The fluids flowing through many pipelines are two phase, i.e., a mixture of liquid and gas. Ideally the two phases separate out with the liquid at the bottom section of the pipe. The two phases flow together to a processing facility that separates the two phases. In practice, however, the two phases often travel at different rates which means that they can form slugs whereby the composition of the stream at different sections is almost all liquid or almost all vapor.
Welding a new nozzle on to a pipe or equipment item normally requires that the system be drained, depressurized and purged. This is a time-consuming and potentially risky process. In order to expedite the change, it is sometimes possible to make the attachment using a technique known as hot tapping or pressure tapping where the welding is carried out while the pipe of equipment item is in service. In the case of a pipe, the flowing fluid will remove the heat.