Safe Handling of Liquid Nitrogen

Transcription

1 Safe Handling of Liquid Nitrogen Guidance for University Departments and Functions January 2001 Safety Services Office

2 Introduction Liquid nitrogen is a colourless, odourless liquid that boils at 196 C at atmospheric pressure. The density of nitrogen gas changes with temperature and below 5.5 C the gas is heavier than air. It is in widespread use in laboratories as a cryogenic liquid. This document covers the small-scale use of liquid nitrogen in laboratories. Departments are strongly advised to obtain a copy of the British Compressed Gases Association Code of Practice CP30, entitled The safe use of liquid nitrogen Dewars up to 50 litres. This is a very comprehensive guide and gives all the information that is needed for risk assessment and safe handling in a laboratory context. Hazards: Asphyxiation in oxygen deficient atmospheres. Small amounts of liquid vaporize into large volumes of gas one litre of liquid nitrogen becomes approximately 700 litres (0.7m 3 ) of nitrogen gas. An oxygen deficient atmosphere thus caused cannot be detected by the human senses. Combustion hazard. Liquid nitrogen can condense air from the atmosphere, which can lead to production of liquid containing a higher oxygen content than that of air and consequently a combustion hazard. This can happen if Dewars and insulated flasks containing liquid nitrogen are left open, allowing oxygen to condense, which gives rise to the risk of explosion or fire on contact with flammable solvents, oil or grease. Cold burns, frostbite and hypothermia from the intense cold. The liquid can freeze human flesh very rapidly. Objects cooled by liquid nitrogen can stick fast to the skin and the flesh may be torn on removal. Eyes are more vulnerable than skin and can be damaged by exposure to the cold gas as well as the liquid. Transient exposure to very cold gas causes discomfort in breathing and can trigger an asthma attack in susceptible people. Overpressurisation from the large volume expansion of the liquid. For example, if liquid nitrogen enters sample vials during storage, the vials when removed from the liquid nitrogen can become rapidly overpressurised and explode in the face of the user. Brittle fracture of materials such as carbon steel. Risk assessments In the past, risk assessment and selection of control measures has tended to concentrate on risk of contact with cryogenic liquid. In October 1999 an incident involving the handling of liquid nitrogen at a UK research unit resulted in the death of a member of technical staff. This incident has focused attention on the risk of asphyxiation. Risk assessments must be carried out for all activities involving liquid nitrogen in accordance with the Management of Health & Safety at Work Regulations These should include an assessment of the risk of handling the liquid nitrogen as part of the assessment of the overall risk of the activity, including the risk of asphyxiation as well as risks from contact with the cryogenic liquid. 1

3 If the same risk assessment applies to the handling of liquid nitrogen in the course of several different activities, a separate risk assessment could be drawn up for liquid nitrogen handling and appended to the risk assessments for the individual activities. Risk assessments should be used as the basis for formulating Standard Operating Procedures, emergency plans and staff training. The following should be considered when assessing the risks and selecting appropriate control measures. Written procedures Heads of Departments must ensure that written procedures such as Standard Operating Procedures or Safe Working Practices are in place. As well as helping to ensure that staff are aware of the correct procedures to be used, these help to demonstrate that there is a safe system of work. It is extremely difficult to demonstrate that safe procedures are in place if there are no written instructions for staff to follow. Procedures should aim to reduce the risk of accidental spillage, and in the event of a spillage to reduce the quantity that might be spilt. The format of these instructions can be decided by each Department, and may form part of the Departmental Laboratory Safety Booklet or Laboratory Safety Manuals as appropriate. Written procedures should not only specify what a member of staff should do, but be explicit about what they should not do. Staff responsibilities, management and supervision The Head of Department is responsible for ensuring that risk assessments are carried out. It is the responsibility of line management (whether or not they are called managers ) to ensure that staff are aware of risk assessments and safe working procedures, and are adequately trained. Therefore, anyone who directs or supervises the work of others must carry out these duties. Overall responsibility rests with the Head of Department, who must ensure that line managers adequately train and supervise their staff. It must be made clear which members of staff are authorised to use liquid nitrogen, what their responsibilities are and what they have permission to do. This should be documented as a list of competent persons, with their signatures to indicate that they have been trained and are competent. Training records should be kept without them, it is difficult to demonstrate that staff are adequately trained. Management should check on a regular basis that staff are following procedures. Lone / out of hours working Clearly, the risks are greater when someone is working alone, especially outside normal working hours when it might be difficult to summon help. Staff should not handle liquid nitrogen alone unless this is absolutely essential; whenever possible they should work in pairs. Departments should put special procedures in place if staff have to work out of hours, including instructions to be followed in case of emergencies. Inexperienced staff and students are especially at risk, and Departments should make it clear whether they are allowed to work alone or out of hours and, if so, what they may and may not do whilst unsupervised. In all cases, a risk assessment is required for lone working. 2

4 Room size and ventilation The risk assessment should take into account the location and room size. Staff should be aware of the increased risk of oxygen depletion in a small room. Room volume, volume of liquid nitrogen, and room ventilation should be considered together. If the room is mechanically ventilated, the possible failure of the air handling system should be taken into account in the risk assessment. In all cases it is recommended that the minimum volume possible be used; this makes handling easier and reduces the hazards from spillages. Cold rooms are poorly-ventilated small rooms and should not be used for the storage of liquid nitrogen vessels. The worst case scenario would be the loss of the entire contents of a liquid nitrogen vessel in a short period of time. The resulting oxygen concentration is calculated using the formula: %O 2 = 100 x V o V r where for nitrogen, V o = 0.21 (V r -V g ) V g = gas release, in litres spilled x 700, divided by 1000 to convert to m 3 V r = room volume in cubic metres. If the oxygen concentration could fall below 18%, suitable measures should be put in place to control the risk, such as filling or keeping vessels in a more suitable (betterventilated) room where practicable, fitting a permanent oxygen monitor, or use of smaller storage vessels. Worked examples can be found in the BCGA Code of Practice CP30 or obtained from the Safety Services Office. Use of lifts All departments must state in their safety policy or operating procedures that accompanying liquid nitrogen vessels in lifts is strictly forbidden. The correct practice is to obtain the lift key from the porters, lock-off the lift and send the vessel unaccompanied to the destination floor, ensuring that another member of staff is waiting at that floor to receive it. Even a small spillage can have a dramatic effect in such a small space as a lift - the evaporation of less than 0.5l of liquid nitrogen will, through displacement, reduce the available oxygen to a dangerously low level in a standard size passenger lift. Pressurised vessels Since the storage of liquid nitrogen under pressure presents a greater hazard, the need to keep vessels pressurised should be considered very carefully and allowed only when genuinely necessary. 3

5 Personal protective equipment (PPE) Gloves must be of a suitable type, e.g. chrome leather or nylon cryogloves. Woollen or other absorbent gloves must not be used. Sleeves should cover the ends of the gloves, and gauntlets should not be used as liquid can drip into them. Shoes or boots should cover the foot properly (sandals should not be worn). Trousers should be worn outside boots, not tucked in. Full-face visors or closed goggles are recommended, not ordinary safety spectacles or prescription spectacles. Where there is a risk from overpressurisation, such as when removing sample vials from a liquid nitrogen refrigerator, a visor must be used. PPE is almost always provided and conveniently located, but not always used. The Personal Protective Equipment Regulations 1992 require that employees use personal protective equipment provided to them, and that employers take all reasonable steps to ensure that the employees use it. The written procedure should make it clear that protective clothing and equipment are mandatory for all staff. Oxygen monitors Oxygen monitors can be helpful in some locations if they are correctly selected, installed, maintained and used. The risk assessment will determine whether an oxygen monitor would be appropriate. The purpose of using an oxygen monitor is to ensure that staff do not unknowingly walk into an oxygen-deficient atmosphere. Oxygen level alarms should therefore sound both inside the area to be monitored and outside. The sensor should be sited such that it is not susceptible to localised pockets of nitrogen, which would give rise to false alarms. Emergency procedures Written procedures should include instructions to be followed in case of emergency, such as a significant spill or the formation of an ice plug. The spillage procedures should include evacuation of personnel from the area, opening outside doors and windows to encourage dispersal, and closing internal doors as appropriate. First aid measures for cold burns and frostbite should also be included, and the department s first aiders should be made aware of these measures. The Material Safety Data Sheet, obtainable from BOC, sets out the correct First Aid procedures. The risk assessment will indicate the factors to be considered in emergency plans for particular areas. Staff should be trained in emergency procedures. Manual handling In some areas, users lift Dewar flasks in order to pour liquid nitrogen into other containers. The risk assessment should therefore include a manual handling risk assessment where necessary. If possible, a liquid withdrawal device (fitted to a compatible Dewar) or a tipping stand should be used. A spillage is most likely to occur when liquid is being poured, so staff should have clear instructions to follow and must wear the correct PPE. 4

6 Filling from bulk tanks and transport of containers Procedures must be in place for the transportation of Dewars around the premises, e.g. to and from filling points. As already stated, they must not be transported in passenger lifts accompanied by personnel. Carrying Dewars up stairs should be avoided where possible; if unavoidable, two people should carry the Dewar together. Care is also needed when transporting Dewars through doorways. When vessels are being filled from a bulk tank, care must be taken to minimise the risk to passers-by, who should be kept at a safe distance. Vessels being filled must not be left unattended. Liquid nitrogen vessels must not be transported in cars, either on or off the University premises. If using the bulk tank which is under the control of the Department of Chemistry, Departments must comply with all rules and procedures for its use issued by the Department of Chemistry. References and further reading 1. Care with Cryogenics, BOC Gases. Available free from BOC Gases. 2. Controlling the risks of inert gases, BOC Gases. Available free from BOC Gases. 3. Safety Bulletin SB : Oxygen-deficient atmospheres, Compressed Gas Association Handling LN 2 Safely, Cryosafe HSE issues warning about use of liquid nitrogen, HSE Press Release E229:99 18 November The Safe Use of Liquid Nitrogen Dewars up to 50 Litres. British Compressed Gases Association Code of Practice CP30, price including postage. Available from British Compressed Gases Association, 14 Tollgate, Eastleigh, Hampshire SO53 3TG, or can be ordered online from their website: The photograph on the cover is reproduced with the kind permission of Wessington Cryogenics. 5