L51548 Criteria for Hot Tap Welding

There is a frequent need in the transmission industry to modify pipe work by welding on additional pipes and branches to existing pipelines. It is very expensive to shut down the pipeline during this operation, and it is therefore, customary to make these welds onto live pipelines. This procedure must be done with great care, since a burn through would allow the leak of pressurized products. However, a greater problem is the rapid cooling of any weld due to the high heat sink effect of the flowing gas. In ferritic steel pipelines, this can lead to excessive hardening of the heat-affected zone (HAZ) of the weld and a high risk of cracking. Although guidelines are available to help welding engineers design safe welding procedures for hot tap welding, these guidelines were developed specifically for high carbon equivalent steels and are excessively conservative for the less hardenable lower carbon steels that have recently been supplied.

This work develops a new criteria for hot tap welding which will allow safe procedures to be developed easily for all ferritic steels. The approach being used is primarily experimental and a series of tests using simulated branch and sleeve connections have been welded with various electrodes (E7018, E6010 and E8010) and cooling rates and the extent of cracking determined. A parallel study is being made to develop a technique in which the heat sink capacity of a pipeline containing flowing pressurized gas can be easily measured in the field. Knowledge of this parameter will enable the welding engineer to select an appropriate welding procedure.