Safetygram 7: Liquid Nitrogen
General inner vessel. Vaporizers convert the liquid nitrogen to its gaseous state. A pressure control manifold Liquid nitrogen is inert, colorless, odorless, non- controls the pressure at which the gas is fed to corrosive, nonflammable, and extremely cold. the process. Processes that use nitrogen as a Nitrogen makes up the major portion of the atmo- liquid do not require the vaporizers and pressure sphere (78.03% by volume, 75.5% by weight). control manifold. Nitrogen is inert and will not support combustion; however, it is not life supporting. Physical and chemical properties are listed in Table 1. Nitrogen is inert except when heated to very high temperatures where it combines with some of the Manufacture more active metals, such as lithium and magne- sium, to form nitrides. It will also combine with Nitrogen is produced at air separation plants by oxygen to form oxides of nitrogen and, when com- liquefaction of atmospheric air and separation of bined with hydrogen in the presence of catalysts, the nitrogen by continuous cryogenic distillation. will form ammonia. The nitrogen is then recovered as a cryogenic liquid. Since nitrogen is noncorrosive, special materials of construction are not required to prevent corro- Uses sion. However, materials of construction must be Nitrogen is the largest volume inorganic chemical selected to withstand the low temperature of liquid sold in the world, supporting a multitude of com- nitrogen. Vessels and piping should be designed to mercial and technical applications. In its liquid American Society of Mechanical Engineers (ASME) state, nitrogen is used for food freezing, plastic specifications or the Department of Transportation and rubber deflashing and grinding, cooling, metal (DOT) codes for the pressures and temperatures treating, biological sample preservation, pulveriza- involved. tion, and other temperature-related applications. Although used more commonly in the gaseous state, nitrogen is commonly stored and transported Health Effects as a liquid, affording a more cost-effective way of Being odorless, colorless, tasteless, and nonirritat- providing product supply. ing, nitrogen has no warning properties. Humans When nitrogen is converted to liquid form it possess no senses that can detect the presence becomes a cryogenic liquid. Cryogenic liquids are of nitrogen. Although nitrogen is nontoxic and liquefied gases that have a normal boiling point inert, it can act as a simple asphyxiant by displac- below –238°F (–150°C). Liquid nitrogen has a ing the oxygen in air to levels below that required boiling point of –320.5°F (–195.8°C). The tem- to support life. Inhalation of nitrogen in excessive perature difference between the product and the amounts can cause dizziness, nausea, vomiting, surrounding environment, even in winter, is sub- loss of consciousness, and death. Death may stantial. Keeping this surrounding heat from the result from errors in judgment, confusion, or loss product requires special equipment to store and of consciousness that prevents self-rescue. At handle cryogenic liquids. low oxygen concentrations, unconsciousness and death may occur in seconds and without warning. A typical system consists of the following com- ponents: a cryogenic storage tank, one or more Personnel, including rescue workers, should vaporizers, a pressure and temperature control not enter areas where the oxygen concentra- system. The cryogenic tank is constructed like, in tion is below 19.5%, unless provided with a principle, a vacuum bottle. It is designed to keep self-contained breathing apparatus or air-line heat away from the liquid that is contained in the respirator. For more information on oxygen-deficient atmo- Table 1 spheres, consult Air Products’ Safetygram #17, “Dangers of Oxygen-Deficient Atmospheres.” Liquid Nitrogen Physical and Chemical Properties
Extensive tissue damage or burns can result Chemical Formula N2 from exposure to liquid nitrogen or cold nitrogen Molecular Weight 28.01 vapors. Boiling Point @ 1 atm –320.5°F (–195.8°C) Containers Freezing Point @ 1 atm –346.0°F (–210.0°C) Liquid nitrogen is stored, shipped and handled in several types of containers, depending upon Critical Temperature –232.5°F (–146.9°C) the quantity required by the user. The types of Critical Pressure 492.3 psia (33.5 atm) containers in use are the dewar, cryogenic liquid cylinder, and cryogenic storage tank. Storage Density, Liquid @ BP, 1 atm 50.45 lb/scf quantities vary from a few liters to many thou- Density, Gas @ 68°F (20°C), 1 atm 0.0725 lb/scf sands of gallons. Since heat leak is always pres- ent, vaporization takes place continuously. Rates Specific Gravity, Gas (air=1) @ 68°F (20°C), 1 atm 0.967 of vaporization vary, depending on the design of Specific Gravity, Liquid (water=1) @ 68°F (20°C), 1 atm 0.808 the container and the volume of stored product. Specific Volume @ 68°F (20°C), 1 atm 13.80 scf/lb Containers are designed and manufactured according to the applicable codes and Latent Heat of Vaporization 2,399 Btu/lb mole specifications for the temperatures and pressures Expansion Ratio, Liquid to Gas, BP to 68°F (20°C 1 to 694 involved.
Figure 1 protect the cylinders from pressure buildup. These Figure 2a containers operate at pressures up to 350 psig A Typical Dewar and have capacities between 80 and 450 liters of Typical Cryogenic Liquid Cylinder, side view liquid. Liquid Level Gauge Product may be withdrawn as a gas by passing Handling Ring liquid through an internal vaporizer or as a liquid Handling Post Annular Space under its own vapor pressure. For more details on Rupture Disk the construction and operation of cryogenic liquid Inner Vessel Vent Tube cylinders, consult Air Products’ Safetygram #27, Gas Use Liquid Tube Vaporizer Float Assembly “Cryogenic Liquid Containers.” (Optional) Outer Vessel Rubber Shock Cryogenic Storage Tanks Mounts A typical customer installation (see Figure 3) Foot Ring includes a tank, a vaporizer, and a pressure control manifold. Tanks may be spherical or cylindrical Dewars in shape. They are mounted in fixed locations as Figure 2b Figure 1 illustrates a typical, vacuum-jacketed stationary vessels or on railroad car or truck chas- dewar. A loose fitting dust cap over the outlet of sis for easy transportation. Sizes range from 500 Typical Cryogenic Liquid Cylinder, top view the neck tubes prevents atmospheric moisture to 420,000 gallons and all tanks are powder- and Vent Pressure Rupture Vent Relief Pressure Rupture from plugging the neck and allows gas produced vacuum-insulated in the annular space. Tanks are Valve Gauge Disk Valve Valve Gauge Disk from vaporized liquid to escape. This type of con- equipped with various circuits to control product tainer is a nonpressurized container. The unit of fill, pressure buildup, pressure relief, product measure for the capacity of a dewar is typically withdrawal, and tank vacuum. Tanks are designed the liter. Five to 200 liter dewars are available. to ASME specifications for the pressures and tem- Product may be removed from small dewars by peratures involved. pouring, while larger sizes will require a transfer Transfer Lines Pressure tube. Cryogenic liquid cylinders that are pressur- Liquid Pressure Building Building Economizer Liquid Gas ized vessels are sometimes incorrectly referred to A liquid transfer line is used to safely remove liquid Valve Regulator Valve Regulator Valve Disk as dewars. product from dewars or cryogenic liquid cylinders. A typical transfer line for dewars is connected to Liquid product is typically removed through Cryogenic Liquid Cylinders a bayonet that provides a means of using product insulated withdrawal lines to minimize the loss Figure 2 shows a typical cryogenic liquid cylinder. vapor pressure buildup or an external pressure of liquid product to gas. Insulated flexible or rigid Cryogenic liquid cylinders are insulated, vacuum- source to remove the liquid. For cryogenic liquid lines are used to withdraw product from storage jacketed pressure vessels. They come equipped cylinders, the transfer line is connected to the tanks. Connections on the lines and tanks vary by with safety relief valves and rupture discs to cylinder’s liquid withdrawal valve. manufacturer. Figure 3 Figure 4 Use only transfer lines and equipment designed for use with cryogenic liquids. Some elastomers and A Typical Customer Station with a Nonflammable Gas Shipping Label metals, such as carbon steel, may become brittle at Cryogenic Storage Tank extremely low temperatures and may easily frac- ture. These materials must be avoided in cryogenic service.
On gas withdrawal systems use check valves or other protective apparatus to prevent reverse flow into the container.On liquid systems pressure relief devices must be used in lines where there is the potential to trap liquid between valves. It is recom- mended that all vents be piped to the exterior of the building.
Liquid containers should not be left open to the atmosphere for extended periods. Keep all valves closed and outlet caps in place when not in use. If NOTE: Liquid cylinders designed to dispense If oxygen-deficient atmospheres are suspected or restriction results from freezing moisture or foreign gaseous nitrogen have valves equipped with can occur, use oxygen monitoring equipment to test material present in openings and vents contact the standard Compressed Gas Association (CGA) for oxygen-deficient atmospheres. Review the liquid vendor for instructions. Restrictions and blockages outlets. Suitable pressure regulating equipment nitrogen Material Safety Data Sheet (MSDS). may result in damgerous over-pressurization. Do not may be attached. Valves provided for the with- attempt to remove the restriction without proper in- drawal of liquid product are also equipped with Buildings structions. If possible move the cylinder to a remote standard CGA outlets, but are different than the location. connections used for gaseous withdrawal. This Because of the large expansion ratio of liquid to gas, is to prevent cross connections between pro- it is very important to provide adequate ventilation For additional information on the storage and cesses using liquid or gaseous product. in areas using liquid nitrogen. A minimum of six air handling of cryogenic liquids refer to Air Products’ changes per hour are suggested in these areas. Safetygram-16, “Safe Handling of Cryogenic Shipment of Liquid Nitrogen Liquids”, and CGA Pamphlet P-12, “The Safe Provide monitoring for areas where oxygen dis- Handling of Cryogenic Liquids”. Containers used for transporting liquid nitrogen placement may occur. at less than 25 psig (40 psia) pressure are OSHA has established 19.5% oxygen concentration Personal Protective Equipment UN/DOT authorized containers. These are contain- as the minimum for working without supplied air. (PPE) ers built to other than DOT specifications but are authorized by the DOT for use in the transport of Remember, nitrogen has no warning properties! Personnel must be thoroughly familiar with proper- approved products. Containers used for transport- ties and safety considerations before being allowed ing liquid nitrogen at pressures greater than 25 Storage and Handling to handle liquid nitrogen and/or its associated psig (40 psia) need to be designed, manufactured, equipment. and tested to DOT specifications. Store and use liquid containers with adequate venti- lation. Do not store containers in a confined area or Eyes are most sensitive to the extreme cold of liquid For air shipments, all packages must be in compli- in area unprotected from the extremes of weather. nitrogen and its vapors. The recommended personal ance with International Air Transport Association/ Cryogenic containers are equipped with pressure protective equipment when handling or using liquid International Civil Air Organization (IATA/ICAO) relief devices designed to control the internal pres- nitrogen is a full faceshield over safety glasses; Dangerous Goods Regulations, as well as DOT sure. Under normal conditions these containers will loose-fitting thermal insulated or leather gloves; and regulations. periodically vent product. Do not plug, remove or long-sleeved shirts and pants without cuffs, espe- tamper with any pressure relief device. cially whenever the possibility of exposure or a spill DOT Hazard Class: 2.2 exists. In addition, safety shoes are recommended Cryogenic containers must be stored, handled and DOT Shipping Label: Nonflammable Gas (Fig. 4) for those involved with the handling of liquid nitro- transported in the upright position. When moving except none for cargo tanks under 23 psig gen containers. never tip, slide or roll containers on their side. Use Identification Number: UN1977 a suitable hand truck for moving smaller contain- In emergency situations, self-contained breathing DOT Shipping Name: Nitrogen, Refrigerated Liquid, ers. Move larger containers by pushing, not pulling. apparatus (SCBA) must be used. 2.2, UN1977 Avoid mechanical and thermal shock. Never allow any unprotected part of the body to come in contact First Aid Safety Considerations with uninsulated pipes or equipment containing cryogenic product. The extreme cold will cause flesh People suffering from lack of oxygen should be The hazards associated with liquid nitrogen are to stick fast and potentially tear on withdrawal. moved to fresh air. If the victim is not breathing, exposure to cold temperatures, which can cause administer artificial respiration. If breathing is dif- severe burns; overpressurization due to expansion If there is any difficulty in operating the container ficult, administer oxygen. Obtain immediate medical of small amounts of liquid into large volumes of gas valve or container connections discontinue use and attention. in inadequately vented equipment; and asphyxi- contact the vendor. Do not remove or interchange ation due to displacement of oxygen in the air in connections. Use only the properly assigned con- confined work areas. nections. Do not use adaptors. Self-contained breathing apparatus (SCBA) In the case of massive exposure, remove clothing may be required to prevent asphyxiation of while showering the victim with warm water. Call rescue personnel. a physician immediately.
For skin contact with cryogenic liquid nitrogen, If the eyes are exposed to the extreme cold of the remove any clothing that may restrict circulation liquid nitrogen or its vapors, immediately warm to the frozen area. Do not rub frozen parts, as the frostbite area with warm water not exceeding tissue damage may result. As soon as practical, 105°F (40°C) and seek immediate medical place the affected area in a warm water bath that attention. has a temperature not in excess of 105°F (40°C). Never use dry heat. Fire Fighting
Frozen tissue is painless and appears waxy with Since nitrogen is nonflammable, special fire fight- a possible yellow color. It will become swollen, ing equipment and instructions are not needed. As painful, and prone to infection when thawed. If the a note of caution, however, water streams must frozen part of the body has been thawed, cover not be directed toward venting nitrogen, as the the area with a dry sterile dressing with a large water will freeze and plug the pressure relief vent bulky protective covering, pending medical care. and may result in a container failure.
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