As we have seen, most valves can be classified
start the flow of fluid. The ball (fig. 6-5) performs
as being either stop valves or check valves. Some
the same function as the disk in the globe valve.
valves, however, function either as stop valves or
When the valve handle is operated to open the
as check valves-depending on the position of the
valve, the ball rotates to a point where the hole
valve stem. These valves are known as STOP-
through the ball is in line with the valve body
CHECK VALVES.
inlet and outlet, When the valve is shut, which
A stop-check valve is shown in cross section
requires only a 90 rotation of the handwheel for
in figure 6-6. This type of valve looks very much
most valves, the ball is rotated so the hole is
like a lift-check valve. However, the valve stem
perpendicular to the flow openings of the valve
is long enough so when it is screwed all the way
body, and flow is stopped.
down it holds the disk firmly against the seat, thus
Most ball valves are of the quick-acting type
preventing any flow of fluid. In this position, the
(requiring only a 90 turn to operate the valve
valve acts as a stop valve. When the stem is raised,
either completely open or closed), but many are
the disk can be opened by pressure on the inlet
planetary gear operated. This type of gearing
side. In this position, the valve acts as a check
allows the use of a relatively small handwheel and
valve, allowing the flow of fluid in only one
operating force to operate a fairly large valve. The
direction. The maximum lift of the disk is
gearing does, however, increase the operating time
controlled by the position of the valve stem.
for the valve. Some ball valves contain a swing
Therefore, the position of the valve stem limits
check located within the ball to give the valve a
the amount of fluid passing through the valve
check valve feature. Ball valves are normally
even when the valve is operating as a check
found in the following systems aboard ship:
valve.
seawater, sanitary, trim and drain, air, hydraulic,
Stop-check valves are widely used throughout
and oil transfer.
the engineering plant. Stop-check valves are
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 of the swing type, lift type, or ball
type.
Figure 6-6.--Stop-check valve.
Figure 6-5.--Typical seawater ball valve.
6-4
