A 300MM Butterfly valve consists of a disc rotating in trunnion bearings. In the open position, the disc is edge-on, offering minimum resistance to flow. In the closed position, it is rotated against a seat. Butterfly valves are available in sizes from DN32-DN3000 (l 1/4" in to 120") diameter and include the popular 300MM butterfly valves. They are particularly attractive because they take up little more room than the pipe flanges. In fact some butterfly valves are designed specifically for insertion between pipe flanges, using through bolts. These are known as wafer butterfly valves. Other types are normally single-flanged or double-flanged, known also as lug-body valves.