Evaluation of stressed state of coker reactor shell with plastic deformation

In the operation of reactors in delayed coking units, plastic deformations accumulate as time passes. In the zone of welding to the support (more rarely in the zone of the upper level of coke), bulges are observed, with a tendency to grow [1-3], sometimes leading to catastrophic failure of the vessel with the formation of cracks through the entire thickness of the shell. Therefore, determination of the critical dimensions of the bulge from the standpoint of crack formation [4] is an urgent task. The accumulation of plastic deformations is influenced by various factors. However, the nature of the deformations and their source are the same in all cases: inconstancy and nonuniformity of temperature distribution in the reactor shell during the coking cycle, The most characteristic mechanisms of deformation of delayed coking reactors can be defined: generation of bending stresses as a result of nonuniform temperature distribution, both through the shell wall and in the cross sections of the vessel. Measurements of the reactor wall temperature during an entire operating cycle have shown that the maximum local rate of temperature change while the reactor is being heated or the coke is being cooled will amount to I00-450~ and the temperature difference in diametrical opposite points of the cross section of the vessel will be 120-150~ With these temperature differences, the stresses as determined from relationships given in [5] will be 3-14 and 113-160 MPa, respectively. When bulges appear, it is necessary to evaluate the stressed state of the coker reactor shell in this zone in order to predict its reliability. The complex character of the deformation and the lack of adequate information on the contribution of each type of load to ahe stressed state, together with the impossibility of modeling the conditions of vessel deformation in the present state of the art, make it impossible to arrive at a phenomenological solution of the problem, This solution is based on known relationships from the theory of plasticity [6]. In obtaining a true picture of the stressed state in the zone of plastic deformation of a cylindrical shell, it is fully acceptable to use the deformation characteric of the material o = ~(ep) (where o is the equivalent stress in the plastic region; ~ is the plastic component of the logarithmic deformation), plotted from the results hydrostatic tests of bulging through a circular opening. The feasibility and desirability of such an approach have been demonstrated in many studies [7, 8], The method used to plot the deformation characteristic curves ensures good agreement between calculated and experimental data for thin sheet steels and is entirely acceptable for any scheme of the stressed state [9]. Since a r reactor can be classed as thin-wall on the basis of its geometric parameters, its stressed state can be regarded as biaxial, neglecting the thinning of the wall. The equivalent stress in the plastic region is related to the deformation by a power function that takes into account the capability of the metal for strain hardening and softening through the hardening (strengthening) index n [9]: