Hazards of Chemicals and Gases
The industry recommends that free acetylene should not ordinarily be handled at pressures
greater than 15 psig because, if handled at higher pressures without special equipment, it can
decompose with explosive violence. Consequently, acetylene cylinders are packed with an inert
porous material that is saturated with acetone. Acetylene charged into the cylinder dissolves in
the acetone and in solution will not decompose at or below the maximum authorized shipping
pressure of 250 psig at 70 F.
Compressed or liquified gases are also often described according to loosely-knit families to
which they belong through common origins, properties, or uses. The major families of gases are
atmospheric gases, fuel gases, refrigerant gases, aerosol gases, and poison gases.
Atmospheric gases comprise one family. Its most abundant member is nitrogen, constituting
78 percent of air by volume; oxygen, constituting 21 percent of air by volume, is its second most
abundant member. Most of the remaining 1 percent of the atmosphere consists of a sub-family
of gases, the inert gases, that share the property of chemical inertness. Inert gases are chiefly
argon, with minute quantities of helium, neon, krypton, xenon and radon. The last four are
frequently called the rare gases due to their scarcity. Hydrogen also occurs minutely in the
atmosphere, as do a large variety of trace constituents, small amounts of carbon dioxide, and
large amounts of water vapor.
Another family of gases are the fuel gases. Fuel gases burned in air or with oxygen to produce
heat make up a large family related through their major use. Its members are notably the
hydrocarbons including liquefied petroleum (LP) gases, propane, butane, methane, and welding
gases such as acetylene and hydrogen.
An opposite application relates members of another large family, the refrigerant gases. A
refrigerant gas liquifies easily under pressure and works by being compressed to a liquid which
then absorbs large amounts of heat as it circulates through coils where it vaporizes back into
gaseous form. Examples of refrigerant gases include ammonia and the fluorocarbons (freon).
Aerosol propellant gases make up a family also related by use through the introduction of
pressure-packaged products used in the form of a spray or a foam. Propellant gases have
moderate vapor pressures at room temperatures (70 psig down to 35 psig, and even lower in
some cases). It is usually agreed that a good propellant should also be nontoxic, chemically
stable, noncorrosive, and inexpensive. The fluorocarbons (freon) and nitrous oxide are the most
commonly used propellant gases.
Gases considered to be members of the poison gas family are generally those that the ICC has
classified as poison gases to ensure public safety in interstate shipments. Two examples of these
gases are hydrogen cyanide and phosgene.