Investigation Of Thermal Stress,Cracking And Bulging Behavior Of Coke Drum

Coke drums are the main equipment for delayed coking in the modern petrochemical industry.It undergoes cyclic temperature changes during operation,resulting in high-value thermal stress on the shell wall,and cracking and bulging deformation generally occur after several years of service.In this paper,a combination of numerical simulation and theoretical analysis is used to analyze the thermoelastic field of the shell wall during the hot feed filling step and water quench step,and the mechanism of cracking and bulging deformation of the shell is studied.The main work and research conclusions are as follows:(1)Based on the two-dimensional thermoelasticity theory and the classical thin shell theory,the thermoelastic fields of the coke drum in the hot feed filling step and water quench step are studied using axisymmetric and quasi-static assumptions.The results show that under the two working conditions,the shell wall near the liquid surface produces a severe axial temperature gradient,which leads to high thermal stress;the axial temperature gradient and radial temperature difference of the shell wall in the water quench step are larger than those in the hot feed filling step;the shell wall under the liquid level in the hot feed filling step and the water quench step expands outward and contracts inward respectively;the stress amplitude of the shell wall in the water quench step is greater than that in the hot feed filling step.Under the two working conditions,the inner wall stress amplitude is significantly larger than the outer wall stress amplitude,and the axial stress amplitude is larger than the hoop stress amplitude.(2)The superposition principle is adopted,and the shell wall stress is regarded as the effect of two factors,the radial temperature difference and the axial temperature difference.On this basis,a method for calculating the inner wall stress based on the outer wall stress is proposed.The research results show that the factors that cause the hoop strain of the shell wall include radial temperature difference and axial thermal shrinkage,so the calculation of hoop stress is related to the length of the transition region of the vase effect caused by the axial temperature difference.The axial stress of the shell wall can be obtained by the superposition of the hoop stress and the bending stress caused by the vase effect.The proposed method for calculating the inner wall stress has no damage to the shell wall and also has high precision,and has a good engineering application prospect.(3)Based on linear elastic fracture mechanics,using the weight function method and numerical simulation,the stress intensity factor(SIF)of the coke drum circumferential crack under a two-dimensional transient temperature field is studied,and the effect of geometry,material properties and operation on SIF are discussed.The results show that the SIF of the circumferential crack increases first and then decreases with the increase of the crack depth;the inner diameter of the coke drum has little effect on the SIF,but the SIF increases significantly with the increase of the shell wall thickness.The SIF of the circumferential crack increases nonlinearly with the rising rate of the cooling water,and increasing the cooling water temperature is conducive to reduce the SIF.The greater the thermal conductivity of the coke drum material,the greater the SIF.(4)Using the local axisymmetric model of the coke drum,finite element numerical simulation is used to explore the bulging deformation mechanism of the coke drum,and the effects of heat transfer coefficient,cooling water rising speed and water temperature on the plastic deformation of the shell wall are analyzed.The study found that when the inner wall was in direct contact with the cold water,the high heat transfer coefficient between the shell wall and the cold-water results in the largest plastic strain.At lower cooling water rising speed and lower cooling water temperature,the plastic deformation is larger.The strength mismatch between the base metal and the girth weld is an important cause of bulging deformation in the middle of the cylinder section.The greater the strength difference between the base metal and the weld,the more significant the cylinder bulge.The bulge has no obvious effect on the stress distribution of the shell wall and the SIF at the crack tip.