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PNEUMATIC GASES
In chapter 1, you learned that many factors are
considered in determining whether to use hydraulics
or pneumatics as a power source in a fluid
power system. Once it is determined that pneumatics
will be used as the source of power, some
of the same factors are considered in selecting
the pneumatic gas.
QUALITIES
The ideal fluid medium for a pneumatic system
is a readily available gas that is nonpoisonous
(nontoxic), chemically stable, free from
any acids that cause corrosion of system components,
and nonflammable. It also will not support
combustion of other elements.
Gases that have these desired qualities may not have
the required lubricating power. Therefore, lubrication
of the components of some pneumatic systems
must be arranged by other means. For example,
some air compressors are provided with a
lubricating system, some components are lubricated
upon installation or, in some cases, lubrication
is introduced into the air supply line. Two
gases meeting these qualities and most commonly
used in pneumatic systems are com-pressed air
and nitrogen.
COMPRESSED AIR
Compressed air is a mixture of all gases contained
in the atmosphere. In this manual, compressed
air is referred to as a gas when it is used
as a fluid medium.
The unlimited supply of air and the ease of compression
make compressed air the most widely used
fluid for pneumatic systems. Although moisture
and solid particles must be removed from
the air, it does not require the extensive distillation
or separation process required in the production
of other gases.
Compressed air has most of the desired properties
and characteristics of a gas for pneumatic
systems. It is nonpoisonous and nonflammable
but does contain oxygen, which supports
combustion. One of the most undesirable qualities
of compressed air as a fluid medium for pneumatic
systems is moisture content. The atmosphere
contains varying amounts of moisture in
vapor form. Changes in the temperature of compressed
air will cause condensation of moisture
in the pneumatic system. This condensed moisture
can be very harmful to the system, as it
increases corrosion, dilutes lubricants, and may freeze
in lines and components during cold weather.
Moisture separators and air driers (dehydrators)
are installed in the compressed air lines
to minimize or eliminate moisture in systems
where moisture would deteriorate system performance.
The supply of compressed air at the required volume
and pressure is provided by an air compressor.
(For information on air compressors, refer
to Naval Ships’ Technical
Manual, chapter 551.)
In most systems the compressor is part of the
system with distribution lines leading from the compressor
to the devices to be operated. In these systems
a receiver is installed in-line between the compressor
and the device to be operated to help eliminate
pulsations in the compressor discharge line,
to act as a storage tank during intervals when the
demand for air exceeds the compressor’s capacity,
and to enable the compressor to shut down
during periods of light load. Other systems receive
their supply from cylinders which must be filled
at a centrally located air compressor and then
connected to the system. Compressed
air systems are categorized by their
operating pressures as follows: high-pressure (HP)
air, medium-pressure (MP) air, and low-pressure (LP)
air.
High-Pressure Air Systems
HP air systems provide compressed air at a nominal
operating pressure of 3000 psi or 5000 psi
and are installed whenever pressure in excess
and
high flow rates of compressed air by the addition
of HP storage flasks to the system. An example
of such a system is one that provides air for
starting diesel and gas turbine engines. Reduction
in pressure, if required, is done by
using specially designed pressure-reducing stations.
Medium-Pressure Air
MP air systems provide compressed air at a nominal
operating pressure of 151 psi to 1000 psi. These
pressures are provided either by an MP air compressor
or by the HP air system supplying air through
an air bank and pressure-reducing stations.
Low-Pressure Air
LP air systems provide compressed air at a nominal
operating pressure of 150 psi and below. The
LP air system is supplied with LP air by LP air
compressors or by the HP air system supplying air
through an air bank and pressure-reducing stations.
LP air is the most extensive and varied air
system used in the Navy, In
addition to being used for various pneumatic
applications, LP and HP compressed air
are used in the production of nitrogen.
NITROGEN
For all practical purposes, nitrogen is considered
to be an inert gas. It is nonflammable, does
not form explosive mixtures with air or oxygen,
and does not cause rust or decay. Due to
these qualities, its use is preferred over compressed
air in many pneumatic systems, especially
aircraft and missile systems, and wherever
an inert gas blanket is required. Nitrogen
is obtained by the fractional distillation
of air. Oxygen/nitrogen-producing plants
expand compressed air until its temperature decreases
to –196°C (–320°F), the boiling point of
nitrogen at atmospheric pressure. The liquid nitrogen
is then directed to a storage tank. A liquid
nitrogen pump pumps the low-pressure liquid
nitrogen from the storage tank and discharges
it as a high-pressure (5000 psi) liquid to
the vaporizer where it is converted to a gas at 5000
psi. Oxygen/nitrogen-producing plants are located
at many naval installations and on submarine
tenders and aircraft carriers.
CONTAMINATION CONTROL
As in hydraulic systems, fluid contamination is
also a leading cause of malfunctions in pneumatic
systems. In addition to the solid particles
of foreign matter which find a way to enter
the system, there is also the problem of moisture.
Most systems are equipped with one or more
devices to remove this contamination. These include
filters, water separators, air dehydrators, and
chemical driers, which are discussed in chapter
9 of this manual. In addition, most systems
contain drain valves at critical low points in
the system. These valves are opened periodically to
allow the escaping gas to purge a large percentage
of the contaminants, both solids and moisture,
from the system. In some systems these valves
are opened and closed automatically, while in
others they must be operated manually. Complete
purging is done by removing lines from
various components throughout the system and
then attempting to pressurize the system, causing
a high rate of airflow through the system. The
airflow will cause the foreign matter to be dislodged
and blown from the system.
NOTE: If an excessive amount
of foreign matter, particularly
oil, is blown from any one system,
the lines and components should be removed
and cleaned or replaced. In
addition to monitoring the devices installed to
remove contamination, it is your responsibility as
a maintenance person or supervisor to control the
contamination. You can do this by using the following
maintenance practices:
1. Keep all tools and the work area in a clean, dirt-free
condition.
2. Cap or plug all lines and fittings immediately
after disconnecting them.
3. Replace all packing and gaskets during assembly
procedures.
4. Connect all parts with care to avoid stripping
metal slivers from threaded areas. Install and
torque all fittings and lines according to applicable
technical instructions.
5. Complete preventive maintenance as specified
by MRCs. Also, you must take care
to ensure that the proper cylinders
are connected to systems being supplied
from cylinders. Cylinders for
compressed air are painted black.
Cylinders containing oil-pumped air have
air
have one green stripe. Oil-pumped air indicates that
the air or nitrogen is compressed by an oil-lubricated
compressor. Air or nitrogen com-pressed by
a water-lubricated (or nonlubricated) compressor
is referred to as water pumped. Oil-pumped
nitrogen can be very dangerous in certain
situations. For example, nitrogen is commonly
used to purge oxygen systems. Oxygen will
not burn, but it supports and accelerates combustion
and will cause oil to burn easily and with
great intensity. Therefore, oil-pumped nitrogen
must never be used to purge oxygen systems.
When the small amount of oil remaining in
the nitrogen comes in contact with the oxygen, an
explosion may result. In all situations, use only
the gas specified by the manufacturer or recommended
by the Navy. Nitrogen cylinders are painted
gray. One black stripe identifies cylinders for
oil-pumped nitrogen, and two black stripes identify
cylinders for water-pumped nitrogen. In addition
to these color codes, the exact identification of
the contents is printed in two locations diametrically
opposite one another along the longitudinal
axis of the cylinder. For compressed air
and nitrogen cylinders, the lettering is white.
POTENTIAL HAZARDS
All compressed gases are hazardous. Compressed air
and nitrogen are neither poisonous nor flammable,
but should not be handled carelessly. Some
pneumatic systems operate at pressures exceeding
3000 psi. Lines and fittings have exploded,
injuring personnel and property. Literally
thousands of careless workers have blown
dust or harmful particles into their eyes by the
careless handling of compressed air outlets. Nitrogen
gas will not support life, and when it
is released in a confined space, it will cause asphyxia
(the loss of consciousness as a result of too
little oxygen and too much carbon dioxide in the
blood). Although compressed air and nitrogen seem
so safe in comparison with other gases, do not
let overconfidence lead to personal injury.
SAFETY PRECAUTIONS
To minimize personal injury and equipment damage
when using compressed gases, observe all practical
operating safety precautions, including the
following:
1. Do not use compressed air to clean parts of
your body or clothing, or to perform general space
cleanup in lieu of vacuuming or sweeping.
2. Never attempt to stop or repair a leak while the
leaking portion is still under pressure. Always isolate,
repressurize and danger tag out the portion
of the system to be repaired. For pressures of
1000 psi or greater, double valve protection is required
to prevent injury if one of the valves should
fail.
3. Avoid the application of heat to the air piping
system or components, and avoid striking a
sharp or heavy blow on any pressurized part of the
piping system.
4. Avoid rapid operation of manual valves. The
heat of compression caused by a sudden high-- pressure
flow into an empty line or vessel can cause
an explosion if oil is present. Valves should be
slowly cracked open until airflow is noted and should
be kept in this position until pressures on both
sides of the valve have equalized. The rate of
pressure rise should be kept under 200 psi per
second, if possible. Valves may then be opened fully.
5. Do not discharge large quantities of nitrogen
into closed compartments unless adequate
ventilation is provided. 6. Do not
subject compressed gas cylinders to
temperatures greater than 130°F. Remember,
any pressurized system can be hazardous
to your health if it is not maintained and
operated carefully and safely.
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