Page 119 - Valve Selection Handbook

106 Valve Selection Handbook

Figure 3-65. Ball Valve with
Trunnion-Supported Ball
and Wedge Seats Forced
into the Valve Body by the
Bonnet, with One-Piece
Top-Entry Body. (Courtesy
of Trufio Limited.)

and bleed, the back of the seat rings must be provided with an elastomer-
ic O-ring.
The fourth seating method in which the seating load is regulated on
closing by an introduced mechanical force is designed to avoid most of
the sliding action between the seatings. In the valve shown in Figure 3-
66, this is achieved by a cam mechanism that lifts the ball out of the seat
prior to opening the valve and forces the ball back into the seat after clos-
ing the valve.

Pressure-Equalizing Connection

Double-seated ball valves may contain a sealed valve body cavity in
both the open and closed valve positions. When the valve is closed, the
sealed cavity extends between the upstream and downstream seats. When
the valve is open, a sealed cavity may exist also between the ball and the
valve body. If these cavities are filled with a liquid of high thermal
expansion, the pressure rise in these cavities may overstress some valve
components because of thermal expansion of the trapped fluid, unless the
excess fluid pressure can be relieved.
The cavity between the ball and the valve body is normally relieved to
the flow passage via a hole in the top or bottom flank of the ball. If the