The Research Of In-service Welding For Modification Of Gas Pipelines

With the wide usage of the buried gas pipeline,people have paid more and more attention on in-service welding of gas pipelines. At present, China's such technology is only based on previous experience which lacks scientific and quantitative data. Therefore, this paper will do research on modification welding of gas pipelines whose topic is supplied from Tianjin Gas Group CO.LTD. It is mainly about the temperature of pipe inside wall and the strength of the welding joint.Firstly, the paper found proper technics parameters for the modification welding of gas pipelines, which includes: welding method, the choice for base metal and welding filler, the size of welding joint the heat treatment method and so on. In order to get these parameters, it needs a few experiments: tensile test, groove test and so on. After confirming the welding technics, the paper used the finite element software, ANSYS, to simulate the modification welding process of gas pipelines. Since the welding process contains two parts, thermal issue and stress issue, the paper chose the thermal-stress coupling method of finite element numerical simulation. Meanwhile, because the welding process is a quite unstable and asymmetric issue for inputting heat, the paper choice indirect coupling method to get the accurate solution. There are three kinds of internal press (3MPa, 4MPa and 5MPa) condition for simulation. In the simulation, the paper utilized the important function of "birth-death element". In the thermal-stress coupling section, in order to coordinate the different element density between thermal part and stress part, the paper harnessed the BFINT function to activate the body force interpolation operation which is the highlight of this research.At last, it is found that under three internal press condition, the temperature of the gas pipeline's inside wall is all less than 982℃. It means it would not burn through during the welding process. At the same, with the increased internal press, the coefficient of thermal transfer is also raised, so the temperature of the gas pipeline's inside wall is reduced. However, raising the internal press leads to the increase of stress and displacement. Therefore, the 3MPa is a proper and safe press for the modification welding of gas pipelines.