Many industrial gases are transported, stored and used in liquid form, a situation that poses significant health and safety risks. This one-page reference highlights good safety practices for preventing personnel injury when working with cryogenic fluids.
High-purity industrial gases, including nitrogen, oxygen, helium, argon and other noble gases, are commonly generated by air-separation units (ASUs), which typically use a cryogenic distillation process. Liquefied industrial gases, especially liquid nitrogen, are often delivered to end-users by cryogenic tankers into onsite vacuum-insulated storage vessels for on-demand use as a liquid or a gas. Among the myriad uses of industrial gases, several involve their use in liquid form. These include: R&D; cooling of process equipment (reactors, crystallizers, storage tanks and so on); lyophilization; recovery of volatile organic compounds; food freezing; cryogenic milling of solids; cryogenic preservation; plastic and rubber deflashing and grinding; and others.
Exposure hazards
The extremely low temperatures and high expansion rates (from a phase change) that characterize liquefied gases present health and safety hazards.
Cryogenic burns and frostbite hazards. Cryogenic liquids and cold gas that come in contact with human skin can cause serious damage to living tissue, including cold burns and frostbite. Damage can occur with exposures of longer than a few seconds, especially to delicate tissues, such as the eyes.
High-pressure gas. Storing cryogenic fluids presents hazards from high-pressure gas, since the liquefied gases are usually stored at or near their boiling points (see Table 1). Large expansion ratios on vaporization can cause a buildup of high pressures. The evaporation rate depends on the fluid, storage-container design and environmental conditions, but all storage containers should allow for the evaporation of the liquid.
| Table 1. Properties of common industrial gases | ||||
| Substance | Chemical symbol | Boiling point, °C (at 1 atm) | Density, kg/m3 | Latent heat of vaporization, kJ/kg |
| Air | — | –194.3°C | 1.29 | 205.0 |
| Nitrogen | N2 | –195.8 °C | 1.25 | 199.1 |
| Helium | He | –268.9°C | 0.18 | 20.28 |
| Oxygen | O2 | –183.0 °C | 1.43 | 213.0 |
| Argon | Ar | –185.9°C | 1.78 | 162.3 |
| Acetylene | C2H2 | –83.3°C | 1.17 | 614.0 |
| Hydrogen | H2 | –252.8°C | 0.09 | 446.0 |
| Ethane | C2H6 | –88.6°C | 1.36 | 489.4 |
| Methane | CH4 | –161.5°C | 0.72 | 509.9 |
| Neon | Ne | –246.0°C | 0.90 | 86.3 |
| Xenon | Xe | –108.2°C | 5.85 | 96.3 |
| Carbon dioxide | CO2 | –78.5°C | 1.25 | 571.3 |
| Ammonia | NH3 | –33.3°C | 0.77 | 1,371.0 |
| Krypton | Kr | –153.4°C | 3.71 | 107.5 |
Displacement of oxygen/asphyxiation. Because of the large expansion when a cryogenic fluid evaporates, it can cause asphyxiation by displacing breathable air. Natural ventilation should be ensured, and transporting and using cryogenic fluids in enclosed spaces should be avoided. In normal dry air, oxygen makes up 20.95% by volume. No one should enter an area where the oxygen concentration is below 19.5% without self-contained breathing equipment.
Flammability. In the case of cryogenic gases that are flammable, including hydrogen, methane and acetylene, ignition sources must be prohibited. Flammable materials must be stored away from liquid oxygen areas, since O2 supports and accelerates combustion. Be aware that liquefied inert gases and extremely cold surfaces can condense oxygen from the atmosphere, causing oxygen entrapment in unsuspected areas.
Safety practices
Tasks with the potential for exposure to cryogenic liquids should only be conducted by competent staff members who have received appropriate training on specific health and safety risks, standard operating procedures and actions to be taken in the event of an exposure.
Personal protective equipment (PPE). Cryogenic exposure is often attributable to inadequate PPE. Where the potential for cryogenic exposure exists, all personnel should use the following PPE:
• Eye protection, with a full-face shield over safety glasses, should be used when transferring liquids to an open container
• Gloves should either be designed to prevent cryogens from flowing into the glove or be loose fitting so the glove can be easily shaken off after accidental contact. Cotton or nylon gloves covered with disposable nitrile or vinyl gloves are recommended for work that requires delicate handling of cold items in close proximity to liquid nitrogen, but are not designed to allow immersion of hands and fingers into the liquid
• Laboratory coats, disposable coveralls or long-sleeved shirts and pants that provide complete coverage of skin not otherwise protected by PPE or attire, should be worn
Contact. Cryogenic liquids and uninsulated cryogenic equipment and pipes should generally not not be directly touched. Tongs can be used to withdraw objects immersed in a cryogenic liquid. All parts of the body must be protected from uninsulated pipes or vessels containing cryogenic liquids, because the extremely cold metal may stick fast to the skin and tear the flesh upon removal. Watches, rings and jewelry should not be worn (metals can become frozen to the skin).
Handling. The following are tips for safe handling of cryogenic liquids.
• Do not overfill containers
• Pour slowly to minimize splashing
• Avoid the path of boil-off gases
• Ensure that cryogenic fluids are stored in appropriately insulated containers, which minimize the loss of product due to boil-off
• Containers of cryogenic liquid should be allowed to vent. Where a special vented stopper or venting tube is used, as on some small portable containers, the vent should be checked regularly to ensure it has not plugged with ice formed from condensed water vapor from the air
• The materials used in cryogenic systems must have the appropriate physical properties to qualify them for use at extremely low temperatures. Acceptable materials include aluminum, copper, brass, fiberglass and stainless steels (304 and 316)
Pressure relief. The follow are tips related to pressure relief:
• All system vents must be directed away from personnel or designated work areas
• Venting fluids should not impinge on any part of the body. Ensure that pressure-relief devices are checked, maintained and sized for maximum backpressure
• Ensure all safety valves and vent valves are unobstructed and functioning properly. Check the safety vents on liquid nitrogen tanks at least twice a week
• Ensure that an oxygen alarm is present in the work area when appropriate
Spills. If cryogenic material is spilled skin contact is involved, immediate medical attention is required. Large spills (especially in a confined space) can lead to an oxygen-deficient atmosphere, so personnel should be evacuated from the area.
References
1. Cryogenic Safety Manual. Argonne National Laboratory, Physics Division. Available at: www.phy.anl.gov.
2. OSHA Laboratory Safety Guidance, OSHA 3404-11R, U.S. Department of Labor. 2011.
3. Virginia Commonwealth Univ., Cryogenics safe work practices. 2009. www.vcu.edu/oehs.