With the continuous expansion of the scale of central heating, large-diameter and high-pressure direct-buried heating pipes have been widely used, and the pipe diameter has reached DN1400. However, there is no clear stress calculation method standard for large-diameter directly-buried pipes in China, especially for elbows, which are simplified to a point by following the design theory of the DN500 specification. The elastic bending hinge analytical method is still used for stress calculation, and a large number of fixed piers and compensators are installed in the project to protect the elbows.
Through the mechanical research of large-diameter directly-buried heating elbow, under the action of complex forces, the accurate calculation of displacement and stress can achieve the purpose of reducing the number of fixed piers and compensators, reducing the engineering cost, accelerating the construction speed, and enhancing the reliability of the pipe network. In this paper, the following work has been done for the large-diameter direct-buried heating elbow: 1. A comprehensive stress analysis of the large-diameter direct-buried heating pipe has been carried out: considering the weight of the pipe and the medium itself, the buoyancy of the groundwater level, the gravity of the soil on the top of the pipe, the ground traffic load and the role of the accumulated load, the theoretical analysis of the vertical load of the large-diameter direct-buried heating pipe has been carried out, and a more practical calculation formula for the stress of the pipe has been selected. 2. Briefly introduced the elbow processing technology and the defects of the elbow during the processing, such as uneven wall thickness and elliptical section. These will inevitably affect the stress size and stress distribution of the elbow, and provide a theoretical basis for the finite element modeling.
3. Further analyzed the stress calculation method of elbows in the domestic direct buried technical specifications below DN500. Through a large number of calculations and analysis of the calculation results, it is shown that the design pressure of the pipe network, the buried depth, the circulating working temperature difference, the radius of curvature of the elbow and the size of the corner have different degrees of influence on the bearing capacity of the elbows in the horizontal and vertical corner pipe sections of the direct buried heating, among which the circulating working temperature difference The radius of curvature of the elbow and the bend angle of the corner pipe section have the most obvious influence. The construction treatment of elbow and fatigue life checking calculation in foreign codes are briefly introduced, which serve as the theoretical basis for finite element numerical calculation. It provides theoretical basis for load application and parameter selection of finite element model. 4. The finite element software is used to simulate the ideal elbow, the elbow with unequal wall thickness, the elliptical elbow, etc. under the action of internal pressure load and displacement load (temperature load). From the simulation results, the quantitative influence relationship of various influencing factors on the maximum equivalent stress of elbow is obtained. Using curve fitting method, the relationship between the maximum equivalent stress of ideal elbow and internal pressure load and displacement load is fitted. Based on this, the accurate numerical calculation results of the actual elbow can be obtained by simple calculation through the correction of the unequal wall thickness and ovalization of the elbow. 5. Finally, the rationality of the simplified formula is proved by an engineering example. The existing DN500 and below direct-buried technical specifications continue to be applied to the fatigue strength checking calculation of large-diameter direct-buried heating elbow, which is too conservative to give full play to the potential of elbow.
Post time: Mar-15-2023