Cryogenic liquids (i.e., cryogens) have unique properties that create a very different set of concerns and requirements when compared to materials like flammable liquids, corrosives, and poisons. The majority of the differences focus on packaging. In this article, we will limit the discussion to smaller packages such as cylinders and Dewar flasks and not worry about bulk shipments or tanks.
Definition and Hazards
The US DOT defines a cryogenic liquid as a refrigerated liquefied gas having a boiling point below –90°C (–130°F) at 101.3 kPa (14.7 psia) absolute. [49 CFR 173.115(g)] In other words, at normal temperatures and pressures, cryogens are gases. When cooled and pressurized, they turn to liquid. All cryogens have two properties in common: they are kept extremely cold, and small amounts of liquid can expand into very large volumes of gas.
Prolonged exposure to cryogens can cause frostbite and severe damage to the lungs. At room temperature, some cryogens expand to over 600 times their liquid volume and can easily displace the oxygen in the room, creating an asphyxiation hazard. Liquid carbon monoxide is especially dangerous, because it rapidly expands if not properly maintained and thus creates a toxic environment. Due to the unique hazards these materials pose, shipments of cryogens are subject to the Hazardous Materials Regulations regardless of whether they meet the regulatory definition of a non-flammable, non-poisonous compressed gas.
Identifying and Naming Cryogenic Liquids
Cryogenic liquids, like all other hazardous gases, are assigned a Proper Shipping Name (PSN), an identification number, and a hazard division, but not a packing group. Cryogenic gases are typically assigned to Hazard Division 2.2, but some gases are flammable or poisonous or have an oxidizing subsidiary hazard. PSNs for cryogenic liquids are typically in the form of “GAS NAME, Refrigerated Liquid, (cryogenic liquid).” In addition to three “generic” (G) shipping names for flammable, oxidizing, and other miscellaneous gases, there are PSN entries in the Hazmat Table for the following cryogenic gases: air, carbon monoxide (North America only), ethylene, methane, hydrogen, nitrogen, oxygen, and the noble gases argon, helium, krypton, neon, and xenon.
Possible Packagings: What Is a Dewar Flask?
A Dewar flask is a type of packaging often compared to a Thermos (vacuum) jug. Created by James Dewar and patented by Thermos L.L.C., this two-layer container consists of a glass or metal bottle that holds the liquid and is surrounded by a sealed vacuum layer that extends the insulating capacity of the flask.
Shipping Cryogenic Liquids in Cylinders
As with any hazardous material, a package containing cryogens must be strong enough to survive a normal trip, be compatible with its contents, and handle the pressures and temperatures incident to transport. When shipping using cylinders, though, these requirements are only the beginning.
Additional requirements for cryogens include keeping the temperatures of the materials within the “design service temperature” of the packaging. When shipping a flammable cryogen liquid in a cylinder, the jacket covering the insulation on the cylinder must be made of steel. A valve or fitting made of aluminum with internal rubbing or abrading aluminum parts that may come in contact with oxygen in the cryogenic liquid form may not be installed on any cylinder used to transport oxygen, cryogenic liquid unless the parts are properly anodized to increase corrosion resistance.
It is critical to keep in mind the high pressure of the material in the cylinder. Each cylinder must be provided with one or more pressure relief devices, which must be installed and maintained in compliance with 49 CFR. Each cylinder containing a cryogenic liquid must have a pressure control system designed and installed so that it will prevent the cylinder from becoming liquid full. A cryogen release, especially of a flammable cryogen like methane, could be catastrophic.
When certain criteria are met, the US DOT allows shippers to vent packages in order to reduce pressure during transit. Per 49 CFR 173.24:
“Venting of packagings, to reduce internal pressure which may develop by the evolution of gas from the contents, is permitted only when-
(1) Except for shipments of cryogenic liquids as specified in §173.320(c) and of [dry ice], transportation by aircraft is not involved.”
Also, the shipper must ensure that the outage and filling limits were not exceeded when preparing the package for shipping. For cylinders, these limits are found in 49 CFR 173.301 through 173.306.
Orientation Arrows for “Open Cryogenic Receptacles”
Unlike toxic, flammable, or compressed gases, orientation markings (i.e., up arrows) are required for “open cryogenic receptacle intended for the transport of refrigerated liquefied gases.” [49 CFR 172.312]
Special Provisions (49 CFR 172.102)
The requirements for shipping any cryogen include tank special provisions the shipper must check to ensure correct package selection. These special provisions cover standards like pressure relief devices and operating temperatures.
For liquid nitrogen shippers, two special provisions exist that may be very helpful in simplifying the shipping process.
Special Provision SP 345 states: “Nitrogen, refrigerated liquid (cryogenic liquid), UN1977 transported in open cryogenic receptacles with a maximum capacity of 1 L are not subject to the requirements of this subchapter. The receptacles must be constructed with glass double walls having the space between the walls vacuum insulated and each receptacle must be transported in an outer packaging with sufficient cushioning and absorbent materials to protect the receptacle from damage.”
Special Provision SP 346 states: “Nitrogen, refrigerated liquid (cryogenic liquid), UN1977 transported in accordance with the requirements for open cryogenic receptacles in §173.320 and this special provision are not subject to any other requirements of this subchapter. The receptacle must contain no hazardous materials other than the liquid nitrogen which must be fully absorbed in a porous material in the receptacle.”
Cryogenic Liquids Exceptions and Reliefs
When shipped by ground at relatively low pressure, some cryogens may qualify for exceptions from the US DOT’s hazardous materials shipping requirements. Specifically, 49 CFR 173.320 refers to atmospheric gases and helium, cryogenic liquids, shipped in Dewar flasks, insulated cylinders, insulated portable tanks, insulated cargo tanks, and insulated tank cars.
Because the hazards posed by cryogens are so great given their potential to expand rapidly, the high pressure involved, and the temperature controls needed for stability, attention to detail is critical for shippers when preparing these materials for transport. Mishandling or incorrectly packaging these shipments can lead not only to fines as high as $75,000 per day, per violation, but to catastrophic incidents in transit. By following the detailed, exacting standards for these materials, shippers can keep their supply chain moving smoothly and safely.
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