Cryogenic Safety

By Eric W. Spencer

Cryogenics may be defined as low temperature technology, or the science of ultralow temperatures. To distinguish between cryogenics and refrigeration, a commonly used measure is to consider any temperature lower than - 73.3°C (- 1OO°F) as cryogenic. Although there is some controversy about this distinction, and some who insist that only those areas within a few degrees of absolute zero may be considered as cryogenic, the broader definition will be used here.

Low temperatures in the cryogenic area are primarily achieved by the liquefaction of gases, and there are more than twenty-five which are currently in use in the cryogenic area; i.e., gases which have a boiling point below -73.3°C (- 1OO°F). However, the, seven gases which account for the greatest volume of use and applications in research and industry are helium, hydrogen, nitrogen, fluorine, argon, oxygen, and methane (natural gas).

Cryogenics is being applied to a wide variety of research areas, a few of which are: food processing and refrigeration, rocket propulsion fuels, spacecraft life support systems, space simulation, microbiology, medicine, surgery, electronics, data processing, and metalworking.

TABLE I- Properties of Cryogenic Fluids.

Gas  Boiling Point Centigrade Boiling Point Kelvin Volume Expansion to Gas Flammable Toxic Odor
Helium-3  -269.9 3.2 757 to 1 No No(a) No
Helium-4 -268.9 4.2 757 to 1 No No(a) No
Hydrogen -252.7 20.4 851 to 1 Yes No(a) No
Deuterium -249.5  23.6 ... Yes Radioactive No
Tritium -248.0 25.1 ... Yes Radioactive No
Neon -245.9 27.2 1438 to 1 No No(a) No
Nitrogen -195.8 77.3 696 to 1  No No(a) No
Carbon monoxide -192.0 81.1 ... Yes Yes No
Fluorine  -187.0 86.0 888 to 1 No Yes Sharp
Argon -185.7  87.4 847 to 1 No No(a) No
Oxygen -183.0  90.1 860 to 1 No No(a) No
Methane  -161.4 111.7  578 to 1 Yes No(a) No 
Krypton -151.8 121.3 700 to 1 No No(a) No
Tetrafluoromethane -128 145 . . .  No Yes No
Ozone  -111.9 161.3  . . .  Yes Yes Yes 
Xenon -109.1 164.0 573 to 1  No No(a) No
Ethylene  -103.8  169.3 . . .  Yes  No(a) Sweet
Boron trifluoride -100.3 172.7  . . .  No Yes Pungent
Nitrous oxide -89.5 183.6 666 to 1 No No(a) Sweet
Ethane  -88.3  184.8 ... Yes No(a) No
Hydrogen chloride  -85.0 188.0 ... No  Yes  Pungent
Acetylene -84.0 189.1  ... Yes  Yes Garlic
Fluoroform  -84.0 189.1 ... No No(a) No
1,1-Difluoroethylene  -83.0 190.0 ...  Yes No(a) Ether
Chlorotrifluoromethane -81.4  191.6 . . . No  Yes Mild
Carbon dioxide  -78 5(b) 194.6 553 to 1 No Yes(a) Pungent

0°K = -273.16°C; -459.69°F.

(a) Nontoxic, but can act as an asphyxiant by displacing air needed to support life. As with most chemicals, even harmless materials can be toxic or poisonous if taken in sufficient quantities under the right circumstances.

(b) Sublimes.

Cryogenic fluids (liquefied gases) are characterized by extreme low temperatures, ranging from a boiling point of -78.5°C (-109°F) for carbon dioxide to -269.9°C (-454°F) for helium. Another common property is the large ratio of expansion in volume from liquid to gas, from approximately 553 to 1 for carbon dioxide, to 1438 to 1 for neon. Table I contains a more complete summary of the properties of cryogenic fluids.

HAZARDS

There are four principal areas of hazard related to the use of cryogenic fluids or in cryogenic systems. These are: flammability, high pressure gas, materials, and personnel. All categories of hazard are usually present in a system concurrently, and must be considered when introducing a cryogenic system or process.

The flammability hazard is obvious when gases such as hydrogen, methane, and acetylene are considered. However, the fire hazard may be greatly increased when gases normally thought to be non-flammable are used. The presence of oxygen will greatly increase the flammability of ordinary combustibles, and may even cause some noncombustible materials like carbon steel to burn readily under the right conditions. Liquefied inert gases such as liquid nitrogen or liquid helium are capable, under the right conditions, of condensing oxygen from the atmosphere, and causing oxygen enrichment or entrapment in unsuspected areas. Extremely cold metal surfaces are also capable of condensing oxygen from the atmosphere.

The high pressure gas hazard is always present when cryogenic fluids are used or stored. Since the liquefied gases are usually stored at or near their boiling point, there is always some gas present in the container. The large expansion ratio from liquid to gas provides a source for the build-up of high pressures due to the evaporation of the liquid. The rate of evaporation will vary, depending on the characteristics of the fluid, container design, insulating materials, and environmental conditions of the atmosphere. Container capacity must include an allowance for that portion which will be in the gaseous state. These same factors must also be considered in the design of transfer lines and piping systems.

Materials must be carefully selected for cryogenic service because of the drastic changes in the properties of materials when they are exposed to extreme low temperatures. Materials which are normally ductile at atmospheric temperatures may become extremely brittle when subjected to temperatures in the cryogenic range, while other materials may improve their properties of ductility. The American Society of Mechanical Engineers' Boiler and Pressure Vessel Code, Section VIII Unfired Pressure Vessels may be used as a specific guide to the selection of materials to be used in cryogenic service. Some metals which are suitable for cryogenic temperatures are stainless steel (300 series and other austenitic series), copper, brass, bronze, monel, and aluminum. Non-metal materials which perform satisfactorily in low temperature service are Dacron, Teflon, Kel-F, asbestos impregnated with Teflon, Mylar, and Nylon. Once the materials are selected, the method of joining them must receive careful consideration to insure that the desired performance is preserved by using the proper soldering, brazing, or welding techniques and materials. Finally, chemical reactivity between the fluid or gas and the storage containers and equipment must be studied. Wood or asphalt saturated with oxygen has been known to literally explode when subjected to mechanical shock. When properties of materials which are being considered for cryogenic uses are unknown, or not to be found in the known guides, experimental evaluation should be performed before the materials are used in the system.

Personnel hazards exist in several areas where cryogenic systems are in use. Exposure of personnel to the hazards of fire, high pressure gas, and material failures previously discussed must be avoided. Of prime concern is bodily contact with the extreme low temperatures involved. A very brief contact with fluids or materials at cryogenic temperatures is capable of causing burns similar to thermal burns from high temperature contacts. Prolonged contact with these temperatures will cause embrittlement of the exposed members because of the high water content of the human body. The eyes are especially vulnerable to this type of exposure, so that eye protection is necessary.

While a number of the gases in the cryogenic range are not toxic, they are all capable of causing asphyxiation by displacing the air necessary for the support of life. Even oxygen may have harmful physiological effects if prolonged breathing of pure oxygen takes place.

There is no fine line of distinction between the four categories of hazards, and they must be considered collectively and individually in the design and operation of cryogenic systems.

GENERAL PRECAUTIONS

Personnel should be thoroughly instructed and trained in the nature of the hazards and the proper steps to avoid them. This should include emergency procedures, operation of equipment, safety devices, knowledge of the properties of the materials used, and personal protective equipment required.

Equipment and systems should be kept scrupulously clean and contaminating materials avoided which may create a hazardous condition upon contact with the cryogenic fluids or gases used in the system. This is particularly important when working with liquid or gaseous oxygen.

Mixtures of gases or fluids should be strictly controlled to prevent the formation of flammable or explosive mixtures. As the primary defense against fire or explosion, extreme care should be taken to avoid contamination of a fuel with an oxidant, or the contamination of an oxidant by a fuel.

As further prevention, when flammable gases are being used, potential ignition sources must be carefully controlled. Work areas, rooms, chambers, or laboratories should be suitably monitored to automatically warn personnel when a dangerous condition is developing. When practical, it would be advisable to provide for the cryogenic system or equipment to be shut down automatically as well as to sound a warning alarm.

When there is a possibility of personal contact with a cryogenic fluid, full face protection, an impervious apron or coat, cuffless trousers, and high-topped shoes should be worn. Watches, rings, bracelets, or other jewelry should not be permitted when personnel are working with cryogenic fluids. Basically, personnel should avoid wearing anything capable of trapping or holding a cryogenic fluid in close proximity to the flesh. Gloves may or may not be worn, but if they are necessary in order to handle containers or cold metal parts of the system, they should be impervious, and sufficiently large to be easily tossed off the hand in case of a spill. A more desirable arrangement would be hand protection of the potholder type.

When toxic gases are being used, suitable respiratory protective equipment should be readily available to all personnel. They should thoroughly know the location and use of this equipment.

STORAGE

Storage of cryogenic fluids is usually in a well insulated container designed to minimize loss of product due to boil-off.

The most common container for cryogenic fluids is a double-walled, evacuated container known as a Dewar flask, of either metal or glass. The glass container is similar in construction and appearance to the ordinary Thermos bottle. Generally. the lower portion will have a metal base which serves as a stand. Exposed glass porions of the container should be taped to minimize the flying glass hazard if the container should break or implode.

Metal containers are generally used for larger quantities of cryogenic fluids. and usually have a capacity of 10 to 100 liters (2.6 to 26 gallons). These containers arc also of double-walled evacuated construction, and usually contain some adsorbent material in the evacuated space. The inner container is usually spherical in shape because this has been found to be the most efficient in use. Both the metal and glass Dewars should be kept covered with a loose-fitting cap to prevent air or moisture from entering the container, and to allow built-up pressure to escape.

Larger capacity storage vessels are basically the same double-walled containers, but the evacuated space is generally filled with powdered or layered insulating material. For economic reasons, the containers are usually cylindrical with dished ends, which approximates the shape of the sphere but is less expensive to build. Containers must bc constructed to withstand the weights and pressures that will be encountered, and adequately vented to permit the escape of evaporated gas. Containers should also bc equipped with rupture discs on both inner and outer vessels to release pressure if the safety relief valves should fail.

Cryogenic fluids with boiling point below that of liquid nitrogen (particularly liquid helium and hydrogen) require specially constructed and insulated containers to prevent rapid loss of product from evaporation. These are special Dewar containers which are actually two containers, one inside the other. The liquid helium or hydrogen is contained in the inner vessel, and the outer vessel contains liquid nitrogen which acts as a heat shield to prevent heat from radiating into the inner vessel. The inner neck as shown in the illustration, should be kept closed with a loose fitting, non-threaded brass plug which prevents air or moisture from entering the container, yet is loose enough to vent any pressure which may have developed (Fig. 1). The liquid nitrogen fill and vent lines should be connected by a length of gum rubber tubing with a slit approximately 2.54 cm ( I in.) long near the center of the tubing. This prevents the entry of air and moisture, while the slit will permit release of gas pressure. Piping or transfer lines should be double-walled evacuated pipes to prevent the loss of product during transfer.

Most suppliers are now using a special fitting to be used in the shipment of Dewar vessels. Also, there is an automatic pressure relief valve, and a manual valve to relieve pressure before removing the device. Dewar vessels of this type must be strictly and regularly maintained to prevent the loss of product, and to prevent an ice plug from forming in the neck.

The liquid nitrogen outer jacket should be kept filled to maintain its effectiveness as a radiant heat shield. The cap must be kept on at all times to prevent entry of moisture and air, which will form an ice plug. The liquid helium fill (inner neck) should be reamed out before and after transfer, and at least twice daily. Reaming should be performed with a hollow copper rod, with a marker or stop to prevent damaging the bottom of the inner container. Some newer style Dewar vessels are equipped with a pressure relief valve, and pressure gauge for the inner vessel.

Transfer of liquids from the metal Dewar vessels should be accomplished with special transfer tubes or pumps designed for the particular application. Since the inner vessel is mainly supported by the neck, tilting to pour the liquid may damage the container, shortening its life, or creating a hazard due to container failure at a later date. Piping or transfer lines should be so constructed that it is not possible for fluids.to become trapped between valves or closed sections of the line. Evaporation of the liquid in a section of line may result in pressure build-up and eventual explosion. If it is not possible to empty all lines, they must be equipped with safety relief valves and rupture discs. When venting storage containers and lines, proper consideration must be given to the properties of the gas being vented. Venting should be to the outdoors to prevent an accumulation of flammable, toxic or inert gas in the work area.

ACKNOWLEDGEMENT: Reprinted from the Journal of the American Society of Safety Engineers. Vol. 11 (8). 15-19. August, 1963, Chicago. Illinois.

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