A cryogenic liquid has a normal boiling point below -238°F (-15O°C). The most-commonly used industrial gases that are transported, handled and stored at cryogenic temperatures are oxygen, nitrogen, argon, hydrogen, and helium. Three rare atmospheric gases-neon, krypton, and xenon-are used in the liquid state. Natural gas, liquefied natural gas (LNG) or liquid methane, and carbon monoxide also are handled as cryogenic liquids, although they are not usually classified as industrial gases. Liquefied ethylene, carbon dioxide, and nitrous oxide are transported and stored as liquids, but are not classified as cryogenic.

Handling cryogenic liquids in large volumes is not new. Liquid oxygen was first shipped by tank truck in 1932, and today it is common to see portable liquid containers, cryogenic trailers and trucks, and railroad tank cars hauling large quantities of liquefied gases across the country. Cryogenic tanker ships transport LNG overseas, and aircraft move other liquefied gases, especially liquid helium, from one place to another.

Many safety precautions that must be taken with compressed gases also apply to liquefied gases. However, some additional precautions are necessary because of the special properties exhibited by fluids at cryogenic temperatures.

Both the liquid and its boil-off vapor can rapidly freeze human tissue and can cause many common materials such as carbon steel, plastic, and rubber to become brittle or fracture under stress. Liquids on containers and piping at temperatures at or below the boiling point of liquefied air (-318°F or -194°C) can cause the surrounding air to condense to a liquid.

Extremely cold liquefied gases (helium, hydrogen, and neon) can evenly solidify air or other gases to which they are directly exposed. In some cases, even plugs of ice or foreign material will develop in cryogenic container vents and openings and cause the vessel to rupture. Following the supplier's operating procedures can help prevent plugging. If a plug should form, contact the supplier immediately. Do not attempt to remove the plug; move the vessel to a remote location.

All cryogenic liquids produce large volumes of gas when they vaporize. For example, 1 volume of saturated liquid nitrogen at 1 atmosphere vaporizes to 696.5 volumes of nitrogen gas at room temperature at I atmosphere. The volume expansion ratio of oxygen is 860.6 to 1. Liquid neon has the highest expansion ratio - 1445 to 1 - of any industrial gas.

Vaporized in a sealed container, these liquids produce enormous pressures. For example, when 1 volume of liquid helium at 1 atmosphere is vaporized and warmed to room temperature in a totally enclosed container, it has the potential to generate pressure of more than 14,500 psig. Because of this high pressure, cryogenic containers usually are protected with two pressure-relief devices; a pressure-relief valve and a frangible disc.

Relief devices should function only during abnormal operation and emergencies or when gas is not being withdrawn from the tank or cylinder. If they are triggered, the system should be checked for loss of insulating vacuum or for leaks. Do not tamper with the safety valve settings. Report leaking or improperly set relief valves to the gas supplier and have them replaced or reset by qualified personnel. Similarly, all safety valves with broken seals or with any frost, ice formation, or excessive corrosion should be reported.

Most cryogenic liquids are odorless, colorless, and tasteless when vaporized to a gas. As liquids, most have no color; liquid oxygen is light blue. However, whenever the cold liquid and vapor are exposed to the atmosphere, a warning appears. As the cold boil off gases condense moisture in the air, a fog that extends over an area larger than the vaporizing gas forms.