require small amounts of air at lower pressures, which
MAINTENANCE OF LP AND HP AIR
is supplied through pressure-reducing stations.
Always use caution with HP systems! When HP air
enters suddenly into pockets or dead ends, the air
temperature in the confined space increases
dramatically. If there is any combustible material in the
space and the air temperature increases to the ignition
point of the material, an explosion may occur.
Explosions of this type may set up shock waves that
travel through the compressed air system. This travel
may cause explosions at remote points. Even a small
amount of oil residue or a small cotton thread may be
sufficient to cause ignition.
Follow the scheduled maintenance of LP or HP air
dehydrators according to the PMS requirements. The
following is a sample LP air dehydrator maintenance
schedule and is for general information only:
- Check applicable power on lights, flowmeter
readings, cooling water temperatures, heater
temperatures, and outlet air temperatures for proper
Some common pressure requirements for HP
systems may be as follows:
600 to 3,000 psig
100, 750, 1,500, 2,000,
and 4,500 psig
Over 3,000 psig
Up to 6,000 psig
- Note any dehydrator filter element pressure
drops for element replacement.
- Periodically blow down and clean the conden-
ser water strainer.
Figures 6-5 and 6-6 show a maintenance index page
(MIP) for one design and make of an HP air compressor
system. This will give you an idea of the differences in
planned maintenance requirements between the LP air
(see figs. 6-2, 6-3, and 6-4 for comparison) and HP air
systems. It is very important that the PMS you are using
are the correct ones.
- Blow down type I and type III dehydrators.
Dump the valve if more than 1/2 pint of water drains
out; the automatic feature is not working.
- Check the purge pressure and the free move-
ment of the purge flowmeter float of type II and type III
Blow down the inlet separator,
prefilter, and trap dump valve of dehydrators by opening
the manual drain valve.
You should inspect air flasks, receivers, separators,
and piping for damage or external corrosion once every
6 months. Enter the inspection date and results in the
Maintenance and Material Management (3-M) Systems
by submitting a work request for any discrepancies
found. You must document completion of all inspection
results through PMS.
- Check the outlet air moisture content. The dew
point should be below -40°F at 80 lb/in2 for both type II
and type III dehydrators, and below 40°F at 80 lb/in for
type I dehydrators. An excessive dew point indicates a
AIR DRYERS OR DEHYDRATORS
- Check the inlet and outlet filters and the purge
filter for the type II and type III dehydrators. Replace
the filter elements if necessary.
- Clean the condenser water tubes.
The Navy uses three types of air dehydrators for
drying LP air. These are the refrigeration (type I),
desiccant (type II), and combination of both
refrigeration and desiccant (type III). HP air application
uses only the type III. The Navy is replacing the various
types of desiccant used in the fleet with activated
alumina beads in 1/8-inch diameter spheres. This type
of desiccant is also intended to reduce dust problems
produced by the other various types. The dust causes
clogged filters and other component malfunctions.
- Disassemble and clean the inlet and interstage
separators and purge the solenoid valves.
- Remove and calibrate the pressure gauges.
Adjust them to give maximum error 2 lb/in2 (1 percent
of full scale).
- Disassemble the desiccant chambers. Clean
the assembly of dust, oil, and dirt. Replace the desiccant