CfA Safety: Cryogenic Liquids
 

Intent: To provide a minimum standard for the safe handling of cryogenic liquids.

Definitions:

Cryogenic fluids - liquid gases at temperatures nominally below -238°F.

Dewars are especially designed double-walled insulated containers having provisions for pressure relief.

Information:

  • All cryogenic liquids are extremely cold. Cryogenic liquids and their cold "boil-off" vapor can rapidly freeze human tissue, and can cause many common materials such as carbon steel, plastics and rubber to become brittle, or even fracture under stress. Liquids in containers and piping at temperatures at or below the boiling point of liquefied air (-318°F, -194°C) can actually condense the surrounding air to a liquid. Extremely cold liquefied gases (liquid helium and liquid nitrogen) can even solidify air or other gases.
  • All cryogenic liquids also produce large volumes of gas when they vaporize. For example, one volume of liquid hydrogen at one atmosphere vaporizes to 700 volumes of hydrogen gas at 70°F and one atmosphere. If these liquids are vaporized in a sealed container, they can produce enormous pressures which could rupture the vessel. For this reason pressurized cryogenic containers are usually protected with multiple devices of pressure relief, usually a pressure relief valve for primary protection and a frangible disc for secondary protection. Vaporization of liquid oxygen in an enclosed area can cause an oxygen-rich atmosphere and could saturate a worker's clothing. Although oxygen is not flammable it will vigorously support and/or accelerate the combustion of other materials. Vaporization of liquid hydrogen in an enclosed work area can cause a flammable or explosive mixture with air.
  • Most cryogenic liquids are odorless, colorless, and tasteless when vaporized to the gaseous state. Most of them have no color as liquids, although liquid oxygen is light blue. However, the extremely cold liquid and vapor have a warning property that appears whenever they are exposed to the atmosphere. The cold "boil-off" gases condense the moisture in the air, creating a highly visible fog. The fog normally extends over a larger area than the vaporizing liquid.

Standard:

  • Supervisors responsible for projects using cryogenic fluids must familiarize themselves and their personnel with appropriate safety procedures.
  • Matches, smoking materials, lighters, etc., and other sources of ignition are prohibited where liquid hydrogen and oxygen are present. All such areas shall be designated as "No Smoking Areas".
  • Safety glasses are required during transfer and normal handling of cryogens. If severe spraying or splashing may occur, a face shield or chemical goggles should be worn for additional protection.
  • Dry cryogenic gloves should always be worn when handling anything that comes in contact with cold liquids and vapor. Gloves should be loose fitting so that they can be removed quickly if liquids are spilled into them. Depending upon the application, special clothing may be advisable. It is preferable to wear trousers outside of boots or work shoes.
  • Keep container (Dewar) vertical at all times. Do not roll the container on its side.
  • Do not enter an area or laboratory where a major spill may have caused an oxygen deficiency, unless you are equipped with a self-contained air supply breathing apparatus.
  • Relief valves on Dewars shall not be tampered with under any circumstances!
  • Electronic gases (including arsine, diborane, dichlorosilane, germane, phosphine and silane) require the following special safety provisions:
    • The use of vented gas cabinets with alarmed monitoring devices is mandatory
    • The exhaust rate of the cabinet must be sufficient to vent a complete cylinder discharge.
    • The height of the exhaust stack must be sufficient to safely disperse the toxic gas before it reaches the ground or a receptor.
    • The use of these gases require special safety review by the Safety Office before use

  • Evacuated glassware (Dewars) must be shielded against implosion. Exposed glass portions of the container must be taped to minimize flying glass hazard.
  • All cryogenic storage vessels shall be chosen to withstand the weights and pressures of the material used, and shall have adequate venting to prevent pressure build-up.
  • All cryogenic liquids with boiling points below that of liquid nitrogen (particularly liquid helium and hydrogen) require specially constructed and insulated containers to prevent rapid evaporation.
 
 

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