Safe Handling of Liquid Nitrogen
Liquid nitrogen has two main properties that are potentially hazardous:-
Extremely low temperatures can freeze flesh very rapidly. When spilled on a surface the liquid tends to cover it completely and intimately, cooling a large area. The gas issuing from the liquid is also extremely cold. Delicate tissue like eyes can be damaged by exposure to cold gas alone which would be too brief to affect skin.
Liquid Nitrogen rapidly vaporises to gas. The gas may has the potential to kill by asphyxiation. When the Oxygen concentration in air is sufficiently low, a person can become unconscious without any warning symptoms.
Because Liquid Nitrogen boils rapidly users must ensure that it is never used in a closed system. Tape exposed glass parts to minimise the hazard of flying glass shards. Therefore do not use thermos flasks, and it may be necessary to punch holes in cryovials.
Cryotubes used to contain samples stored under liquid nitrogen may explode without warning. Tube explosions are thought to be caused by liquid nitrogen entering the tube through minute cracks and then expanding rapidly as the tube thaws
• Wear heavy gloves.
• Wear a lab coat and trousers or long skirt.
• Place the Cryotube in a heavy-walled container (e.g., a desiccator) or behind a safety shield while it is thawing.
Many ordinary materials cannot withstand cryogenic temperatures. Never dispose of cryogenic liquids down the drain. Materials exposed to cryogenic temperatures for long periods or which have undergone periodic warming and freezing should be examined for cracks and crazing.
It is unlikely that a Dewar will spill its contents whilst in a lift thus putting the handler at risk of injury or death. It’s also unlikely a lift will breakdown whilst one is being transported. There is a small risk that should a person remain in a closed lift for a prolonged time the venting gases may reduce the Oxygen level sufficiently to cause harm. However to eliminate these risks the following practice should be followed when transporting Dewars.
• One person should send and another should receive the Dewar from the lift.
Storage of Dewars
Dewars should not be stored in sealed rooms (e.g. walk in refrigerated rooms) because the reduced ventilation may be inadequate to mitigate against spillage and general evaporation.
Use only containers designed for low-temperature liquids.
• Do not use any stopper or other device that would interfere with venting of gas.
When using or decanting Liquid Nitrogen a face shield or safety goggles must be used.
Safety precautions must be followed to avoid potential injury or damage. Do not attempt to handle liquid nitrogen until you fully understand the potential hazards, their consequences, and the related safety precautions.
Never overfill Dewars. Spillage damages flooring and may cause injury. Insert pipes and funnels slowly to avoid splashing. Great care should be exercised to ensure that space is left to replace lids/tops on Dewars especially those that insert a considerable distance into the vessel. Spills and splashes can set off oxygen monitors
Condensed moisture or frost on the outer shell of a refrigerator or Dewar and abnormally rapid evaporation of the liquid nitrogen are indications of vacuum loss. If vacuum loss is evident or suspected, transfer the materials stored in the unit to another refrigerator as soon as possible and remove the unit from service.
Use a phase separator or special filling funnel to prevent splashing and spilling when transferring Liquid Nitrogen into or from a Dewar or refrigerator. The top of the funnel should be partly covered to reduce splashing. Use only small, easily handled Dewars for pouring liquid. When liquid cylinders or other large storage containers are used for filling, follow the instructions supplied with those units and their accessories.
Keep Dewars upright at all times.
Never dispose of cryogenic liquids down the drain. Allow waste Liquid Nitrogen to evaporate naturally in a fume hood or, preferably, pour the liquid slowly on gravel or bare earth, from which other people are excluded, where it can evaporate without causing damage.