Hazards of Chemicals and GasesDOE-HDBK-1015/2-93COMPRESSED GASESRev. 0CH-05Page 17Because 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 simplifythe above definition. Accordingly, a compressed gas is one that gives a pressure reading of:1.either 25 psig (pounds per square inch gage) at 70 F; or 125 psig at 130 F; or2.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 twomajor groups that differ in physical state when contained.1.Non-liquified gases, which are gases that do not liquify in containers at ambienttemperatures and under pressures attained in commercially used containers thatrange to 2000 to 2500 psig.2.Liquified gases, which are gases that do become liquids to a very large extent incontainers at ordinary ambient temperatures and at pressures from 25 to2500 psig.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 boilingpoints. 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 atmospherictemperatures (from about -130 F to 30 F). The liquified gases solidify at cryogenictemperatures.Oxygen, helium and nitrogen are examples of gases in wide use both as non-liquified gases andcryogenic fluids. With respective boiling points of -297 F, -425 F, and -320 F, they arecharged into high pressure steel cylinders at more than 2000 psig at 70 F for shipment and useas non-liquified gases. However, when shipped as cryogenic fluids, they are cooled down toliquid form and charged into special insulated containers that keep them below their boilingpoints 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 thatis dissolved into a solution.
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