The Effect of a Discontinuous Welding Technique on Stress Levels of a Hot Tap Tee

With the development of petrochemical industry, the hot tapping technology has been widely used. A hot tapping is a technique that allows establishing a branch connection to a live pipeline. As it is a technology without halting transportation, it can avoid large economic losses and serious air pollution. When an operation of hot tapping is performed, a technology of discontinuous welding is sometimes used to reduce the localized high temperature that may cause burn-through of pipe. The purpose of this paper is to investigate whether the structure fabricated by discontinuous welding can work safely. Simulation of discontinuous welding was conducted to compare the temperature and stress distribution between continuous and discontinuous welding. It can be concluded that the temporary high temperature can be reduced effectively by discontinuous welding. However, due to frequent arc starting, a larger residual stress is resulted in. The stress level under internal pressure was studied by experiment using stress-gauging method. And the internal pressure was imposed by a high-lift pump. The failure load of the test model was 24.0MPa. Finite element simulation was also conducted to study the stress level under internal pressure. Then a comparison between experimental and FEA results was made. The experimental and FEA results indicate that the maximal stress occurs at the intersection area of the branch and run pipe. And it also can be seen that the results of radial deformation in the cross section are in good agreement. The metallurgical structure study shows that the welding and heat-treating procedures were proper. It can be concluded from this study that although the stress level is high under operating pressure, the structure can undergo the operating pressure safely.