Plastic limit analysis of high temperature and high pressure pipeline elbows based on ANSYS Li Xinghua, Cao Leisheng, Nie Lincheng (Shenzhen Zhongguang Nuclear Engineering Design Co., Ltd., Shenzhen, Guangdong, 518057) Load is an indispensable important parameter. In the design of the piping system of a nuclear power plant, pressure and temperature are the working loads of the pipe elbow and the main load form considered in the design of the pipe elbow structure. In this paper, the ANSYS finite element analysis software is used to simplify the elbow material to ideal elastoplasticity. At the same time, considering the geometric nonlinearity, the plastic limit load distribution law of a pipe elbow under the pressure and temperature load in the safety injection system of the nuclear power plant is analyzed. 1 Introduction Pipe elbow is one of the important pipe components in the piping system of nuclear power plants. Pipe elbows may be subject to loads such as self-weight, internal pressure, in-plane bending moment, out-of-plane bending moment, torque, axial force, etc., making it a weak and critical component in pressure piping systems that is prone to failure. In the design of the piping system of a nuclear power plant, temperature and pressure are the working loads of the pipe elbow and the main load form considered in the design of the pipe elbow structure. In this paper, the ANSYS finite element analysis software is used to simplify the elbow material into an ideal elastoplastic material; under the action of temperature and pressure load, the plastic limit analysis of a pipe elbow in the safety injection system of the nuclear power plant is carried out to obtain the stress distribution of the pipe elbow The law, the expansion law of the plastic yield surface with increasing load, the final failure failure law of the elbow and the plastic limit pressure provide the theoretical basis for the design of the nuclear power plant piping system. 2 Nonlinear finite element analysis of elbows under high temperature and high pressure The nonlinear finite element numerical analysis technique is used to analyze the elastoplastic behavior of nuclear pipe elbows under high temperature and high pressure in the safety injection system of nuclear power plants. The geometric size of the elbow is 0219.1mmx10mm; the bending radius ruler = 305mm; the maximum temperature of the medium in the elbow is 120C; the ambient temperature at the elbow is 20C. The geometric model is as shown. 2.1 The finite element model elbow and the straight pipe section connected to it use the 8-node geometric model Fig. SOLID185 element. Considering the actual situation of the elbow connected with the straight pipe in the pipeline system, straight pipe sections are added at both ends of the elbow. It can be seen that with the increase of the length of the constrained straight pipe section, the maximum error of the limit load of the elbow does not exceed 3%. Therefore, when building the pipe elbow model, the length of the straight pipe sections at both ends is taken as 5 times the diameter of the elbow (1 = 5 Merit. 2.2 Material properties In the finite element analysis, the material is assumed to be an ideal elastoplastic material, and the strain hardening effect of the material is not considered. According to the existing test experience at home and abroad, the average value of the yield strength and tensile strength of the real material is selected as the flow stress CTf of the finite element model material. The mechanical parameters of the elbow material are shown in Table 1. The main research object is the plastic deformation characteristics of the pipe elbow. In order to eliminate the influence caused by the constraints, in the analysis of the two elbows, the large deformation technique can be used to obtain the numerical solution of the elbow plastic load that meets the engineering needs. The meshing diagram of the finite element model Fig. Since the yield limit of the elbow is different at different temperatures, in order to accurately obtain the plastic limit load of the elbow and the expansion and distribution law of the plastic area of ​​the elbow, the focus is on the analysis of the typical position of the elbow. Stress-strain relationship, plastic zone expansion law and elbow deformation law. The typical locations are as follows. Table i Material performance parameters-temperature; five-elastic modulus;-Poisson's ratio; a thermal expansion coefficient; one-by-one thermal conductivity coefficient. Typical position distribution of the elbow Fig. Add a straight pipe section at the end, constrain the displacement of the straight pipe section, bend When applying internal pressure to the head, apply evenly distributed pressure directly on the inner surface of the elbow. Nonlinear finite element analysis of the ultimate load of the structure requires the load to be applied in increments. In order to ensure the accuracy of the final calculation result, a reasonable load step size should be given. In this paper, a two-step loading scheme is used: the first step is loaded with a trial algorithm, which is loaded before the maximum equivalent stress of the structure reaches the yield limit; the second step is divided into 50 sub-steps to ensure that each sub-step does not exceed 1% of the limit load .3 Plastic limit load analysis In order to truly reflect the deformation and plastic bearing capacity of the elbow under external load, the nonlinear limit of the high-temperature and high-pressure pipeline elbow is determined using ANSYS software At the outer wall of the inner arch, that is, at 4 typical locations, in order to determine the plastic limit load of the elbow, the pressure-strain curve of the center point of the outer wall of the inner arch of the pipe elbow is selected, combined with the limit load determination method defined by the ASME code to finally determine the plasticity of the pipe elbow Ultimate load. The method of determining the limit load defined by the ASME code is the 2 times elastic slope criterion, that is, to make a load curve at the origin of the overload-deformation curve, so that the angle between the line and the ordinate and the elastic segment of the load-displacement curve and the ordinate The angle 0 has the following relationship: after processing the results of the finite element analysis, the pressure-strain curve at the midpoint of the outer wall of the inner arch of the elbow is obtained. According to 2 actual grate / pull force change curve 2 times elastic slope grate 1 1 1 1 1 line displacement / mm pressure-strain curve at the outer wall of the inner arch of the pipe bend elbow stress distribution law double elastic slope rule under pressure load, It can be calculated that the plastic limit pressure of the elbow is 27MPa. When the pressure exceeds this limit, it is considered that the elbow structure becomes a geometrically variable mechanism, the plastic deformation can be developed freely, and the elbow cannot bear a greater load, so that the pipeline The elbow loses its carrying capacity. 3.2 The stress distribution law of the pipe elbow Since the pipe elbow is in an elastic state at the initial loading, the stress of the elbow gradually increases with the increase of internal pressure, and the maximum pressure is outside the inner arch of the elbow; as the pressure increases, the inner arch of the pipe elbow The outer wall first enters the yielding state, and the stress at the outer wall of the elbow outer arch no longer increases with the increase of the internal pressure and reaches a constant state; as the internal pressure further increases, the area where the stress reaches a constant expands from the outside to the inside; when the pressure reaches the elbow When the plastic limit pressure is reached, the elbows all reach the plastic limit state, and the stress no longer increases with the increase of pressure. The change law of the stress distribution of the elbow with the increase of pressure is shown in and. 3.3 The plastic deformation law of the pipe elbow is known from the results of the finite element analysis of the plastic limit of the elbow. First, the change law of the stress with the pressure increase at each typical position is the yielding part is the outer wall of the inner arch of the pipe elbow. Point on both sides. The stress that has been yielded no longer increases, but the deformation progresses, that is, plastic deformation occurs; as the pressure increases, the yield surface expands inward along the wall thickness direction of the elbow, and the position of maximum plastic deformation extends from the midpoint of the outer wall The midpoint of the outer wall expands; when the elbow reaches the plastic yield limit state, the maximum plastic deformation is located at the midpoint of the outer wall of the elbow. In the process of increasing pressure, the plastic changes of the elbow are seen. Conclusion of plastic deformation law of elbow under pressure load 4 Conclusion After the plastic limit analysis of the elbow of high temperature and high pressure pipeline, the following conclusion can be drawn: For the pressure pipe elbow of nuclear power plant, due to the large temperature difference between the inner and outer walls of the elbow, the yield surface is first It occurs at the outer wall of the inner arch of the pipe elbow, and gradually expands inward along the wall thickness direction of the elbow. As the pressure increases, the area of ​​maximum plastic deformation of the elbow gradually expands from the two sides of the center of the elbow to the middle; when the elbow enters the plastic limit state, the maximum plastic deformation is located at the midpoint of the outer wall of the elbow. Under the action of internal pressure and temperature, the pipe elbow is compressed along the bend radius of the elbow and stretched in the direction perpendicular to the elbow plane. In this paper, the plastic limit load of the elbow obtained based on the finite element numerical solution can be applied to the strength design of the elbow. Product name:Bedbug and Dust Mite KillerBrand:YIQUN AEROSOL
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