Processes involving extremely low temperatures present unique process design and safety challenges. This one-page reference outlines considerations for low-temperature operations.
Extremely low temperatures can rapidly freeze human tissue. Contact between a worker’s bare skin and a low-temperature vapor, liquid or solid can result in cryogenic burns. Contact is most likely when objects are being moved into or out of a low-temperature zone, such as a liquid-nitrogen-storage bath, during maintenance activities, or when low-temperature fluids are being transferred. If cryogenic fluids are involved, workers should wear long sleeves, long pants, thermally insulating gloves, and face and eye protection (a full face shield over safety glasses is advisable). Pants should not have cuffs, and gloves should be loose so they can be quickly removed. Even when low-temperature liquids are not handled directly, it is important to identify uninsulated pipes or vessels that contain them. If unprotected skin comes into contact with these surfaces, the skin may stick to them.
Many materials embrittle at cold temperatures. This can be useful for size reduction of materials that would otherwise be too soft, oily or volatile to grind. However, many materials commonly used for ambient temperature systems, such as carbon steel or galvanized steel, lose their ductility as their temperature is lowered. This can result in a catastrophic failure of equipment or piping made from these materials if subjected to excessive stress at low-temperatures. Piping and pressure vessel design and fabrication codes, such as ASME B31.3 (Process Piping) and B31.5 (Refrigeration Piping and Heat Transfer Components), address this hazard by specifying minimum temperatures for materials of construction, plus materials testing and design restrictions for selecting and using materials at low temperatures. Materials that remain ductile at low temperatures include austenitic stainless steel (including types 304, 316 and 321), copper, red brass and many copper alloys and aluminum.
Most materials of construction will shrink as their temperatures decrease. For example, a stainless steel or copper pipe that is 100 ft. (30 m) long will contract linearly by about 3.5 in. (90 mm) as it cools down from 70°F to –320°F (20°C to –195°C). This thermal contraction is independent of the diameter of the pipe. The stresses generated by thermal contraction are large and will severely damage an improperly designed pipeline or piece of equipment.
Liquid nitrogen (Figure 1) expands to over 700 times its liquid volume when warmed to 68°F (20°C). This expansion property is used commercially to purge, inert and pressurize containers housing foods, drugs and chemicals that are sensitive to air or moisture by dropping a small amount of liquid nitrogen into the container during packaging. Low-temperature systems may need to be designed to accommodate the pressures that can be generated whenever a liquid refrigerant is trapped in a closed volume. For example, liquid nitrogen or liquid trifluoromethane can become trapped in a pipeline between two closed valves. As the cold liquid warms, the increase in vapor pressure can spring flanged joints and burst pipes. In liquid-nitrogen piping systems, this expansion is usually managed by installing a pressure relief valve, in pipe segments that can trap liquid. All thermal relief valves in a liquid nitrogen system should discharge to a safe location, ideally outdoors. In mechanical systems, the higher unit cost and other properties of the refrigerants used, such as trifluoromethane, means that thermal relief valves are not feasible. Instead, these systems typically incorporate expansion vessels.
Condensing nearby materials
The ability of cryogenic temperatures to liquefy substances with low boiling points is useful in many process operations. For instance, cryogenic temperatures can condense volatile compounds that cannot be separated from exhaust-air streams at ordinary refrigeration temperatures. The condensed material can often be reused and recycled instead of incinerated. If cryogenic liquids contained in vessels or piping are colder than the oxygen dewpoint of the surrounding air, an oxygen-enriched liquefied-air condensate will form on uninsulated surfaces. This can drip onto surrounding equipment and personnel, causing cryogenic burns. As the condensate warms and re-evaporates, the resulting local raised oxygen levels can create a serious fire hazard. In the case of H2 and He, the surrounding air can even be solidified. This frozen air can block the discharge ports of pressure relief valves, preventing them from operating correctly.
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