Study On Stress Performance Of Large Diameter Directly Buried Heating Pipe Sections With Ellipse Angle

With the rapid development of urbanization in my country,the scale of central heating is expanding day by day,and the direct burial laying method is widely used because of its advantages of short construction period,small footprint and cost saving.Due to the terrain and obstacles and other problems in the actual laying of the directly buried heating pipeline,it needs to be folded.However,the bending angle of the directly buried heating pipeline is easy to cause stress concentration,resulting in high local peak stress,which becomes the weak link of the force and brings security risks to the heating pipeline network.In the process of processing or operation,the bending angle of the directly buried heating pipeline will inevitably appear the phenomenon of section ovalization,which will affect its stress distribution.Therefore,it is necessary to study the stress characteristics of the elliptical cross-section directly buried heating pipes.Based on this,this paper conducts a study on the stress performance of the elliptical cross-section directly buried heating pipeline under different structural parameters,different loads and different soil parameters through the three-dimensional numerical simulation method.Fatigue analysis is also carried out in this paper.The main research contents and results are as follows:(1)Based on the nonlinear theory,this paper establishes a solid element with an elliptical cross-section of a directly buried heating pipe.The pipe body adopts the bilinear isotropic strengthening model,the soil body adopts the Drucker-Prager material model,and the interaction between the pipe and soil adopts the contact element.The mathematical model under actual working conditions was established,the calculation grid was generated and the independence assessment was carried out.The validity of the numerical model is verified.(2)The bending stress performance of large-diameter direct-buried heating pipes with different ovality and pipe diameter was studied by numerical simulation.The results show that:when the bending angle of the directly buried heating pipe is less than or equal to 4°,the peak stress of the bending angle increases with the increase of the ellipticity.When the bending angle is 5°～10°,the peak stress shows a trend of decreasing or first decreasing and then increasing with the increase of ellipticity.For DN800,DN1000,DN1200 large diameter direct buried heating pipes,the peak stress at the corner weld decreases with the increasing ovality;and under the same ellipticity,the peak stress increases with the increase of pipe diameter.(3)The research on the corner stress performance of large-diameter directly buried heating pipes with different cyclic temperature differences and internal pressures was carried out.The results show that: in the research range of this paper,when the cyclic temperature difference is 70℃ and 80℃,the peak stress of the bending angle decreases first and then increases with the increase of ellipticity.When the temperature difference is 90℃,100℃,110℃,the peak stress of the bending angle decreases continuously decreasing.the increase of temperature difference and ellipticity will cause stress concentration in the angled pipe.Under different internal pressures,the peak stress of the bending angle decreases continuously with the increase of the ellipticity;when the ellipticity is the same,when the ovality is less than2%,the peak stress first increases and then decreases with the increase of the internal pressure.When the ellipticity is within 2% ～ 10%,the peak stress decreases continuously with the increase of the internal pressure;with the increase of the internal pressure,the peak stress generally decreases gradually,however,With the increase of internal pressure,the peak stress generally decreases gradually,but the stress of the whole pipe section of the bending angle will cause stress concentration with the increase of internal pressure and the increase of ellipticity,so a certain degree of ellipticity is allowed,but it is still necessary to controls the pipe internal pressure and ovality.(4)The research on the corner stress performance of large-diameter directly buried heating pipes with different friction coefficients and elastic modulus was carried out.The results show that: Under different soil friction coefficients,the peak stress of the bending angle decreases continuously with the increase of the ellipticity;under the same ellipticity,with the increase of the friction coefficient,the soil’s constraint on the bending angle is strengthened,and the lateral compression reaction force of the soil increases,Peak stress is reduced.Under different elastic moduli,the peak stress decreases continuously with the increase of the ellipticity.When the ellipticity is within 3%,the peak stress decreases with the increase of the elastic modulus under the same ellipticity.When the ellipticity exceeds 4%,the peak stress increases with the increase of the elastic modulus.(5)The influence of the ellipticity of the large-diameter directly buried heating pipe on the contact state of the pipe and soil under different bending angles was analyzed,and the fatigue analysis was also carried out.With the increase of the ellipticity and the angle of the bending angle,the separation of the pipe and soil is more obvious,and the pipe is more easily damaged.The bending angle studied in this paper meets the design fatigue life requirements.