A Butterfly valve is a quarter-turn rotational motion valve, that is used to stop, regulate, and start flow. ... When the butterfly valve is fully closed, the disk completely blocks the line. When the butterfly valve is fully opened, the disc is at a right angle to the flow of gas or liquid.
A 90° rotation of the handle provides a complete closure or opening of the valve. Large Butterfly valves are usually equipped with a so-called gearbox, where the handwheel by gears is connected to the stem. This simplifies the operation of the valve, but at the expense of speed.