Page 166 - Valve Selection Handbook

Check Valves 153

spective of minor flow fluctuations, or as a pulsating-flow check valve in
which the valve opens and closes with each pulse of the flowing gas.
Constant-flow check valves are used after centrifugal, lobe-type, and
screw compressors, or after reciprocating compressors if the flow pulsa-
tions are low enough not to cause plate flutter. Pulsating-flow check
valves are used after reciprocating compressors if the flow pulsations
cause the valve to open and close with each pulsation. The valves are
designed on the same principles as compressor valves and, therefore, are
capable of withstanding the repeated impacts between the seatings. The
manufacturer will advise whether a constant-flow or pulsating-flow
check valve may be used for a given application.
The valves owe their operational characteristics to their design princi-
ple, based on minimum valve lift in conjunction with multiple ring-
shaped seat orifices, low inertia of the plate-like closure member, fric-
tionless guiding of the closure member, and the selection of a spring that
is appropriate for the operating conditions.
The valve shown in Figure 4-7 is a combined lift check and stop valve.
The valve resembles an oblique pattern globe valve in which the closure
member is disconnected from the stem. When the stem is raised, the
valve acts as a lift check valve. When the valve is lowered and firmly
pressed against the closure member, the valve acts as a stop valve.
Lift check valves must be mounted in a position in which the weight
of the closure member acts in the closing direction. Exceptions are some
spring-loaded low-lift check valves in which the spring force is the pre-
dominant closing force. For this reason, the valves shown in Figure 4-1
and Figure 4-5 may be mounted in the horizontal flow position only,
while the valve shown in Figure 4-2 may be mounted in the vertical
upflow position only. The valves shown in Figure 4-3, Figure 4-4, and
Figure 4-7 may be mounted in the horizontal and vertical upflow posi-
tions, while the valve shown in Figure 4-6 may be mounted in any flow
position, including vertical downflow.

Swing Check Valves

Conventional swing check valves are provided with a disc-like closure
member that swings about a hinge outside the seat, as in the valves
shown in Figure 4-8 and Figure 4-9. Travel of the disc from the fully
open to the closed position is greater than in most lift check valves. On
the other hand, dirt and viscous fluids cannot easily hinder the rotation of
the disc around the hinge. In the valve shown in Figure 4-9, the closure