Page 134 - Valve Selection Handbook

Manual Valves 121

metal-to-metal seatings as illustrated, or the disc may be provided with a
variety of soft sealing elements that seal against the metal seat provided
by the body. With this choice of seating constructions, the valve may be
2
used for differential pressures up to 35 bar (500 lb/in. ) and operating
temperatures between -200°C (-350°F) and 650°C (1200°F).
The concept of the high-performance butterfly valve shown in Figure
3-78 may be likened to that of a ball valve that uses a wafer section of
the sphere only in connection with a single seat. The illustrated seat con-
sists of a U-shaped plastic seal ring that is mounted in a T-slot of the
valve body and backed up by an elastomeric O-ring. The O-ring imparts
some initial compression stress to the seat. As the disc moves into the
seat with slight interference, fluid pressure acting on the O-ring forces
the seat ring into closer contact with the rim of the disc. The valve is
made with a variety of seat ring constructions, including metal construc-
tion to suit a variety of operating conditions.
The concept of the high-performance butterfly valve shown in Figure
3-79 is also borrowed from the ball valve, but with a difference from the
valve shown in Figure 3-78 that the axis of the disc is offset not only
from the plane of the seat, but also by a small amount from the centerline
of the valve. In this way, the disc moves into the seat in a camming
action, thereby moving progressively into intimate contact with the seat.
Conversely, the seatings rapidly disengage during opening so that there is