Page 165 - Valve Selection Handbook

152 Valve Selection Handbook

shut-off of the reverse flow can then be calculated as described in Chap-
ter 2, pages 39 through 43.
It is important for the valve user to know that valve manufacturers can
use mathematics in designing check valves for given critical applications
and predicting surge pressure.

DESIGN OF CHECK VALVES

Lift Check Valves

The check valves shown in Figure 4-1 through Figure 4-7 represent a
cross section of the family of lift check valves.
Lift check valves have an advantage over most other types of check
valves in that they need only a relatively short lift to obtain full valve
opening. This lift is a minimum in lift check valves in which the flow
passage at the seat is ring-shaped, as in the valves shown in Figure 4-4
and Figure 4-6. Lift check valves are, therefore, potentially fast closing.
In the majority of lift check valves, the closure member moves in a
guide to ensure that the seatings mate on closing. However, such guiding
also has a disadvantage in that dirt entering the guide can hang up the
closure member, and viscous liquids will cause lazy valve operation or
even cause the closure member to hang up. These types of lift check
valves are therefore suitable for low viscosity fluids only, which are
essentially free of solids. Some designs overcome this disadvantage, as in
the valve shown in Figure 4-5, in which the closure member is ball-
shaped and allowed to travel without being closely guided. When the
valve closes, the ball-shaped closure member rolls into the seat to
achieve the required alignment of the seatings.
The check valve shown in Figure 4-2 is specifically designed for
applications in which a low surge pressure is critical. This is achieved in
two ways, first, by providing the closure member with a conical exten-
sion that progressively throttles the flow as the valve closes, and second,
by combining the closure member with a dashpot that comes into play in
the last closing moments. A spring to assist closing of the valve has been
purposely omitted, as breakage of the spring was considered a hazard for
the service for which the valve is intended.
The check valve shown in Figure 4-6 is designed for gas service only.
Depending on flow conditions, the valve may serve either as a constant-
flow check valve in which the valve remains fully open in service irre-