China cast iron butterfly valve are used for isolating and regulating flow. The closing mechanism is a disc that allows for quick shut off, much like a ball valve. Butterfly valves are light in weight so they require less support. Unlike the ball valve, the disc is always present within the flow, therefore a pressure drop is always induced during flow, regardless of valve position. The disc is typically rotated a quarter turn when the valve is fully opened, but the valve may also be opened incrementally in order to throttle flow.