Butterfly valves consist of a rotating disc on a shaft. There are three types concentric, double offset and triple offset. These different types correlate to the way that the disc connects with the sealing surface of the seat. Typically butterfly valves are of a wafer design (insertion between two pipe flanges), lugged types offer the advantage of not requiring the second flange. An undesirable feature of the butterfly valve is that, in all positions, the disc remains in the fluid, resulting in pressure drops and low CV.
General purpose butterfly valves (concentric design) requires a resilient material to form the seal at the disc and seat in the closed position. This type has good isolating characteristics in small size, low flow, and clean fluid applications. Where the limitations of the concentric valves end, the double and triple offset valves offer improved seat performance on higher temperature and pressure applications.