Butterfly valves are rotary valves in which a disc-shaped closure member is rotated through 90 , or approximately, to open or close the flow passage. The butterfly valves shown in Figure 3-69 through Figure 3-83 represent a cross section of the many variations available.

The original butterfly valve is the simple pipeline damper that is not intended for tight shut-off. This valve is still an important member of the butterfly valve family.

The advent of elastomers has initiated the rapid development of tight shut-off butterfly valves in which the elastomer serves as the sealing element between the rim of the disc and the valve body. The original use of these valves was for water.

As more chemical-resistant elastomers became available, the use of butterfly valves spread to wide areas of the process industries. The elastomers used for these purposes must not only be corrosion-resistant but also abrasion-resistant, stable in size, and resiliency-retentive, that is, they must not harden. If one of these properties is missing, the elastomer may be unsuitable. Valve manufacturers can advise on the selection and limitations of elastomers for a given application.

Efforts to overcome some of the limitations of elastomers led to the development of butterfly valves with PTFE seats. Other efforts led to the development of tight shut-off butterfly valves with metal seatings. By these developments, butterfly valves became available for a wide range of pressures and temperatures, based on a variety of sealing principles.

Butterfly valves give little resistance to flow when fully...