Page 147 - Valve Selection Handbook

134 Valve Selection Handbook

If the fluid is chemically active, the surface of highly stressed rubber is
more readily attacked than that of unstressed rubber. The corrosives may
also form a thin, highly resistant film of corrosion products, which is
much less flexible than the parent rubber. This film will crack on severe
flexing and expose the rubber to further attack. Repeated flexing will
finally lead to cracking right through the rubber.
Some of the synthetic rubbers are considerably less subject to this form
of attack than natural rubber. These rubbers extend, therefore, the applica-
tion of pinch valves for corrosive fluids. In developing such compounds,
manufacturers aim not only at corrosion resistance but also at high tensile
strength combined with softness for abrasion resistance and ease of closure.
To minimize the severity of flexing, the inside of the valve body may be
provided with groove-like recesses on opposite sides, along which the
valve body folds on closing. The valve shown in Figure 3-90 reduces the
severity of flexing by providing tear drops inside the valve body on oppo-
site sides. This design permits the valve body to be made entirely of PTFE.

Limitations

The flexible body puts some limitations on the use of pinch valves.
These limitations may be overcome in some cases by special body
designs or by the method of valve operation.
For example, fluid-pressure-operated pinch valves tend to collapse on
suction duty. If mechanically pinched valves are used for this duty, the
body must be positively attached to the operating mechanism.
Pinch valves on the downstream side of a pump should always be
opened prior to starting up the pump. If the valve is closed, the air between
the pump and the valve will compress and, upon cracking the valve, escape
rapidly. The liquid column that follows will then hit the valve body with a
heavy blow—perhaps severe enough to burst the valve body.
Pinch valves may also fail if the flow pulsates. The valve body will
pant and finally fail due to fatigue.
There is a limitation when using pinch valves as the main shut-off
valve in liquid-handling systems in which the liquid can become locked
and has no room to move. Because the valve must be able to displace a
certain amount of liquid when closing, the valve cannot be operated in
this situation. Any effort to do so may cause the valve body to burst.

Standards Pertaining to Pinch Valves

National standards that apply specifically to pinch valves do not exist.