Page 152 - Valve Selection Handbook

Manual Valves 139

Figure 3-98. Pressure/Temperature Relationship of Straight-Through Diaphragm Valves.
(Courtesy of Sounders Valve Company Limited.)

Valve Pressure/Temperature Relationships

Figure 3-97 and Figure 3-98 show typical pressure/temperature rela-
tionships of weir-type and straight-through diaphragm valves. However,
not all construction materials permit the full range of these relationships.
The valve user must therefore consult the manufacturer's catalog for the
permissible operating pressure at a given operating temperature.

Valve Flow Characteristics

Figure 3-99 shows the typical inherent and installed flow characteris-
tics of weir-type diaphragm valves, though the shapes of the curves may
vary to some extent between valve sizes. In the case of automatic control,
these characteristics may be modified using a variable cam positioner.

Operational Limitations

Like pinch valves, diaphragm valves displace a certain amount of fluid
when operated; that is, diaphragm valves are positive-displacement type
valves. For this reason, diaphragm valves must not be installed as shut-
off valves in lines containing an incompressible fluid where the fluid has
no room to move.