Lug Butterfly Vlave We are largest valves manufacturers in China and 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 turn so that it allows unrestricted passage. The valve may also be opened incrementally to regulate flow. There are different kinds of Butterfly Valves, each adapted for different pressures and different usage.When the butterfly valve is closed, the disc is turned so that it totally shut off the passageway. When the Butterfly Valves is fully open, the disc is rotated at a quarter turn so that it allows unrestricted passage. The butterfly valve may also be opened incrementally to regulate flow. There are different kinds of butterfly valves, that are adapted for different pressures and different usage. A Butterfly valve is a rotational motion valve with a quarter turn. It is used to stop, regulate, and start flow.