There are various designs and types of
pressure-reducing valves. The spring-loaded
reducer and the pilot-controlled valve are
discussed in this text.
The spring-loaded pressure-reducing valve
(fig. 6-18) is commonly used in pneumatic
systems. It is often referred to as a pressure
The valve simply uses spring pressure against
a diaphragm to open the valve. On the bottom
of the diaphragm, the outlet pressure (the pressure
in the reduced-pressure system) of the valve forces
the diaphragm upward to shut the valve. When
the outlet pressure drops below the set point of
the valve, the spring pressure overcomes the outlet
pressure and forces the valve stem downward,
opening the valve. As the outlet pressure increases,
approaching the desired value, the pressure
under the diaphragm begins to overcome spring
pressure, forcing the valve stem upwards, shutting
the valve. You can adjust the downstream
pressure by turning the adjusting screw, which
varies the spring pressure against the diaphragm.
This particular spring-loaded valve will fail in the
open position if a diaphragm rupture occurs.
Pilot-Controlled Pressure-Reducing Valve
Figure 6-19 illustrates the operation of a
pilot-controlled pressure-reducing valve. This
valve consists of an adjustable pilot valve, which
controls the operating pressure of the valve, and
a spool valve, which reacts to the action of the
The pilot valve consists of a poppet (1), a
spring (2), and an adjusting screw (3). The valve
Figure 6-19.Pilot-controlled pressure-reducing valve.