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. For tanks the heat is removed by the movement of the heated fluid (with tanks containing liquid, the work should always be done at least 1 meter below the liquid level).
The technique relies on having enough flow in the pipe or liquid volume in the vessel or tank to safely carry the heat away from the weld. Hot taps are inherently risky because a naked flame is being applied to a system that has not been cleared of flammable or toxic chemicals. If the heat associated with the flame is not removed quickly enough a large leak could be created.
Hot tapping is inherently hazardous and should only be done when:
- Continuity of service is essential.
- Shutdown of the system in impractical.
- Documented procedures are followed.
- Equipment is used which will provide proven effective protection for employees.
The principles of hot tapping are illustrated in a number of YouTube videos. These include:
Before allowing a hot tap, the following risks should be evaluated.
- The integrity of the equipment that is being worked on — in particular the thickness of the metal to which the hot tap is being applied.
- The adequacy of the heat sink (either volume of liquid in a tank or fluid flow through a pipe). For work on tanks, a general rule is that the hot work should be at least one meter below the liquid level. Hot work should not be allowed on the shell or roof of a tank above the liquid level.
- Vessels or tanks should have adequate venting capacity so that pressure does not build up.
The procedure for carrying out this operation generally runs on the following lines.
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