Stress Analysis Of Pipe Fittings In Piping System Of Directly Buried Heating Pipeline

As the main type of urban district heating pipe network,directly buried heating pipes are very important for the safety of operation.The bursting of directly buried heating pipes seriously endangers the lives and property safety of residents,so it is of great significance to investigate the operation safety of directly buried pipes.Pipe fittings such as elbows and tees,which lead to discontinuity of the pipeline structure,are prone to stress concentration.enduring repeated plastic deformation for many cycles,fatigue damage would occur;the pipeline tends to move in axial direction due to the thermal expansion effect.Under the restrict of the anchor,excessive axial compression will be generated in the local area of the pipeline.When the axial compression exceeds the critical value,the local deformation of the pipeline exceeds the limit and local instability occurs.In this paper,the finite element method was used to study the stress characteristics of elbows and tees under the pipe-soil interaction and the buckling problem of straight pipe sections.In this paper,the Start-Prof software is used to calculate the full-scale pipelines to obtain the axial forces and the displacements of the arm end in pipe fittings.The above axial forces and displacements value were used as the boundary condition in the ANSYS models to analyze the local stress state of the pipelines.For the problem of local instability,this paper uses ANSYS software to study the local buckling of the pipelines.Taking into account the influence of the initial defects of the pipeline geometry,determine the appropriate defect proportional coefficient and the temperature load in the critical buckling state to analyze the influence of the buried depth and wall thickness on the local stability.Finite element analysis shows that: internal pressure,wall thickness,buried depth and corner angle affect the primary stress of elbows;internal pressure,wall thickness,buried depth,radius of curvature,corner angle and temperature affect the secondary stress of the elbow;internal pressure,wall thickness,buried depth and branch pipe diameter affect the primary stress of the tee;internal pressure,wall thickness,buried depth,branch pipe diameter and temperature affect the secondary stress of the tee.Specifying the appropriate defect proportional coefficient,when the temperature exceeds a certain value,the pipeline will be partially destabilized,and the deformation in the central depression of the pipeline will suddenly increase.Thus,the operating temperature difference of the pipeline is the critical buckling load.The buried depth and wall thickness of the pipeline have significant influence on the local buckling stability of pipelines.