【作者】 宁志华；

【导师】 刘人怀；

【作者基本信息】 暨南大学 ， 工程力学， 2010， 博士

【摘要】 焦炭塔是炼油工业中延迟焦化的关键设备。由于延迟焦化工艺的特点,使焦炭塔经历循环的升温、降温,16至48小时内在室温与495℃之间循环变化,同时容器承载的介质由气态到液态至固态,工作环境复杂、恶劣,致使焦炭塔在运行若干年后普遍存在以下两方面的问题：筒体鼓胀以及筒体、焊缝和裙座等开裂。现场调查及有限元分析的研究结果表明塔壁内产生的循环热梯度是引起上述问题的主要原因。对焦炭塔问题进行合理的理论建模分析将有助于进一步弄清焦炭塔问题产生的热力学机理。本文以焦炭塔的热弹性问题为研究对象,对焦炭塔(单层/含衬里)的轴向、径向二维瞬态温度场、环焊缝和衬里与筒体材料之间的热机性能失配,以及焦炭塔热应力强度因子等关键问题进行了理论分析。首先,基于二维热传导理论,采用迭代法模拟了焦炭塔在进油和进水阶段由于液面不断上升引起的动态热边界条件,获得了轴向及径向二维瞬态温度场的解析解。数值计算中,分析了焦炭塔的温度场分布规律,并讨论了焦炭塔的半径及厚度、液体升速以及冷却水水温、塔壁热传导参数等对温度分布及温度梯度的影响。分析结果表明,液面升速对温度场的影响最大。基于经典的薄壳理论与热弹性理论,建立了焦炭塔的组合圆柱壳模型,研究了焦炭塔的筒体与环焊缝之间的相互作用。数值计算中,分析了含环焊缝焦炭塔的位移与应力随液面高度的变化规律,焊缝对应力的影响规律,以及筒体与焊缝材料的弹性模量、热膨胀系数等热机性能参数对塔壁应力分布的影响规律。分析结果表明,材料的热膨胀系数对应力分布的影响非常显著。基于二维热弹性理论与经典的层合薄壳理论,采用准静态假设研究了含衬里焦炭塔的热弹性问题,获得了解析解答。数值结果中,分析了衬里和基础金属层温度场的分布规律,以及衬里厚度对温度分布的影响,并与有限元结果进行了比较。讨论了含衬里焦炭塔的塔壁应力分布规律,以及衬里和基础金属材料的弹性模量及热膨胀系数对热应力的影响规律。基于线弹性断裂力学,分别采用权函数法、Reissner型板壳弯曲断裂理论对二维瞬态温度场下焦炭塔裂纹的热应力强度因子进行了初步的研究。首先,采用权函数法,提出了对焦炭塔含环向完全圆周裂纹的热应力强度因子进行分析的计算模型。同时,利用Duhamel-Neumann原理,将二维瞬态温度场下焦炭塔的热弹性问题转化为相应的等温弹性问题,根据Reissner型板壳弯曲断裂理论建立了二维瞬态温度场下Reissner型圆柱壳的基本方程,并应用摄动法给出了二维瞬态温度场下含环向穿透裂纹焦炭塔裂纹尖端位移及应力场的一般解。更多还原

【Abstract】 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.更多还原