the hole is perpendicular to the flow openings of
the valve body, and flow is stopped.
Most ball valves are of the quick-acting type
(requiring only a 90-degree turn to operate the
valve either completely open or closed), but many
are planetary gear operated. This type of gearing
allows the use of a relatively small handwheel and
operating force to operate a fairly large valve. The
gearing does, however, increase the operating time
for the valve. Some ball valves contain a swing
check located within the ball to give the valve a
check valve feature. Ball valves are normally
found in the following systems aboard ship:
seawater, sanitary, trim and drain, air, hydraulic,
and oil transfer.
Check Valves
Check valves are used to allow fluid flow in
a system in only one direction. They are operated
by the flow of fluid in the piping. A check valve
may be the swing type, lift type, or ball type.
As we have seen, most valves can be classified
as being either stop valves or check valves. Some
valves, however, function either as stop valves or
as check valves—depending on the position of the
valve stem. These valves are known as STOP-
CHECK VALVES.
A stop-check valve is shown in cross section
in figure 9-23. This type of valve looks very much
like a lift-check valve. However, the valve stem
is long enough so when it is screwed all the way
down it holds the disk firmly against the seat, thus
preventing any flow of fluid. In this position, the
valve acts as a stop valve. When the stem is raised,
the disk can be opened by pressure on the inlet
side. In this position, the valve acts as a check
valve, allowing the flow of fluid in only one
direction. The m a x i m um lift of the disk is
controlled by the position of the valve stem.
Therefore, the position of the valve stem limits
the amount of fluid passing through the valve even
when the valve is operating as a check valve.
Stop-check valves are widely used throughout
the engineering plant. Stop-check valves are used
in many drain lines and on the discharge side of
many pumps.
Special-Purpose Valves
There are many types of automatic pressure
control valves. Some of them merely provide an
escape for pressures exceeding the normal
pressure; some provide only for the reduction of
pressure; and some provide for the regulation of
pressure.
Figure 9-23.—Stop-check valve.
Figure 9-24.—Typical relief valve.
9-16
