【作者】 徐书根；

【导师】 王威强； 陈学东；

【作者基本信息】 山东大学 ， 过程装备工程， 2010， 博士

【摘要】 在设计理念上,层板包扎容器一度被认为是非常安全的结构。然而,在使用历史上,出现了许多失效、泄漏乃至爆破等事故。尤其是平阴尿塔爆炸事故以来,尿塔塔体严重爆破的原因受到了高度重视。不同专家对尿塔爆炸的原因提出了不同的观点。为了探索尿塔爆炸的真实原因,作者对尿塔塔体爆破相关的科学问题展开研究：一是液相区泄漏时多元物料蒸气爆炸的机理和爆炸载荷大小；二是带间隙多层圆筒结构的力学行为,包括层板裂纹的启裂与扩展和结构完整层板结构对冲击载荷的力学响应。只有弄清这两个问题,才能解决尿塔等层板包扎容器爆炸的根本原因,进而提出有针对性的爆炸防范措施。针对带间隙多层圆筒的线弹性和弹塑性应力状态,已有一些计算公式。但是,这些计算方法存在一些错误。作者通过修正的带间隙多层圆筒的应力计算公式,计算了多层圆筒在存在层板间隙的情况下,层板线弹性和弹塑性应力沿半径的分布规律,并且用于层板上轴向裂纹的临界长度的确定。结果表明,在正常工作状态下,多层圆筒层板的各向应力状态呈现出较大的不连续性。内部层板应力提高,外部层板应力降低。同时,作者基于波动方程的解的结构定理,提出了求解圆筒在动态载荷作用下结构响应的广义的解的结构定理,可以方便的求解不同边界条件和初值条件下的关于结构动力响应的偏微分方程。在求解单层筒体的结构动力响应的基础上,提出了求解带间隙的多层筒体的动力结构响应的方法。该方法的求解结果能够反映出因为层板间隙的存在,外部层板在结构响应时的滞后效应。基于统计热力学能量涨落理论,对在用压力容器液相区泄漏的蒸气爆炸进行了理论计算。分析了在蒸气爆炸过程中,蒸气爆炸压力、均相成核速率、液相温度和气相温度等热力学参数随时间的变化规律,以及各参数之间的内在联系。结果表明,盛装高温高压介质的容器,液相区的泄漏能够引发蒸气爆炸。蒸气爆炸能否发生以及爆炸载荷峰值的大小与泄漏面积密切相关。泄漏面积越大,就越容易出现蒸气爆炸压力反弹,反弹幅度也就越大。以平阴尿塔为例,在容器液相区壁面出现900mm的轴向穿透裂纹时,如果不考虑裂纹的扩展,蒸气爆炸压力峰值会达到31.40MPa；如果考虑轴向裂纹的扩展,在裂纹半长由376mm扩展为1235mm时,蒸气爆炸压力峰值会达到32.9 MPa。将带间隙多层圆筒的应力计算公式引入层板轴临界向裂纹长度的确定。分析表明,多层圆筒的各层板轴向裂纹的临界尺寸不同,内部层板临界裂纹尺寸较短,外部临界裂纹尺寸较长。以尿塔为例,在正常的操作压力下,盲层上轴向裂纹的临界半长为为185mm,最外层裂纹临界半长为866mm。使用ABAQUS的XFEM模块,对一个三层带间隙的圆筒上的裂纹扩展路径进行分析,外部层板裂纹在接近环焊缝处发生转向,变成近似环向裂纹。这种现象说明分段多层包扎容器在焊根处存在较大间隙时,沿轴向扩展的层板裂纹在接近环焊缝转变为环向裂纹。针对化学爆炸对带间隙多层圆筒的影响,作者使用理论计算和有限元计算方法分析了平面应力状态下带间隙多层圆筒的弹性动力响应。使用有限元方法,计算了局部冲击载荷作用下多层圆筒的动力响应。分析表明,冲击载荷对多层圆筒结构的影响存在局部效应。直接作用区域的应力峰值最大。离冲击载荷作用区域越远,受到的影响越小。对一带上下球形封头的多层圆筒的上封头区域施加1OOMPa冲击载荷,其他区域施加20MPa的冲击载荷。多层圆筒的内半径为700mm,长度为7410mm。接近上封头区域的圆筒内壁的应力峰值为653.56MPa,轴向方向离开该点1.9m处,应力峰值减小为490MPa；离开该点7.4m处,应力峰值骤减为310MPa。分析了裂纹等导致尿塔泄漏的缺陷是导致尿塔塔体发生灾难性破坏的根本原因,深环焊缝结构和焊根附近的间隙是容器断为三段的根本原因。根据对爆炸原因的分析,提出了尿塔爆炸防范措施。对于新造尿塔,提出了将现在的深环焊缝结构改进为兼有分段包扎和整体包扎优势的焊缝结构。对于在用尿塔,提出按照863课题——“层板包扎高压容器剩余寿命试验评价技术”的最新成果进行无损检测、缺陷评定和寿命预测。总之,通过对带间隙多层圆筒的层板应力分析和蒸气爆炸载荷的计算,层板轴向裂纹的启裂和扩展分析,层板结构在冲击载荷下的力学响应,确认了尿素合成塔的塔体爆破机理为层板的原始缺陷和蒸气爆炸的发生,并提出了相应的爆破防范措施。深化了对尿素合成塔等层板包扎容器爆炸原因、过程和最后破坏形态的认识,对于采取有效的措施去防范爆炸事故的发生,提高容器的安全性,具有重要意义。此外,层板包扎容器爆炸事故非常复杂,要想彻底对爆炸的原因进行溯源,尚需进行小尺寸容器的爆破模拟实验来确认本文的分析结果。更多还原

【Abstract】 The layered vessel is considered as a safe structure in design concept. However, in the history, the layered vessels were involved in some failure accidents, such as leakage and explosion. From the Pingyin Urea reactor explosion which happened in 2005, the catastrophic explosion reasons related to urea reactor and the same style layered vessels attracted more and more eyes of vessel experts. Different experts gave us different viewpoints on the rupture reasons. In order to figure out the true reason of the explosion, the scientific issues related to urea reactor shell rupture are investigated. There are two key issues behind the explosion phenomena, one is the mechanism of vapor explosion and the explosion pressure which triggered by the liquid leakage, and the other one is mechanical behavior of the layered cylinder with interlayer gaps, including the initiation and propagation of the longitudinal cracks in layers, and the structural response of the layered vessel under the impact loading.The formulae for the linear elastic and elastoplastic stress calculation for layered cylinder with gaps have been modified in this paper. The elastic stress and elastoplastic stress has been calculated. It can be used for calculating the crack critical length in layers. The results show that the hoop stress along the radial direction is not continuous. Compared with monobloc cylinder, the inside stress is increased, and the outside stress is decreased. Based on the solution structure theorem for wave equation, the generalized solution structure theorem has been proposed. It can be use for solving the partial differential equation with different initial values and boundary conditions for structural response. Once the structural response of monobloc cylinder has been obtained, the structural response for the layered cylinder with gaps can be solved easily with the method proposed in this paper. The result shows that the response of outside layers is delayed due to gaps.According to the probability distribution of energy fluctuation in statistical thermodynamic fluctuation theory, the theoretical model for the vapor explosion trigged by the liquid release in the in-service pressure vessel was established. The vapor explosion pressure, homogeneous nucleation rate, liquid temperature, and vapor temperature are analyzed. The relationship between the vapor explosion amplitude and the crack length is also analyzed. The results show that the vapor explosion is possible when the liquid release for a vessel with high temperature and pressure contents. It depends on the liquid leakage mass flux rate. The crack opening area is bigger, and the possibility of vapor explosion is bigger, and the pressure amplitude is also bigger. For example, in Pingyin urea reactor, when the longitudinal crack length is 900mm, the peak explosion pressure is 34.4MPa. In this process, if the crack propagation is considered, the peak pressure is 32.9 MPa.The elastic stress calculation formulae are introduced to the longitudinal crack critical length determination. The crack critical length in different layer is different. The crack length in inside layers is short, and in the outside layers is long. For example, in Pingyin urea reactor, the crack critical half length is 185mm in the dummy layer, and is 866 in the outermost layer. The crack propagation path in the outside layer in a double layered vessel with gaps is analyzed in ABAQUS. The XFEM method is used for the crack modeling. The longitudinal crack will change to an almost circumferential crack when it propagated to heavy circumferential weld. The reason is the gap near the weld root is cannot removed.In order to study the effect of chemical explosion to layered cylinder, the elastodynamic response of layered cylinder with gaps is calculated theoretically and numerically. The layered cylinder is considered to be infinite long, and in the plane strain status. The structural response under the local impact pressure is also calculated by finite element method. The results indicate that the impact loading have a local effect to the layered structure, that is, the peak hoop stress is biggest in the zone under the pressure directly. The farther from the loading applied zone, the effect is smaller. For example, a layered vessel is constructed by layered section and two hemisphere head. The layered section is constructed by 14 layers. All the interlayer gaps are equal to 0.05mm. The inner radius is 700mm, and the length is 7410mm. An impact pressure which applied into the inner surface of top head is 100MPa, and the pressure which applied into the inner surface of top head and layered section is 20MPa. The peak hoop stress in the inner surface near top head is 653.56MPa. The peak hoop stress is 490MPa in the point which is 1.9m far from the top head. The peak hoop stress is reduced to 310 MPa in the point which is 7.4m far from the top head.The basic reason for the catastrophic rupture of urea reactor is analyzed using the conclusions obtained above. The vapor explosion which trigged by liquid leakage is the basic reason for the rupture. The heavy circumferential weld and the big gap near the weld root is reason for the urea reactor ruptured in to three sections. According to the explosion analyzed above, the prevention method for the urea reactor explosion is proposed. For the new designed urea reactor, a modified circumferential weld structure is proposed. This structure has the advantage of both segment layered vessel and integral layered vessel. For the in-service urea reactor, the nondestructive testing, defect assessment, and the life prediction should accord to the 863 Program-Experimental Remaining Life Evaluation Technology for Layered High Pressure Vessels.In a word, the urea reactor explosion reason was focused on, and the basic issues were researched. The following topic is discussed intensively:the stress calculation of layered cylinder with gaps, the mechanism of vapor explosion and the pressure calculation, the initiation and propagation of longitudinal cracks in layers, the elastodynamic response of layered cylinder with gaps, the confirmation of the urea reactor mechanism, and the proposal for the prevention of urea reactor explosion. This research enhances the recognition level to the layered urea reactor explosion. It is meaningful for the explosion prevention and the vessel safety. However, the mechanism of the layered urea reactor explosion is so complicated that the small scale vessel explosion testing is required to confirm the analysis result in this paper.更多还原