In the gas transmission industry there is a frequent need for modification of existing pipework. Modifications are made primarily by welding on additional pipes and branch connections to existing pipelines. The associated costs of shutting down a pipeline creates a situation whereby it becomes economically feasible to weld onto pipelines containing flowing, pressurized products. The risk of burn through and in particular, the susceptibility to cracking are greatly increased by the rapid cooling rates associated with the high heat sink effect of the flowing gas and in turn can result in excessive...
In the gas transmission industry there is a frequent need for modification of existing pipework. Modifications are made primarily by welding on additional pipes and branch connections to existing pipelines. The associated costs of shutting down a pipeline creates a situation whereby it becomes economically feasible to weld onto pipelines containing flowing, pressurized products. The risk of burn through and in particular, the susceptibility to cracking are greatly increased by the rapid cooling rates associated with the high heat sink effect of the flowing gas and in turn can result in excessive hardening of the heat affected zone in ferritic steels. Edison Welding Institute developed new criteria for safe hot tap welding procedures on all ferritic steels. Their approach is based on correlating field measurements of the ability of the pipe to conduct heat (i.e., its heat-sink capacity) to the cooling rate of the weld. Laboratory welding trials with simulated branch and sleeve connections on live pipelines containing natural gas were used to validate the new criteria. The results indicate that structurally sound hot tap repairs can be made with a high level of confidence.