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).
General design criteria
Standard pressure class ratings / end connections