This type of valve may be classified base on multiple parameters:
design: centric, double eccentric, triple eccentric
end connection type: wafer, lug (semi or full), flanged and double flanged
seat material: soft (example: Teflon, Buna, rubber, etc) or metal-to-metal (SS304, SS316)
body and disc materials (from cast iron to high nickel alloys)
actuation: manual (lever, gear, a worm gear) and actuated (electric, pneumatic, hydraulic and gas-over-oil types
A butterfly valve is used to shut off or modulate the flow of a fluid (isolation and regulation). API 609 Centric butterfly valves (soft seated) are preferred to gate and ball valves for low-pressure and non-critical applications as they are cheaper, lighter and easier to install. Eccentric butterfly valves (double offset and triple offset valves) with metal seats have surged in popularity and compete with globe and ball valves for some applications.