Page 127 - Valve Selection Handbook

114 Valve Selection Handbook

metallic. For applications in which a lower leakage rate is required, but-
terfly valves that have a piston ring on the rim of the disc are available.
The intimate contact between the seatings of tight shut-off butterfly
valves may be achieved by various means. Some of the ones more fre-
quently used are:

1. By interference seating that requires the disc to be jammed into the
seat.
2. By forcing the disc against the seat, requiring the disc to tilt about a
double offset hinge in a manner comparable to tilting disc check
valves, as described on page 154.
3. By pressure-energized sealing using sealing elements such as O-
rings, lip seals, diaphragms, and inflatable hoses.

The majority of butterfly valves are of the interference-seated type in
which the seat has a rubber liner, as in the valves shown in Figure 3-70
through Figure 3-72. Where rubber is incompatible with the fluid to be
sealed, the liner may be made of PTFE, which is backed-up by an elas-
tomer cushion to impart resiliency to the seat.

Figure 3-69. Butterfly Valves. (Courtesy ofGEC-Elliot Control Valves Limited.)