A butterfly valve is from a family of valves called quarter-turn valves, which can be used for isolating or regulating flow. The closing mechanism takes the form of a disk.   In operation, the valve is fully open or closed when the disc is rotated a quarter turn. The "butterfly" is a metal disc mounted on a rod. When the valve is closed, the disc is turned so that it completely blocks off the passageway. When the valve is fully open, the disc is rotated    a quarter turn so that it allows an almost unrestricted passage of the fluid. The valve may also be opened incrementally to throttle flow.   Butterfly valve has wafer, lug, flanged, butt welded end connections as ASME, DIN,GOST, ISO JIS criterions. 

There are different kinds of butterfly valves, each adapted for different pressures and different usage.   

1.Concentric butterfly valves Called zero-offset butterfly valve, which uses the flexibility of rubber, has the lowest pressure rating.

2.Doubly-eccentric butterfly valves Called high-performance butterfly valves or double-offset butterfly valves– different type of materials is used for seat and disc.

    used in slightly higher-pressure systems, is offset from the center line of the disc seat and body seal (offset one), and the center line of the bore (offset two).

3.Triply-eccentric butterfly valves Called triple-offset butterfly valves – the seats are either laminated or solid metal seat design.    The valve best suited for high-pressure systems is the triple offset butterfly valve. In this valve the disc seat contact axis is offset, which acts to virtually eliminate sliding contact between disc and seat. In the case of triple offset valves the seat is made of metal so that it can be machined such as to achieve a     bubble tight shut-off when in contact with the disc.