TYPES OF VALVES
Reducing valves automatically reduce supply pressure to a preselected pressure as long as the
supply pressure is at least as high as the selected pressure.
As illustrated in Figure 16, the
principal parts of the reducing valve are the main valve; an upward-seating valve that has a
piston on top of its valve stem, an upward-seating auxiliary (or controlling) valve, a controlling
diaphragm, and an adjusting spring and screw.
Figure 16 Variable Reducing Valve
Reducing valve operation is controlled by high pressure at the valve inlet and the adjusting screw
on top of the valve assembly.
The pressure entering the main valve assists the main valve
spring in keeping the reducing valve closed by pushing upward on the main valve disk.
However, some of the high pressure is bled to an auxiliary valve on top of the main valve. The
auxiliary valve controls the admission of high pressure to the piston on top of the main valve.
The piston has a larger surface area than the main valve disk, resulting in a net downward force
to open the main valve. The auxiliary valve is controlled by a controlling diaphragm located
directly over the auxiliary valve.