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GLOBE VALVES
Globe valves are probably the most common valves
in existence. The globe valve gets its name
Figure 6-5.—Rising stem gate
valve.
Figure 6-6.—Types of globe valve
bodies.
from the globular shape of the valve body. Other types
of valves may also have globular-shaped bodies.
Thus, it is the internal structure of the valve
that identifies the type of valve. The
inlet and outlet openings for globe valves are
arranged in a way to satisfy the flow requirements.
Figure 6-6 shows straight-, angle-, and
cross-flow valves. The moving parts
of a globe valve consist of the
disk, the valve stem, and the handwheel. The stem
connects the handwheel and the disk. It is threaded
and fits into the threads in the valve bonnet.
The part of the globe valve that controls flow is
the disk, which is attached to the valve stem. (Disks
are available in various designs.) The valve is
closed by turning the valve stem in until the disk is
seated into the valve seat. This prevents fluid from
flowing through the valve (fig. 6-7, view A). The
edge of the disk and the seat are very accurately
machined so that they forma tight seal when
the valve is closed. When the valve is open (fig.
6-7, view B), the fluid flows through the space between
the edge of the disk and the seat. Since the
fluid flows equally on all sides of the center of
support when the valve is open, there is no unbalanced
pressure on the disk to cause uneven wear.
The rate at which fluid flows through the valve
is regulated by the position of the disk in relation
to the seat. The valve is commonly used as
a fully open or fully closed valve, but it may be
used as a throttle valve. However, since the seating
surface is a relatively large area, it is not suitable
as a throttle valve, where fine adjustments are
required in controlling the rate of flow. The
globe valve should never be jammed in the
open position. After a valve is fully opened, the
handwheel should be turned toward the closed position
approximately one-half turn. Unless this is
done, the valve is likely to seize in the open position,
making it difficult, if not impossible, to close
the valve. Many valves are damaged in this
Figure 6-7.—Operation of a globe valve.
manner. Another reason for not leaving globe valves
in the fully open position is that it is sometimes
difficult to determine if the valve is open
or closed. If the valve is jammed in the open position,
the stem may be damaged or broken by someone
who thinks the valve is closed, and attempts
to open it.
It is important that globe valves be installed with
the pressure against the face of the disk to keep
the system pressure away from the stem packing
when the valve is shut.
NEEDLE VALVES
Needle valves are similar in design and operation
to the globe valve. Instead of a disk, a
needle valve has a long tapered point at the end of
the valve stem. A cross-sectional view of a needle
valve is illustrated in figure 6-8. The
long taper of the valve element permits a
much smaller seating surface area than that of the
globe valve; therefore, the needle valve is more suitable
as a throttle valve. Needle valves are used to
control flow into delicate gauges, which might
be damaged by sudden surges of fluid under
Figure 6-8. —Cross-sectional view of a needle valve.
pressure. Needle valves are also used to control the
end of a work cycle, where it is desirable for motion
to be brought slowly to a halt, and at other points
where precise adjustments of flow are necessary
and where a small rate of flow is desired.
Although many of the needle valves used in fluid
power systems are the manually operated type
(fig. 6-8), modifications of this type of valve are
often used as variable restrictors. This valve is constructed
without a handwheel and is adjusted to
provide a specific rate of flow. This rate of flow will
provide a desired time of operation for a particular
subsystem. Since this type of valve can be
adjusted to conform to the requirements of a particular
system, it can be used in a variety of systems.
Figure 6-9 illustrates a needle valve that was
modified as a variable restrictor.
HYDRAULIC AND PNEUMATIC
GLOBE
VALVES
The valve consists of a valve body and a stem
cartridge
assembly. The stem cartridge assembly includes
the bonnet, gland nut, packing, packing retainer,
handle, stem, and seat. On small valves (1/8
and 1/4 inch) the stem is made in one piece, but
on larger sizes it is made of a stem, guide, and
stem retainer. The valve disk is made of nylon and
is swaged into either the stem, for 1/8- and 1/4-inch
valves, or the guide, for larger valves. The
bonnet screws into the valve body with left-hand
threads and is sealed by an O-ring (including
a back-up ring).
Figure 6-9.—Variable restrictor.
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.
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