Pipe fitting is defined as a part used in a piping system, for changing direction, branching or for change of pipe diameter, and which is mechanically joined to the system. There are many different types of fittings and they are the same in all sizes and schedules as the pipe.
Fittings are divided into three groups:
Buttweld (BW) fittings whose dimensions, dimensional tolerances et cetera are defined in the ASME B16.9 standards. Light-weight corrosion resistant fittings are made to MSS SP43.
Socket Weld (SW) fittings Class 3000, 6000, 9000 are defined in the ASME B16.11 standards.
Threaded (THD), screwed fittings Class 2000, 3000, 6000 are defined in the ASME B16.11 standards.