Cryogenic Liquids

Cryogenic liquids are liquefied gases that are kept in their liquid state at very low temperatures; examples include liquid nitrogen and helium. Severe cold burns or frostbite may be inflicted if exposed skin comes in contact with cryogenic fluids, vapor, or surfaces cooled by cryogenic fluids. Also, in the event of a large spill or system failure, large volumes of gases may be released in the atmosphere resulting in asphyxiation. For instance, 1 liter of nitrogen expands to approximately 700 liters in volume when going from liquid to gas at ambient temperature. Asphyxiation and chemical toxicity are the primary hazards; these risks can be mitigated by proper ventilation/purging techniques.

Cold burns and Frostbite

  1. Skin contact with liquid nitrogen can cause tissue freezing, resulting in severe burns. The burns are caused by the extremely low temperature of the cryogenic liquid rather than chemical action. Skin may appear red with the formation of blisters. In cases that involve prolonged or severe exposure, tissue may freeze and have a waxy or yellow appearance.
  2. Contact with non-insulated parts or equipment or vessels containing cryogenic liquids can produce similar damage. Unprotected parts of the skin may stick to low-temperature surfaces and flesh may be torn upon removal.
  3. Inhalation of cold vapor can cause damage to the lungs and may trigger an asthma attack in susceptible individuals.

FIRST AID: Remove any clothing that may restrict circulation to frozen area. Do not rub frozen parts as tissue damage may result. The affected area should be placed in a warm water bath which has a temperature not to exceed 105°F/40°C. Never use dry heat. Call a physician as soon as possible.

Pressurization and Explosion

As mentioned above, cryogenic liquids vaporize with a volume change ratio of 700-900 which may cause large changes in pressure, which may in turn cause an explosion. Cryogenic liquid must be stored in a well-ventilated area with precautions against rapid changes in pressure.

Hazards:

  1. Ice forming on the venting tube, plugging it and preventing gas release
  2. Damaged equipment resulting causing cryogenic fluids to leak in small areas, subsequent vaporization of the liquid causes pressure build up
  3. Loss of vacuum inside a cryostat or Dewar
  4. Liquid helium–cooled superconducting magnet "quench" (changing spontaneously from a superconducting state to a normal state)
  5. Liquid nitrogen having permeated through sealed cryotubes containing samples which and returning to room temperature
  6. Direct contact of the cryogenic liquid with water in a tube resulting in rapid vaporization of the cryogenic liquid (can cause the tube to explode)

Liquid oxygen hazard and other liquid oxidizing agents: Liquid oxygen has 4,000 times more oxygen by volume than normal air. Therefore materials that are usually considered non-combustible can become flammable in the presence of liquid oxygen. There can be explosive reaction in organic materials if a flammable mixture is produced. Flammable materials such as oil, grease, aerosols, paint, gasoline and solvents could ignite and burn when introduced to high concentrations of oxygen. Never use wax or polish on or near any oxygen equipment.

Personnel Safety

  1. Face shields and goggles shall be worn during the transfer and normal handling
  2. Gloves that have been specifically designed to provide protection to the hands and arms at ultra-cold (-160°C/-256°F) temperatures must be worn.
    • These gloves are not designed to be totally immersed in liquids
  3. A laboratory coat will be worn in order to prevent liquid from spraying or spilling inside gloves. Pants must be worn when working with cryogenic liquid.

Safety Practices

Cryogenic fluids must be handled and stored only in containers and systems specifically designed for these products and in accordance with applicable standards, procedures, or proven safe practices.

  1. Transfer of cryogenic liquids must be conducted slowly to minimize boiling and splashing of the cryogenic fluid. Transfer of cryogenic fluids from open containers must occur below chest level of the person pouring liquid.
  2. Transfer of cryogenic liquids shall be conducted only in well ventilated areasto prevent possible gas or vapor accumulation.
  3. All cryogenic systems must be equipped with pressure-relief devices to prevent pressure build-up.
  4. Never refill Dewars or transfer cryogenic liquids alone
  5. Remove watches, rings and other metal jewelry on hands and wrists to prevent rapid freezing of metal which may cause damage to property or tissue damage.
  6. Wear required Personal Protective Equipment:
    • Cryogenic gloves
    • Lab coat with sleeves pulled over cuffs of cryogenic gloves
    • Full length cuffless pants that extend over shoes tops
    • Closed toed shoes must be worn
    • Chemical splash goggles and a full face shield
  7. Use Dewars rated for the cryogen being transferred.
    • Glass Dewars must be taped solidly around the outside. The plastic mesh which comes with some small thermos bottles primarily provides some protection for the Dewar, but does not necessarily protect against glass shards resulting from implosion.
    • Dewar lids should be placed on loosely to provide adequate pressure relief.
  8. Dewars larger than 20 liters will be lifted and poured by two people
  9. Ensure the receiving vessel is dry.
  10. Cool the receiving vessel by dispensing a small amount of cryogenic fluid, then continue the dispensing process. Delivery should be conducted slowly to minimize splashing, spilling and thermal shock to the receiving vessel.
  11. Do not leave a filling operation unattended.

Liquid nitrogen has the ability to condense oxygen from surrounding air, when most of the nitrogen has evaporated from such a vessel there is a risk that liquid oxygen remaining can react violently. Users should attempt to limit air exposure as much as possible by:

  1. Reducing the amount of time they use liquid nitrogen
  2. Never leaving liquid nitrogen completely uncovered for long periods of time
  3. Covering openings of containers of liquid nitrogen as much as possible without stopping all venting of gas.

If you notice that the liquid nitrogen has a slightly blue color, this is an indication that liquid oxygen is present. The material should be kept away from all ignition sources and any flammable materials and allowed to evaporate.


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