the equipment used to service aircraft pneumatic and
compressor was used in the processing. This term is
life support systems. There are many similarities
used only when the gas is normally supplied in both O.F.
between hydraulic and pneumatic systems.
and O.T. grades. A gas cylinder that is not labeled with
O.F. or O.T. is presumed to be O.T. Oil-tolerant nitrogen
For all practical purposes, nitrogen is considered
to be an inert gas. (Inert is defined as chemically
may be used when there is no potential danger of
inactive, not combining with other chemicals.)
contaminating systems with oil. Class 2 gaseous
However, nitrogen is not totally inert, like helium or
nitrogen is stored in cylinders that are gray with one
a rg o n , f o r t h e r e a r e m a ny n i t r o g e n d e r iva t ive
black band near the top.
compounds. One such compound is nitrate, which is
Class 2 (oil-pumped) nitrogen can be dangerous in
used in fertilizers and explosives. Under natural
certain situations. For example, if oil-pumped nitrogen
c o n d i t i o n s , n i t r o g e n i s ve r y s l ow t o c o m b i n e
were used to inflate a tire, an oil film (hydrocarbon)
chemically with other elements. There are, however,
may build up on the inside walls of the tire, soaking
chemical and electrical processes by which nitrogen
into the pores of the rubber. This should not hurt the
can be combined with other substances. Such
tire, and by itself does not present a combustion hazard.
processes are called nitrogen fixation.
However, if at a later time, nitrogen is not available and
Although 80 percent of our atmosphere is pure
compressed air (the approved alternative) is used to
nitrogen, as a gas it does not support life. When
inflate the tire, the hydrocarbon film will come in
released in a confined space, nitrogen causes asphyxia
c o n t a c t w i t h c o m p r e s s e d a i r. A m i x t u r e o f
(the loss of consciousness as a result of too little
hydrocarbons and compressed air is definitely
oxygen and too much carbon dioxide in the blood).
combustible.
Nitrogen is not poisonous, but unless oxygen is mixed
with it, it cannot support life. Also, nitrogen gas does
WARNING
not support fires, cause rust, or decay most of the
materials with which it comes in contact. Because of
Oil-pumped nitrogen must NEVER be used to
these properties, nitrogen is preferred over compressed
purge oxygen systems. An explosion could result.
air in pneumatic systems, especially aircraft and
missile servicing systems.
Nitrogen is obtained by fractional distillation of
When oil-pumped nitrogen is available, there is
air. On some ships, particularly aircraft carriers,
always the chance that someone will unwittingly use it
nitrogen is obtained as a by-product of oxygen
to purge an oxygen system. And although oxygen does
production. In addition to being used in pneumatic
not burn, it does support and accelerate combustion
systems, nitrogen is used extensively in ships as an
and causes oil to burn more easily and with greater
inert blanket over aviation fuel and other special fuels
intensity. For this reason, oil-pumped nitrogen is
in their storage tanks.
never used to purge oxygen systems. If the small
amount of oil remaining in the nitrogen were to come
There are two classes of gaseous nitrogen, and
in contact with the oxygen, an explosion could result.
both are available in military supply.
C l a s s 1 , o i l - f r e e ( O . F. ) ( f o r m e r l y
Safety rules concerning the use of gaseous
nitrogen are similar to those cited for other
water-pumped). Most nitrogen gases are manufactured
nonflammable, nonoxidizing, and nontoxic
from liquid nitrogen as a dry, oil-free gas. The term dry,
compressed gases.
when applied to a gas, means that the gas has a moisture
content of less than 0.02 milligrams per liter. Oil-free
Q9-2. Which of the following qualities of nitrogen
means that the gas has not been in contact with oil
makes it desirable to be used for servicing
during processing, and that it was either compressed by
aircraft systems?
a water-lubricated compressor or is produced from a
1. It contains oil for proper lubrication of
liquid that is oil-free. Most oil-free gases are also dry.
aircraft pneumatic systems
Class 1 gaseous nitrogen is stored in cylinders that are
2. It is considered an inert gas and is
gray with two black bands near the top.
chemically inactive
Cl a s s 2 , o i l - t o l e r a n t ( O . T. ) ( f o r m e r l y
3. It does not contain corrosion by-products
4. It is less compressible than atmospheric
oil-pumped). The term oil-tolerant means that traces of
air and is moisture free
oil may be carried in the gas because an oil-lubricated
9-2
