Cryogen and cryogenic hazard information for consideration when undertaking risk assessments

A1.1 Cryogenic Liquids

The low viscosity of cryogenic liquids means that they will penetrate woven or other porous materials much faster than for example water

A1.2 Solid Carbon Dioxide

Solid carbon dioxide, either dry or in a solvent, is used as a refrigerant for trapping water vapour, oil vapour etc, in systems under vacuum and for other purposes such as the cooling of metal parts to cause contraction where its vaporising temperature, 194.5 K (-78.50 °C), is sufficiently low.

A1.3 Liquid Nitrogen

Liquid nitrogen is used as a refrigerant for trapping water vapour, oil vapour etc., in systems under vacuum, and for other purposes such as the cooling of metal parts to cause contraction. Its normal boiling point is 77K (-196°C). Ice which is formed from frozen atmospheric water vapour can form on exposed liquid nitrogen systems.

The vacuum-jacketed vessels commonly used to store and transport liquid nitrogen, can typically lose upwards of 0.5% per day by evaporation.

The evaporation of one litre of liquid nitrogen produces some 696 litres of gas at standard temperature and pressure.

A1.4 Liquid Helium

Liquid helium is used for cooling superconducting materials and for experiments involving very low temperatures.  Its normal boiling point is 4.2K (-269°C). An additional common danger associated with using liquid helium can be the formation of liquid oxygen in localised areas, see Section 5.9.

The vacuum-jacketed low-pressure vessels commonly used to store and transport liquid helium lose upwards of 1% per day by evaporation.

The evaporation of 1 litre of liquid helium produces some 757 litres of gas at normal temperature and pressure.

The low temperature of liquid helium can solidify any other gas. Solidified gases and liquids allowed to form and collect can plug pressure-relief passages and foul relief valves. Users should endeavour to store and handle liquid helium under positive pressure and in closed systems to prevent the infiltration and solidification of air or other gases. Where this is not possible there are measures which can be taken e.g. use of a helium bottle or installation of a small heater in the helium, to maintain a positive pressure. The pressure relief systems installed should be positioned in such a way that they will not be blocked by freezing of any other gases leaking into the system. The pressure relief systems must be well maintained.

A1.5 Liquid Hydrogen

Liquid hydrogen at 20K is used as one of the moderators for the ISIS neutron spallation source. Its normal boiling point is -252.7°C.

Condensed air could result in oxygen enrichment and explosive conditions near a liquid hydrogen storage system. Further guidance on explosives atmospheres can be found in the Controlling Explosive and Flammable Gases and Dusts code (Safety Code 20).

Although it is non-corrosive hydrogen can cause some metals to undergo a process of hydrogen embrittlement. This is a process by which various metals become brittle and fracture following exposure to hydrogen, and particularly important where high-strength steels have been used because of their susceptibility to hydrogen.

If a liquid hydrogen leak or spill occurs, a hydrogen cloud will rise and could flow horizontally for some distance or even downward, depending on the terrain and weather conditions. Hydrogen is a very small molecule with low viscosity, and therefore prone to leakage. In a confined space, leaking hydrogen can accumulate and reach a flammable concentration. Hydrogen's flammability range is between 4% and 75% in air, which is very wide compared to other gases/vapours and the energy required to initiate combustion is much lower than for other common fuels. Hydrogen combustion is more rapid than combustion of other fuels but produces little overpressure in an open environment. In confined spaces hydrogen ignition can result in flame acceleration and generation of high pressures capable of exploding buildings and ejecting shrapnel.

A1.6 Liquid Oxygen

Liquid oxygen is not currently used on STFC facilities for any cryogenic purposes. Its normal boiling point is 90K (-183°C).

Organic materials such as oil, grease and hydrocarbons can react explosively, while other materials that are usually considered non-combustible may burn in contact with liquid oxygen.

A1.7 Liquid Methane

Liquid methane at 100K is used as one of the moderators for the ISIS neutron spallation source. Its normal boiling point is -161.5°C.

It has no odour and is therefore difficult to detect a leak. An odorant could be added but at cryogenic temperatures it is extremely difficult or impossible to add an odorant. However, it may also be undesirable to add the odorant impurity to the system.

The flammability range for liquefied methane is between 5% and 15%.

A1.8 Ice Formation

Accidental air leakage into a liquid cryogen storage vessel (e.g., from inadequate purging) will result in the introduction of moisture. The water may form ice plugs in the neck of open Dewars and cause a build-up of pressure.  As the pressure rises within the Dewar, the ice plug may be expelled at high velocity or in extreme cases the pressure may build up sufficiently to rupture the vessel.

Should an ice plug be found, extreme caution should be exercised and the area immediately vacated since the pressure built up within the system is unknown.  Clearing ice-blockages is a specialist job and under normal circumstances only carried out by someone with the appropriate knowledge and training. Personnel dealing with the incident should do so from a safe position.  Methods of clearing ice-blockages can vary but a hole can be pierced through the ice with a heated metal tube to release the pressure, since this will allow the pressure to be released without the tube becoming a projectile.

Ensure that the Dewar is examined by the manufacturer or a competent person before returning it to service.

Ice plugs can be prevented by diligent use of the correct Dewar stopper.

A1.9 Pressure build-up

Continuous evaporation generates a gaseous atmosphere and an increase in pressure inside a liquid cryogen storage vessel, which, if not properly controlled and released by suitable measures, can result in significant build-up of pressure. A pressure relief valve (PRV) of suitable specification, which has been registered for statutory inspection, should be used to prevent over pressurisation of the vessel or system.

A1.10 Oxygen enrichment

Operators should be trained to recognise that the low temperatures of liquid nitrogen and helium, having a lower boiling point than oxygen, can cause oxygen to condense out of atmospheric air.  This can occur around cold pipework, valves and in open Dewars.  This oxygen enrichment may result in increased flammability and explosion risk.  Oxygen enriched liquid must not be allowed to come into contact with oils or grease, or flammable materials as spontaneous combustion can occur. These contact areas should be cleaned to oxygen clean standards. The selection of appropriate insulating materials should be made based on the possibility of oxygen enrichment.

A1.11 Manual handling/ergonomic issues

The manual movement of storage Dewars, storage tanks and equipment can give rise to manual handling injuries to back, neck, hands and fingers and proper lifting and moving procedures should be adopted.

The layout of the work area should be ergonomically designed to ensure a safe system of working and to minimise or eliminate manual handling requirements as far as is reasonably practicable.  Consideration should be given to factors such as:

• Static storage tanks/equipment and piped liquid/gas supplies should be used in preference to mobile storage tanks/equipment wherever possible;
• Where mobile storage tanks/equipment are used, then it is essential that the handles and/or wheels on the storage tanks/equipment/trolleys are secure, and that all wheels turn freely. Wheels and axles employed on such equipment should, be subject to routine inspection and maintenance;
• Where large Dewars are used and maneuvered into position using the mobility handles, it is important that these handles should NOT be used for lifting the Dewar for which they are specifically NOT designed for this purpose.
• Wheels or platforms should be of adequate construction, and tested to withstand the weight of a full tank;
• To avoid impact damage to the tank, the base should be extended where possible to provide bumpers or similar protection; and
• If any manual handling is required, this must be subject to an appropriate risk assessment to ensure it is carried out safely without any risk of personal injury (e.g. back strains, hernias, sprains, cuts or fractures).
• Some Dewars should not be tipped or stored on their sides since they are not sufficiently robust to withstand this without damage being caused. There may be no indication of this on the Dewar.

A1.12 Cold contact burns

Liquid or low-temperature gas from any of the specified cryogenic materials will produce effects on the skin similar to a burn. This will vary with temperature and exposure time. Cryogenic fluids that are allowed to come into contact with human skin can cause severe damage to living tissue. Similarly, contact with uninsulated pipes etc. will cause contact burns and may result in the skin freezing to the uninsulated pipework etc.

Similarly the gases released as cryogenic liquids vapourise can permanently damage delicate tissues e.g. the eyes can be damaged by an exposure to cold gases too brief to affect the skin.

Never allow any unprotected part of your body to touch un-insulated pipes or vessels that contain liquefied gases since the extreme cold may cause the point of contact to stick to the metal by virtue of the freezing of the available moisture and tear the flesh when you attempt to separate the contact.

A1.13 Frostbite and exposure

Individuals not suitably protected against low ambient temperatures often associated with cryogenic handling may suffer cold exposure, slowing their reactions and capabilities and could lead to hypothermia. Continued exposure is likely to result in frostbite.

A1.14 Physiological effects

Effect of Cold on Lungs.

Short exposure may produce discomfort, whereas prolonged exposure and inhalation of vapour or cold gas, whether respirable or not, can produce serious lung effects.

A1.15 Toxicity

Most liquefied gases have low toxicity; however, in high concentration may cause nausea or dizziness. Even prolonged breathing of pure oxygen can result in harmful physiological effects.

A1.16 Asphyxiation

Liquid nitrogen is colourless, odourless, exists at -196°C (at atmospheric pressure), and is widely employed in cold storage applications. Virtually all common liquefied gases have no odour, and so cannot be detected by smell, exceptions being ethane and ethylene.

An increase in temperature through spillage, release or even simple exposure to surrounding air causes the liquid to boil and the cryogen to return to the gaseous phase. A large expansion in volume accompanies this change in physical state; one litre of liquid producing hundreds of litres of gas (see Section 3.3 for specific gases). This can result in a problem of asphyxiation through displacement of atmospheric oxygen, and will be applicable to cryogenic liquids in general.

Any reduction in the normal content of the oxygen in the breathing atmosphere must be considered a hazard. In sudden asphyxia, such as that from inhalation of pure nitrogen, unconsciousness is immediate. Other degrees of asphyxia will occur when the oxygen content of the working environment is less than 20.9% by volume.  Effects from oxygen deficiency become noticeable at levels below 18% and sudden death may occur at 6% oxygen content by volume. This decrease in oxygen content can be caused by a failure / leak of the cryogenic vessel or transfer line and subsequent vaporisation of the cryogen.

Asphyxia symptoms for low oxygen levels:

Oxygen depletion sensors are typically set to register an alarm at 19.5% oxygen.

A1.17 Explosion - pressure

Heat flux into the cryogen from the environment will vaporise the liquid and potentially cause pressure build up in cryogenic containment vessels and transfer lines.  Adequate pressure relief must be provided to all parts of a system to permit this routine out-gassing and prevent an explosion.

Liquefied​ gases are usually stored at or near their boiling points, and hence there is always some gas present in the container. Due to the large expansion ratio of a cryogenic liquid when vapourised a build up of high pressure can occur when the liquid evaporates, and as such there should be sufficient pressure relief valves in all lines between valves and between shut-off valves and downstream equipment. The evaporation rate will depend on the fluid, storage container design and environmental conditions, but the container capacity must include an allowance for the evaporation of the liquid into the gaseous state. The pressure relief devices should be maintained and checked regularly for leaks or damage.

Containers of cryogenic liquids must never be closed so that they cannot vent. Where a special vented stopper or venting tube is used the vent must be checked regularly to ensure it has not plugged with ice formed from water vapour condensed from the air.

Pressure relief valves must be installed on all vessels and piping which contain cryogenic liquids or might under some failure conditions contain the cryogenic liquid e.g. cryostat vacuum vessels. The maximum safe working pressure for all piping and vessels should be determined and used to identify the flow requirements for the relief valve in a worst-case failure scenario e.g. failure of a cryostat insulating vacuum to atmosphere; trapping of a cryogenic liquid due to valve failure or operator error etc. Selection of a pressure relief valve should not be just on the pressure value but also allow adequate flow rate to prevent further pressure build up occurring.

All system vents must be directed away from personnel or designated working areas.

A1.18 Explosion - chemical

Cryogenic fluids with a boiling point below that of liquid oxygen are able to condense oxygen from the atmosphere.  Repeated replenishment of the system can thereby cause oxygen to accumulate as an unwanted contaminant.  Similar oxygen enrichment may occur where condensed air accumulates on the exterior of cryogenic piping.  Violent reactions, e.g. rapid combustion or explosion, may occur if the materials which make contact with the oxygen are combustible.

Helium, Hydrogen and Nitrogen will all condense air creating conditions for a potential explosion hazard by causing oxygen entrapment in unsuspected areas. In addition, extremely cold surfaces are also capable of condensing oxygen from the atmosphere.

An atmosphere that contains more than 21% of oxygen by volume creates a dangerous fire hazard. Higher concentrations of oxygen will increase both the chance of a fire and its intensity and can involve materials normally regarded as being non-flammable. There should be no sources of ignition in areas where oxygen enrichment is likely to occur.

If there is any use of cryogenic oxygen itself this would fall within the scope of DSEAR and further guidance can be found in STFC SHE Code 20: Controlling explosive and flammable gases and dusts.

A1.19 Explosion - radiation

Under irradiation conditions, such as in a proton beam, liquid nitrogen can explode when the cryogen is warmed. This is thought to be due to reactions with ozone formed from trace amounts of oxygen in the cryogen.

A1.20 Embrittlement of materials​

When materials are cooled the Young’s modulus of the material will typically increase by around 20% down to liquid helium temperatures. This will increase the material’s strength and stiffness, but also the brittleness which could also cause failure of parts due to a change in this property.

A wide variety of materials become brittle at very low temperatures associated with cryogenic liquids, e.g. plastics, rubber, Teflon and carbon steels; and as such are unsuitable for use in such environments. However, there are a number of suitable metals and these include austenitic stainless steels, 9% nickel steel, copper and its alloys and aluminium alloys. Suitable plastics are reinforced plastics such as G-10 and G-11 (glass-epoxy composites).

A1.21 Fire

Think about the method of extinguishing fires in relation to the cryogen etc. e.g. CO₂ extinguishers can cause static discharge of sufficient magnitude to ignite some gas mixtures.

A1.22 Combustibles storage

Flammable substances and combustible materials should not be stored or allowed to accumulate in the vicinity of cryogenic liquid installations, and is particularly important where cryogenic oxygen is being stored due to issues of spontaneous combustion.

A1.23 Material stresses

Not all materials will respond well to contact with liquid cryogens or accidental cryogen spillages resulting in subsequent failure of the component’s integrity. The sudden contraction of materials that have become exposed to the cryogens could be sufficiently large to cause high internal stresses resulting in the failure of critical parts.