motor drive
valve (encased in
insulation)
position indicator
c. Pneumatic Valve
This type of valve has a flexible diaphragm clamped between an upper and lower
housing. The valve stem is attached to the diaphragm. By injecting air pressure
into the upper housing, the diaphragm will push the valve stem. An opposing
spring force on the valve stem will also resist this movement. Therefore, by varying
this air pressure, valve positioning can be achieved.
For this to work, an external pneumatic converter/positioner must be used to
regulate the air pressure. A thermostat may be used together for such a purpose.
This cost for this type of valves is also high.
upper housing
diaphragm
lower housing
spring
valve body
air pressure inlet
Section 3 Page 46
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