Page 164 - Valve Selection Handbook

Check Valves 151

Rapid fluctuating movements of the closure member must be avoided
to prevent excessive wear of the moving valve parts, which could result
in early failure of the valve. Such movements can be avoided by sizing
the valve for a flow velocity that forces the closure member firmly
against a stop. If flow pulsates, check valves should be mounted as far
away as practical from the source of flow pulsations. Rapid fluctuations
of the closure member may also be caused by violent flow disturbances.
When this situation exists, the valve should be located at a point where
flow disturbances are at a minimum.
The first step in the selection of check valves, therefore, is to recog-
nize the conditions under which the valve operates.
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Assessment of Check Valves for Fast Closing

In most practical applications, check valves can be assessed only quali-
tatively for fast closing speed. The following criteria may serve as a guide:

1. Travel of the closure member from the fully open to the closed posi-
tion should be as short as possible. Thus, from the point of speed of
closing a smaller valve is potentially faster closing, than a larger
valve of otherwise the same design.
2. The inertia of the closure member should be as low as possible, but
the closing force should be appropriately high to ensure maximum
response to declining forward flow. From the point of low inertia,
the closure member should be of light construction. To combine
light-weight construction with a high closing force, the closing force
from the weight of the closure member may have to be augmented
by a spring force.
3. Restrictions around the moving closure member that retard the free
closing movement of the closure member should be avoided.

Application of Mathematics to the Operation of Check Valves

The application of mathematics to the operation of check valves is of
40
relatively recent origin. Pool, Porwit, and Carlton describe a calculation
method for check valves with a hinged disc that involves setting up the
equation of motion for the disc and applying to that the deceleration
characteristic of the flowing fluid within the system. Before the equation
of motion for the disc can be written, certain physical constants of the
valve must be known. The calculation determines the reverse flow veloc-
ity at the instant of sudden shut-off. The surge pressure due to the sudden