In-service welding is the key operation of hot tapping which allows establishing a branch connection to a live pipeline. As there is no need to halt transportation, it can avoid large economic losses and serious air pollution. However, the operation is made much more difficult than normal welding by the pressure, temperature and flowing of contents. Firstly, the local high temperature causes a loss of material strength during in-service welding. Burn-through may occur if the region with elevated temperature can not support the stress it suffered. Secondly, the flowing media create a large heat loss through the pipe-wall, resulting in accelerated cooling of the weld, which increases hardness of HAZ and possibility of HAZ cracking. In this paper, temperature distribution of in-service welding under variable parameters onto SS304 pipeline with tap water flowing in the pipes were simulated by FEM. Temperature dependent material properties and effects of convection and radiation were considered. Internal heat generation and amplitude loading were used to simulate the moving heat source. The effect of the flowing of internal media was regarded as forced convection. Based on the results of temperature field, RSF and limit pressure of the pipe can be obtained and burn-through can be predicted. It can be concluded that the limit pressure decrease with the increasing of heat input. RSF increases with the increasing of pipe-wall thickness. While the thickness increases to an extent, RSF show little increase. RSF and limit pressure of the pipe increase with the increasing of flow rate. There is a range that the increase changes greatly. While in-service welding, the range should be adequately considered to determine operating condition optimally. According to the results, the maximum of heat input and minimum of pipe-wall thickness can be obtained.

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