sufficiently cooled, as by dowsing it with water, it will go
out. If a carbon dioxide fire extinguisher is used to
remove the oxygen around the fire, the fire goes out.
But, as the fraction and partial pressure of oxygen
increase, many materials that do not burn under normal
conditions will burn if ignited. Also, any fuels will
ignite more easily. Materials that are of little concern in
air may become quite flammable in an oxygen-rich
environment and even more so in a high-pressure oxygen
rich environment. Petroleum-based products and
other hydrocarbons are of special concern because they
ignite extremely easily in a pure oxygen environment.
Other materials, although they may not become flammable,
will oxidize and degrade much more rapidly in
the presence of hyperbaric oxygen. When high pressures
of oxygen are going to be present, extra care must be
taken to prevent the fire triangle from occurring.
EQUIPMENT CONSIDERATIONS
The ways that nitrox is prepared will be discussed
later, but the most common mixing method involves
introduction of pure oxygen at pressure. All equipment
systems that will be so exposed must be specially prepared
in order to withstand the oxygen. Hydrocarbons
and petroleum-based products must be avoided. This
includes not only petroleum-based compressor lubricants
but also the silicone lubricants normally used in
scuba air systems. Neoprene, silicone “rubber,” plastic
and metal shavings, even finely divided particulate matter
all become potential fuels for a fire in an oxygenrich
environment, especially one at high pressure. A
spectacular laboratory demonstration is to burn steel
wool in a high-oxygen environment.
OXYGEN CLEANING
All equipment to be used with pure oxygen must be
cleaned for oxygen service. The equipment itself must
be “oxygen clean,” and “oxygen compatible”
Chapter 6 - Oxygen Precautions and Preparing Nitrox
parts must be used in order to minimize the risk of fire
or destruction by the oxygen. Oxygen clean means that
any potentially flammable contaminants have been
removed. Oxygen compatible means that the materials
used in the various parts in the system are not flammable
or readily oxidizable in the presence of high-pressure
or pure oxygen. This special oxygen cleaning can be
complex, involving an initial cleaning using solvents
that remove all hydrocarbons and then the use of special
oxygen-compatible lubricants, o-rings, seats, seals, flexible
tubing, etc. as the system is reassembled. Even
though certain materials may not be flammable in the
presence of high-pressure oxygen, more durable materials
that do not so rapidly degrade will be substituted.
“Formal oxygen cleaning,” which is required in many
industrial and governmental (e.g., NASA) applications,
requires adherence to very strict procedures as well as
careful documentation. After cleaning, future contamination
must be carefully avoided, or the whole oxygen
cleaning process will have to be repeated. Even skin oil
from a finger can contaminate formally oxygen-cleaned
equipment and necessitate re-cleaning.
Care must also be taken to avoid sources of ignition,
the third side of the fire triangle. A sudden increase
in system pressure could elevate the temperature in the
system sufficiently to cause ignition of any contaminants.
Valves should be opened slowly to prevent a sudden
increase in pressure with an accompanying rapid
temperature rise. Even a static spark from a rug or opening
a Velcro® closure could cause ignition in a pure
oxygen environment.
THE 40% RULE
When is oxygen cleaning necessary? As a recreational
nitrox diver, you will dive with an ordinary, wellmaintained
regulator, but your cylinder will have been
cleaned for designated service as a nitrox cylinder, and it
will be clearly marked as an enriched-air nitrox cylinder.
The general rule of thumb is that any equipment
that is to be used with pure oxygen or an oxygen level
that is above 40% (and at a pressure above 200 psi)
Oxygen Precautions and Preparing Nitrox
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