2.0 Classification of Gases

Major Groups of Compressed Gases

"Compressed Gas" is a generic term used for describing 1) compressed gases, 2) liquefied compressed gases, 3) refrigerated liquefied gases (cryogenic gases), and 4) dissolved gases.

Compressed Gases

Compressed gases are non-liquefied gases. Compressed gases do not become liquid at normal temperatures, even at very high pressures. Examples include helium, nitrogen, oxygen, and argon.
A compressed gas is either:

  • A gas or mixture of gases having, in a container, an absolute pressure exceeding 40 pounds per square inch (psi) at 70° F (21° C);
  • A gas or mixture of gases having, in a container, an absolute pressure exceeding 104 psi at 130° F regardless of the pressure at 70° F(21° C); or
  • A liquid having a vapor pressure exceeding 40 psi at 100° F (38° C); as determined by ASTM D-323-72.

Compressed gases are stored in heavy-walled metal cylinders that are manufactured to withstand high pressure.  Compressed gas cylinders are available in different sizes (1 to 6 feet length). Compressed gases are filled under high pressure exceeding 2000 psi.

Gases are classified to meet US department of Transportation (DOT) and IATA regulations. The DOT has adopted GHS guidelines for classification, labeling/signage and safety data sheets (SDSs), and compressed gases are classified as Class 2, Class 5, and/or Class 6 gases. OSHA and DOT pictograms that are used for the classification of compressed gases are given below.

OSHA and DOT Pictograms for Compressed Gases

The intrinsic properties of a gaseous chemical inside the cylinder may present a hazardous situation when released into the atmosphere or work areas.

Hazardous properties of compressed gases are classified as below:

(1) Corrosive reactions to equipment and human tissue;
(2) Generation of flammable gas and acutely toxic intermediates (poison);
(3) Oxidizing reactions; and
(4) Generation of an asphyxiating environment;

The GHS classification of commonly used general and specialty gases, along with the intrinsic hazardous properties and respective pictograms, are summarized in Appendix A.

Liquefied Gases

Liquefied gases are gases that become liquid at normal temperatures when they are pressurized inside a gas cylinder. The cylinder is initially filled as a liquid. The liquid then evaporates to a gas and saturates the head space above the liquid and maintains liquid-vapor equilibrium. As gas is released from the cylinder, enough liquid evaporates to the head space, thus keeping the pressure in the cylinder constant. Examples of liquefied gases include ammonia, carbon dioxide, chlorine, methane, natural gas, and propane.

Refrigerated Liquefied Gases (Also Known as Cryogenic Liquids)

Refrigerated liquefied gases also known as cryogenic liquids are kept in their liquid state at very low temperatures. Refrigerated liquefied gases are extremely cold with boiling points below - 150° C. Refrigerated liquefied gases are heavier than air under cold temperature conditions and can accumulate near the floor. The vapors and gases released from refrigerated liquefied gases can be extremely cold and can result in frost bites and blisters. Small amounts of refrigerated liquefied gases liquid can expand into very large volumes of gas.

Examples of refrigerated liquefied gases include liquid helium, liquid nitrogen, and liquid argon. Carbon dioxide and nitrous oxide, which have slightly higher boiling points, are also included in this category.

Dissolved Gases

Acetylene is the only commonly used dissolved gas. It is chemically very unstable. Even at atmospheric pressure, acetylene can explode. Yet, acetylene is routinely stored and used safely in cylinders at high pressures, up to 250 psi at 21° C. If an acetylene cylinder has been accidentally left on its side, set the cylinder in upright position for at least an hour before use. Otherwise, dissolved gases have the potential to emit a burst of liquid solvent instead of gas when the valve is opened.