Page 170 - Valve Selection Handbook

Check Valves 157

dashpot, designed to come into play during the last closing movements,
can considerably reduce the formation of surge pressure.

SELECTION OF CHECK VALVES

Most check valves are selected qualitatively by comparing the
required closing speed with the closing characteristic of the valve. This
selection method leads to good results in the majority of applications.
However, sizing is also a critical component of valve selection, as dis-
cussed in the following. If the application is critical, a reputable manu-
facturer should be consulted.

Check Valves for Incompressible Fluids

These are selected primarily for their ability to close without introduc-
ing an unacceptably high surge pressure due to the sudden shut-off of
reverse flow. Selecting these for a low pressure drop across the valve is
normally only a secondary consideration.
The first step is qualitative assessment of the required closing speed for
the check valve. Examples of how to assess the required closing speed in
pumping installations are given in Chapter 2, page 41.
The second step is the selection of the type of check valve likely to
meet the required closing speed, as deduced from page 151.
Check Valves for Compressible Fluids

Check valves for compressible fluids may be selected on a basis simi-
lar to that described for incompressible fluids. However, valve flutter can
be a problem for high lift check valves in gas service, and the addition of
a dashpot may be required.
Where rapidly fluctuating gas flow is encountered, compressor-type
check valves such as that shown in Figure 4-6 are a good choice.
Standards Pertaining to Check Valves

Appendix C provides a list of USA and British standards pertaining to
check valves.