A butterfly valve, another type of quarter-turn valve, is named for the wing- like action of the flow-controlling disc inside the valve. The flat, circular disc is mounted on a rod which is connected to a handle or actuator which is used to turn the disc either parallel or perpendicular to the flow of fluid.

When the valve is closed, the disc is turned so that it completely blocks off the passageway.

When the valve is fully open, the disc is rotated a quarter turn so that it allows an almost unrestricted passage of the fluid.

The valve may also be opened incrementally to regulate flow.

It operates similar to a ball valve, however, the disc is always present within the flow of fluid, causing a slight pressure drop regardless of it's position. Butterfly valves have relatively easy maintenance due to the small number of moving parts.