valves are also commonly called transfer valves,
maintenance instructions should be consulted to
determine filter element requirements.
selector valves, or control valves.
Filter assemblies used in support equipment vary
It is beyond the scope of this course to cover all of
in physical size according to their maximum flow
the many varieties of valves in use today. However,
rating. Common sizes in use are 10, 20, and 30 gal/min,
since more elaborate valves are combinations or
with the flow rating determining the overall length of
variations of basic types, only the basic types are
the elements used and the filter bowl. To minimize
covered here. For a more detailed study of the
supply requirements, at least one manufacturer
operation of the various types of valves, consult Fluid
provides non-cleanable primary stage elements in the
Power, NAVEDTRA 14105.
form of 10 gal/min cartridges capable of being
Valves, like pumps, are precision made. Usually,
connected together with a simple coupling device. By
no packing is required to prevent leaking between
joining the required number of 10 gal/min elements
valve elements and valve seats because of their closely
together, 20 or 30 gal/min cartridges can be locally
machined clearances. This is one of the reasons why
assembled. All of the support equipment filter
minimizing system contamination is so important, as
assemblies discussed are of the non-bypass type and
even the most minute particles of dirt, dust, and lint can
are equipped with differential pressure indicators to
defeat the seal between valve elements and valve seats.
warn of a loaded element. The differential pressure
indicators are, in most cases, preset to activate with an
Flow Control Valves
element pressure drop of 100 15 psi, which represents
a p p r o x i m a t e l y 9 0 p e r c e n t o f t h e fi l t e r 's t o t a l
The ordinary water faucet is a good example of a
dirt-holding capacity.
flow control valve. Turning the faucet handle changes
the opening of the valve, which controls the amount of
Unlike most filter elements, 3-micron,
water that passes. Many of the flow control valves used
high-pressure support equipment filters are not
in fluid power systems are similar to the water faucet.
normally replaced on a prescribed periodic basis.
Because of their large dirt-holding capacity and nature
Many flow control valves are operated manually,
of service, it is more effective to replace such elements
similar to the operation of a water faucet. For example,
only when indicated as being loaded by their
the release valve in a hydraulic jack system is a flow
associated differential pressure indicators. Element
control valve of this type. The release valve is closed
replacement procedures vary with the particular type,
during the raising operation, and is then opened during
and applicable maintenance instructions should be
the lowering operation. The speed of the lowering
consulted for specific procedures.
operation depends upon the rate of flow of the fluid
through the release valve, which, in turn, depends upon
VALVES
the amount that the release valve is opened.
A restrictor is another type of flow control valve
Once the pump has begun to move the fluid in a
used in fluid power systems. Its purpose is to limit the
hydraulic system, valves are usually required to
speed of an actuator. (An actuator is a device that
control, monitor, and regulate the operation of the
transforms fluid pressure into mechanical force to
system. While the pump is recognized as the heart of a
move some mechanism. A hydraulic motor is one type
system, the valves are very important devices for
of actuator, as it converts fluid under pressure into
providing flexibility.
rotary motion. Another example of an actuator is the
A valve is a device that controls the flow or
lift cylinder of a forklift.)
pressure of fluid in a fluid power system. Valves are
A two-way restrictor restricts the flow of fluid in
usually classified as to the type of control they provide:
both directions. This type of restrictor is simply a valve
Flow control valves--Control the volume of
containing a small orifice for the passage of fluid. The
fluid going to various parts of the system.
rate of flow through the valve is determined by the size
of the orifice. In some two-way restrictors, the size of
Pressure control valves--Regulate pressure in
the orifice is adjustable. This type is called a variable
the system.
restrictor.
Directional control valves--Direct a flow of
In some cases it is necessary to control the speed of
fluid to and from an actuator. Directional control
an actuator in only one direction. For example, the lift
8-21
