| 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. |
PDF Full Text Request