Research On Fatigue Life Of Buried Hot Water Pipe Fitting Based On Finite Element Analysis

At present,people pay close attention to the safety and stability of large diameter buried pipes.Pipe fittings with large local stress concentration,such as elbows and three-way connections,are the weakest links in the pipe network.There is no corresponding theoretical basis for stress analysis and checking calculation of straight buried pipe fittings with large diameter,so it is impossible to accurately predict and control the stress and fatigue life of pipe fittings.Therefore,this paper mainly studies the stress and fatigue life of straight buried pipe fittings with large diameter.Numerical simulation of straight pipe section,elbow and three-way pipe with ANSYS finite element analysis software is carried out to simulate the stress state of directly buried thermal pipe fittings under different influence factors.As well as fatigue life,determine the fatigue life of pipe fitting,determine the optimization scheme of direct-buried heating pipeline.Aiming at the problem of stress and fatigue life of directly buried pipe,the following work has been done:Firstly,the stress criterion of direct-buried hot water pipeline is analyzed in detail,and the method and formula that is more suitable for stress calculation and checking calculation of direct-buried hot water pipeline are determined.On this basis,the thermoelastic model formula of direct-buried hot water pipeline is established.The model of tube-soil interaction and the physical model of buried pipeline are the theoretical basis of finite element simulation.Then the finite element analysis software ANSYS and the element used in the simulation are introduced in detail.The simulation method of thermal stress and the basic theory of finite element are discussed.The straight pipe section is established by using ANSYS.Based on the finite element model of the local pipe and soil interaction between the elbow section and the three-way pipe section,the correctness of the model is verified by calculation,and the results of the thermal stress simulation of the elbow section with the pipe trench and buried directly are compared and analyzed.The optimum size of the circular corner at the intersection line of three links is determined.By using the ANSYS simulation study of the bend pipe and the three way pipe section,it can be concluded that the primary equivalent stress and the second equivalent stress of the elbow with internal pressure.The variation of primaryequivalent stress with internal pressure,branch pipe diameter and wall thickness with internal pressure,secondary equivalent stress with internal pressure,temperature difference,branch pipe diameter,The length of branch pipe and the wall thickness of three-way;the maximum region of thermal stress produced by temperature has not changed at the bend,but has changed at the three-way;the thermal stress is much larger than the primary stress under the action of internal pressure.The effect of internal pressure on the secondary equivalent stress of pipe fittings is small,and the thermal stress is the main factor affecting the maximum stress,and the fatigue life of the three-way pipe segment is simulated by formula checking and finite element method,which shows that the finite element method can be accurate.Determine the fatigue life of the fittings quickly.Finally,the design of the uncompensated three-way pipe system in the direct buried hot water pipeline project is optimized.Through the simulation optimization,the specific position of the stress weak zone of the three-way pipe system is determined.The natural compensation ability of parallel branches of three links and Z shape is fully brought into play,which accords with the requirement of fatigue life in engineering and provides a certain reference value for the future design of direct-buried heating without compensation engineering.