Section 154.528 - Piping joints: Flange type(a) A flange must be one of the following types:(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) Slip-on type, if the nominal pipe size is 100 mm (4 in.) or less;(2) Socket weld, if the nominal pipe size is 50 mm (2 in.) or less; or(c) If the piping is designed for a temperature lower than -55 °C (-67 °F), the pipe flange must be a welding neck type.