The valve is available with either a rising stem
or a non-rising stem. The rising stem valve uses
the same port body design as does the non-rising
stem valve. The stem is threaded into the gland
nut and screws outward as the valve is opened.
This valve does not incorporate provisions for
tightening the stem packing nor replacing the
packing while the valve is in service; therefore,
complete valve disassembly is required for
maintenance. Figure 6-10 illustrates a rising stem
hydraulic and pneumatic globe valve. Additional
information on this valve is available in Standard
Navy Valves, NAVSHIPS 0948-012-5000.
PRESSURE CONTROL VALVES
The safe and efficient operation of fluid
power systems, system components, and related
equipment requires a means of controlling
pressure. There are many types of automatic
pressure control valves. Some of them merely
provide an escape for pressure that exceeds a set
pressure; some only reduce the pressure to a lower
pressure system or subsystem; and some keep the
pressure in a system within a required range.
Some fluid power systems, even when operat-
ing normally, may temporarily develop excessive
pressure; for example, when an unusually strong
work resistance is encountered. Relief valves are
used to control this excess pressure.
Relief valves are automatic valves used on
system lines and equipment to prevent over-
pressurization. Most relief valves simply lift (open)
at a preset pressure and reset (shut) when the
pressure drops slightly below the lifting pressure.
They do not maintain flow or pressure at a given
amount, but prevent pressure from rising above
a specific level when the system is temporarily
Main system relief valves are generally
installed between the pump or pressure source and
the first system isolation valve. The valve must
be large enough to allow the full output of the
hydraulic pump to be delivered back to the
reservoir. In a pneumatic system, the relief valve
controls excess pressure by discharging the excess
gas to the atmosphere.
Figure 6-10.Hydraulic and pneumatic globe valve (rising stem).