Why are all butterfly valves called double flanged and not single flanged?
Because butterfly valves are usually used for throttling application, which mostly means they are sandwiched between two flanges. Thus they are always manufactured as a wafer or sandwich type, double flanged.
However, in certain cases, butterfly valves are also used for relieving by mounting them at the end of the pipeline where it will be termed as ‘dead-end service valve’.
Butterfly valves are called as double flanged because of its structure or body design as the disc is installed within the body and is fitted to the pipe line like any other conventional valve. With a low operating torque these valves have very sturdy and robust design.