Large-Diameter Directly Buried Heating Pipe 90° Elbow Fatigue Life Of The Finite Element Analysis

With the expansion of the scale of central heating, the buried heating pipes of large-diameter and high-pressure have been widely used in engineering, the dimension of pipe diameter has reached to DN1400. But there is no clear standard method of the force calculation for the design of large-diameter buried pipes in our country, especially the design of elbow below DN500 still follow the theory of the specification which simplify the elbow to a point and adopt the elastic bending hinge analytical method to calculate the stress, besides a large number of fixed piers and compensators have been installed to protect the elbow in the engineering.Based on the mechanics study of the large-diameter buried heating elbows, this article has made an accurate calculation of the complex force, displacement and stress, so that we can reduce the volume of the fixed piers and compensators, reduce the engineering costs, accelerate the construction speed and enhance the reliability of the pipelines. In this paper, we do the following aspects of the work about the large-diameter buried heating elbow:To conduct a comprehensive mechanical analysis for the large-diameter buried heating pipes:we should consider the weight of the pipeline and the medium, the buoyancy of the ground-water table, the gravity of the top casing pipe, ground transportation load and the effect of accumulation of load, then we conduct a theoretical analysis of vertical load carried by the large-diameter heating pipe, and advance the pipeline force computational formula which is more closer to the reality.1. Briefly describe the processing technology as well as the existent defects of elbow during the processing:such as the uneven wall thickness of elbow there and the cross-sectional ellipse-oriented, which will inevitably affect the elbow stress and stress distribution, and provide theoretical basis for establishing the model of the finite element.2. The paper further analysis the stress calculation methods of elbow which below domestic DN500 directly buried heating pipeline in the technical specification, through a large number of calculation and analysis of the results, it showed that the pipe network design pressure, buried depth, temperature of work cycle, curvature radius of elbow and the bending size have different effect on the elbow carrying capacity of the horizontal and vertical angle pipes. And the work cycle temperature, bend radius of curvature and the angle of pipe segment corner's impact is obvious mostly. Foreign norms regarding the construction of elbow treatments and the fatigue life of checking were carried out to make a brief instruction, which can act as theoretical basis for the finite element numerical calculation and also provide the theoretical basis for the parameter selection and the load imposed by the finite element model.3. Using finite element software, we do numerical simulation for the ideal elbow, unequal wall thickness of elbow and oval elbow which is under the action of internal pressure load, displacement load (temperature load). From the simulation results, we can find the ration influence relationship of the maximum equivalent stress of elbow under various factors.4. Using the method of curve fitting, we also can fit the relation formula between the maximum equivalent stress, internal pressure load and displacement load. Based on the results above, through correcting the unequal wall thickness and oval-oriented of elbows, we can obtain the numerical calculation results accurately of the actual elbow by simple calculation.5. Finally, we can prove the rationality of the simplified formula through engineering examples. Now the existing domestic procedures applied to large-diameter buried heating bends fatigue strength checking below DN500 is too conservative, which cannot fully develop the potential of elbow.