Safe Handling of Liquid Nitrogen

Handling Liquid Nitrogen

 

Hazards

Liquid nitrogen has two main properties that are potentially hazardous:-

• It’s extremely cold. At atmospheric pressure, liquid nitrogen boils at -196°C.

• Small amounts of liquid vaporise into large amounts of gas. Roughly 700 times expansion.

Cold Burns

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.

Unprotected body parts contacting objects cooled by liquid nitrogen may stick fast. This may result in flesh injuries whilst attempting to withdraw from the object.

It is often stated that small splashes of Liquid Nitrogen will run off bare skin due to a vapour layer forming between the skin and the liquid. This must never be relied upon.

Asphyxiation

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.

Over Pressure

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.

Cryotube Explosions

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

When thawing Cryotubes take the following precautions :

• Wear a face shield, or at least safety goggles.

• 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.

Embrittlement.

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.

Lifts

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.

• No one should accompany the Dewar.

• One person should send and another should receive the Dewar from the lift.

Precautions

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.

Containers

Use only containers designed for low-temperature liquids.

Cryogenic containers (eg Dewars) are designed to withstand the rapid changes and extreme temperature differences encountered in working with Liquid Nitrogen. However, these special containers should be filled SLOWLY to minimise the internal stresses that occur when any material is cooled. Excessive internal stresses can damage the container.

• Do not cover or plug the entrance opening of any Liquid Nitrogen refrigerator or Dewar.

• Do not use any stopper or other device that would interfere with venting of gas.

Cryogenic liquid containers are generally designed to operate with little or no internal pressure. Inadequate venting can result in excessive gas pressure which could damage or burst the container. Check the unit periodically to be sure that venting is not restricted by accumulated ice or frost.

Protective Clothing

When using or decanting Liquid Nitrogen a face shield or safety goggles must be used.

Always wear appropriate cryogenic gloves when handling anything that is, or may have been, in immediate contact with Liquid Nitrogen. Use tongs to withdraw objects immersed in the liquid, and handle the object carefully. Do not put hands (even in the best gloves) into Liquid Nitrogen.

Inadequate protective clothing can absorb the Liquid Nitrogen and result in even more severe burns than would otherwise have resulted.

Training

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.

Decanting of Liquid Nitrogen

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

Maintenance of Dewars

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 correct equipment

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.

Never use hollow rods or tubes as dipsticks. When a warm tube is inserted into liquid nitrogen, liquid will spout from the bottom of the tube due to vaporisation and rapid expansion of liquid inside the tube. Wooden or solid metal dipsticks are recommended.

Transport

Keep Dewars upright at all times.

Rough handling can cause serious damage to Dewars and refrigerators. To protect the vacuum insulation system, handle containers carefully.

Do not place Liquid Nitrogen containers in closed vehicles where the nitrogen gas that is continuously vented can accumulate.

Disposal

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.