Page 145 - Valve Selection Handbook

132 Valve Selection Handbook

The open construction is used if it is desirable to inspect the valve
body visually and physically while the valve is in service. Bulges and
soft spots that appear in the valve body are signs that the valve body
needs to be replaced.
If the escape of the fluid from an accidentally ruptured valve body
cannot be tolerated, the valve body must be encased. The casing is nor-
mally split along the axis of the flow passage for convenient access, but
casings of unit construction are also being made.
Flow Control with Mechanically Pinched Valves

Pinch valves give little flow control between the fully open and the
50% pinched position because of the negligible pressure drop at these
valve positions. Any further closing of the valve gives good flow control.
Mechanically pinched valves for flow control duty are therefore often
50% prepinched.
If the fluid is severely erosive, flow control near the closed valve posi-
tion must be avoided to prevent grooving of the valve body. For this rea-
son also, pinch valves for erosive duty must always be closed fluid-tight
to prevent grooving of the valve body as a result of leakage flow.

Flow Control with Fluid-Pressure Operated Pinch Valves

Fluid-pressure operated pinch valves are not normally suitable for
manual flow control because any change in the downstream pressure will
automatically reset the valve position. The flow control characteristic of
fluid-pressure operated pinch valves, such as the one shown in Figure 3-
91, is otherwise similar to that of mechanically pinched valves.
An exception to this flow control characteristic is the valve shown in
Figure 3-92, in which the closing action of the valve body is iris-like.
This closing action gives equal-percentage flow control throughout.
The iris-like closure allows the valve to pass larger particles through in
any valve position than is possible with any other valve. This ability con-
siderably reduces the tendency of some slurries to bridge in the partly
closed valve. The circular throttling orifice tends to eliminate also the
grooving of the valve body that develops in other types of pinch valves
when severely throttling abrasive fluids. On the debit side, the valve can-
not be fully closed.