In this paper, the effect of three main parameters: a) welding speed, b) cooling rate of fluid flow through the main pipe; and c) number of welding passes, have been studied to obtain an effective method to reduce the burn-through risk during the in-service welding of AISI-316 pipe branch connection to perform hot-tapping. In addition, important patents regarding the new methods of hot-tapping have been reviewed. To carry out numerical simulation, a 3D Finite Element (FE) based thermo-mechanical model has been developed. Using this model, thermo-mechanical stresses and temperature distribution along the main-pipe wall-thickness have been obtained with maximum and minimum allowable welding speeds; and with two high and low level of steam flow rate through the main pipe. The Von-Mises yield criterion using the temperature dependent yield stress has been used to check the main pipe failure during the welding process. The results show that current techniques, including API recommendations, which only rely on the observation of the main-pipe inner wall temperature, do not take into account the effect of mechanical or thermal stresses due to the inline pressure or other working parameters which have significant role in burn-through. In addition, the results show that the increase of welding speed reduces the risk of burn-through but it increases the risk of hot cracking. On the other hand, decreasing the steam flow rate has the opposite effect. It has also been shown that using smaller electrode size is the most effective way to decrease burn-through risk.
In-service welding, burn-through, 3D FE model, branch connection, &, welding parameters
Department of Mechanical Engineering, University of Tabriz, Tabriz, Iran.