Figure 7-24 shows atypical main system relief
valve and its component parts breakdown. The relief
valve consists of a cylindrical housing that contains a
poppet valve and piston assembly. Each end of the
housing is fabricated to include a wrench-holding
surface and a threaded port for installation of a
hydraulic fitting, and the housing is stamped to
identify the ports as PRESS (pressure) and RET
A coil spring at one end of the piston retains it
against a stop on the valve housing; and the poppet
valve, which is located just inside the pressure port, is
spring seated over a passage through the valve. When
fluid pressure at the pressure port reaches 3,650 psi,
the pressure forces the piston to depress the coil
spring and move clear of the poppet valve. Thus, the
passage through the piston is opened, and fluid flows
through the valve into the return line. When pressure
at the pressure port is reduced to 3,190 psi, the coil
spring reseats the piston against the poppet valve, and
fluid flow through the relief valve ceases. Should the
pressure at the outlet port exceed the pressure at the
inlet port, the poppet valve will unseat, and fluid from
the return line will flow through the valve into the
Thermal Relief Valves
Thermal relief valves are usually smaller as
compared to system relief valves. They are used in
systems where a check valve or selector valve
prevents pressure from being relieved through the
main system relief valve.
Figure 7-25 shows a typical thermal relief valve.
As pressurized fluid in the line in which it is installed
builds up to an excessive amount, the valve poppet is
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 7-25. When
system pressure decreases to a predetermined
pressure, spring tension overcomes system pressure
and forces the valve poppet to the closed position, as
shown in view A.
Relief valve maintenance is limited to adjusting
the valve for proper relieving pressure and checking
the valve for leakage. If you think a relief valve is
leaking internally, a flexible hose maybe connected to
the return 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 external source of rower is used.
Figure 7-24.System relief valve.