A butterfly valve is a type of quarter-turn valve that uses a disc to regulate or shut off flow in a pipeline. The disc, from which comes the moniker, “butterfly,” rotates about an axis perpendicular to the direction of flow. Used in America in the 1870s as damper valves to control air and steam, innovations throughout the 20th century have rendered the butterfly valve suitable for regulating flow of many fluids and even solids.

The butterfly valve usually has flanges to facilitate mounting to pipe flanges, which come in two styles, lug and wafer. The lug-style flanges have tapped holes which make them suitable for ending a pipeline (“dead-end” service), and when bolted between two pipes, either pipe can be removed for maintenance without disturbing the other. The wafer-style valve’s flanges serve only to position the valve during installation; the bolts connecting the two pipe flanges firmly clamp the valve in place once tightened. Some wafer-style butterfly valves don’t even have flanges.