Ethylene Cracking Tube Computer Simulation, Time-varying Stress Field And The Damage Process

The stress distribution and damage accumulation with time of ethylene cracking tubes were computed through the finite element method (FEM). Carbonization is one of the root causes of failure of cracking tubes, and thus it is the key of simulation of stress distribution. In this paper, carbon concentration profile in cross section of cracking tubes is described by using modified error function method. Carbonization has an obvious effect on stress of tubes due to dynamic change in density during the tube operation process. After defining a linear expansion coefficient caused by carbonization, we can conveniently compute the carbonization stress in steady temperature stage according to similarity between thermal expansion and expansion caused by carbonization. Thermal stress results from different thermal expansion coefficient in the radial direction after carbonization in the changed temperature stage. Internal pressure is a kind of surface force. The influence of creep deformation on the stress distribution can be solved using the initial strain method. Creep damage is calculated according to Larson-Miller抯 master curve and time dependent stress distribution in the tube. Stress distribution of tubes only under the internal pressure shows that both hoop stress and radial stress caused by the internal pressure are very little, and therefore, they don抰 have a significant effect on the creep damage of the tubes. So it is concluded that the internal pressure should not be a main parameter for the design of tube wall thickness. The results computed also indicates that compressive stress occurs in the inside layer of the tubes, at the same time, tensile stress which is the main cause of failure of cracking tubes occurs in the outside layer due to increase of linear expansion coefficient caused by carbonization. The hoop stress curves under actual conditions keep the same shapes as those under ideal conditions without creep at high temperature, which demonstrates that the carbonization is the main factor of stress distribution indeed. Difference in thermal expansion coefficient in the radial direction gives rise to cyclic stress as temperature changes cyclically. The stress range on the inner surface is greater than that on the outer surface, which is probably the reason why the fatigue failure mainly occurs on the inner surface. The result computed also shows that stress relaxes due to creep deformation to low levels under which plastic formation delay happening. The result computed about HK-40 austenitic heat resistant steel reveals that creep damage results from hoop stress in the outer wall of cracking tubes, where II creep failure occurs after the operation of about one and half years. These results are compared with the actual results, and considerably good agreement is obtained.