Health & Safety Guidance
Enhanced Good Chemical Practice for Work with Cryogenic Liquids
GUIDANCE/08/EGCPCRYO/20
Table of Contents 1 Introduction ...... 4 2 Responsibilities ...... 4 3 Hazards of Cryogenic Liquids ...... 4 3.1 Asphyxiation ...... 4 3.2 Cold burns and frostbite ...... 6 3.3 Explosions due to trapped, expanding gas ...... 6 3.4 Condensation of liquid oxygen ...... 6 3.5 Effects on materials ...... 6 4 Risk Assessment ...... 7 5 Ventilation and Oxygen Monitoring ...... 7 6 General Precautions for Small-Scale Use ...... 8 6.1 Cryogenic containers ...... 8 6.2 Labelling ...... 9 6.3 Filling ...... 9 6.4 Handling ...... 9 6.5 Transport ...... 9 6.6 Sample storage containers ...... 10 6.7 Disposal ...... 10 7 Precautions for Bulk Scale Use ...... 10 8 Information, Instruction and Training ...... 11 9 Personal Protective Equipment ...... 11 9.1 Eye / face protection ...... 11 9.2 Hand protection ...... 11 9.3 Foot protection ...... 12 9.4 Body protection ...... 12 10 Fill Procedure ...... 12 10.1 Pre-fill checks ...... 12 10.2 Filling ...... 12 11 Emergency Procedures ...... 13 11.1 Spillage ...... 13 11.2 First aid ...... 14 11.3 Ice plug formation ...... 15 12 Further Sources of Information ...... 15
Appendix 1 – Oxygen Depletion Calculations ...... 16 Assessment of Ventilation Requirements ...... 16 Calculating the Potential Oxygen Depletion in a Room due to Liquid Gas Filling and Spillage .... 16 Oxygen Depletion Example for Liquid Nitrogen ...... 18 Appendix 2 – Physical Properties of Cryogenic Liquids ...... 21 Appendix 3 – Liquid Nitrogen Transit Sign ...... 22
2020 GUIDANCE/08/EGCPCRYO/20
1 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: i. Very low temperatures and the risk of serious personal injury or material damage. ii. 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.
This Enhanced Good Chemical Practice guidance sets out requirements additional or alternative to Good Chemical Practice for control of the exceptional risks from cryogenic liquids. This document contains further information on the properties of cryogenic liquids and the requirements for meeting the policy, and applies to the use and storage of all cryogenic liquids in connection with University activities.
2 Responsibilities The following responsibilities are in addition to those within the University’s Health and Safety Policy, and specifically relate to the arrangements for work with cryogenic materials.
Senior managers 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 - UHSP/16/SIET/01 Statutory Inspection, Examination and/or Testing of Specified Equipment. • Oxygen monitoring equipment and warning systems are suitably sited and maintained. • The provisions within this guidance document are observed. • Sufficient information, instruction and training is provided to users to enable them to understand the dangers associated with cryogenic liquids and how to use them safely. • Appropriate emergency procedures are in place in the event of a liquid spill.
3 Hazards of Cryogenic Liquids The hazards of cryogenic liquids are largely related to the large volume of gas produced on evaporation and to the liquid’s low temperature. The very low viscosity means that they rapidly and completely cover surfaces on which they are spilt and easily penetrate cracks and voids. This means that any spillage on clothing will penetrate much more readily than, say, water. Large spillages on other surfaces may affect areas beneath the surface, by damaging materials or even by causing oxygen depletion in areas remote from the spill.
3.1 Asphyxiation On boiling, cryogenic liquids produce hundreds of times their volume of gas. For example, 1 litre of liquid nitrogen produces approximately 700 litres of nitrogen gas. The resulting displacement of oxygen from the atmosphere may be sufficient to cause asphyxiation.
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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.
In these incidents, 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. Smaller leaks or spills, or normal boil-off from liquid nitrogen containers in confined spaces (e.g. poorly ventilated small rooms, or cold rooms) may give rise to lesser reductions in oxygen content, but they may still carry a risk of asphyxiation.
The risk of asphyxiation must be assessed wherever cryogenic liquids are used or stored, taking into account the volume present in relation to the room volume, the likelihood of leakage or spillage, the normal evaporative losses that occur with liquid nitrogen use and any ventilation arrangements.
Appendix 1 shows how to calculate the oxygen depletion arising from normal evaporative losses and from spills. As an approximation, if the volume of nitrogen gas (m3) produced from the complete loss of the contents of the largest container in the room is > 0.15 x room volume (m3), then this corresponds to an oxygen content of around 18% (air normally contains 21% oxygen) and further action must be taken to control the risk of asphyxiation.
The physiological effects of reduced oxygen are shown in Table 1. Note that exposure to an atmosphere containing less than 18% oxygen poses a significant risk.
Oxygen Concentration in Air Physiological Effects 18 – 19.5% Oxygen May affect physical and intellectual performance without person’s knowledge. 15-18% Oxygen Decreased ability to work strenuously. May impair co- ordination and may induce symptoms in persons with coronary, pulmonary, or circulatory problems.
12-15% Oxygen Respiration deeper, increased pulse rate, and impaired co-ordination, perception and judgment.
10-12% Oxygen Further increase in rate and depth of respiration, further increase in pulse rate, performance failure, dizziness, poor judgment, blue lips.
8-10% Oxygen Mental failure, nausea, vomiting, fainting, ashen face, blue lips.
6-8% Oxygen Fainting within a few minutes. Resuscitation possible if carried out immediately. Potential for brain damage.
0-6% Oxygen Fainting almost immediately. Death ensues fairly promptly. Brain damaged even if resuscitated.
Table 1 Physiological effects of a reduced oxygen atmosphere.
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3.2 Cold burns and frostbite Skin contact with cryogenic liquids or cold cryogen 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, or short exposures to cold vapour or gas, may cause instant freezing of eye tissues and permanent damage.
These injuries can be avoided by ensuring that users always wear appropriate personal protective equipment (PPE) as described in Section 9. First aid treatment for cold injuries is described in Section 11.2.
3.3 Explosions due to trapped, expanding gas If a cryogenic liquid is trapped inside a container that is sealed, then expansion on warming above their boiling point (-196°C for liquid nitrogen) may cause an explosion, giving rise to danger from contamination by the vessel’s contents as well as injury from fragments of the vessel itself.
This is most likely to happen if sample storage vials have been immersed in the liquid. The shrinkage and embrittlement of materials at these temperatures render any sealing system ineffective and the relatively low surface tension of the liquid also makes it likely to seep into the vial. Similar explosions have been reported with glass vessels - the low temperature caused microscopic cracks or holes to open, which then resealed on warming. Great care must be taken to avoid injury from such explosions.
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. Domestic vacuum flasks must not be used for cryogenic liquids.
3.4 Condensation of liquid oxygen The boiling point of oxygen is -183°C; therefore liquid oxygen may condense in open containers of liquid nitrogen, neon and helium or in open vessels cooled by liquid nitrogen (e.g. cold traps). Liquid oxygen will accumulate if the cryogenic liquid 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 oxidisable material is present, then liquid oxygen may react explosively with it.
Cryogenic liquids are normally handled in insulated vessels, often with narrow necks in which the gas acts as a barrier against oxygen contamination. However, oxygen contamination can arise in a wide mouthed vessel which is not closed. Oxygen can also condense out in vacuum systems to which cold traps are applied before pumping down.
3.5 Effects on materials Many materials become brittle when cooled by cryogenic liquids and may be irreparably damaged. Other materials (e.g. glass Dewars) may fail due to temperature stresses.
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Use only articles or materials designed for use with cryogenic liquids. Glass domestic vacuum flasks must not be used as they may fail due to thermal shock on filling.
4 Risk Assessment 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 (Section 3), the physical properties of the cryogenic liquid in use (Appendix 2) and consider all relevant risks, including the risk of asphyxiation (see Appendix 1). The risk assessment should include the storage location (i.e. ensuring cryogenic material is not allowed to evaporate in enclosed areas such as fridges, cold rooms etc.), take transport routes into account (e.g. use of lifts and stairs), and details of any specific PPE required. Emergency procedures must be included (Section 11), as should the names of those authorised to carry out certain safety related 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.
5 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 1 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).
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Where false alarms persist, then the sensors must be resisted 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: The upper level set at 19.5% O2 (if this alarm is triggered, then there should be urgent investigation and corrective action). The lower level set at 18% O2 (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. Written instructions of the action to take in the event of monitor activation must be prepared and displayed outside the facility.
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).
6 General Precautions for Small-Scale Use 6.1 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.
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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.
6.2 Labelling All vessels containing cryogenic liquids must be clearly labelled showing basic safety-related information.
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.
6.3 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 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 1.5bar, 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 Section 10 should be followed.
6.4 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. Vessels should be kept upright at all times, except for when pouring liquids from Dewars specifically designed for that purpose. Always protect the vessel from severe jolting and impact. Manual handling assessments (UHSP/6/MHO/95) will be needed for larger Dewars (> 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 (> 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.
6.5 Transport When transporting Dewars, the following aspects should be considered: Is the Dewar likely to pose a manual handling problem? Is the destination ready to accept it? Does the route take you through populated work areas? Is the route passable (steps, kerbs etc.)? Are there any slip/trip hazards (including stairs) that could result in spillage? Is the Dewar going to be transported by lift (Section 6.5.2)? The correct PPE should be worn.
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.
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6.5.1 Transportation by road If cryogenic liquids are to be transported by road vehicle the arrangements must conform to University Hazardous Substances Policy – Transport of Hazardous Substances. Transportation of cryogenic substances is covered by the Carriage of Dangerous Goods by Road Regulations 2009. These regulations cover volumes of dangerous goods that may be transported, duties of responsibility, packaging and labelling of goods, vehicle usage and driver training. The University's Insurers have imposed further requirements on the carriage of liquid nitrogen by road, 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.
6.5.2 Transportation by lift If a container of cryogenic liquid, no matter how small, is transported by lift: Only a lift authorised by the local 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. Always place a sign in the lift with the Dewar, warning individuals not to enter until the lift reaches its destination. A copy of such a sign is given in Appendix 3. The unaccompanied transportation of cryogenic liquids in lifts must be supervised/monitored outside the lift by a competent person.
6.6 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 way that they might breathe this atmosphere and collapse into or over the container, resulting in asphyxiation.
6.7 Disposal Ensure that the area in which the cryogenic liquid is left to vaporise is well ventilated. Do not store cryogenic substances or allow them to vaporise in enclosed areas such as fridges, cold rooms, sealed room etc. Do not pour cryogenic liquids down the sink – they will likely crack waste pipes, potentially causing leaks.
7 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
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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 University Safety Services. All such installations must comply with BCGA Code of Practice CP 36 and be designed specifically for use.
They must display hazard-warning signs to alert people of the presence of cryogenic liquids, along with warnings of ‘no smoking’ and ‘no naked flames’.
Bulk storage tanks of 500L capacity and above should be located outside the building in an area acceptable to the supplier. They should have a safe means of escape.
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.
8 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. The following mandatory training must be completed for all those undertaking work with cryogenic materials: • Chemical Safety training • Cryogenic Materials Canvas course • Local, hands-on training by a competent person
A record of all training should be kept.
9 Personal Protective Equipment 9.1 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.
9.2 Hand protection For filling operations, non-absorbent, insulated gloves to BS EN 511:2006 (Cold protection) must be worn. Coat sleeves should cover the ends of the gloves. Gauntlets are not recommended as liquid may run down inside them, instead gloves should be securely banded at the wrist or arm. These gloves will also protect the skin from contact with objects that have been cooled by liquid nitrogen. The material should be rough to give good grip whilst handling and not increase the chance of spillage. Gloves are not intended to protect the hands against immersion in liquid nitrogen. Instead, tongs or forceps should be used for such operations.
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9.3 Foot protection Open toed shoes must not be worn. If boots are worn then trousers should be worn outside of them, not tucked into them.
9.4 Body protection As a minimum, a lab coat or overall should be worn. There should be no pockets that liquid could get trapped in. 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.
All metallic jewellery should be removed because metal will quickly spread the cold from any contact with the cryogenic material, possibly extending burns.
10 Fill Procedure Dewar filling shall be carried out by properly trained personnel wearing appropriate protective clothing (see Section 9).
When filling Dewars that are for sample storage, there is a risk of cross contamination of the samples via the fill hose. If this is a possibility, the user should include in the procedure a method for preventing cross-contamination.
10.1 Pre-fill checks Check that the supply vessel is in an appropriate location and at the correct operating pressure. If the pressure is too high ensure that someone trained to do so vents the tank. Check that the Dewar is labelled for liquid nitrogen service. Do not fill a Dewar which is labelled for another product. Check that the filling equipment is clean and free from damage. Do not attempt to use blocked or damaged filling equipment. Ensure that the Dewar is not fitted with a liquid withdrawal device. Initiating the fill with the device in place may lead to over filling or over pressurisation of the Dewar. Excessive pressure may result in the device detaching from the Dewar at high speed. Before removing the device ensure that the Dewar is vented to atmospheric pressure by opening the vent-valve fully and ensuring that the pressure gauge is reading zero. Check that the Dewar is in good condition. Ensure that there is no neck damage or twisting. Ensure that the insulating bung under the protective cap has not detached. If it has, fit a new cap before filling. If the bung has fallen into the Dewar then it must be removed. Do not fill a Dewar which is damaged or has the bung inside. Do not fill the Dewar if: o there is water inside; o there is ice inside; o there is excessive frosting around the neck.
10.2 Filling The filling procedure shall include the essential elements listed below: • Purge the hose to clear any excess atmospheric moisture or dust. This can be done by securing the hose and cracking the decant valve slightly for a short period. Close the valve as soon as frosting appears. • Insert the fill hose into the Dewar and ensure it is secure. • Initiate the fill slowly by cracking open the fill-valve. If the Dewar has warmed the liquid will boil and turn to gas immediately on contact.
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• When the Dewar has cooled the fill-valve can be opened to establish a steady flow of liquid. If liquid is spitting back out of the Dewar then the flow should be reduced. • For Dewars with neck tubes, stop the fill when the liquid reaches the bottom of the neck. The “sound” of the fill will change, indicating that it has happened. Do not fill past the bottom of the neck. • For Dewars that do not have neck tubes, stop the fill when the liquid reaches the required level, which shall be a level below that which the insulating bung will reach when placed onto the Dewar after filling. Never overfill a Dewar. • When the Dewar is full, replace the protective cap. If the cap rattles, this is evidence that the Dewar is over filled and liquid is boiling at a greater rate than is normal. Leave the Dewar in the open air until there is no excessive boiling. • If fitting a liquid withdrawal device, fit it immediately after the fill, ensuring that the Dewar has not been overfilled. (Rapid gas boiling should indicate overfilling). Check the pressure indicator on the device to ensure the pressure rise has stabilised at 0.1-0.2 bar. If the pressure is rising towards 0.5 bar, open the vent valve on the device and reduce the pressure. Check the pressure indicator again and repeat the venting cycle as many times as is necessary to obtain a steady pressure reading. Inability to achieve a steady pressure reading is an indication of loss of vacuum from the insulating jacket. Test that the liquid line is clear of ice blockage by operating the liquid valve momentarily, allowing liquid to issue out. • Check that the labelling has not been damaged by liquid spills during the fill. Replace if necessary.
11 Emergency Procedures As part of the risk assessment for use of cryogenic materials, written emergency procedures should be held, detailing what to do in the event of a spillage, accident or unforeseen event. These procedures should include what action to take if the oxygen depletion alarm sounds, evacuation procedures etc. Accidents and incidents should be reported to Safety Services via the accident/incident form.
11.1 Spillage The method described in Appendix 1 allows the effects of a cryogenic liquid spill on the oxygen content of a room to be assessed. The results of the calculations may identify a need for evacuation if a spillage above a certain size occurs. If this is the case, then the risk assessment should identify this limit and should also specify what to do if such a spill occurs, or if the low oxygen alarm (18 vol. %) is triggered. It should take into account: who may be affected by the spill; the means of raising an alarm; possible escape routes; the means of isolating the supply of liquid nitrogen, especially if supplied from a bulk tank; the means of preventing access to the area until the oxygen content returns to normal; the possibility of liquid nitrogen affecting other areas (e.g. by penetrating floors, or by accumulating in ducts).
The assessment should specify what to, and what not to do, if someone has collapsed in an area of low oxygen concentration. Attempts at rescue by poorly equipped and untrained rescuers are likely to lead to more casualties. Rescue should not be attempted if this is likely to put the rescuers in danger (such attempts have led to deaths or casualties amongst rescuers). Instead, the supply of cryogenic liquid must be isolated and attempts must be made to ventilate the area (e.g. by opening external doors and windows, but without entering the oxygen-deficient atmosphere). The Fire Service must be called as fire-fighters are the only persons likely to be suitably prepared for such rescues.
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In the event of a minor spillage (< 1 litre): Allow the liquid to evaporate, ensuring adequate ventilation. Following return to room temperature, inspect area where spillage has occurred and notify any equipment or infrastructure damage to your local school/departmental contact. Notify your local Health and Safety co-ordinator and report the incident to Safety Services via the accident/incident form.
In the event of a major spillage (> 1 litre): Evacuate all personnel from the area likely to be affected by the liquid and the evolved gas. Try to prevent the gas flowing along the ground into pits, basements, cellars and stairwells by closing doors. The cold gas will collect in those areas. Take appropriate action to ensure that the ventilation system does not spread the gas to other areas. Open exterior doors and windows to encourage evaporation of the liquid and safe dispersal of the gas. Allow the liquid to evaporate naturally. The evolved gas will be very cold and will create a cloud of condensed water vapour restricting visibility. Do not allow anyone to enter this cloud. Do not allow anyone to enter the area until you are sure that the gas has all dispersed and that the air is safe to breathe. If in doubt, use an oxygen monitor to check oxygen levels. Notify your local Health and Safety co-ordinator and report the incident to Safety Services via the accident/incident form.
11.2 First aid 11.2.1 Contact with cryogenic liquids Contact between cryogenic liquids and eyes or skin should be treated immediately by flooding the affected area with large quantities of cold water, followed by the application of cold compresses. Never use dry heat.
If the skin is blistered or the eyes are affected, medical attention must be obtained as soon as possible.
11.2.2 First aid - cold burns Cold contact burns will require medical attention as soon as is practicable. Frozen tissues are painless and appear waxy with a pallid discoloration. During thawing, frozen tissues become painful, swollen, and are very prone to infection when thawed. Thawing should be induced slowly with the aim of completion after arrival at hospital or of medical attention.
Until medical attention is available, the following first aid measures may be employed: Remove any clothing that may constrict the blood circulation to the frozen area. Clothing which has stuck to frozen tissue must not be removed until completely thawed. Do not permit smoking or the consumption of alcoholic drinks as these will decrease blood flow to the frozen tissue. Thawing is commenced by flushing the area with tepid water to return the tissue to normal body temperature. Never use dry heat as this and wet temperatures above about 45°C may cause further burns. A massive exposure to cryogenic liquid which has caused the general body temperature to be depressed, will require re-warming by total immersion in a bath. Precautions against shock must be taken following any accident involving cryogenic liquids. If the thawing of frozen tissue is complete before the arrival of medical attention or before arrival at hospital, the affected area must be well-covered with dry sterile dressings.
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11.2.3 First aid – eye contact Flush the eye with running water for at least 15 minutes and ensure the casualty is taken to the Eye Hospital for assessment.
11.2.4 Inhalation of inert gases Dizziness or loss of consciousness while working with cryogenic liquids must be treated by moving the affected person immediately to a well ventilated area. Artificial respiration and treatment for shock should be given as necessary.
11.3 Ice plug formation If an ice plug forms there is danger that: It will detach at high velocity when the Dewar pressure rises. It will cause sufficient pressure build up in the Dewar to cause rupture.
Extreme caution must be exercised if an ice plug is found. All personnel, except the minimum number required to deal with the incident, should be evacuated from the area.
The recommended method for dealing with the plug is to insert a copper tube into the neck and blow warm nitrogen gas onto the blockage. Compressed air is not recommended as it contains moisture. To do this safely, sandbag the Dewar before approaching it. Carefully insert a copper tube into the neck. Do not make contact with the ice blockage. The gas supply should be set up so that the defrosting process can be initiated in a remote or protected position. Once the defrost has been initiated the operator can retire to a safe place whist the blockage is being cleared.
The pressure build up may have damaged the inner wall of the Dewar. Therefore, ensure that the Dewar is examined by a competent person before returning it to service.
12 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, 2008. • British Compressed Gas Association (BCGA), Cryogenic Liquid Storage at Users’ Premises, Code of Practice 36, Rev 1, 2011. • British Compressed Gas Association (BCGA), Bulk Liquid Carbon Dioxide Storage at Users’ Premises, Code of Practice 26, 2004. • 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: bio stores: guidance on design and operation, Guidance Note 19, 2012. • MRC & Cryo Service, Standards for Liquid Nitrogen Supply, 2008.
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Appendix 1 – Oxygen Depletion Calculations Assessment of Ventilation Requirements The following guidance is based on that provided in British Compressed Gases Association Code of Practice 30.
The rate at which a room is ventilated is usually expressed in the number of air changes per hour.
In locations above ground level with no special ventilation openings, natural ventilation provides typically 1 change per hour. However, a lower value will apply where windows are sealed with tight seals. For underground rooms with small windows 0.4 changes per hour is assumed.
Mechanical ventilation is considered necessary where the ventilation requirement is for more than 2 changes per hour.
Calculating the Potential Oxygen Depletion in a Room due to Liquid Gas Filling and Spillage
Five cases are considered: a) Evaporative loss from the storage tank b) Filling losses which always occur when a Dewar is being filled c) Spillage of the contents of the Dewar d) The 'next worst case' where the entire contents of the vessel are lost to the room immediately after the Dewar is filled e) The “worst case” where there is catastrophic failure of the full storage tank. a) Evaporative loss from storage tank
Evaporation is a continuous process, hence the oxygen concentration in the air (C∞) can be calculated over a long period using:
푉푟 × 0.21 × 푛 퐶∞ = 퐿 + (푉푟 × 푛) where: 3 Vr = room volume, m 0.21 = the normal concentration of oxygen in air, 21% n = air changes per hour L = gas release m3/hr
In order to allow for a deterioration in the insulation performance over the life of the tank it is prudent to double the manufacturer’s quoted evaporation rate.
b) Filling A value of 10% of the volume of the product in the Dewar is used to estimate the losses to atmosphere during filling.
0.1 × 푉퐷 × 푓푔 푉표푙푢푚푒 표푓 표푥푦푔푒푛, 푉 = 0.21 [푉 − [ ]] 푂 푟 1000
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100 × 푉표 푅푒푠푢푙푡𝑖푛푔 표푥푦푔푒푛 푐표푛푐푒푛푡푟푎푡𝑖표푛, 퐶표푥 = 푉푟 where: 0.21 = the normal concentration of oxygen in air, 21% 3 Vr = room volume, m 0.1 = 10% volume loss during filling VD = Dewar capacity, litres fg = liquid to gas expansion ratio 3 VO = volume of oxygen, m c) Spillage For the spillage of the entire contents of a Dewar:
푉퐷 × 푓푔 푉 = 0.21 [푉 − [ ]] 푂 푟 1000
100 × 푉표 푅푒푠푢푙푡𝑖푛푔 표푥푦푔푒푛 푐표푛푐푒푛푡푟푎푡𝑖표푛, 퐶표푥 = 푉푟
where: 0.21 = the normal concentration of oxygen in air, 21% 3 Vr = room volume, m VD = Dewar capacity, litres fg = liquid to gas expansion ratio 3 VO = volume of oxygen, m d) Filling and spillage together The 'next worst case', where the entire contents of a Dewar are lost to the room immediately after filling, equivalent to 110% of vessel contents to allow for the 10% filling losses prior to spillage:
1.1 × 푉퐷 × 푓푔 푉 = 0.21 [푉 − [ ]] 푂 푟 1000
100 × 푉표 푅푒푠푢푙푡𝑖푛푔 표푥푦푔푒푛 푐표푛푐푒푛푡푟푎푡𝑖표푛, 퐶표푥 = 푉푟
where: 0.21 = the normal concentration of oxygen in air, 21% 3 Vr = room volume, m 1.1 = 110% volume loss during filling and spillage VD = Dewar capacity, litres fg = liquid to gas expansion ratio
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3 VO = volume of oxygen, m 3 Vr = room volume, m e) Catastrophic failure of storage tank For the release of the entire contents of a tank:
푉푡 × 푓푔 푉 = 0.21 [푉 − [ ]] 푂 푟 1000
100 × 푉표 푅푒푠푢푙푡𝑖푛푔 표푥푦푔푒푛 푐표푛푐푒푛푡푟푎푡𝑖표푛, 퐶표푥 = 푉푟 where: 0.21 = the normal concentration of oxygen in air, 21% 3 Vr = room volume, m Vt = tank capacity, litres fg = liquid to gas expansion ratio 3 VO = volume of oxygen, m
Oxygen Depletion Example for Liquid Nitrogen A basement room contains two 25 litre and three 10 litre Dewars. Room dimensions: 7 x 8 x 2.5 metres =140m3, 5 litre Dewar: loses 0.2 litres per day through evaporation 10 litre Dewar: loses 0.15 litres per day through evaporation (Dewar manufacturers' quoted evaporation rates).
Normal evaporation losses
Evaporation is a continuous process, hence the resulting oxygen concentration in the air (C∞) can be calculated over a long period using: 푉푟 × 0.21 × 푛 퐶∞ = 퐿 + (푉푟 × 푛)
Whilst manufacturers will quote the evaporation rate for their Dewar, it is prudent to double it when calculating the rate of nitrogen release, L. This allows for a deterioration in the insulation performance over the life of the Dewar. The nitrogen gas factor of 683 at 15C has to be used to calculate the volume of gaseous nitrogen released through evaporation, as the Dewar manufacturer's figures relate to the volume of liquid nitrogen lost.
Thus: 2 × 683 × (2 × 0.2 + 3 × 0.15) 퐿 = = 0.048푚3/ℎ 24 × 1000
Assume there is an average of 0.4 air changes per hour in the room. The oxygen concentration is, therefore: 18 | P a g e
140 × 0.21 × 0.4 퐶 = = 0.2098 = 20.98% 표푥 0.048 + (140 × 0.4)
Thus, in this case, evaporation from the five Dewars in the circumstances described would reduce the oxygen concentration by some 0.02%.
In this example, normal nitrogen evaporation from the Dewars has only a small effect in increasing the nitrogen concentration, and thus reducing the oxygen concentration, in the room. If, however, far more Dewars were stored in the same room used in the above example, or if a much smaller room was used for the five Dewars mentioned, then the nitrogen concentration would increase by a much larger factor. If Cox in such a case was calculated to be 0.20 (i.e. 20%), then forced ventilation would be recommended since this would reduce the oxygen concentration in the room by 1%, which is at the level where the safety margin has been virtually used up.
Losses due to filling
First calculate the volume of oxygen in the room, Vo, using:
0.1 × 푉퐷 × 푓푔 푉 = 0.21 [푉 − [ ]] 푂 푟 1000
The same Dewars and room size are used (140m3), but here the largest nitrogen release is during the filling of the largest (25 litre) Dewar. Again the nitrogen factor of 683 must be used to convert liquid to gaseous nitrogen. Thus:
0.1 × 25 × 683 푉 = 0.21 [140 − [ ]] = 29.04푚3 푂 1000
The resulting oxygen concentration in the room (Cox) can then be calculated using:
100 × 푉표 퐶표푥 = 푉푟
Thus:
100 × 29.04 퐶 = = 20.7% 표푥 140
Clearly, this is acceptable. As a guide it is recommended that the combined effect of normal evaporation and filling processes should give rise to alarm if the oxygen level falls to 19.5%.
Losses due to filling and spillage
Following the same process as above, calculate the volume of oxygen in the room (Vo) as a result of the spillage of the entire contents following filling, using:
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1.1 × 푉퐷 × 푓푔 푉 = 0.21 [푉 − [ ]] 푂 푟 1000
3 Again we have a 140m room and again the largest release is from the 25 litre Dewar. Thus:
1.1 × 25 × 683 푉 = 0.21 [140 − [ ]] = 25.5푚3 푂 1000
Then calculate the resulting room oxygen concentration (Cox) after the spillage, using:
100 × 푉표 퐶표푥 = 푉푟
Thus:
100 × 25.5 퐶 = = 18.2% 표푥 140
This is just above the level (set at 18%) at which oxygen monitors are usually set to give an emergency alarm, leading to immediate evacuation.
In this example, it is recommended an oxygen monitor be fitted with two levels of alarm: 19.5% should lead to urgent investigation and corrective action 18% should cause immediate evacuation - assuming that this level results from spillage.
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Appendix 2 – Physical Properties of Cryogenic Liquids
Oxygen Nitrogen Argon Neon Krypton Xenon Helium Boiling point / °C -183 -196 -186 -246 -169 -109 -269 Melting point / °C -218 -210 -189 -248 -152 -140 -272 Liquid-to-gas at 15°C 1:842 1:683 1:824 1:1415 1:689 1:533 1:739 expansion at 20°C 1:857 1:695 1:838 1:1440 1:701 1:542 1:752 ratio Liquid density kg m-3 1142 807 1395 1206 2415 2942 71 Gas density relative to 1.12 0.98 1.4 0.7 2.93 4.61 0.07 dry air at 15°C % (v/v) gas in air 20.9 78 1 0.0015 0.0001 0.00008 0.0005
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Appendix 3 – Liquid Nitrogen Transit Sign
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