Hazardous Substances Policy - Control Measures POLICY

Transcription

1 Hazardous Substances Policy - Control Measures Aims of the policy Enhanced Good Chemical Practice for Work with Cryogenic Liquids INTRODUCTION Cryogenic liquids are liquefied gases which have very low boiling points such as oxygen, nitrogen, helium, etc. ALL cryogenic liquids present two principal hazards: Very low temperatures and the risk of serious personal injury or material damage Very high liquid-to-gas expansion ratios resulting in the risk of over pressurisation of holding vessels and with the exception of oxygen the risk of oxygen deficiency and therefore possible asphyxiation. Individual cryogenic liquids may also exhibit particular properties, some of which present quite severe hazards, for example, the fire risk associated with liquid oxygen. Since the year 2000 there have been 3 recorded fatalities in research institutions resulting from asphyxiation by liquid nitrogen. This Enhanced Good Chemical Practice policy sets out requirements additional or alternative to Good Chemical Practice for control of the exceptional risks from cryogenic liquids. The Code of Practice and Appendices associated with this policy contain further information on the properties of cryogenic liquids and the requirements for meeting the policy. Scope of the Policy This policy applies to the use and storage of all cryogenic liquids in connection with University activities. POLICY Heads of Budget Centres must make arrangements in areas under their control to ensure: a written assessment covers the risks relevant to the circumstances of use or storage the risk assessment describes, where necessary; control measures, emergency procedures, and individuals authorised to carry out various tasks associated with the use of liquid nitrogen only suitably constructed and labelled dewars or transportable liquid cylinders are used for cryogenic liquids; this equipment must be maintained according to the manufacturer s instructions and must comply with any requirements imposed by the University Health and Safety Policy on Pressure Systems (Statutory Inspection, Examination and/or Testing of Specified Equipment UHSP/16/SIET/01) oxygen monitoring equipment and warning systems are suitably sited and maintained the provisions within the Code of Practice are observed sufficient information, instruction and training is provided to users to enable them to understand the dangers associated with cryogenic liquids and how use them safely appropriate emergency procedures are in place in the event of a liquid spill 1

2 Risk assessment Schedule 3.12 CODE OF PRACTICE A written risk assessment must be prepared wherever cryogenic liquids are used or stored, describing any control measures required to minimise its dangers. The assessment must take into account the hazards associated with cryogenic liquids (Appendix 1), the physical properties of the cryogenic liquid in use (Appendix 2) and consider all relevant risks, including the risk of asphyxiation (see Appendix 3). Where appropriate, emergency procedures must be included (see Appendix 4), as should the names of those authorised to carry out certain safetyrelated or higher risk activities (e.g. inspection or maintenance work on dewars, or filling dewars or sample storage vessels from bulk supply tanks). Generic risk assessments (e.g. School risk assessments for liquid nitrogen) can usually adequately cover the risks of cold burns or explosion, but they are unlikely to consider the asphyxiation risk for individual areas in sufficient detail. Ventilation and oxygen monitoring In order to control the risk of asphyxiation, the following conditions are to be met for rooms where cryogenic liquids are stored or used. Rooms should be sufficiently well ventilated, or sufficiently large, to ensure that the oxygen concentration does not fall below 19.5% due to the routine conditions of use, i.e. due to: the normal evaporation losses from all cryogenic containers in the room the losses caused by filling the largest container from a warm condition. In addition, the loss of the contents of the largest container immediately after filling from a warm condition should not cause the oxygen concentration to fall below 18%. Appendix 3 details how to assess the likelihood of oxygen depletion and the effect of ventilation, and this must be done as part of the risk assessment. In most rooms, natural ventilation will generally provide around one air change per hour. For basement rooms, cold rooms, or where there are well-sealed windows, less than half an air change per hour will be achieved. Because they are tightly sealed, cold rooms are particularly unsuitable as storage areas for cryogenic liquids and they must not be used for this purpose. (In any case, there is no benefit to be gained from keeping a cryogenic liquid in a cold room the small temperature reduction relative to the laboratory has an insignificant effect on the evaporation rate of a liquid that is around -196 C). Cold gas accumulates at low level, so basement rooms, rooms with ventilation openings only at high level, or rooms with floor ducts or pits may pose particular danger in the event of a spill. Where natural ventilation openings are provided, they are to be at both high and low level and ideally have a total area of around 1% of the floor area. Where mechanical ventilation is provided, then air should be extracted from low level and supplied at high level. Where ventilation is insufficient to control the build-up of the cryogen gas, or where leaks or spills would reduce the oxygen content to below 18%, then fixed oxygen monitoring equipment must be used. Care should be taken in siting the oxygen sensors in order to avoid persistent false alarms caused by nuisance triggering (e.g. by direct exposure to gas issuing from containers as they are being filled). Where false alarms persist, then the sensors must be resited in order to prevent any consequent complacency in the response to alarms. In a well publicised incident in the UK, a worker was killed by asphyxiation following an incident that occurred while filling dewars. He had no warning of his fate - the alarms had been turned off because they gave continual false readings while dewars were being filled. The monitors should be positioned at a height of between 1m and 1.2m above the ground. This equipment normally has two alarm levels: 2

3 The upper level set at 19.5% O 2 (if this alarm is triggered, then there should be urgent investigation and corrective action) The lower level set at 18.5% O 2 (if this alarm is triggered, then the area should be evacuated immediately). Alarms must be visible and audible both inside and outside of the area monitored, in order to give adequate warning of oxygen depletion. The lower level alarm should produce a distinct visual and audible alarm signal. In some circumstances, personal oxygen monitors may usefully supplement fixed ones. All oxygen monitoring equipment must be installed, operated, serviced, and calibrated in line with the manufacturer s instructions. (Users should be aware that the working life of the common electrochemical cell oxygen detectors is only about one year). General precautions for small-scale use (a) Cryogenic containers Generally speaking, in quantities up to about 50 litres, cryogenic liquids are stored and distributed in simple open-topped vessels, designed to operate at atmospheric pressure ( tulips or dewar flasks). They are of lightweight construction and should be handled with care to avoid damage to the insulation. The smaller flasks may be easily knocked over. Larger quantities (up to 250 litres) are generally held in transportable liquid cylinders that may be designed to deliver liquid or gas. They operate at above atmospheric pressure, so they are fitted with safety devices to allow them to vent excess pressure. The manufacturer s recommended intervals for inspection and replacement of the safety devices must be observed. Care must be taken to ensure that any venting takes place safely (as supplied, many such cylinders have safety devices discharging horizontally at eye level) and venting may need to be directed to a safe place outside of the storage area. Transportable cylinders should be handled with care. In particular, trolleys used for moving them, or the trolley bases fitted to some cylinders, must be suitably designed and in good condition to avoid accidents resulting in the cylinder tipping over. The correct vessels must be used, designed and constructed in accordance with the relevant Code. The outlet of vessels that are not designed to be sealed must be kept free of obstruction (e.g. ice) to prevent pressurisation. Outlets may be loosely covered, but must never be stoppered. Vessels must be maintained in good condition. In addition to the checks carried out before filling and before transportation the appropriate maintenance procedures must be carried out on a regular basis, or at least at intervals not exceeding six months. Vessels operating at above 0.5 bar are subject to statutory examination in accordance with University Policy Users should be alert to the signs of insulation failure (the need for frequent topping-up, or excessive condensation on the dewar) as the high boil-off rate increases the risk of oxygen depletion. (b) Labelling All vessels containing cryogenic liquids must be clearly labelled showing basic safety-related information, using a label of the type shown in Appendix 6. Once labelled, a vessel must be used only for the indicated substance, unless it is ascertained that the vessel is free of the original substance and it is relabelled. (c) Filling Only those who have been suitably trained may fill dewars using a hose from a transportable container or bulk tank. This is a potentially dangerous operation and appropriate PPE must be used. Care must be taken to secure the hose, to purge the line of excess moisture or dust, and to initiate the fill slowly. If an excessively high fill rate allows an unsecured hose to whip out of the dewar, then the 3

4 situation may rapidly get out of control, with a high probability of injury or death from cryogenic burns and asphyxiation. Bulk storage vessels must be fitted with appropriate decanting equipment that includes a device for venting excess gas before it reaches the dewar. If operating above 1.5barg, the decant valve on the vessel should be a slow opening type, e.g. a globe valve, not a ball valve. The filling procedure identified in appendix 5 should be followed. (d) Handling Dewars should be handled with care and not walked, rolled or dragged along the floor - rough handling may damage them, as may severe impacts. Manual handling assessments (UHSP/6/MHO/95) will be needed for larger dewars (say > 20 litres) and these may identify a need for trolleys or tipping trolleys. Stairs and doorways present an added risk of spillage due to tripping, or colliding with someone. If a large dewar (say > 20 litres) must be carried on stairs, then two people should carry it, the use of additional body protection (e.g. an apron) is recommended and access to the stairway should be restricted. (e) Transport If cryogenic liquids are to be transported by road vehicle the arrangements must conform with University Hazardous Substances Policy for transport. The University's Insurers have imposed the following further requirements on the carriage of liquid nitrogen by road, and which apply equally to other cryogenic liquids: Dewars must be transported separately from the driver and passengers. There must be a separating bulkhead that gas cannot leak through, across the full height and width of the vehicle, separating the cryogenic liquid from the occupants of the vehicle. Where a van is used at least one window of the cab must be fully open while full containers are being carried. The vehicle must be clearly marked that it is carrying liquid nitrogen. Dewars should be checked for damage before transportation. Do not transport a full damaged dewar or a full dewar that has lost vacuum. Dewars shall be secure during transport to prevent spillage or damage. The driver shall carry a document with the following information: product UN number, product designation, i.e. NITROGEN, REFRIGERATED LIQUID, product classification code, i.e. Class 2.2, the volume of each dewar and number of dewars, the consignor and consignee names and addresses. N.B. A private motor car is unlikely to meet these requirements To safely transport samples at liquid nitrogen temperatures Dry Shippers can be used. These are dewars designed for the shipment of samples without the risk of a spillage, as when prepared correctly the Dry Shipper does not contain any liquid nitrogen. These can be purchased from a number of suppliers such as BOC and VWR. If a container of cryogenic liquid, no matter how small, is transported by lift: Only a lift authorised by the Budget Centre Health and health and safety co-ordinator must be used; The cryogenic liquid must not be accompanied by passengers; The carrier must ensure that others do not enter the lift; The unaccompanied transportation of cryogenic liquids in lifts must be supervised/monitored outside the lift by a competent person. (f) Sample storage containers Users should be aware that there is an oxygen-deficient atmosphere inside large storage containers. Care must be taken to ensure that people retrieving samples cannot lean over the containers in such a 4

5 way that they might breathe this atmosphere and collapse into or over the container, resulting in asphyxiation. Precautions for Bulk Scale Use Where bulk supply tanks are used, the consequences of an accident are potentially much more serious because of the quantity of cryogenic liquid present. Unless steps are taken to prevent it, the entire contents of the bulk tank may be lost. When such incidents occur, there is a high risk of loss of life. Therefore special care must be taken in the design and operation of such systems. Planned new installations, or alterations to existing installations, may not take place without first consulting the University Health and Safety Unit. All such installations must comply with BCGA Code of Practice CP 36 (copies available from the Health and Safety Unit). Bulk storage tanks of 500L capacity and above should be located outside the building in an area acceptable to the supplier. Oxygen monitoring must be provided where liquid nitrogen take-off points from a bulk supply tank are inside a building, whether for manual operation or for automatic filling of storage tanks. In these cases, the low oxygen (18%) alarm must be linked to an automatic valve that cuts off the supply from the bulk tank in the event of the alarm being set off. This link must operate in a fail-safe mode and be capable of operating in the event of mains power failure. Additional mechanical ventilation linked to this alarm should also be considered, with low-level extract and high-level air make-up. Only suitably trained and experienced individuals should be allowed to operate the system (e.g. to fill dewars or liquid nitrogen refrigerators) or to carry out installation or maintenance work on the system. Information, instruction and training All users of cryogenic liquids must have received information, instruction and training to enable them to understand the dangers associated with them and how this relates to their own work. A record of all training should be kept. Personal Protective Equipment Eye/face protection As a minimum, safety spectacles with side shields must be worn whenever handling liquid nitrogen. A face shield to BS EN 166 must be worn where there is a risk of splashing the face or eyes, e.g. during filling operations. Models with brow guard and chin guard offer the best protection. Hand protection For filling operations, non-absorbent, insulated gloves to BS EN 511, or loose fitting leather gloves (for ease of removal in case of spillage) must be worn and coat sleeves should cover the ends of the gloves. Gauntlets are not recommended as liquid may run down inside them. These gloves will also protect the skin from contact with objects that have been cooled by liquid nitrogen. Gloves are not intended to protect the hands against immersion in liquid nitrogen. Foot protection Open toed shoes must not be worn. Body protection As a minimum, a lab coat or overall should be worn. If boots are worn then trousers should be worn outside of them, not tucked into them. A splash resistant apron will give added protection where dewars are being lifted or carried, or wherever there is a high risk of splashing, e.g. during filling operations. 5

6 FURTHER SOURCES OF INFORMATION British Compressed Gas Association (BCGA), The Safe Use of Liquid Nitrogen Dewars up to 50 Litres, Code of Practice 30, Rev 1, British Compressed Gas Association (BCGA), Cryogenic Liquid Storage at Users Premises, Code of Practice 36, Rev 1, British Compressed Gas Association (BCGA), Bulk Liquid Carbon Dioxide Storage at Users Premises, Code of Practice 26, British Compressed Gas Association (BCGA),Transportable Vacuum Insulated Containers of not more than 1000 litres volume, Code of Practice 27, Rev 1, 2004 British Compressed Gas Association (BCGA), Cryogenic sample storage systems: biostores: guidance on design and operatio, Guidance Note 19, MRC & CryoService, Standards for Liquid Nitrogen Supply,