The primary function of a butterfly valve is for isolation i.e. on or off. Butterfly valves are mainly supplied in wafer pattern and this allows the on / off control of a process to be achieved in a shorter face to face and with less weight than for example a ball valve.
There is however a sacrifice to be made, in so far as 100% full bore flow is not achievable, since the disc remains in the middle of the process at all times, even when in the open position.
Isolation Butterfly valves should not be used for throttling (i.e. using the disc in any semi open / closed position) as this can result in erosion of the sealing surface. Butterfly valves which have been used for throttling will generally show a wear pattern on the leading edge of the disc (i.e. on the upstream side).
Butterfly valves are quarter turn operation. Operation can be achieved by either manual hand lever, gearbox or other actuator. By truning the stem through 90 Degrees, a disc rotates to achieve fully open or fully closed valve positions.
Energisation of the seat to seal interface is by the mechanical force transferred through the stem by the hand lever, however certain designs of butterfly valves also use the process pressure to assist in the energisation of the seal.