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Cryogenic fluids

 

  The hazards arising from the use of low temperature liquefied gases (cryogenic fluids) are:
· Asphyxiation from oxygen deficient atmospheres (This is a problem particularly in poorly ventilated areas);
· Cold burns from the intense cold (The damage to the skin is similar to that caused by heat burns.  The eyes are particularly vulnerable to damage.);
· Overpressurisation from the large volume expansion ratio from liquid to gas on evaporation.;
· Explosion of vials stored in liquid nitrogen if the cap on the vial leaks - see guidance
· Fire in oxygen enriched atmospheres (While obviously a problem with liquid oxygen, it can also be a problem with liquid helium and sometimes with liquid nitrogen.  Near the surface of the cryogenic fluid is possible for oxygen to be condensed from the atmosphere causing localised oxygen enrichment.);
· The effects of very low temperatures on materials, (For example, normally ductile materials can become brittle and methods of jointing need careful consideration because of possible differential rates of contraction.)

Asphyxiation hazards
The ratio of volume of gas to volume of liquid for low temperature liquefied gases can be relatively high.  At 15 oC and 101.3 kPa the ratios are:
Nitrogen         682
Helium           738
Argon             822

Thus 1 litre of liquid nitrogen will produce 682 litres of gas.  Air normally contains 20.9% oxygen.  If liquid nitrogen is spilled in a room it is possible for an oxygen deficient atmosphere to be produced which can be hazardous to anyone in the room.  Oxygen deficiency initially leads to loss of mental alertness and distortion of judgement and performance.  This happens within a relatively short time, without the person's knowledge and without prior warning. 

Oxygen content
(Vol %)

Effects and symptoms
(at atmospheric pressure)

14-20.9 Increasing pulse rate and tiredness
11-14 Diminution of physical and intellectual performance without person's knowledge
8-11 Possibility of fainting after a short period without prior warning.
6-8 Fainting within a few minutes; resuscitation possible if carried out immediately.
0-6 Fainting almost immediately, death ensues; brain damage even if rescued

 It is important therefore that cryogenic fluids which could give rise to oxygen deficient atmospheres are stored and used in adequately ventilated rooms.  In determining whether ventilation is adequate consideration should be given to:
- Normal evaporation of the cryogenic fluids in the room;
- Filling losses from filling vessels from a warm condition;
- Spillage of the contents of the largest container in the room or boiling off due to failures such as loss of vacuum insulation or rupture of a bursting disc.

The following simple calculation can be used to establish a threshold for the amount of liquid nitrogen held in a room beneath which it is unlikely to be necessary to investigate further the possibility of an oxygen deficient atmosphere developing:
- 1 litre of liquid nitrogen can produce up to approximately 700 litres of nitrogen gas.  As a rule of thumb, a spillage or instantaneous release of 1/7th litre of liquid nitrogen per cubic metre of room volume, even in an unventilated room, may be dismissed as it will reduce the oxygen content by only 2%.
- Above this threshold more detailed scrutiny will needed and the adequacy and reliability of the ventilation in the room assessed.  Ventilation should prevent oxygen content falling below 18%.  If ventilation is not adequate consideration should be given to the use of oxygen monitors.  (It should also be remembered that cold nitrogen gas will be heavier than air and may accumulate at a low level.)
- Even if a room is adequately ventilated, consideration should be given to the possibility of a failure of the ventilation system and whether those working in the room would be aware of the failure.  Installation of alarms to indicate failure of ventilation or, in some circumstances, installation of oxygen monitors might be necessary.

Use of cryogenic fluids

1)   No one should work with cryogenic fluids until they have been thoroughly instructed and trained in the nature of the hazards and the precautions to be taken.  The hazards associated with cryogenic fluids will not be readily apparent to someone who has not received appropriate training.

2)   If contact with the cryogenic fluid is possible, goggles (or preferably a full face visor) must be worn.  Other protective clothing should be worn depending on the risk associated with the work.  Clothing should be non-absorbent and not have features which could trap a spillage of fluid (e.g. open pockets, turn-ups on trousers).  Trousers should be worn outside boots.  Watches and jewellery which could trap cryogenic fluid close to the skin should not be worn. 

If gloves are worn they should ideally by "Cryo-gloves".  (These are gloves which are specially designed for low temperature work.)  If other gloves are used they should be non-absorbent leather gloves and they should be a loose fit so they can easily be removed if fluid should splash into them.  Sleeves should cover the ends of the gloves.  Gauntlet gloves should not be used as they increase the likelihood of a spillage going into the glove

Those working with cryogenic fluids must be aware of the first aid treatment for cold burns

3)   Cryogenic fluids should be dispensed only into vessels of a type which have been designed for use with the particular fluids concerned. The vessels must have vented lids (unless they are of the self-pressurised type in which case they should be fitted with safety relief valves). The lid on a vessel should be secured down only if the securing method is integral to the manufacturer's design of the lid and permits the vessel to vent even when the lid is secured in place.  Vessels should be clearly labelled to identify the cryogenic fluid which they contain.

4)   When using liquid oxygen it is vital to ensure that equipment is scrupulously clean as dirt, oil or grease can pose a serious fire or explosion hazard.  Any jointing materials must also be oxygen compatible.

When using other cryogenic fluids it is good practice to use oxygen compatible materials in these cases as well.

5)   Materials used in experiments involving cryogenic fluids must be chosen with care.  The most significant considerations are that of brittle fracture and ensuring that joints are suitable for the temperatures which will be established.

Storage of bulk stocks of cryogenic fluids

1)   Someone should be responsible for ensuring that
- the storage areas are maintained in a satisfactory condition
- adequate ventilation is provided
- if necessary, oxygen monitors are installed and maintained according to manufacturer's recommendations
- necessary personal protective equipment is available
- appropriate warning notices are posted
- containers used for storage are suitable for purpose and are maintained in good condition

2)   A face visor and appropriate gloves must be worn when dispensing from or otherwise handling bulk stocks.

3)  If an oxygen monitor is fitted there should be arrangements for its operation which should include a prohibition on entering the area if the alarm has activated and a requirement to leave the area immediately if the alarm should activate.  Staff must receive training so they know the action to take should the alarm activate.

Transport of cryogenic fluids

Cryogenic fluids must never be transported in lifts accompanied by people.  They must always travel unaccompanied.  A lift cage is a confined space.  If the lift were to become trapped between floors for a period it is possible that rapid evaporation of the fluid due to failure of the container could cause the air to become unbreathable. 

 


GUIDANCE:  Vials stored in liquid nitrogen - risk of explosion