Hazards of Chemicals and Gases
Because we often deal in gage pressures, and absolute pressure is equal to atmospheric pressure
(14.7 psi at sea level) plus the pressure that would be read on an ordinary gage, we can simplify
the above definition. Accordingly, a compressed gas is one that gives a pressure reading of:
either 25 psig (pounds per square inch gage) at 70 F; or 125 psig at 130 F; or
if the contained substance is flammable, 25 psig at 100 F.
This simplified definition and the range of boiling points among gases classify gases into two
major groups that differ in physical state when contained.
Non-liquified gases, which are gases that do not liquify in containers at ambient
temperatures and under pressures attained in commercially used containers that
range to 2000 to 2500 psig.
Liquified gases, which are gases that do become liquids to a very large extent in
containers at ordinary ambient temperatures and at pressures from 25 to
The first group, commonly called non-liquified gases, have relatively low boiling points,
approximately -150 F or lower.
Non-liquified gases do, however, become liquids if cooled to temperatures below their boiling
points. Those that liquify at "cryogenic" temperatures (from absolute zero [-459.7 F] to around
-240 F) are known as cryogenic fluids.
The second group, or liquified gases, have boiling points relatively near atmospheric
temperatures (from about -130 F to 30 F). The liquified gases solidify at cryogenic
Oxygen, helium and nitrogen are examples of gases in wide use both as non-liquified gases and
cryogenic fluids. With respective boiling points of -297 F, -425 F, and -320 F, they are
charged into high pressure steel cylinders at more than 2000 psig at 70 F for shipment and use
as non-liquified gases. However, when shipped as cryogenic fluids, they are cooled down to
liquid form and charged into special insulated containers that keep them below their boiling
points and are contained at pressures normally less than 75 psig.
A third physical state in the container is represented by only one widely used gas, acetylene.
Acetylene is sometimes referred to as a dissolved gas. A dissolved gas is defined as a gas that
is dissolved into a solution.