Page 120 - Valve Selection Handbook

Manual Valves 107

Figure 3-66. Ball Valve with
Cam Mechanism to Seat and
Unseat Ball. (Courtesy of Orbit
Valve Company.)

valve is closed, the excess pressure in the cavity between the seats may
be relieved in various ways.
In the case of ball valves with floating seats, as shown in Figure 3-64,
excess pressure in the valve body will open the upstream seat seal where the
least pressure differential exists. This permits the excess pressure to escape.
In other double-seated ball valves, however, the fluid pressure must
overcome the prestress between the ball and the upstream seat. If the seat
rings are provided with some springing action, as in the valves shown in
Figure 3-60 through Figure 3-62, the fluid pressure may be able to open
the upstream seat seal without becoming excessively high. On the other
hand, if the seat rings are of a more rigid construction, thermal expansion
of the trapped fluid may create an excessively high pressure in the sealed
cavity, depending on the prestress between the upstream seatings. In this
case, the upstream flank of the ball is usually provided with a pressure-
equalizing hole, thus permitting flow through the valve in one direction
only. If the valve catalog does not advise on the need for a pressure-
equalizing connection, the manufacturer should be consulted. The provi-
sion of a pressure-equalizing connection is not normally standard with
ball valves except for cryogenic service. The pressure-equalizing connec-
tion is necessary in that case because of the rigidity of normally soft plas-