Cryogenic Liquids

A cryogenic liquid is defined as a liquid with a boiling point below -130oF (-90oC).  Common cryogens and their properties are summarized in Table 1. 

Table 1

Cryogen Boiling point (1 atm) oC (oF) Critical pressure psiga Liquid density, g/L Gas density (27oC), g/L Liquid-to-gas expansion ratio Type of gas
Ar -186(-303) 710 1402 1.63 860 Inert
He -269(-452) 34 125 0.16 780 Inert
H2 -253(-423) 188 71 0.082 865 Flammable
N2 -196(-321) 492 808 2.25 710 Inert
O2 -183(-297) 736 1410 1.4 875 Oxidizerb
CH4 -161(-256) 673 425 0.72 650 Flammable

apound per square inch gauge; bAlthough oxygen does not burn, it will support combustion. Oxygen-enriched atmospheres may lead to violent reactions, such as rapid combustion or explosions, with incompatible materials.  

Hazards

Hazards may include fire, explosion, embrittlement, pressure buildup, frostbite and asphyxiation.

Many of the safety precautions observed for compressed gases also apply to cryogenic liquids. Two additional hazards are created from the unique properties of cryogenic liquids:

  • Extremely Low Temperatures –The cold boil-off vapor of cryogenic liquids rapidly freezes human tissue. Most metals become stronger upon exposure to cold temperatures, but materials such as carbon steel, plastics and rubber become brittle or even fracture under stress at these temperatures. Proper material selection is important. Cold burns and frostbite caused by cryogenic liquids can result in extensive tissue damage.
  • Vaporization - All cryogenic liquids produce large volumes of gas when they vaporize. Liquid nitrogen will expand 696 times as it vaporizes. The expansion ratio of argon is 847:1, hydrogen is 851:1 and oxygen is 862:1. If these liquids vaporize in a sealed container, they can produce enormous pressures that could rupture the vessel. (See Anecdotes for an account of such an incident.) For this reason, pressurized cryogenic containers are usually protected with multiple pressure relief devices.

Vaporization of cryogenic liquids (except oxygen) in an enclosed area can cause asphyxiation. Vaporization of liquid oxygen can produce an oxygen-rich atmosphere, which will support and accelerate the combustion of other materials. Vaporization of liquid hydrogen can form an extremely flammable mixture with air.