46 C.F.R. § 154.528 – Piping joints: Flange type.

(a) A flange must be one of the following types:
1. (1) Welding neck.
2. (2) Slip-on.
3. (3) Socket weld.
(b) If the piping is designed for a temperature between −10 °C (14 °F) and −55 °C (−67 °F), the pipe flange may be a:
1. (1) Slip-on type, if the nominal pipe size is 100 mm (4 in.) or less;
2. (2) Socket weld, if the nominal pipe size is 50 mm (2 in.) or less; or
3. (3) Welding neck.
(c) If the piping is designed for a temperature lower than −55 °C (−67 °F), the pipe flange must be a welding neck type.