Numerical simulation using ANSYS finite element software to analyze the influence of bend angle, radius of curvature, pipe diameter, wall thickness and circulating working temperature difference on the stress law of 40°～90° heating direct buried bend. According to the calculation results, the maximum stress curves of different influencing factors and bending bends are plotted. The results show that: with the increase of bending angle, radius of curvature, pipe diameter and wall thickness, the maximum stress value of the bend is decreasing; with the increase of circulating temperature difference, the maximum stress value of the bend gradually increases.
The corner bend of the direct buried heating pipeline is one of the weakest links of the heating pipeline. However, the stress influencing factors of bending corners in pipelines have not been studied systematically. There are few studies on 40°-90° bends in the literature, but they are used in many practical projects, so it is necessary to analyze the stresses and influencing factors of 40°-90° bends. ANSYS is a widely used finite element analysis software. It integrates structural, thermal, fluid, electromagnetic, and acoustics, and can intuitively realize the force analysis of complex structures. It has the functions of electromagnetic analysis, contact analysis, optimization design, highly nonlinear analysis of structure, adaptive meshing, large strain/finite rotation, etc. In view of the excellent performance of ANSYS finite element software in structural force analysis, this paper uses ANSYS software to explore the factors influencing the load-bearing capacity of 40°~90° heating direct buried corner bends.
1 Mathematical model
In order to represent the displacement, strain and stress of a cell in terms of nodal displacements, certain assumptions must be made about the unique distribution in the cell when analyzing the continuum. The displacement is assumed to be some simple function of the coordinates, and this function is called the displacement mode or displacement function (form function)
2 Establishment of finite element model
2.1 Physical model
The bent corner bend is shown in Figure 1. Figure 1: φ is the bending angle of the horizontal bend. The range of the study in the paper: the bending angle φ is 40° to 90° (10° step); both arm lengths L1, L2 are taken as 20m. The entity of the bending bend adopts 20-node three-dimensional structure SOLID95 unit. The coupling between the pipe and the soil is simulated by the spring? damping unit. Spring? The COMBIN14 unit is used for the damping unit, and its integrated machine tool coefficient is determined by the insulation layer, expansion bedding and soil properties.
Pipe material characteristics (steel grade Q235B)
2.3 Grid division
A free partitioning approach is adopted and the grid independence is assessed to ensure accurate calculation results and to save computational resources. Five sets of meshes are created and the stress values are calculated for different mesh cell sizes.
2.4 Boundary conditions and applied load
Bend pipe load includes two forms of force and deformation. The force is mainly caused by the pressure of the medium inside the pipe, which is achieved by applying pressure load to the inner surface of the pipe, and the deformation is caused by the temperature change of the medium inside the pipe, which is accomplished by applying temperature load to the pipe as a whole, and the boundary conditions of the two straight arms of the bend are set to be fixed in the axial direction, and the results are shown in Fig.
3 Simulation and Analysis
At present, the design temperature is usually 130 ℃, the design pressure has reached 2.5 MPa, and the pipe diameter has reached 1400 mm. Therefore, in order to make the results of the research and analysis more practical and relevant, and to provide direct reference and reference for the design of 40°-90° bends, the basic parameters of the analysis and the working parameters of the pipe network are defined within the range commonly used in practical engineering.
3.1 Effect of bend radius of curvature on the maximum stress of the bend
As the angle of bend angle increases, the maximum value of bend stress gradually decreases. At the same time, the maximum stress value of the bend gradually decreases with the increase of the radius of curvature for the bend angle of 40° to 90°, and the larger the bend angle, the smaller the decrease of the maximum stress value of the bend. The reason is that when other conditions are fixed, the radius of curvature increases, the size coefficient of the bend increases, resulting in a reduction in the stress reinforcement coefficient of the bend, the bend stress under the action of the bending moment will be reduced accordingly, ultimately resulting in a reduction in the maximum stress value of the bend. For 40 ° ~ 90 ° heating right buried corner bend, increase its radius of curvature, can effectively improve the bend's load-bearing capacity.
3.2 The influence of the bend pipe diameter on the maximum stress of the bend
Bend stress maximum with the increase in bend angle is decreasing situation, in the same bend angle, the larger the bend pipe diameter, the smaller the maximum stress value. The reason is that when other conditions are certain, the diameter of the bend increases, the bend stress reinforcement factor and the bending moment of the elbow section are increased, but the bending moment increases faster than the increase in stress reinforcement factor, the bend stress will be reduced accordingly, which eventually leads to a reduction in the maximum value of the bend stress.On the one hand, the maximum stress value of the bend is decreasing with the increase in the angle of the bend; on the other hand, at the same angle of the bend, the larger the diameter of the bend, the smaller the maximum stress value. But the difference is that, for any angle, when the bend to make the same specification adjustment, the stress change is similar.
3.3 The influence of the wall thickness of the bend on the maximum stress of the bend
When other conditions are certain, the wall thickness of the bend increases, the stress reinforcement coefficient of the bend decreases, and the bending moment of the elbow section increases, so that the stress in the bend will be reduced accordingly, which eventually leads to a reduction in the maximum stress in the bend.
3.4 Effect of cyclic working temperature difference on the maximum stress of the bend
The maximum stress value of the bend decreases with the increase of the bending angle. In the same bend angle, the greater the cycle work temperature difference, the greater the maximum stress value of the bend. The reason is that other conditions are certain, the increase in the cyclic working temperature difference will increase the axial force of the bend, which will lead to a greater maximum stress value of the bend under the same conditions. The effect of the circulating working temperature difference on the maximum stress value of the bend does not vary significantly with the angle of the bend. Therefore, for the design of high-temperature pipe network, pay more attention to the load-bearing capacity of the horizontal bend.
4 Conclusion
ANSYS finite element software was applied to simulate the maximum stress value of 40° to 90° bending angle bends, and the change law of bending angle φ with different radius of curvature, different pipe diameter, different wall thickness and different cyclic temperature difference was studied.
(1) When 40 ° ≤ φ ≤ 90 °, with the increase in the bending angle φ, the compensation capacity of the horizontal bend gradually enhanced, the maximum value of the bend stress gradually decreased.
(2) When 40 ° ≤ φ ≤ 90 °, increase the radius of curvature of the bend, can effectively improve the bend's load-bearing capacity. When 40 ° ≤ φ ≤ 60 °, the improvement effect is particularly obvious.
(3) Increase the diameter of the pipe will reduce the stress value of the horizontal bend. Increase the wall thickness can improve the strength of the pipe network, but also reduce the stress value of the horizontal bend. However, when the bending angle φ > 60 °, increase the wall thickness of the bend on the maximum stress value of the reduction effect is weakened.
(4) high-temperature hot water heating network on the strength of the horizontal bend higher requirements, you can adjust the bend radius of curvature or increase its wall thickness to reduce the bend stress value, thereby improving the safety performance of the pipe network.