External Pressure Evaluation for Hot Taps

Abstract Welded hot taps are a commonly used method by which a branch connection or nozzle can be added to a pipe, pipeline, or vessel while the equipment is operating. ASME PCC-2 Repair of Pressure Equipment and Piping 1 includes this technique in Article 216. This paper focuses on “set on” style hot taps in which the new nozzle is welded directly to the run pipe or vessel. One step in the hot tap process occurs after the new connection is welded to the equipment but prior to cutting the hole through which the fluid will flow. This step involves ensuring that the cutting machine and welded-in nozzle has been installed correctly and can withstand the fluid pressure without a loss of containment. When the assembly is tested at pressures greater than the internal operating pressure of the equipment, the area of the equipment which is exposed to the test pressure is in effect operating under an external pressure condition. This area is vulnerable to a failure often characterized by buckling. A review of Article 216 led to some changes proposed in the wording within the Article. Specifically, clause 216-3.13(b)(2) which discusses determining the maximum allowable external pressure (MAEP) of set-on hot taps to prevent buckling of the underlying pipe or vessel shell was revised. This revision has passed Committee balloting and is anticipated to be published in the 2026 edition of PCC-2. This paper explains the derivation of the original closed form solution, describes the basis of the MAEP evaluation as originally published in the 2018 edition of PCC-2, and presents the changes which are approved for publication in the 2026 edition and others which are proposed but not yet approved. Several comparisons are made to the current methodology using Finite Element Analysis. An investigation into MAEP sensitivity for various set-on hot tap geometric parameters is presented as well as an updated geometric range of validity for the current equations within the Article.