What are cryogenic liquids?
Cryogenic liquid are liquefied gases that are kept in their liquid state at very low temperature. All cryogenic liquids are cold. Cryogenic liquids have boiling points below -150 °C (- 238 °F). At normal temperatures and pressures, all cryogenic liquids are gases . They must be cooled below room temperature before an increase in pressure can liquefy them. Different cryogens become liquids under different conditions of temperature and pressure, but they have two common properties, they are extremely cold, and small amounts of liquid can expand into very large volumes of gas.
The vapours and gases released from cryogenic liquids also remain very cold. They often condense the moisture in air, creating a highly visible fog. Some cryogenic liquids actually condense the surrounding air in poorly insulated containers and forming a liquid air mixture. Cryogenic liquids are classified as "compressed gases". Everyone who works with cryogenic liquids must be aware of their hazards and know how to work safely with them.
Different types of cryogenic liquids?
Inert Gases Inert gases do not react chemically and do not burn or support combustion. Examples of this group are nitrogen, helium, neon, argon and krypton.
Flammable Gases
Some cryogenic liquids can burn in air. The most common examples are hydrogen, methane and liquefied natural gas.
Oxygen
Many materials considered as non-combustible can burn in the presence of liquid oxygen. Organic materials react explosively with liquid oxygen. The hazards and handling precautions of liquid oxygen must therefore be considered separately from other cryogenic liquids.
How are cryogenic liquids stored?
Cryogenic liquids are used in thermally insulated containers. The containers are specifically designed to withstand rapid temperature changes and extreme differences in temperature.
Liquid Dewar Flasks
Liquid dewar flasks are non-pressurized, vacuum-jacketed vessels, somewhat like a "Thermos bottle". They should have a loose fitting cap that prevents air and moisture from entering, yet allows excess pressure to vent. Flasks containing helium, hydrogen and other low- boiling liquids have an outer vessel of liquid nitrogen for insulation.
Laboratory Liquid Dewar Flasks
Laboratory liquid dewars have wide-mouthed openings and do not have covers. These small containers are primarily used in laboratories for temporary storage only.
Liquid Cylinders
Liquid cylinders are pressurized containers. They are specifically designed for cryogenic liquids. This type of container has valves for filling and dispensing the cryogenic liquid, and a pressure-control valve with a frangible (bursting) disk as backup protection. There are three types of liquid cylinders which are designed for dispensing:
- Liquid or gas
- Only gas
- Only liquid
Health hazards of cyrogenic liquids?
There are three groups of health hazards associated with cyrogenic liquids:
- Toxicity
- Extreme cold
Asphyxiation
Each gas can cause specific health effects. For example, large quantities of carbon monoxide gas can be released from liquid carbon monoxide which can cause death almost immediately.
Cryogenic liquids and their associated vapours and gases can produce effects on the skin similar to a thermal burn. Brief exposures that would not affect skin on the face or hands can damage delicate tissues such as the eyes.
Unprotected skin can stick to metal that is cooled by cryogenic liquids. The skin can then tear when pulled away. Even non-metallic materials are dangerous to touch at low temperatures. Prolonged breathing of extremely cold air may damage the lungs.
When cryogenic liquids form a gas, the gas is very cold and they are heavier than air. This cold, heavy gas does not disperse very well and can accumulate near the floor. Even if the gas is non-toxic, it displaces air. When there is not enough air or oxygen, asphyxiation and death can occur. Oxygen deficiency is a serious hazard in enclosed or confined spaces.
