Page 169 - Valve Selection Handbook

156 Valve Selection Handbook

The check valve shown in Figure 4-12 consists of a perforated cone-
shaped basket that supports a matching diaphragm. This assembly is
mounted in the pipeline between two flanges or clamped between pipe
unions. Flow passing through the cone lifts the diaphragm off its seat and
lets the fluid pass. When forward flow ceases, the diaphragm regains its
original shape and closure is fast. One application worth mentioning is in
purge-gas lines, which feed into lines, handling slurry or gluey sub-
stances. Under these conditions, diaphragm valves tend to operate with
great reliability, while other valves hang up very quickly.
The check valve shown in Figure 4-13 uses a closure member in the form
of a pleated annular rubber diaphragm. When the valve is closed, a lip of the
diaphragm rests with the pleats closed against a core in the flow passage.
Forward flow opens the pleats, and the lip retracts from the seat. Because
the diaphragm is elastically strained in the open position, and travel of the
lip from the fully open to the closed position is short, the diaphragm check
valve closes extremely fast. This valve is well-suited for applications in
which the flow varies within wide limits. However, the pressure differential
2
for which the valve may be used is limited to 10 bar (145 lb/in ), and the
operating temperature is limited to about 70°C (158°F).
The closure member of the diaphragm check valve shown in Figure 4-
14 consists of a flexible sleeve that is flattened at one end. The flattened
end of the sleeve opens on forward flow and recloses against reverse flow.
The sleeve is made in a large variety of elastomers, and is externally
reinforced with plies of nylon fabric similar in construction to an auto-
mobile tire. The inside of the sleeve is soft and capable of embedding
trapped solids. The valve is therefore particularly suitable for services in
which the fluid carries solids in suspension or consists of a slurry.
Figure 4-15 shows an interesting application of this check valve as a
tidal gate.
The valve is available in sizes as small as DN 3 (NFS 1/8) and as large
as DN 3000 (NFS 120) for tidal gates.

Dashpots

The purpose of dashpots is to dampen the movement of the closure
member.
The most important application of dashpots is in systems in which
flow reverses very fast. If the check valve is unable to close fast enough
to prevent a substantial reverse-flow buildup before sudden closure, a