46 CFR §154.528

1. (a)A flange must be one of the following types:
   1. (1)Welding neck.
   2. (2)Slip-on.
   3. (3)Socket weld.
2. (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.
3. (c)If the piping is designed for a temperature lower than −55 °C (−67 °F), the pipe flange must be a welding neck type.