The valve is used to isolate or regulate flow. The disc is positioned in the center of the pipe and, passing through the disc, is a rod connected to an actuator on the outside of the valve. Rotating the actuator turns the disc either parallel or perpendicular to the flow. Unlike a ball valve, the disc is always present within the flow; therefore, a pressure drop is always induced in the flow, regardless of valve position.
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 fully open, the disc is rotated a quarter of a turn so that it allows almost unrestricted passage. The valve may also be opened incrementally to regulate product flow.