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BiteSize SHE (Code summary)
|1||Initial release||April 2012|
|1.1||Modifications to take into account pressure build up in systems||October 2012|
|1.2||Amendments to audit checklist||May 2013|
|1.3||Added document retention policy appendix||Feb 2014|
|1.4||Added reference to treatment of cold burns||Sept 2017|
|1.5||Minor changes to reflect the launch of SHE Assure||October 2018|
Across the STFC cryogens are widely used to cool equipment, for example, magnets and detectors, and to cool or store samples and materials. The hazards associated with the use of cryogenic liquids include:
- causing contact burns (by the liquid), frostbite or cold exposure (by the vapour);
- the ability to wick in woven materials, due to their low viscosity, making contact with the skin and entrapping cryogenic liquids within clothing;
- the potential for the liquid to rapidly convert to a large quantity of gas, which, especially in a confined space, can present a suffocation/asphyxiation or over pressurisation hazards; and
- they may be flammable and/or explosive.
This code is aimed at those using cryogenic materials and provides information regarding hazards, safe methods of work and provision of suitable personal protective equipment to reduce the risk of occupational exposure to cryogens. Many of the safety precautions observed for compressed gases also apply to cryogenic liquids with the addition of extremely low temperatures and vapourisation.
While there is no specific cryogenic safety legislation it is addressed through related general safety requirements under:
- The Health and Safety at Work etc. Act, 1974
- The Management of Health and Safety at Work Regulations 1999
- The Provision and Use of Work Equipment Regulations 1998
- The Control of Substances Hazardous to Health (COSHH) Regulations 2002 (as amended)
- The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR)
- Pressure Equipment Directive (PED) and the Pressure Systems (Safety) Regulations, 2000 (PSSR 2000);
- The Confined Spaces Regulations 1997; and
- Personal Protective Equipment at Work Regulations 1992.
This code applies to the use of all cryogenic materials such as; helium, nitrogen, hydrogen, methane, oxygen, neon and solid carbon dioxide. See Appendix 1 for hazard information.
Additional hazards arise when using flammable cryogenic materials such as hydrogen and methane where more stringent safety precautions are required to prevent fire and explosion; see STFC SHE Code 20 Controlling Explosive and Flammable Gases and Dusts.
The introduction of the Pressure Equipment Directive (PED) and the Pressure Systems (Safety) Regulations, 2000 (PSSR 2000) details the safety and certification requirements for of pressurised cryogenic storage vessels, seeSTFC SHE Code 33: Safety of pressure and vacuum systems.
This code does not address the design of containers for liquefied gases or transfer lines and pipe work but does include the siting of standard commercially manufactured storage facilities, vessels and equipment. However, in-house designed systems which are required to have suitable pressure relief systems must have all pressure relief valves registered for statutory inspection. Reference to STFC SHE Code 33 Safety of pressure and vacuum systems, where appropriate.
This Code addresses the specific hazards associated with cryogenic materials but does not address the many related hazards especially the treatment of cryogenic cold burns (see Appendix 8 of Code 36 – Management and provision of first aid). These are addressed by separate STFC SHE Codes which include:
STFC SHE Code 1 Lone Working (under development);
STFC SHE Code 11 Work in Confined Spaces;
STFC SHE Code 12 Manual Handling;
STFC SHE Code 20 Controlling explosive and flammable gases and dusts;
STFC SHE Code 33 Safety of pressure and vacuum systems;
STFC SHE Code 36 Management and provision of first aid; and
STFC SHE Code 37 COSHH: safe use of chemicals / hazardous substances.
The temperature at which refrigeration ends and cryogenics begins is not well defined. The National Institute of Standards and Technology (NIST) in the US have suggested that cryogenic temperatures are defined as those below 93.15K (-180°C). However, in the context for STFC we should consider all low temperature liquified gases. This will include the most commonly used cryogenics within STFC: liquid helium and liquid nitrogen, and also cover liquified natural gas which is produced at a temperature of approximately -162°C, as well as solid carbon dioxide.
3.2 Cryogenic Dewar
Cryogenic storage Dewars are specialised flasks and can take several different forms e.g. open flasks with loose-fitting stoppers and self-pressurising storage tanks. All Dewars have walls constructed of two or more layers with either a high vacuum maintained between them to provide good thermal insulation, or where there is a need to avoid a vacuum barrier the space is filled with a material such as perlite or similar insulating material.
3.3 Expansion Ratio
This is the ratio of the volume of the cryogenic liquid from the boiling point to normal ambient temperature and atmospheric pressure. Example expansion ratios (gas volume from liquid phase) and relative densities of the gas compared to air at normal temperature and pressure are given below:
|Gas volume from liquid||Relative Density|
|Solid carbon dioxide||1:554||1.520|
It should be noted that the conditions of comparison are standard temperature and pressure. Freshly evaporated gaseous cryogens can stay below ambient temperature for a considerable time. Nitrogen in particular will fill a room from the bottom up and displace the air upwards assuming there is minimal turbulence to the atmosphere in the environment of the room.
3.4 Cryogenic Storage Tank
This is an assembly constructed of an inner vessel and an outer jacket for insulation purposes. The insulation space is normally under vacuum.
4.1 Department Directors shall:
4.1.1 be aware of all cryogenic operations requiring oxygen depletion alarms.
4.1.2 ensure that their staff have sufficient competent resources to manage this hazard such that any identified control measures are implemented in line with this SHE code.
4.2 Line Managers (Group Leaders) and Supervisors shall:
4.2.1 ensure that their staff and others within their area of responsibility are aware of the requirements of this SHE Code and consider these issues when using and storing cryogenic liquids.
4.2.2 ensure that a suitable and sufficient documented risk assessment is undertaken as required by SHE Code 6 Risk Management. and that all control measures identified are implemented through the establishment of documented operating instructions / procedures for handling cryogenic materials, including their movement and transport, see Appendices 1 and 2.
4.2.3 ensure that risk assessments include calculations for oxygen depletion, see Appendix 2 and Appendix 3, and decisions with respect to the installation of oxygen depletion monitors are recorded. Installed oxygen depletion monitors shall be regularly tested and maintained, according to the maintenance regime, and records kept.
4.2.4 with reference to the above, ensure that adequate ventilation is provided in areas where cryogenic liquids are used or stored.
4.2.5 ensure that the cryogenic strorage tanks/equipment are inspected, tested and maintained according to a schedule based on risk assessment.
4.2.6 ensure that their staff and others using cryogenic materials in areas where they are responsible have been given information, instruction and training as to the hazards associated with these cryogens. This shall include the specific equipment to be employed, the precautions required, appropriate Personal Protective Equipment (PPE) and actions to take in the event of an emergency, specific guidance on responses to cryogenic emergencies can be found in Appendix 4.
4.2.7 ensure that the operator has suitable and sufficient supervision appropriate to his / her level of competence.
4.2.8 ensure a sufficient number of personnel, typically first aiders, are trained in the treatment of asphyxia and cold burns in the event of an incident, specific guidance on cryogenic first aid can be found in Appendix 4.
4.2.9 ensure that documented emergency procedures are in place in the event of a cryogenic liquid spillage.
4.2.10 ensure that all pressurised Dewars purchased, and valves used for pressure release on cryogenic systems, are registered for statutory inspection with the specific on-site group responsible for pressure systems.
4.2.11 ensure that all monitors and alarms are calibrated and maintained in accordance with a planned schedule and records of these activities maintained.
4.2.12 ensure that any vessel containing a cryogenic material is clearly marked with the name of the materials and the vessel shall only be used for that material, and that suitable hazard warning sings are affixed to the vessel.
4.3 Cryogen Users shall:
4.3.1 perform all work with cryogenic liquids in accordance with this code, local procedures / standard operating instructions for the equipment and good laboratory practices.
4.3.2 not store cryogenic storage vessels in confined or restricted spaces for example corridors or stairwells or accompany vessels in lifts.
4.3.3 only undertake work involving cryogenic materials after completing defined training for the safe use of cryogenic materials and instruction in the use of the equipment employed, see Appendix 5.
4.3.4 adhere to the controls and employ the PPE, defined in the risk assessment for the work they undertake with cryogenic materials, and ensure that warning devices, such as portable oxygen monitors, where required, are used properly.
4.3.5 only use Dewars for the specific liquids for which they were designed.
4.4 SHE Group shall:
4.4.1 provide advice and guidance on the implementation of this code and maintain a list of competent advisors for Cryogenic Risk Assessment.
British Compressed Gas Association (BCGA) Codes of Practice:
CP4 Industrial Gas Cylinder Manifolds and Gas Distribution Pipework (excluding acetylene) Revision 3: 2005.
CP23 Application of the Pressure Systems Safety Regulations 2000 to Industrial and Medical Pressure Systems Installed at User Premises Revision 1: 2002.
CP25 Revalidation of Cryogenic Static Storage Tanks Revision 2: 2004.
CP27 Transportable Vacuum Insulated Containers of not more than 1,000 litres volume. Revision 1. 2004.
CP30 rev 1 2008 The safe use of liquid nitrogen Dewars up to 50 litres.
CP36 Bulk Cryogenic liquid storage at user’s premises Revision 1. 2011.
Guidance Note GN11 Reduced oxygen atmospheres. The management of risks associated with reduced oxygen atmospheres resulting from the use of gases in the workplace 2007.
Cryogenics Safety Manual, a Guide to Good Practice. Safety Panel, British Cryogenics Council, 1982.
MRC Standards for Liquid Nitrogen Supply.
European Industrial Gases Association (EIGA) publications.
Useful list of websites relating to cryogenic materials.