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SAFETY
PRECAUTIONS FOR THE SAFE HANDLING AND
STORAGE OF LIQUEFIED GASES
WARNING
Read
and completely familiarize all personnel with handling procedures.
Introduction
The category
of substances known as liquefied gases includes liquid nitrogen,
oxygen, argon, helium and carbon dioxide but those are only
a few of the most common ones. There are many others. Liquefied
gases are extremely cold liquid. For example, at atmospheric
pressures liquid oxygen exists at -297.3°F. Liquefied
gases, because of their peculiar nature, require special attention.
The following paragraphs outline properties, precautions and
safe handling for liquefied gases.
General
To insure
safe control of liquefied gases in laboratories, test stations
or wherever these liquids are used, all orders for these materials
should be cleared through a responsible person. This indiividual
will insure that the potential user is aware of the danger
involved and will follow recommended procedures.
Liquefied
gases should never be used in combination with other substances
without knowing what the result may be. When in doubt, consult
a competent authority.
Purity
Liquid
nitrogen, when placed in the container at the manufacturer's
plant is of a definite purity but this purity is subject to
change since the nitrogen evaporates in preference to the
very small oxygen impurity. If liquid nitrogen remains in
the container until a large portion of the liquid is evaporated,
an analysis of the remaining liquid should be made before
it is used for any purpose where a high oxygen impurity or
a high oxygen content would be dangerous.
Toxicity
Oxygen
is nontoxic. Nitrogen, helium and argon are also nontoxic,
but if allowed to accumulate in sufficient quantities they
may act as asphyxiates. This is because these gases lower
the concentration of oxygen that is normally present in the
air. (For this reason, liquid nitrogen, helium and argon should
never be stored or used in small closed compartments, rooms
or excavations without added ventilation. Well-ventilated
storage and working space should be provided.)
Combustibility
While
oxygen itself does not burn, it does provide an atmosphere
that sustains combustion. Oxygen in liquid form can promote
intense combustion of explosive violence. For this reason,
liquid oxygen must never be stored or used in containers contaminated
with oil, grease or carbonaceous materials of any kind. A
serious fire may result from disregarding these guidelines.
In the
presence of an appreciable oxygen concentration, a spark on
certain materials may cause them to burst into flame, whereas
in air, fire would not result. (For this reason, liquid oxygen
should never be stored or used in small closed compartments,
rooms or excavations without added ventilation. Well ventilated
storage and working space should be provided.) Materials that
should be of special concern in this respect are wood, plastic,
powdered metals, combustible rags and clothing. Anyone working
with liquid oxygen should never allow one's clothing to become
saturated with liquid or gaseous oxygen, as a spark may cause
the clothing to burst into flames.
Pressure
Buildup
The heats
of vaporization of most liquefied gases are low. In addition,
a small quantity of liquid produces a large volume of gas
at atmospheric pressure. One cubic foot of oxygen, for example,
will produce 860 cubic feet of oxygen gas. Small heat flow
from the atmosphere into the liquid, therefore, will produce
an appreciable volume of gas. For this reason, all storage
vessels should be provided with pressure relief devices unless
the container is vented properly to provide escape of evaporating
gases. All lines and vessels in which the liquid may be trapped
between closed valves should be equipped with pressure relief
valves. If there is any likelihood that the relief valve may
freeze, as for instance, from ice formed from dripping water
or condensed moisture, such vessels and lines should be equipped
with rupture discs. Both pressure relief valves and rupture
discs should be placed and protected so that water cannot
splash or condense upon them. In addition, it is desirable
and sometimes necessary, to vent relief valves and rupture
discs to the outside atmosphere.
Liquefied
gases should be transported only in suitable insulated containers
that provide means for the escape of gas as liquid evaporates.
Never cork or plug the outlet to such containers.
The use
of liquefied gases may require other precautions to be taken.
It is imperative that all persons using that material be made
aware of these precautions.
Handling
Personnel
handling liquefied gases should be thoroughly instructed as
to the nature of the materials. Training is essential to minimize
accidental spilling. This is to prevent damage from the coldness
of the liquid or from the fire hazard of the oxygen enriched
air.
Small
amounts of liquefied gases are frequently handled in glass
dewar flasks which occasionally collapse, particularly if
the liquid oxygen is splashed on the joint at the neck. These
flasks should always be kept behind protective shields while
in use.
Liquefied
gases, because of their extremely low temperature, will "burn"
the skin like hot liquids. Never permit liquefied gases to
come into contact with the skin or allow liquid oxygen or
liquid nitrogen to soak clothing. Serious burns may result
from careless handling.
When personnel
are handling liquefied gases, they are advised to protect
themselves by wearing goggles or face shields and leather
gloves large enough to allow quick removal. Rubber aprons
and high-topped shoes worn with trouser legs outside the tops
are also desirable.
Liquid
oxygen must never be poured upon clothing, fabrics, rags,
waste or other readily combustible materials, nor the gaseous
oxygen arising from liquid oxygen be allowed to penetrate
clothing. Combustible substances in the presence of oxygen
are highly flammable. A spark can start a serious fire and
may cause serious personal injury.
Liquid
oxygen should never be poured or demonstrated in close proximity
to a source of ignition. A spark coming into contact with
a combustible material in an oxygen-enriched atmosphere can
burst into flames and immediately cover the surface of the
combustible material.
When pouring
liquefied gases from one container to another, the receiving
container should be cooled gradually to prevent thermal shock.
The liquid should be poured slowly to avoid spattering. The
receiving vessel should always be vented to the atmosphere
and high concentrations of gaseous oxygen and/or nitrogen
should not be allowed to collect.
Introduction
of a substance that is at normal room temperature into a liquefied
gas is always somewhat hazardous. There is a violent evolution
of gas, and there is likely to be considerable splashing of
the liquid. Personnel doing this work should be instructed
of the hazard and should always wear full-face shield and
protective clothing.
In the
event a person is burned by liquefied gas, the following first
aid treatment should be given pending the arrival and care
of a physician:
1. If
any liquid gas contacts the skin or eyes, immediately flood
that area of the body with large amounts of unheated water
and then protect frozen parts with loose, bulky, dry, sterile
dressings.
2. If
the skin is blistered or there is any chance that the eyes
have been affected, get the patient immediately to a physician
for treatment.
Material
Limitations
The physical
properties of many materials at extremely low temperatures
may be quite different from the properties of the same materials
at normal temperatures. Therefore, materials that have been
cooled to the temperatures of liquid oxygen or liquid nitrogen
should be carefully handled until their properties, under
these conditions, are known.
Metals
to be used for equipment in liquid oxygen or liquid nitrogen,
must possess satisfactory physical properties at the low operating
temperatures. Since ordinary carbon steels, and to a lesser
extent most alloy steels, lose their ductility when subjected
to the low temperatures of liquid oxygen or liquid nitrogen,
they are considered unsatisfactory for such service. The austenitic
nickel-chromium alloys have good ductility at the low service
temperatures under consideration, and the most widely known
is 18-8 stainless steel. Cooper, monel, brass and aluminum
are also considered satisfactory materials for low temperature
use.
Each new
use for these liquids should be carefully considered before
it is instituted and safety precautions should be completely
outlined.
WARNING
Inert
gases released in a confined space can displace sufficient
air to make the atmosphere incapable of sustaining life. Entering
an oxygen deficient atmosphere may cause unconsciousness without
warning. Purge the space completely with air and test before
entry. Wear an air respirator and have helper stand by also
equipped with an air respirator.
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