SAFETY IN THE USE OF CRYOGENS

Thomas J. Bruno

Cryogens (liquified gases or cryogenic liquids) are used extensively in laboratories and in analytical instruments. A cryogen is typically defined as a liquefied gas with a normal boiling temperature of no higher than -90 °C (-130 °F, different sources cite differing upper limits). Dry ice (solid carbon dioxide, which sublimes at 194.65 K, -78.5 °C) is also used to achieve low temperatures in the laboratory but is not considered to be a cryogen. The hazards associated with the use of cryogens are actually twofold. There are hazards associated with the cryogenic fluids themselves, and there are hazards associated with the containers used to store and transport the cryogenic fluids. Here, unlike other sources, we will treat these separately (Refs. 1-7.

1. Lemmon, E.W., Properties of Cryogenic Fluids, in CRC Handbook of Chemistry and Physics, 100^th Edition, 2019.
2. Safetygram 27, Cryogenic Liquid Containers, Air Products and Chemicals, Allentown, PA. <900-13-080-US-cryogenic-liquid-containers-safetygram-27.pdf>
3. Safetygram 8, Liquid Argon, Air Products and Chemicals, Allentown PA. <900-13-080-US-argon-safetygram-8.pdf>
4. Safetygram 16, Safe Handling of Cryogenic Liquids, Air Products and Chemicals, Allentown PA. <900-13-080-US-safe-handling-of-cryo-liquids-safetygram-16.pdf>
5. Safetygram 7, Liquid Nitrogen, Air Products and Chemicals, Allentown, PA. <900-13-080-US-liquid-nitrogen-safetygram-7.pdf>
6. Laboratory Safety Chemical Hygiene Plan (CHP), OSHA Fact Sheet, U.S. Occupational Safety and Health Administration, Washington, DC. <www.osha.gov/Publications/laboratory/OSHAfactsheet-laboratory-safety-chemical-hygiene-plan.pdf>
7. Laboratory Safety and Chemical Hygiene Plan, Queensborough Community College of City University of New York, Bayside, NY, 2012. <www.qcc.cuny.edu/ehs/docs/ChemicalHygienePlan2.pdf>

Cryogens

The major hazards associated with the use of cryogens stem from their low temperatures, their high liquid-to-gas expansion ratios, toxicity, and air displacement. Table 1 has some important properties germane to the handling of common cryogens. The gas-to-liquid expansion ratios have a variability of up to ±5 depending on the local ambient temperature. The boiling temperatures are at atmospheric pressure (101.325 kPa). The column definitions are as follows.

TABLE 1. Properties of Common Cryogens

Low Temperature

The primary purpose of the use of a cryogen stems from the low boiling points. The potential of severe frostbite burn is very high if a cryogen comes into contact with skin. While a thin layer of vapor formation will protect the skin initially, if a cryogen is allowed to pool (such as in clothing), the frostbite danger is very high. Protective clothing is essential and should include cryogen gloves and safety glasses. A full face shield is strongly recommended. In addition, canvas shoes are discouraged because pooling can occur in the case of spillage. Pooling can also occur in cuffs on pant legs, which should be avoided. Pants should not be tucked into shoes. A lab coat or shirt cuffs should be tucked under glove gauntlets. If skin should come in contact with a cryogen, it should be rinsed with cold water; do not apply dry heat to the affected area. If clothing has frozen to an individual due to cryogen exposure, cold water should be used to free the clothing, and emergency personnel must be summoned.

Another hazard due to the low temperature results from the ability of these fluids to embrittle materials including hoses, floor mats, and other laboratory surfaces. Of special concern is the embrittlement of electrical insulation, which can lead to a fire hazard.

Because the boiling temperature of liquid nitrogen is below that of liquid oxygen, it is possible for oxygen to condense on all surfaces or vessels cooled by liquid nitrogen. Liquid oxygen is an oxidizer that can enhance the flammability characteristics of liquids and solids that it contacts. The liquid air that is seen dripping from lines transferring liquid nitrogen can be up to 50% oxygen. If a blue tint is observed in a vessel being used with liquid nitrogen, the presence of liquid oxygen must be assumed. Additional hazards of liquid oxygen are discussed below.

Asphyxiation

The high liquid-to-gas expansion ratios listed in Table 1 show that when a cryogen is vaporized, it has the potential to displace air in a laboratory. While most cryogens are not toxic per se, they can act as simple asphyxiants. Laboratories that use cryogens must be adequately ventilated. In labs with large containers of cryogen, it is important to have an oxygen monitor with an audible alarm. Personnel, including rescue workers, should not enter areas where the oxygen concentration is below 19.5% (vol/vol) unless provided with a self-contained breathing apparatus or air-line respirator. The safe range as indicated on an oxygen monitor is between 19.5% and 23% (vol/vol). Personnel in an area of low oxygen concentration may be unaware of the condition, thus monitoring is critical.

If a person seems to become dizzy or loses consciousness while working with cryogens, they should be moved to a well-ventilated area immediately. If breathing has stopped, apply CPR and emergency personnel must be summoned.

Overpressure

Related to the liquid-to-gas expansion ratio is the overpressure that can result if a cryogen is allowed to warm within an enclosure. Indeed, no cryogen can remain liquid within a container; some venting must be provided. If a vent becomes disabled or is not present, a warming cryogen vaporizes and produces very high pressures based on the PVT surface of the fluid.

Reactivity and Toxicity

Some cryogens pose specific hazards or handling requirements based on their chemistry.

Liquid oxygen (LOx) cannot be permitted to contact organic materials; common organic materials include solvents and vacuum pump oil. Organic materials can be readily ignited by spark or shock after exposure to LOx, including fingerprints on a surface. Clothing saturated with oxygen is readily ignitable and vigorously burns. If LOx spills on an asphalt surface, do not walk over or roll equipment over that surface for at least one hour. While not having specific toxicity issues, if LOx is exposed to high-energy electromagnetic radiation, it can produce ozone, which solidifies at LOx temperatures. Solid ozone is unstable and toxic (upon vaporization), and explodes if disturbed.

Liquid hydrogen handling requires all of the precautions used for hydrogen gas. Liquid hydrogen should not be transferred in an atmosphere of air as it readily condenses in the liquid hydrogen, resulting in a potential explosive mixture. Liquid hydrogen must be transferred by helium pressurization in properly designed vacuum-insulated transfer lines pre-purged with helium or gaseous hydrogen. Liquid hydrogen, like liquid helium, can solidify air, which can block vents and safety relief devices. Dewars and other containers made of glass should not be used for liquid hydrogen service. Breakage makes the possibility of explosion too hazardous to risk.

Cryogenic Liquid Containers

Several different types of cryogenic liquid containers are encountered in the laboratory during routine chemical analyses, and each has their own associated hazards and precautions. It is common parlance to refer to all of these containers as Dewars, but this terminology is imprecise. The small portable containers used to assemble laboratory cold baths and the small transport containers (with loose-fitting lids and carry handles) are also known as Dewars. These containers are used at ambient pressure. The larger supply containers (from which Dewars are filled) are called liquid cylinders. These containers are pressurized, with different pressure ratings available.

Liquid Cylinders

Liquid cylinders are large heavy containers with integrated casters or a dolly to facilitate movement. At least two of these casters should be equipped with a braking mechanism. Typical volumes and weights are provided below for liquid cylinders for nitrogen, oxygen, and argon.

Liquid helium cylinders, which often incorporate a liquid nitrogen jacket, are usually heavier than those for the common cryogens listed in Table 2. Moreover, there is a larger variety of available sizes, ranging from 50 to 500 L. The weights listed in Table 2 are typical as-filled weights. Some losses occur in transport, and the losses for helium liquid cylinders can be considerable.

The weight of these cylinders can make them challenging to handle. The personal protective equipment discussed above (cryogens) must be used when handling liquid cylinders. Cylinders should be moved by pushing, not pulling, to reduce the potential of an upset. In locations of frequent transport of liquid cylinders, bottom door sills should be removed to eliminate the potential of bouncing or rough handling. If a cylinder must be transported by elevator, a freight elevator is preferred. Personnel should not ride in the elevator car with the cylinder. The cylinder should be transported in the elevator with no personnel and be met at the receiving floor. A placard reading “CRYOGEN TRANSFER - DO NOT ENTER ELEVATOR” should be posted facing the door if the elevator is to travel more than one story.

If liquid cylinders must be transported between buildings, it is critical to use ramps and to ensure that there are no large cracks in paving that must be traversed. Note also that the casters commonly found on liquid cylinders are not rated for travel along long distances of pavement. In the event loss of control of a liquid cylinder occurs during transport, such as the cylinder begins to fall, it is usually best to simply let it go and summon qualified help as defined in the organization‘s standard operating procedures.

Liquid cylinders are pressurized and can contain up to 350 psi (2411 kPa), depending on the cylinder specifications. Pressure specifications on cylinders are sometimes confusing. The commonly encountered specifications are:

• psia (pounds-force per square inch absolute) gauge pressure plus local atmospheric pressure
• psid (psi difference) difference between two pressures, specified on the cylinder label
• psig pounds (force) per square inch, gauge
• psi-vg (psi-vented gauge) difference between the measuring point and the local pressure
• psi-sg (psi-sealed gauge) difference between a chamber of air sealed at atmospheric pressure and the pressure at the measuring point

Pressure relief devices are integral to all liquid cylinders and must remain unobstructed with frost. If the outlet fitting on a pressure relief valve is facing the same direction as the liquid or gas-dispensing valve, a fitting directing vented gas away from users should be added to protect personnel. Over-pressurization of liquid cylinders is a serious hazard; cylinders can rupture if a pressure relief valve becomes impaired or inoperative.

Dewar Flasks

Dewar flasks or simply, Dewars, are small cryogen containers used at atmospheric pressure, with or without a loose-fitting cover or cap. Smaller Dewars are usually made from an evacuated silvered-glass insert set into a metal jacket. Any exposed glass should be taped to prevent flying glass in the event of a catastrophic rupture.

When filling a small Dewar from a liquid cylinder, it is best to pre-cool the interior of the Dewar with a small amount of cryogen first, before completing the fill. Boiling and splashing generally occur when filling a warm container, so all personnel should stand clear and wear appropriate personal protective equipment as discussed above. The flask should be clean and dry before filling.

A Dewar flask should not be filled to beyond 80% of its capacity. Overfilling increases the risk of splashing and spillage. A beverage thermos bottle is NOT a substitute for a Dewar flask in the laboratory under any circumstances.

When carrying a small Dewar flask, it must be the only item being carried. Dewar flasks should be held as far away from the face as possible. Be aware of other personnel in the area.

Small Dewar flasks with liquid nitrogen are often used as cold traps in the laboratory. When instrument components are placed in the filled Dewar cold bath, it is important to insert components slowly to avoid splashing and excessive boiling. Any Pyrex wool insulation placed around the flask must not dip into the cryogen or become a vapor barrier. If liquid nitrogen acquires a blue tint, it has become contaminated with liquid oxygen, and the discussion of LOx hazards above applies.

When a Dewar cold trap is used in association with a vacuum pump, the trap must be carefully emptied periodically to avoid exposure to toxic chemicals and to prevent over pressurization should the trap run dry. Note also that venting liquid nitrogen near a vacuum pump v-belt can embrittle the belt and shorten its service life. Likewise, if the venting is near electrical cables, embrittlement of the electrical insulation can result in a fire hazard.