LIQUID NITROGEN (LN2) NDMRB LN2 handling training
Before use of Liquid Nitrogen… • You must have read: • University Policy Statement UPS S4/03 • TDI-SOP-005 Handling, Storage and Use of LN2 • TDI-RA-005 Handling, Storage and Use of LN2 Please sign off on your training forms
Access to Liquid Nitrogen room NDMRB • Standard access is 8am – 6pm • No lone working with LN2 • When working in the LN2 room it is advised that you work in pairs where possible. • You must make a colleague aware of when you will be going to the LN2 room • Never give access to someone else – only use your card to let yourself into the room • Access to the NMR room may be granted after this training session with approval from Paul Brennan
Liquid Nitrogen (LN2) • Liquid nitrogen is a colourless, odourless liquid of boiling point -196°C, density 0.8 kg/litre, and very low viscosity. • As the liquid changes to gas at ambient temperature and pressure, the expansion ratio (the gas factor) is approximately 700. • The resulting cold gas is heavier than air, so it accumulates at low level.
Main Hazards • Asphyxiation • Cold burns and frost bite • Explosions due to trapped expanding gas • Condensation of liquid Oxygen • Effects on materials
Asphyxiation There is no preliminary warning of oxygen deficiency caused by the addition of nitrogen. This is a significant hazard, which has been responsible for a number of deaths in research institutions over the past few years.
Asphyxiation • On boiling, liquid nitrogen produces approximately 700 times its volume of gas. The resulting displacement of oxygen from the atmosphere may be sufficient to cause asphyxiation. • Asphyxiation is usually sudden. The victims inhale air with little or no oxygen content, causing immediate collapse into a layer of dense, cold, nitrogen-enriched air. Unconsciousness followed rapidly by death is inevitable without immediate rescue and resuscitation. Rescue attempts often result in the rescuers being overcome as well.
Asphyxiation • Because Nitrogen is a heavy gas it will accumulate below waist level • Four oxygen level sensors are positioned around the room at just over waist height. • Before entering the room the panel (pictured in the next slide) must read oxygen levels >19% at all four positions
Asphyxiation • Evacuate the room as soon as the alarm sounds • NEVER enter the room if the alarms are sounding – even if someone is inside • Always check the oxygen level display before entering the room and ensure that O2 levels are >19% at all four sensors • Never use LN2 in small enclosed spaces, for example cold rooms • Never transport LN2 in a lift
Cold burns and frostbite • Skin contact with liquid nitrogen or cold nitrogen gas may cause severe cold burns, comparable with those caused by boiling water. • Unprotected skin may freeze onto surfaces cooled by the liquid, causing severe damage on removal. • Prolonged skin exposure to cold may result in frostbite, while prolonged inhalation of cold vapour or gas may cause serious lung damage. • The eyes are particularly susceptible – even small splashes of liquid nitrogen, or short exposures to cold vapour or gas, may cause instant freezing of eye tissues and permanent damage.
Cold burns and frostbitePrevention is better than cure • Before using LN2: • Use the thermal protection gloves • Always use a face shield • Always wear a lab coat and closed top shoes NO SANDALS or STRAPPY SHOES • Always check that the PPE is in good repair before use and report any failures to the lab manager or buildings team via email • There is CCTV in place – primarily this is for your safety but please note it will be known if you are not using PPE and your access to the room revoked.
Cold burns and frostbiteif the worst happens…. • First aid - cold burns and frostbite: Flush the affected skin with copious amounts of tepid water, or place the affected part in tepid water, and do not apply any form of direct heat. If possible, move the casualty to a warm room. A first aider must assess all cold burns, or medical attention should be sought to confirm the extent of any tissue damage. • First aid – liquid nitrogen splashes in the eye: Flush the eye with running water for at least 15 minutes and ensure the casualty is taken to the Eye Hospital for assessment. ALL accidents and near misses MUST be reported to the DSO using the accident report form
Explosions due to trapped, expanding gas • LN2 boils and vaporises into Nitrogen gas at room temperature • 1 litre of LN2 will produce approx. 700 litres of nitrogen gas • LN2 must only be decanted into a dewar specifically designed for cryogenic substances and MUST have a “Floating” lid • Vessels may also become sealed due to ice plug formation (e.g. in the necks of dewars where the wrong type of stopper has been used, or on the pressure relief devices of dewars stored in damp conditions). Pressure rise may cause the plug to be ejected, or the vessel may rupture. • If glass domestic vacuum flasks are used for liquid nitrogen, its low viscosity may allow it to penetrate the seal between the glass inner and the outer casing, causing an explosion as it warms and expands. Glass domestic vacuum flasks must not be used for liquid nitrogen. • Special care must be given when handling vials which have been submerged in LN2. There is the potential for the vial to have LN2 inside and this will warm up and expand once moved to room temperature
Explosions due to trapped, expanding gas A member of staff recently escaped serious injury when a sample vial she was removing from a liquid nitrogen container exploded in her face. During sample storage liquid nitrogen had seeped into the vial and when it was removed from the vessel the rapid expansion of the trapped liquid into gas caused the vial to explode. The researcher was not wearing any face protection and sustained facial abrasions. She was very fortunate indeed to have avoided serious eye injury. Had the vial contained harmful biological agents there was also the potential for the release of infectious aerosols. The procedure for storing samples in liquid nitrogen is documented in memo M12/01. To reiterate, the memo states that all new samples, or samples being returned to cryo-storage, must be stored in vapour phase nitrogen. Existing samples already stored in liquid phase nitrogen must be placed in a secondary container immediately after retrieval from the storage vessel or, alternatively, the sample vials must be placed in vapour phase nitrogen for at least 24 hours prior to removal. In all cases face shields must be worn when samples are being handled.
Condensation of liquid Oxygen • The boiling point of oxygen is -183°C • Therefore liquid oxygen may condense in open containers of liquid nitrogen or in open vessels cooled by liquid nitrogen (e.g. cold traps). Liquid oxygen will accumulate if the liquid nitrogen is constantly replenished, so this type of open cooling system should be avoided where possible. • The unsuspected presence of liquid oxygen may give rise to explosions caused by increased pressure if the vessels are subsequently sealed and allowed to warm up. If oxydizable material is present, then liquid oxygen may react explosively with it.
Use of Cryofreezer • Localise your sample using database/spreadsheet kept by your group. • Check Oxygen levels before entering the facility. • Put on adequate PPE (labcoat, visor, cryogloves). • Turn the key to release the lid of the cryofreezer. • Accompany the lid when opening or it will hit the wall/fixtures. • The tank will automatically de-fog itself. • Once de-fogged you can identify the tower of interest, pull it out and retrieve your samples. Take care as towers can be heavy. • Keep the lid open for as little time as necessary to prevent cold traps. • Replace the towers in its place, close the lid and lock it. • Remove PPE and leave the facility.
Effects on materials Many materials become brittle when cooled by liquid nitrogen and may be irreparably damaged. Other materials (e.g. glass dewars) may fail due to temperature stresses. Glass dewars should be enclosed in a metal can or wrapped in tape to give protection against flying glass fragments in the event of such failure. Glass dewars are not recommended for use and are not provided in the NDMRB. Use only articles or materials designed for use with liquid nitrogen. Glass domestic vacuum flasks must not be used as they may fail due to thermal shock on filling. Any containers which are to be used with LN2 must be in good repair before they are used. End users are encouraged to check dewars before use and report any failures to the lab manager
Dewar filling • Restricted to trained personnel only • Only containers designed for cryogenic liquids can be used to transport LN2 • Only fill to approx 500ml • Never fill an air tight container with LN2 • Use containers with floating lids only • NEVER transport LN2 in a lift • NEVER transport LN2 in a vehicle
Transporting LN2 in a small dewar • Persons transporting LN2 in a small dewar must only do so via the stairs. • You must travel holding the dewar in one hand and the stair rail in the other • This is to protect you if you fall and to try and ensure that the dewar does not get dropped. • If you drop a dewar you must inform the lab manager so that the area can be checked and the dewar checked for damage
NMR Filling • Only trained personnel may fill the NMR or the NMR dewar • This must be done in pairs • Due to distance travelled and volume of LN2 • Only engineers from Bruker are to fill and handle the liquid He
Excess LN2 • Any left over LN2 must be poured into a cryo freezer • This is done by removing two racks and pouring into the vacant space • Then return the racks to the freezer • Excess LN2 must not be left in a dewar and must not be disposed of via any other channels