Butterfly valves are quick opening valves that consist of a metal circular disc or vane with its pivot axes at right angles to the direction of flow in the pipe, which when rotated on a shaft, seals against seats in the valve body. They are normally used as throttling valves to control flow. Butterfly valves control the flow of gas or liquid by means of a disk, which turns on a diametrical axis inside a pipe or by two semicircular plates hinged on a common spindle, which permits flow in only one direction.  These valves offer a rotary stem movement of 90 degrees or less, in a compact design.  Unlike ball valves, butterfly valves do not have any pockets in which fluids may become trapped when the valve is closed.

Butterfly valves are available in several body styles and seal types.  The flange type or lug style can be held between flanges of any type. Wafer valves are installed between two flanges using bolts or nuts and studs, while lug style valves have metal inserts installed in the valve's bolt holes and the valve is installed between two flanges using a separate set of bolts for each flange.  If the wafer style is used, avoid placing it between either slip-on or screwed flange types. Some wafer body styles will not form a proper seal. The wafer style is used more commonly and is cheaper than the lug style.

Butterfly valves offer a number of advantages when used for the proper application.  They may be used in a wide variety of chemical services, they are available with small dimensions allowing for use in areas where space is limited, and they allow a high coefficient of flow.  Conversely, the design of butterfly valve creates the disadvantage of not being easy to clean, and therefore should be avoided in situations that call for sterile, medical or food processing applications.  Additionally, some styles may have difficulty dispensing slurries.