Study On Several Problems Of Bulging And Cracking In Coke Drums

Coke drums are important pressure vessels of the delayed coking process in the petroleum industry. During the delayed coking process, coke drums experience cyclic temperature variation from room temperature to 495℃in 16-48 h, with the content inside varying from gas to fluid and solid. Typically, after some years of operating thermal and pressure cycles, coke drums are subject to the following degradation:1) bulging of cylinder. 2) cracking in cylinder, weld seams and skirt. Studies based on the field survey and finite element analysis showed that the cyclic thermal gradient in the drum wall was believed to be the predominant mechanism for the above problems. The analysis based on theoretical models will help to increase our understanding of the thermal and mechanical mechanism. In this dissertation, with focus on the themoelasticity problem in a coke drum, the two-dimensional transient temperature distribution in both radial and axial directions, the thermal and mechanical properties mismatch between materials of the vessel and the circumferential weld or the cladding, as well as the themal stress intensity factors for a crack in a coke drum are studied theoretically.First, the transient temperature distribution in both radial and axial directions is derived analytically based on the two-dimensional heat conduction theory. The iteration technique is applied to simulate the dynamic boundary condition caused by a fluid surface level rising continuously during both heated feed filling and water quench steps. The temperature distribution in the drum wall is discussed numerically. Meanwhile, effects of the geometry of the coke drum, the temperature and rising velocity of quench water, the thermal properties of the vessel material on the axial temperature gradient are also discussed. The results show that the rising velocity of the fluid has the most significant effect on the temperature distribution in the drum wall.Based on the thermoelasticity and classical thin shell theory, a combined cylindrical shell model is presented to investigate the interation between the vessel and the circumferential weld. The variation of the displacement and thermal stresses with the fluid surface level rising, the effects of the circumferential weld, the Young's modulu and thermal expansion coefficient of the materials on the thermal stresses are discussed. The results show that, compared with the other parameters, the thermal expansion coefficient has more significant effect on the thermal stresses.The two-dimensional thermoelasticity and classical laminated thin shell theory are applied to solve the thermoelasticity problem in a coke drum with cladding based on the quasi-static assumption. In the numerical calculations, the temperature distributions in the cladding and base metal layers are analysized. The proposed solution is compared with finite element analysis. The thermal stresses in the drum wall, the effects of the Young's modulu and the coefficient of thermal expansion of the materials on the thermal stresses are discussed in detail.The theoretical analysis is carried out for the thermal stress intensity factor for a crack in a coke drum based on the weight function method and Reissner's cylindrical shell theory, respectively. First, the formulation is presented for the stress intensity factor evaluation of a circumferential crack in a coke drum by weight function. Second, a transient thermal fracture analysis based on Reissner's theory is developed by applying the Duhamel-Neumann analogy to replace thermoelastic problems with isothermal problems. The general solution for the displacement and stress fields at the crack tip of a coke drum with a circumferential through crack is obtained with the perturbation method.