Page 150 - Valve Selection Handbook

Manual Valves 137

Weir-type diaphragm valves may also be used in general and high vac-
uum service. However, some valve makers require a specially reinforced
diaphragm in high vacuum service.
Because the diaphragm area is large compared with the flow passage,
the fluid pressure imposes a correspondingly high force on the raised
diaphragm. The resulting closure torque limits the size to which
diaphragm valves can be made. Typically, weir-type diaphragm valves of
the type shown in Figure 3-93 are made in sizes up to DN 350 (NFS 14).
Larger weir-type diaphragm valves up to DN 500 (NFS 20) are provided
with a double-bonnet assembly, as shown in Figure 3-95.
Weir-type diaphragm valves are also available with a body of T-con-
figuration, as shown in Figure 3-96, in which a branch connects to the
main flow passage without impeding pipeline flow. The main function of
these valves is for sampling duty where the taking of a true sample from
the flowing fluid must be assured.
The stem of the valve shown in Figure 3-93 is of the rising type. To
protect the external stem thread from dust and immediate outside corro-
sive influences. The handwheel carries a shroud that covers the stem
thread while sliding over a yellow lift-indicator sleeve. The yellow lift-
indicator sleeve, in turn, carries a prepacked lubrication chamber to lubri-
cate the stem thread for long life.
If required, the stem may be provided with an O-ring seal against the
bonnet to prevent fluid from escaping into the surroundings of the valve
should the diaphragm break in service.
Conventional weir-type diaphragm valves may also be used in hori-
zontal lines that must be self-draining. Self-draining is achieved by
mounting the valve with the stem approximately 15° to 25° up from the
horizontal, provided the horizontal line itself has some fall.

Straight-Through Diaphragm Valves

Diaphragm valves with a straight-through flow passage require a more
flexible diaphragm than weir-type diaphragm valves. For this reason, the
construction material for diaphragms of straight-through diaphragm
valves is restricted to elastomers.
Because of the high flexibility and large area of these diaphragms,
high vacuum will tend to balloon the diaphragm into the flow passage.
The degree of ballooning varies thereby with make, causing either a
small and acceptable reduction in flow area only or a collapse of the
diaphragm. In the latter case, the bonnet must be evacuated to balance