Section 154.524 - Piping joints: Welded and screwed couplingsPipe lengths without flanges must be joined by one of the following:
(a) A butt welded joint with complete penetration at the weld root except that for design temperatures colder than -10 °C (14 °F) the butt weld must be double welded or must be welded using:(1) A backing ring that for design pressures greater than 979 kPa gauge (142 psig) must be removed after the weld is completed;(2) A consumable insert; or(3) An inert gas back-up on the first weld pass.(b) A slip-on welded joint with sleeves and attachment welds is allowed for an open ended pipe with an external diameter of 50 mm (2 in.) or less and a design temperature of -55 °C (-67 °F), or warmer.(c) A socket weld fitting with attachment welds is allowed for pipe with an external diameter of 50 mm (2 in.) or less and a design temperature of -55 °C (-67 °F) or warmer.(d) Screwed couplings are allowed for instrumentation and control piping that meets § 56.30-20 and § 56.50-105 (a)(4) and (b)(4) of this chapter.(e) A method or fitting specially approved by the Commandant (CG-ENG).CGD 74-289, 44 FR 26009, May 3, 1979, as amended by CGD 82-063b, 48 FR 4782, Feb. 3, 1983