A risk assessment will be required for the initial design of facilities, processes and equipment handling cryogenic materials. A subsequent risk assessment will then be required for their use. General guidance on the Risk Assessment process can be found in STFC SHE Code 6 Risk Management.
A risk assessment will be required for handling cryogenic materials at the initial design and for their subsequent use. General guidance on the Risk Assessment process can be found in STFC SHE Code 6 Risk Management. Factors that should be taken into account include:
- The cryogen being used.
- Maximum volume of cryogen required to be held at point of use.
- Maximum volume of cryogen required to be held in storage vessel.
- Maximum number of samples to be stored.
- Type and quantity of sample storage vessels to be used (including back-up systems).
- Operations to be performed at point of use.
- Lone working.
- Location or options for location (if there is more than one possibility).
- Manual handling and ergonomic considerations.
- Maintenance of plant.
- Confined spaces.
- Restricted access.
Suitable control measures should be identified as part of this risk assessment and should include controls for:
- Design and layout of the room and equipment.
- Ventilation requirements.
- Oxygen depletion monitors and alarms.
- Safe systems of work.
A2.1 Experimental Design (Large Scale)
The siting and installation of any storage tank, associated transfer lines and equipment should be carried out by a trained and competent specialist contractor, taking into account the findings of the risk assessment. It is the responsibility of the person carrying out the siting and installation of the storage tank to ensure that the work is carried out in accordance with the appropriate regulations / codes of practice, and that all parts / components used are safe and suitable for the intended application.
The British Compressed Gases Association (BCGA) Codes of Practice provide guidance and a framework to assist with fulfilling the legal duties and achieving best practice (see references in Section 5).
External storage tanks must have sufficient clearance on all sides to comply with BCGA CP36.
The design of piped gas delivery systems for cryogenic liquids needs to conform to the requirements of the Pressure Systems Safety Regulations 2000 (as amended) and internal STFC guidance and is dealt with in Safety Code 33.
A2.2 Experimental Design (Small Scale)
Small scale laboratory use of cryogens should only be conducted in well-ventilated areas and / or where oxygen depletion sensors are installed. The control unit for the detector should be installed outside the area where the cryogen(s) and detector are situated such that it can be checked before entering the area. Only the minimum amount of cryogen should be used, taking into account the calculations in Appendix 3 which determine the maximum cryogen quantities permitted such that the percentage content of oxygen of the room would not be sufficiently reduced. Excess quantities should not be stored in the laboratory.
Ensure ergonomic design of the work area to minimise or eliminate movement of the tanks/equipment and manual handling requirements.
Ensure a safe location for the cryogenic storage tanks/equipment is provided, and where possible site storage tanks/cylinders outside buildings and avoiding confined spaces and the liquid cryogen piped into the workplace.
STFC recommendation is that nitrogen storage tanks of 500 litres capacity and above must be located outside, in a location acceptable to the supplier, and designated a “No Parking” area.
Where necessary, the storage tanks/cylinders should be secured against accidental impact damage by vehicles using the installation of suitable safety rails or bollards.
Careful consideration should be given to external venting of all gas exhausts/pressure relief lines from the cryogenic storage tanks/equipment; for example where the gases are flammable they should not be vented near ignition sources, or where the relative density is greater than air venting in proximity to confined spaces should be avoided.
The specification, installation, use, inspection and maintenance of the cryogenic storage tanks/equipment should be correct for the usage.
The requirement to provide adequate ventilation to avoid or minimise the accumulation of asphyxiant gases and atmospheres in the workplace is addressed in section 4.2.3. This should take into account the following:
For rooms above ground level with no special ventilation openings, natural ventilation will provide typically one air change per hour. With well sealed windows (e.g. double glazing), this will be less. Basement rooms only average 0.4 air changes per hour.
For general handling of transportable cryogenic vessels in locations at or above ground level, natural ventilation is generally sufficient, provided the room is large enough, or the outdoor area is not enclosed by walls.
An indoor location should have ventilation openings, e.g. door and wall vents, with a total area of at least 1% of the floor area. The openings should be at low level (e.g., 0.5 to 1.0m from floor) and be positioned diagonally across the room.
Cold nitrogen gas is heavier than air and will accumulate at low level. Where possible liquid nitrogen should not be handled in basement rooms, rooms with ventilation at high level only, and rooms where the gas can be trapped in pits or gullies.
Helium is lighter than air and as such will rise and fill a room from the ceiling downwards. Providing an enclosed space is not completely full of helium a person suffering from helium asphyxiation will tend to collapse to the floor where oxygen levels will be higher and the likelihood of survival greater.
Either natural ventilation (air vents, windows etc.) or forced ventilation (mechanical extraction system) may be used depending upon the number of air changes needed per hour; a forced ventilation system is usually necessary for more than 2 air changes per hour.
Consideration must be given to the use of oxygen level meters and alarm systems in areas where the ventilation is poor.
A2.5 Fixed Oxygen Monitors/Alarms
Additional control measures may also be necessary to warn of the development of an unsafe atmosphere and / or assist in ensuring people do not remain in an unsafe area where an asphyxiating atmosphere may exist. These may include any of the following:
- Oxygen depletion monitors must be fitted wherever the risk assessment indicates that oxygen levels may be depleted to less than 19.5%.
- The monitors are installed to test the atmosphere in the workplace e.g. laboratory before entry, and give clear warning during occupancy of that area.
- The alarm should activate at 19.5% and produce a visible and audible alarm signal.
- Any activation of the alarm should give rise to concern and immediate action and any person inside the room should isolate the source of asphyxiant if safe to do so and leave immediately.
- Where the risk assessment indicates that oxygen levels could be depleted to below 18.5%, there should be two levels of alarm, at 19.5% and 18.5%.
- The second alarm should produce a distinct visual and audible alarm signal.
- Fixed monitors are preferable to personal monitors as they protect all personnel in the workplace, rather than just an individual person, and the monitors should normally be positioned near to the potential asphyxiant exposure points, at a height in the order of 1m above the ground.
- External alarm repeaters linked to the fixed monitors should also be installed where necessary, and sited so that they are clearly visible and audible to personnel before entering the workplace.
- The oxygen depletion monitors may also be used to initiate any of the following actions on reaching the second action level of 18.5%:
- To activate a solenoid operated safety shut-off valve to isolate the liquid nitrogen supply from the storage tank.
- To raise the air extraction rate (typically from 10 - 15 air changes per hour to 20 - 30 air changes per hour).
- To activate a door interlock system to prevent access (but allow escape). For rescue purposes override facilities should be provided.
- Where a mechanical extraction system is used and if the ventilation fails for any reason, the system can also be linked to a solenoid operated safety shut-off valve system to isolate the liquid supply from the storage tank.
Restoration of normal operation following an emergency shut-down, following oxygen depletion to below 18.5%, should only be carried out by a competent person.
A2.6 Safe Systems of Work
The following safe working practices should be considered and incorporated into a suitable written safe system of work for use with cryogenics:
Safe working practices:
- Any vessel containing a cryogenic material must be clearly marked with the name of the materials and the vessel shall only be used for that material.
- If cryogenic vessels are to be transported to another area the route should be assessed prior to movement; and the assessment should include: obstructions and clutter; floor surfaces, lifts, stairs, kerbs, ramps and steep inclines, moving through populated areas, suitable rest stops for those moving vessels etc.
- Wherever possible avoid transporting liquid cryogen containers up or down slopes, steps or stairways;
- Wherever possible avoid transporting liquid cryogen containers through densely populated areas;
- Wherever possible avoid transporting liquid cryogen containers over uneven ground or in areas where there is poor lighting or any risk of slips, trips or falls;
- Wherever possible avoid transporting liquid cryogen containers through restrictive access ways;
- Liquid cryogen containers must be regularly inspected and maintained to ensure they are in good condition, in particular the wheels, bearings and axles;
- If the use of a stairway to move a liquid cryogen container is unavoidable, then the following additional safety precautions should be considered:
- If the container is large or bulky, it is recommended that two people carry the container;
- Ensure that access to the stairway is restricted to only the people carrying the container;
- Consider wearing additional personal protection against spillage;
- Consider the installation of a stair lift where practicable.
- Open transfers of cryogenic liquids and venting or purging operations should be carried out in well ventilated areas.
- Care should be taken when handling cryogenic liquid storage Dewars. Avoid mechanical shock and damage to the vessel’s vacuum insulating jacket. Breakdown of the insulation will cause rapid boil-off of the liquid contents, producing large quantities of gas and a possible increase in pressure within the vessel.
NOT store vessels in confined or restricted areas for example corridors or stairwells.
- Transport of vessels and Dewars in lifts must be covered by a risk assessment and operating instructions / procedures. Vessels and Dewars should only be transported in service lifts or lifts for which there is manual key control to prevent passengers entering the lift on intermediate floors if possible. However, if this is not possible alternate controls must be employed e.g. multiple persons, barriers and signage to prevent lift use during the movement of cryogens; the vessels and Dewars must
NOT be accompanied in the lift.
- Specific care should be taken when open ended hollow dipsticks for determining cryogen depth in non-pressurised containers are used because they will produce a spout of liquid cryogen due to the rapid expansion of the liquid and gasification inside the tube.
- Ensure that there are suitable control measures in place if there is a potential for any lone working directly with cryogenic materials.
- Liquid helium Dewars should be of an approved multi-layered insulation and wide necked type.
- Liquid helium Dewars should not be completely drained before returning to the helium plant.
- Liquid helium Dewars should not be tilted.
A2.7 Personal Protective Equipment (PPE)
Exposure to the hazards arising from cryogenic materials is often attributable to the wearing of inadequate PPE. The following guidance should aid selection of items of PPE to be used when handling cryogenic materials.
- Suitable eye protection, such as safety glasses or a full face safety visor, is essential whenever handling or transferring cryogens to protect against unexpected liquid splashes. Full face shields should be used when transferring liquids to an open container.
- Appropriate hand protection should be worn when working with liquid cryogens to protect against possible liquid splashes, or touching equipment that has been in direct contact with the liquid. Gloves should be non-absorbent insulated safety gloves which are designed to prevent cryogens from flowing into the gloves. Tongs should be used to remove objects immersed in cryogenic liquids. Non-absorbent gloves should always be worn when handling solid carbon dioxide or anything that is or may have been in contact with cryogenic liquids or vapours. Gloves are particularly useful in preventing burns from associated cold pipework and valves.
- Where possible, the body should be covered to protect against possible liquid cryogen splashes, or equipment that has been in contact with the liquid such as un-insulated pipes or vessels. The clothing should be made without pockets or turn-ups where liquid can collect. Trousers, which should be cuffless, should be worn outside the footwear. Shorts should not be worn.
- Where liquid nitrogen is being used above floor level (e.g. on a bench or being carried up and down steps at chest height), consideration should also be given to wearing additional splash protection such as a splash-resistant apron.
- Suitable footwear to prevent the ingress of cryogenic liquids in the event of a spillage should be worn. Avoid open-toed shoes or sandals and those made of porous materials e.g. suede, which would allow the liquid to penetrate the surface.
- If there is a likely requirement for an oxygen depletion monitor and there is no fixed installation present then a portable oxygen depletion alarm may be appropriate.
Metal objects e.g. watches, jewellery, rings etc. should not be worn as metals can become frozen to the skin. Objects that are soft and pliable at room temperature usually become very hard and brittle at the temperatures of these liquids and can be prone to being broken easily producing sharp edges etc.
Any confined area where cryogenic liquids are to be used must have appropriate oxygen depletion systems. They must be maintained and suitable to warn all personnel either working in the area or before they enter an area where oxygen depletion is possible.