Study On The Pressure Wave Propagation In A Vessel With Long Vertical Multilayer Cylinder Resulting From The Chemical Explosion In Gas Space And The Dynamic Response Of The Vessel Wall

The vessels with long vertical multilayer cylinder are widely used in large scale chemical plants. Explosion accidents of such vessels happen sometimes and cause serious loss of property and casualty. But, some accident causes have huge dispute, especially the dispute over Pingyin urea reactor accident is chemical explosion and physical explosion. And this situation goes against taking effective measures to prevent such accidents happening. In order to confirm the real explosion causes of such vessels, the author takes the relative study on the explosion accidents of vessels with long vertical multilayer cylinder. The main research topics are as follows:the first question is to study the pressure wave propagation in a vessel with long vertical multilayer cylinder resulting from the chemical explosion in gas space, including the influencing factors of the wave propagation process; the second one is the response of the multilayer cylinder structure, including the dynamic response under dynamic load and the mechanical response of defect shell under static load. Only these two questions are solved, the real causes of urea reactor explosion accidents can be confirmed and it can provide reference idea of similar accident analysis.The structure of a urea reactor and the synthesis process of urea production are analyzed at first. And then, the chemical explosion conditions of combustive gases in the urea reactor are analyzed. The chemical explosion characters are summarized occurred in gas space of the urea rector. The results show that the gas space can gather these combustive gases and the oxygen gases easily. Under the excitation of external energies, such as electrostatic sparks, there exists the explosion possibility. Because of the characters of vessels like a urea reactor, the chemical explosion occured in gas space has following characters:space airtightness, local explosion and sudden change of media character in the vessel.The pressure wave propagation process in a vessel with long vertical cylinder resulted from the gas space chemical explosion is studied. The combustive gas explosion source is compared with the solid explosive source at first. Equivalent analysis of two explosive sources is carried out. The explosive energy is calculated. The explosive model and the motion equations are introduced. The finite element model of such vessel is built. Based on these works, the pressure wave propagation process is simulated by using the LS-DYNA software. The chemical explosion pressure field evolution law is obtained. Meanwhile, the explosion pressure evolution law on vessel wall of different times and pressure evolution law versus time on vessel wall in different locations are obtained. The research results shows that the pressure wave propagation process can be described clearly by the pressure field evolution law, the explosion pressure evolution law on vessel wall of different times and pressure evolution law versus time on vessel wall in different locations. The explosion pressure evolution law of gas space is mainly formed by the gas motion. And it has persistence and irregularity. The first peak value of the pressure curve on the top of the top head is the maxim pressure value and the value is about400MPa. The pressure wave in the liquid space is formed by the gas-liquid interface vibration caused by the gas motion. It also has persistence and irregularity. Wave overlap and wave separation process will occur repeatedly during the pressure wave propagates in the liquid space. The first peak value of the pressure curve is no longer the maximum value for most of the time and the appearance time of the maximum pressure value is irregular. The pressure wave will propagate upward after it is reflected by the bottom head and it will superpose with the following pressure waves which propagate downward. The wave intensity increased after the wave superposition.The effects on the pressure wave propagation process of the influencing factors, such as the volume change of the gas space, the trays, the explosive energy and the gas space shape, are studied. The results show that the maximum pressure value of the whole cylinder appears at the top of the top head and the value is five times the maximum value of other positions. The maximum pressure value of the liquid space appears at the position nears the gas-space interface and it is about20MPa bigger than the pressure value of other positions. The exact position of the maximum value of liquid space changes a little as the influencing factors change. The maximum value of other positions of liquid space is at the same level. In most situations, the specific impulse value at the top position of the top head is the maximum value of the whole vessel. The specific impulse value of the pressure of "strengthen area" is even larger than the one of the top position of the top head when the explosive source is eccentric settled. As the distance from the explosive center increases, the specific impulse value decreases gradually. The specific impulse value of the element on the upside of the tray is a little larger than the value of the adjacent element on the downside of the tray. As the gas space volume decreases and the explosive energy increases, the specific impulse value of different positions increases. As the gas space volume increases, the specific impulse value of different positions decreases. The maximum pressure value of the top position of the top head changes a lot with the explosive center changes, but the value of other positions is almost unchanged. The maximum pressure value increases with the increase of the explosive energy and the decrease of the gas space volume. The maximum pressure value decreases with the increase of the gas space volume. The pressure curve becomes flat after the trays are settled. The maximum pressure distribution law and the specific impulse curve when the top head is flat head are similar to the spherical head.The dynamic response of multilayer cylinder vessel under dynamic load is simulated by the LS-DYNA. The effects of the load curve, the layer gaps and the plane stress state are considered during simulation. The load curves include a single peak curve and two kinds of double peak curve. The layer gaps include0.02mm,0.1mm,0.2mm,0.3mm and0.4mm. The results show that the layer stress increase process is divided into several stages such as oscillation change stage, yield development stage, quick increase simultaneously stage and stress decease stage. During the oscillation change stage, the layer stress shows a significantly change oscillation; at the yield development stage, the inner layer yield at first, and then the outer layers yield layer by layer, the stress increase rate of yield layers decrease a lot; at the quick increase simultaneously stage, all the layer stress increase simultaneously and quickly at the same time; during the stress decease stage, the stress decrease rate of inner layers is larger than the outer layers’. During the whole process, the layer stress show discontinuous distribution between layer and the layer stress is uneven distribution in a layer; the inner layer stress is not guarantee larger than the outer layer stress all the time, the outer layer stress is larger than the inner layer stress when the load returns to the balance pressure with the gap of0.02mm. The layer velocity changes during the loading. The final layer displacement decreases a little as the load decreases. The final plastic deformation does not change with the load decreases. Under the double peak load curve, the layer stress changes under the second pressure peak on the basis of the stress distribution when the first pressure peak comes to an end. The double peak pressure curve load has little influence on the final layer stress, the final plastic deformation and the final displacement. With the increase of gaps, the oscillation change stage time becomes shorter; the final layer stress and the final plastic deformation of inner layers grow larger, while the relative values of the outer layers grow smaller. The inner layer stress is almost equal to the outer layer stress while the load returns to the balance pressure with the gap of0.1mm, but the inner layer stress is still larger than the outer layer stress with gaps of0.2mm,0.3mm and0.4mm. With the same gaps, the layer stress and the plastic deformation under plane stress state are larger than those under plane strain state.The mechanism of the explosion urea reactor is reanalyzed. The rupture possibility of urea reactor caused by the chemical explosion of gas space is discussed and the pressure wave propagation law is analyzed. The results show that the initial fracture of Pingyin urea reactor is not induced by the chemical explosion of gas space. The defects type is summarized in urea reactor. The effect caused by the inner layer cracks is analyzed. The results show that the inner layer stress grows smaller when the crack length grows and the outer layer stress grows larger. There is a stress strengthening area on the outer layer just behind the crack tip, where the stress corrosion cracks is priority to generate. The physical model of the multilayer cylinder with inner layer cracks is built. And the author proposes a method to calculate layer stress and critical crack length of other completed layers when the crack layers effect is considered. Take the Pingyin urea reactor as an example, the change law of the stress and the critical crack length of other completed layers is analyzed. The results show that the stresses of other completed layers grow larger and the critical crack length grow smaller when the inner crack is taken into consideration, which intensifies the risk of urea reactor running.In a word, through the simulation of the pressure wave propagation process in a vessel with long vertical multilayer cylinder, the propagation process is shown intuitionisticly. The research results proved that the initial fracture of Pingyin urea reactor is not caused by the chemical explosion of gas space. The dynamic response of multilayer structures under dynamic load is obtained can provide direct reference for explosion-proof design of similar vessels. The research on the defect multilayer structures explains the distribution law of stress corrosion cracks. The research enhances the recognition level to the multilayer urea reactor explosion. It is meaningful for the explosion prevention and the vessel safety running. However, the mechanism of such vessels is so complicated that the small scale vessel explosion test with large aspect ratio is required to verify the analysis and the results in this paper.