A butterfly valve is from a family of valves called quarter turn
valves. The "butterfly" is a metal disc mounted on a rod. When the
valve is closed, the disc is turned so that it completely blocks
off the passageway. When the valve is open, the disc is rotated a
quarter turn so that it allows unrestricted passage. The position
of the disc is effected from outside the valve.
gas butterfly valve is one kind of butterfly valve, the main
feature is that all the functions are supported by gas just as the
Electric Butterfly Valve which supported by electric.