Page 167 - Valve Selection Handbook

154 Valve Selection Handbook

member is an integral part of the rubber gasket between the valve body
halves. It is steel-reinforced, and opens and closes by bending a rubber
strip connecting the closure member and the gasket.
As the size of swing check valves increases, weight and travel of the
disc eventually become excessive for satisfactory valve operation. For
this reason, swing check valves larger than about DN600 (NFS 24) are
frequently designed as multi-disc swing check valves, and have a number
of conventional swing discs mounted on a multi-seat diaphragm across
the flow passage in the valve.
Swing check valves should be mounted in the horizontal position, but
may also be mounted in the vertical position, provided the disc is pre-
vented from reaching the stalling position. In the latter case, however, the
closing moment of the disc, due to its weight, is very small in the fully
open position, so the valve will tend to close late. To overcome slow
response to retarding flow, the disc may be provided with a lever-mount-
ed weight or spring loaded.
The check valve shown in Figure 4-10 is a double-disc swing check
valve with two spring-loaded D-shaped discs mounted on a rib across the
valve bore. This design reduces the length of the path along which the
center of gravity of the disc travels; it also reduces the weight of such a
disc by about 50%, compared with single-disc swing check valves of the
same size. Coupled with spring loading, the response of the valve to
retarding flow is therefore very fast.

Tilting-Disc Check Valves

Tilting-disc check valves such as the one shown in Figure 4-11 have a
disc-like closure member that rotates about a pivot point between the
center and edge of the disc and is offset from the plane of the seat. The
disc drops thereby into the seat on closing, and lifts out of the seat on
opening. Because the center of gravity of the disc halves describes only a
short path between the fully open and the closed positions, tilting-disc
check valves are potentially fast closing. This particular valve is, in addi-
tion, spring-loaded to ensure quick response to retarding forward flow.
Reference may be made also to the valve shown in Figure 3-77 that
can serve as a butterfly valve, a tilting-disc check valve, or a combined
tilting-disc check and stop valve, depending on the design of the drive.
Tilting-disc check valves have the disadvantage of being more expen-
sive and also more difficult to repair than swing check valves. The use of
tilting-disc check valves is therefore normally restricted to applications
that cannot be met by swing check valves.