Figure 4-6.Lift-check valve.
Ball-check valves handle viscous fluids and are
very efficient in lines that contain scale and other
debris. Because the ball-check valve operates quietly,
it is recommended for use in lines that contain fluids
where pressure changes rapidly.
As we have seen so far, most valves are classified
as either stop valves or check valves; however, some
valves function either as a stop valve or as a check
valve, depending upon the position of the valve stem.
These valves are known as STOP-CHECK VALVES.
The cross section of two stop-check valves is
shown in figure 4-7. As you can see, this type of valve
looks much like a lift-check valve. The valve stem is
long enough so when it is screwed all the way down,
it holds the disk firmly against the seat, thereby
preventing the flow of any fluid. In this position, the
valve acts as a stop valve. When the stem is raised, the
disk can then be opened by pressure on the inlet side.
In this position, the valve acts as a check valve and
allows the flow of fluid in one direction only. The
amount of fluid allowed to pass through is regulated
by the opening. The opening is adjusted by the stem.
Pressure-reducing valves are automatic valves
used to provide a steady pressure lower than that of the
supply pressure. Pressure-reducing valves can be set
for any desired discharge pressure that is within the
limits of the design.
Several types of reducing valves are used in the
Navy; however, you will be working mostly with
those in the water service system. These are normally
single-seated, direct-acting, and spring-loaded, as
shown in figure 4-8. Water passing through this valve
Figure 4-7.Step-check valve.