The term “cryogenic” means producing, or related to low temperatures. Cryogenic liquids are liquefied gases that are kept in their liquid state at very low temperatures. These liquids have boiling points below -238° F (-150° C) and are gases at normal temperatures and pressures. Different cryogens become liquids under different conditions of temperature and pressure, but all have two common properties: they are extremely cold and small amounts of the liquid can expand into very large volumes of gas.
Most cryogenic liquids and the gasses they produce can be placed into one of three groups:
- Inert gases – These gases do not react chemically to any great extent and do not burn or support combustion. Examples of this group are nitrogen, helium, neon, argon and krypton.
- Flammable gases – Some cryogenic liquids produce a gas that 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 can react explosively with liquid oxygen.
Cryogenics have many applications. Cryogenic liquids are often used in industrial and medical applications. The electrical resistance of most metals decreases as temperature decreases. Other applications include fast freezing of some foods and the preservation of some biological materials. The freezing of portions of the body to destroy unwanted or malfunctioning tissue is known as cryosurgery.
Physical Effects of Cryogens
The extreme temperatures of cryogenic liquids cause most solids to become more brittle. Materials such as carbon steel, plastics and rubber should not be used with cryogenic liquids because they can fracture or shatter extremely easily.
Flammable gases such as hydrogen, methane and liquefied natural gas can burn or explode. Hydrogen is particularly hazardous.
Liquid hydrogen and liquid helium are both so cold that they can liquefy air on contact resulting in an oxygen-enriched atmosphere.
Materials that are usually considered non-combustible, such as aluminum, carbon steel, cast iron, stainless steel and zinc may burn in the presence of liquid oxygen.
Without adequate ventilation or pressure-relief devices on cryogenic containers, enormous pressure can build up. The pressure can cause a boiling liquid expanding vapor explosion (BLEVE).
Health Hazards of Cryogens
There are three groups of health hazards associated with cryogenic liquids – extreme cold, asphyxiation and toxicity.
Contact with cryogenic liquids and their cold gases can produce effects on the skin similar to a thermal burn. Brief exposures that may not affect the hands can damage delicate tissues such as the eyes. Prolonged exposure of the skin or contact with cold surfaces can cause frostbite. Tissues that have been frozen will be painless while still frozen and might look waxy yellow. Thawed frostbitten skin will be very painful, red and swollen and can become infected.
Unprotected skin can stick to materials that are cooled by cryogenic liquids, similar to the way some children stick their tongues to flagpoles in the winter. However, where cryogenics are involved, metallic materials are not the only ones that cause this risk. It is important to remember that even nonmetallic materials are extremely dangerous to touch. Removing skin from any material can cause a tearing of the flesh.
Prolonged breathing of extremely cold air may damage the lungs. Also, when cryogenic liquids form a gas, the gas is very cold and usually heavier than air. The cold, heavy gas does not disperse well, tends to accumulate near the floor and displaces air. When there is not enough air, asphyxiation and death can occur. This is a serious hazard especially in enclosed or confined spaces.
Small amounts of cryogenic liquids can evaporate into very large volumes of gas. For example, one liter of liquid nitrogen vaporizes into 695 liters of nitrogen gas when warmed to room temperature.
Each cryogenic liquid/gas can cause specific health effects. Refer to the manufacturer’s Safety Data Sheet (SDS) for information about the toxic hazards of a specific cryogen.
Contact with cryogenic liquids, their gases and any cooled surfaces should be avoided. If contact does occur, immediately flush the area with large quantities of warm water (108°F). Do not rub the affected area as rubbing can cause further damage. Obtain medical attention as quickly as possible.
If oxygen loss overcomes a person working with cryogenic liquids, move the victim to a well-ventilated area. Apply artificial respiration if the victim's breathing has stopped. Supply oxygen if the victim has difficulty breathing and summon emergency medical help.
Personal Protective Equipment for Cryogenic Safety
Personal protective equipment (PPE) is critical to cryogenic safety. It is essential to choose the right PPE for each job. Consult each material’s SDS for specific guidance. Generally, chemical-splash goggles and face shields should always be worn during the transfer and handling process to guard against splashes, bubbles and flying debris.
Loose-fitting, insulated cryogenic gloves should also be worn. The gloves must be loose so they can be thrown off quickly if liquid spills into them.
To protect all parts of the skin, long-sleeve shirts and trousers without cuffs are suggested. Pant legs should go over the tops of footwear so spills cannot get into boots or shoes and cause extreme tissue damage before the footwear can be removed. Cryogenic liquids flow very freely and can penetrate woven or other porous clothing. Wearing a cryogenic apron is suggested to help prevent this from occurring.
If working in an oxygen-deficient atmosphere, you must use an oxygen-supplying respirator, such as an airline respirator with an egress bottle or a fire-fighting or industrial self-contained breathing apparatus (SCBA). Air purifying respirators do not protect against oxygen-deficient environments.
Cryogenic liquids are shipped and used in thermally insulated containers specifically designed to withstand rapid temperature changes and extreme differences in temperature.
Liquid dewar flasks are non-pressurized, vacuum-jacketed vessels, somewhat like a thermos bottle. They have a loose fitting cap or plug that prevents air and moisture from entering, yet allows excess pressure to vent. Some flasks have an outer vessel of liquid nitrogen for insulation.
Liquid cylinders are pressurized containers designed for cryogenic liquids. This type of cylinder has valves for filling and dispensing the cryogenic liquid and a pressure-control valve with a bursting disk as backup protection.
Commonly Asked Questions
Compressed Gas Association
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National Institute of Standards and Technology
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National Fire Protection Association
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