Research On Stress Characteristics Of Asymmetrical Large Diameter Z Shaped Directly Buried Heating Elbow

With the rapid development of urbanization in China,the scale of central heating is expanding gradually.As an important part of urban livelihood engineering,the reasonable design and laying of pipe network are very important to the safe operation of heating system.Among them,direct buried laying method has been widely used in central heating project because of its advantages of low cost,energy saving,long life and fast construction.However,due to the uncertainty of the laying environment,and the direction of the pipeline will change when encountering obstacles,it is inevitable to appear asymmetric Z shaped directly buried heating elbow.Although the asymmetric Z shaped directly buried heating elbow is an urgent need in practical engineering,the stress design method for the large-diameter Z shaped directly buried heating elbow is rarely reported in the specification at present.In addition,the widely used anti-bending hinge analytical method is no longer suitable for the analysis of the stress characteristics of the asymmetric Z shaped directly buried heating elbow.In view of this,through three-dimensional numerical simulation,this paper studies the stress size and distribution law of asymmetric Z shaped directly buried heating elbows with a turning angle of 95° ～ 140° and diameter of DN800 ～ DN1000,and analyzes the distribution mechanism of local stress concentration and the influence of asymmetry on its characteristics.The effects of structural parameters and operating parameters on the stress performance of asymmetric Z shaped directly buried heating elbow were studied,and the coupling compensation mechanism under different mating conditions was revealed,in order to provide reference and basis for the practical application,safe operation and related regulations of asymmetric Z shaped directly buried heating elbow.This paper carries out relevant research work from the following aspects.Firstly,the different load and stress classification methods of large diameter asymmetric Z shaped directly buried heating pipe are analyzed.And the different failure forms of directly buried heating pipe are summarized.A mathematical model of large diameter asymmetric Z shaped directly buried heating elbow was established to apply pressure load,temperature load and freedom limit,and the validity of the numerical model was verified.Then,through three-dimensional numerical simulation,the stress performance of large diameter asymmetric Z shaped directly buried heating elbow under different angles is obtained.The results show that the maximum stress value of the asymmetric Z shaped directly buried elbow is located at the intersection of the symmetrical section of the elbow and the neutral plane.The maximum stress of asymmetric Z shaped directly buried heating elbow with smaller angle is larger than that of larger angle.When the angle increases,the maximum stress of small-angle elbow increases gradually because the elbow part cannot fully absorb the thermal elongation of pipeline,and the thermal displacement accumulates.In the scope of this paper,compared with the symmetrical Z shaped directly buried heating elbow,the maximum stress value of the small angle elbow increases by 42.28%.In addition,the increase of the degree of asymmetry of the elbow will increase the amplitude of the change of the maximum stress of the two elbows,especially when the two angles of the asymmetric Z shaped directly buried heating elbow are large,the maximum stress of the elbow is too large.Secondly,the stress characteristics of asymmetrical Z shaped directly buried heating elbow with different pipe diameters,pipe wall thickness,curvature radius,compensating arm length and compensated arm length are studied in detail.The results show that the maximum stress increases with the increase of the elbow diameter,but the increase of the maximum stress of the elbow with a larger diameter becomes slower as the asymmetry increases.The maximum stress of the elbow decreases with the increase of the wall thickness and curvature radius.In engineering,the maximum stress of the elbow can be reduced by increasing the wall thickness or curvature radius appropriately.With the increase of the length of the elbow compensating arm,the maximum stress at the elbow decreases first and then increases.The maximum stress of small-angle elbow 1 turns at 1.0Le,and the maximum stress of large-angle elbow 2 turns at1.5Le.When the length of compensating arm increases,the maximum stress value of elbow increases.When the length of compensated arm increases from 20 m to 35 m,the maximum stress of elbow be increased by more than 24%.Finally,this paper studies the influence of different operating parameters(working pressure,temperature difference)on the stress characteristics of asymmetric Z shaped directly buried heating elbow.The results show that the maximum stress of small angle elbow 1 and large angle elbow 2 increases with the increase of working pressure and temperature difference.When the working pressure increases from 1.0MPa to 2.5MPa,the maximum stress value of the elbow increases by about 60 MPa.When the cycle temperature difference increases,the difference between the maximum stress values of the two elbows will also increase,and the elbow with the larger stress value will be the first local yield,and at 75℃,the difference between the two elbows is more than 21 MPa.