Page 138 - Valve Selection Handbook

Manual Valves 125

consists of a T-shaped PTFE member that embeds a pliant membrane. A
second fire-tested version consists of a U-shaped metal member with a
PTFE insert that is carried on a metal membrane. The PTFE insert pro-
vides the seat seal under normal operating conditions. If the PTFE insert
is destroyed in a fire, the metal seat takes over the sealing function.
The high-performance butterfly valve shown in Figure 3-83 differs
from the previously described high-performance butterfly valve in that it
relies on interference seating for seat tightness, using a double offset
disc. The seat is available either in plastic, elastomer, metal, or plastic-
metal fire-tested construction to suit the duty for which the valve is to be
used. The performance of the plastic and metal seats relies on a back-up
wire winding that is designed to provide absolute seat rigidity when the
valve is closed but to avoid imposing a load on the seat when the valve is

Figure 3-83. High-Performance Butterfly Valve Designed for Interference Seating. Scrap
Views Show Plastic Seat, Elastomer Seat, Metal Seat, and Plastic/Metal Composition
Seat for Flammable Liquid Service. (Courtesy of Keystone International, Inc.)