What is Hot Tapping?

Hot tapping strictly refers to the installation of connections to pipelines while they remain in service. In the welding context, it is commonly used for any welding onto in-service equipment. Hot tapping is frequently used in order to repair areas that have undergone mechanical damage or corrosion, or to add branches for system modifications. There are distinct economic and environmental advantages to performing this welding without removing a pipe from service and possibly venting quantities of greenhouse gases, such as methane.

Problems can occur with hot tapping when the risk of burn-through, unstable decomposition of the flowing product and the risk of hydrogen cracking are not considered adequately.

Burn-through occurs when welding onto a pressurised pipe if the unmelted area beneath the weld pool is not strong enough to contain the internal pressure of the pipe. The greater the wall thickness of the pipe, the less the risk of burn-through. Unstable decomposition of the flowing product can cause violent reactions when heated under pressure; this is prevented by taking special precautions to prevent the internal wall temperature exceeding a critical temperature. This critical temperature is dependent on the nature of the flowing product.

The risk of hydrogen cracking is increased for hot tapping in comparison to other welding situations, due to the flowing product increasing the rate of heat flow from the weld region. This leads to shorter cooling times, with an associated increased risk of forming hard microstructures and thus greater susceptibility to hydrogen cracking. The cracking risk can be reduced by the use of low-hydrogen electrodes, and through careful selection of heat input. Preheat control is sometimes possible, but the cooling capacity of the flowing contents can make this an inefficient approach. Temper bead deposition sequences can be used to control hardness.

Safe procedures for hot tap welding can be devised using computer heat flow modelling techniques, and qualified using flow loops. Part of this approach is to match and/or control the cooling capacity of the live pipeline, for which measurement techniques are available.

The following references provide more detail about hot tap operations:

Bruce, W.A., 'Welding onto In-service Pipelines: A Review', Pipeline Welding '98 - International Symposium on Pipeline Welding - May 11-13 1998, Istanbul. 

Keifner, J.F., Fischer, R.D.,'Repair and Hot Tap Welding on Pressurised Pipelines' Proceedings of the Pipeline Engineering Symposium during the 11th Annual Energy Sources Technology Conference and Exhibition. New Orleans, 10-13 January 1988. p1-10.

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