Figure 12-24.--System relief valve.
pressure from being relieved through the main system
Do not attempt to adjust a relief valve while it is
installed on an aircraft. This action will result in an
Figure 12-25 shows a typical thermal relief valve.
incorrect pressure setting. The valve must be
As pressurized fluid in the line in which it is installed
removed and adjusted on a test stand to ensure
builds up to an excessive amount, the valve poppet is
proper pressure settings.
forced off its seat; this allows excessive pressurized
fluid to flow through the relief valve to the reservoir
return line, as shown in view B of figure 12-25. When
system pressure decreases to a predetermined
pressure, spring tension overcomes system pressure
All hydraulic systems do not have shutoff valves
and forces the valve poppet to the closed position, as
incorporated; however, in some systems a shutoff valve
shown in view A.
is installed in the fluid supply line between the
reservoir and the engine-driven pumps, and other
valve for proper relieving pressure and checking the
places where shutting off the fluid is desirable. These
valve for leakage. If you think a relief valve is leaking
valves, like other valves, may be electrically or
internally, a flexible hose may be connected to the return
manually controlled, depending upon the design of the
port of the valve and the drippings, if any, caught in a
container. The opening and closing pressure of the
valve may also be checked in this manner provided an
The purpose of shutoff valves differ according to
external source of power is used.
their installation. All shutoff valves control the flow of
fluid; however, they may isolate troubles by shutting
To adjust the opening pressure of a relief valve,
off a complete system or subsystem, or they may
turn the adjusting screw clockwise to increase opening
control the speed a component moves by partially
pressure and counterclockwise to decrease opening
closing the valve (manual type).