of 1000 psi is required. HP compressed air plants
support functions which require high pressures
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
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
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.
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.
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,
2. Cap or plug all lines and fittings
immediately after disconnecting them.
3. Replace all packing and gaskets during
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