16.10 Cryogenic Material Safety
According to the Compressed Gas Association, a cryogenic fluid is a material that has a boiling point of less than -130°F (-90°C). Examples of cryogenic materials include the liquids nitrogen, argon, and helium, and solid carbon dioxide (dry ice). Hazards associated with cryogenic fluids include:
Extreme Low Temperature - These liquids and their boiled off vapors are extremely cold and can cause severe cold contact burns. They also make many materials brittle, such as the epoxy or phenolic resin that laboratory benchtops and sinks are made of.
Asphyxiation - When liquid and solid cryogenic materials vaporize they greatly expand, some by a factor of ~700:1. This displaces oxygen and creates an oxygen depletion hazard. This is especially dangerous when confined in poorly ventilated space like an elevator.
Oxygen makes up 20.9% of the air we breathe. The environment is considered to be oxygen deficient below 19.5%. Two breaths of air with no oxygen can be enough to render a person unconscious.
Typical background levels of carbon dioxide are about 400 ppm. Vaporization of even small quantities of Dry Ice in an unventilated area can exceed the permissible exposure limit of 5000 ppm.
Exposure to oxygen-deficient atmospheres produce dizziness, nausea, vomiting, loss of consciousness, and death. Such symptoms may occur in seconds without warning.
Death may result from errors in judgment, confusion, or loss of consciousness that prevents self-rescue. Working with cryogenic substances in confined spaces, such as walk-in coolers, can be especially hazardous. Where cryogenic materials are used, a hazard assessment is required to determine the potential for an oxygen-deficient condition. Controls such as ventilation and/or gas detection systems may be required to safeguard employees. Asphyxiation and chemical toxicity are hazards encountered when entering an area that has been used to store cryogenic liquids if proper ventilation/purging techniques are not employed.
Toxicity - Many of the commonly used cryogenic gases are considered to be of low toxicity, but still pose a hazard from asphyxiation. Check the properties of the gases you are using because some gases are toxic, for example, Carbon monoxide, Fluorine, and Nitrous oxide.
Flammability and Explosion - Fire or explosion may result from the evaporation and vapor buildup of flammable gases such as hydrogen, carbon monoxide, or methane. Liquid oxygen, while not itself a flammable gas, can combine with combustible materials and greatly accelerate combustion.
Oxygen clings to clothing and cloth items, and presents an acute fire hazard.
High Pressure - In cryogenic systems, high pressures are obtained by gas compression during refrigeration. Warm temperatures surrounding containers cause a constant expansion of the liquid as it turns back into gas. Sudden release through a rupture or break in a line may be violent. Over- pressurization of cryogenic equipment can occur due to improper venting and expansion during the phase change from liquid to gas. All cryogenic fluids produce large volumes of gas when they vaporize.
Materials and Construction Hazards - The selection of materials calls for consideration of the effects of low temperatures on the properties of those materials. Some materials become brittle at super low temperatures. Brittle materials fracture easily and can result in almost instantaneous material failure.
Low temperature equipment can also fail due to thermal stresses caused by differential thermal contraction of the materials. Over-pressurization of cryogenic equipment can occur due to the phase change from liquid to gas if not vented properly.