Precaution to Liquid Nitrogen Usage
There are two broad types of hazard arising from liquid nitrogen (hereafter referred to as LN2) – those related to temperature and those related to vapour.
LN2 has a temperature of -196oC.
Severe cold burns can result from skin contact with the liquid or objects cooled by the liquid or vapour.
The skin will freeze to cold metal surfaces.
Many materials become brittle at this temperature and can shatter or crack.
Liquid oxygen may condense in containers of LN2 because oxygen has a higher boiling point than nitrogen. This raises the possibility of an explosive reaction with oxidisable material.
LN2 expands around 700-fold when it vaporises at room temperature (eg 1 litre of liquid produces nearly 700 litres of gas).
Closed vessels containing LN2 may explode because of the build-up in pressure caused by the evaporation.
In poorly-ventilated rooms there is a danger that air will be displaced by the nitrogen, leading to an oxygen-deficient atmosphere and death by asphyxiation.
The danger is one of asphyxiation from nitrogen gas displacing the air.
Outside the building, LN2 dewars are stored inside an enclosure with mesh walls. Hence there is ample natural ventilation. Storage inside the building is more of concern. The greatest danger is in the morning, because there could have been significant build-up of asphyxiant overnight and especially over the
weekend. Although the oxygen deficiency is unlikely to be hazardous under normal conditions (see above table ), there is still uncertainty about abnormal conditions, for instance the degradation of insulation leading to a larger-than-expected release of gas.
The following are all undesirable features for a room in which LN2 is stored:
· poor ventilation
· small size
· large volume of LN2
· windowless door
· no oxygen deficiency alarm
PERSONAL PROTECTIVE EQUIPMENT
It should be remembered that PPE is not designed to withstand immersion in, or prolonged contact with, cryogenic liquids.
Splashing should be regarded as likely during any pouring operation. The PPE options are:
2. full-face visor
3. full-face visor with a chin guard.
Hands should be protected by special gloves. The possibility of LN2 being splashed into the glove via the cuff area needs to be considered in the risk assessment. Loose, standard-length, gloves are more susceptible to splashing, especially if one hand is holding the receiving vessel while the other tips the storage dewar. Alternatives are long-length gloves or standard gloves with elasticated cuffs.
If splashing on to the body is likely, an apron should be worn. It should be from non-woven fabric
and not have pockets. It should be long enough to protect the lower part of the legs.
Feet also need protection. Open-toed shoes and sandals are not suitable. If boots are worn,
trousers should be worn outside the boots, to prevent liquid running into the top of the boot.