiaxial state, the fundamentals of which have not been studied well, is one of the most complex parts in the fatigue study. In this paper, notched specimens made of material taken from coke drum in service were used to carry out in-phase TMF test and crack propagation test, then the commercial FEA software ANSYS was employed to acquire the stress and strain history of the specimen in a cycle of loads. Based on the numerical results, the local stress-strain method was employed to establish the life estimation equations which use the equivalent plastic strain and equivalent plastic strain energy density as the governing parameters. By applying the empirical equations, the remaining life of the investigated coke drum is figured out as approximately 24 years.The crack propagation under multiaxial stress state has also been studied in this paper. The opening and closing process of crack in a cycle of loads has been simulated successfully with the use of rigid-soft contact pair in ANSYS, and the J-integral was calculated on the local crack tip of the finite element model directly, then the range of the J-integral in a cycle of loads was employed as the key parameter describing the crack propagation regularity. The empirical equation is expected to be helpful to estimate the fatigue crack propagation life of the coke drum.