pressure and to remain open until a specific reset
used in many drain lines and on the discharge side
pressure is reached-at which time they shut.
of many pumps.
Many different designs of these valves are used,
but the same result is achieved.
Special-Purpose Valves
SPRING-LOADED REDUCING
There are many types of automatic pressure
VALVES.--Spring-loaded reducing valves, one
control valves. Some of them merely provide an
type of which is shown in figure 6-8, are used in
escape for pressures exceeding the normal
a wide variety of applications. Low-pressure air
pressure; some provide only for the reduction of
reducers and others are of this type. The valve
pressure; and some provide for the regulation of
simply uses spring pressure against a diaphragm
pressure.
to open the valve. On the bottom of the
diaphragm, the outlet pressure (the pressure in the
RELIEF VALVES.--Relief valves are
reduced pressure system) of the valve forces the
automatic valves used on system lines and equip-
disk upward to shut the valve. When the outlet
ment to prevent overpressurization. Most relief
pressure drops below the set point of the valve,
valves simply lift (open) at a preset pressure and
the spring pressure overcomes the outlet pressure
reset (shut) when the pressure drops only slightly
below the lifting pressure. Figure 6-7 shows a relief
and forces the valve stem downward, opening
valve of this type. System pressure simply acts
the valve. As the outlet pressure increases,
under the valve disk at the inlet of the valve. When
approaching the desired value, the pressure under
the diaphragm begins to overcome spring
system pressure exceeds the force exerted by the
pressure, forcing the valve stem upwards,
valve spring, the valve disk lifts off its seat, allow-
ing some of the system fluid to escape through
shutting the valve. You can adjust the downstream
the valve outlet until system pressure is reduced
pressure by removing the valve cap and turning
to just below the relief set point of the valve. The
the adjusting screw, which varies the spring
spring then reseats the valve. An operating lever
pressure against the diaphragm. This particular
is provided to allow manual cycling of the relief
valve or to gag it open for certain tests. Virtually
all relief valves are provided with some type of
device to allow manual cycling.
Other types of relief valves are the high-
pressure (HP) air safety relief valve and the bleed
air surge relief valve. Both of these types of valves
are designed to open completely at a specified lift
Figure 6-7.--Typical relief valve.
Figure 6-8.--Pressure-reducing (spring-loaded) valve.
6-5
