Elbow Stress Analysis For Z-shape Compensation (DN150-DN500) In The Buried Heating Pipeline

In contemporary society,urban heating has become one of the vital infrastructure,constructions of a city.With the increasingly serious environment pollution and the crisis of energy and resources,the central heating mode has the advantages of saving coal resources,improving the utilization of energy,reducing smoke and dust,decreasing the emissions of pollutants and the transportation of coal and ash and also it is easy to be managed in a unified and scientific way.So it has been an important national policy to replace the small dispersed coal heating mode by the urban central heating.Among the methods of pipe network laying of the central heating,the direct burial laying method has replaced the traditional trench laying method for its advantages of low project cost,less heat loss,energy saving,anti-corrosion,good insulation,long service life and its advantages of less area occupied,fast construction,environment protection and less disturbing others etc.In the pipelines of direct buried,if they are designed reasonably to replace the compensator devices with the natural compensator,we can not only greatly reduce the settings of the fixed piers and investment,but also increase the security of the pipelines,the accident point and the maintenance point.The pity is that the range of application of natural compensator in our country and stress calculation theory are imperfect and especially the theoretical researches of Z-shape bend natural compensator which can be directly used for engineering design and application are very few.In the current buried pipe network engineering design of the central heating,the most commonly used basis are "Technical specification for directly buried heating pipeline engineering in city(CJJ/T 81-98)"(hereinafter referred to as the "Specification")and "The design of direct buried heating pipes engineering"(hereinafter referred to as" Engineering ").However,these two sayings are not completely consistent,and they do not distinguish the L-shape compensator from Z-shape compensator with one limit.Moreover,the applicable conditions in the "Specification" are only in sizes up to DN500 and below while in "Engineering" the ranges extend to DN1000.But both require the condition that the brachium of checking calculation should not be less than the elastic brachium.Neither provides a design method for the Z-shape compensator which does not meet the checking calculation condition.Although some authors use solid elements in ANSYS finite element simulation software to analyze the force on the elbow,the elbow model taken from these articles is the one the very short piece of the pipe near the elbow.For the longer straight pipes,the forces they are on the elbow are to be replaced only by applying a displacement load.However,we have to look for other methods to obtain the length of the straight pipe which is equal to the displacement load applied.For these reasons,there is an urgent need for more study of the Z-shape compensator elbow and thus to provide guidance for the currently a large number of engineering design and follow-up study of this field and to improve the strength calculation theory of the Z-shape bend.The main topic of this paper is for pipes whose diameters are between DN150 and DN500,whose top depth is 1.0m and pressure is 1.6MPa and whose loop temperature difference is 120 ? and for the L-shape bend tube and Z-shape bend whose elbow angle is 90 °.And by the ANSYS software,the author will use solid elements as pipeline unit to establish the long brachium model which is in need to simulate the forces of the elbow.According to the "Elastic flexural heave" analytical method,the author will get the L-shape bend analytic solutions which meet the necessary conditions.According to the comparison between the simulation solutions and theoretical solutions by the L-shape bend,then it can verify the reliability of the ANSYS analysis software model.With the comparison of the simulation solutions of the L-shape bend and Z-shape bend,it explains the inconsistent statement in "Regulations" and "Engineering" and it divides the boundaries of L-shape compensator elbow and Z-shape compensator bend.With the simulation analysis,it provides algorithm for the brachium of the short arm does not meet the restricted conditions of Z-shape bend in"Regulations" and "Engineering",which provides the design basis for engineering design and provides a theoretical reference for the follow-up study of the Z-shape compensator elbow.