Cryogen safety

Liquefied atmospheric gases, also referred to as cryogenic liquids, present several safety hazards. This article provides an overview on potential risks and precautions to be taken when handling cryogens.

These gases — including oxygen, nitrogen, argon and helium — are liquefied by cooling to low temperatures, and therefore present a number of potential hazards.

Risks

Over-pressurisation — When vaporised into gas,  liquefied gases increase in volume. This results in a large pressure increase if the volume change is restricted. Cryogenic systems must therefore be designed with adequate pressure relief on storage vessels and anywhere where liquid may be trapped, such as pipework between valves.

Embrittlement — The most significant consideration when selecting equipment and materials for low temperature use is that of possible brittle fracture. Metals used in any equipment should satisfy the impact test requirements of the design code being used.

Fire hazards — If the atmosphere is enriched with oxygen the likelihood and potential intensity of fire is increased. Combustible materials that are not usually combustible in air will burn fiercely in an enriched oxygen atmosphere. Clothing saturated with oxygen will burn vigorously with potentially fatal results.

Dense cold vapour — Due to the relatively high density of the cold vapour of the liquids, the gases may collect and persist in low-lying areas, posing an oxygen deficiency or enrichment hazard. Manholes, trenches, basements, drainage systems, underground service ducts and any low-lying, poorly ventilated areas may pose such a hazard.

Cold burns and frostbite — Due to the low temperatures of liquefied atmospheric gases, the liquid, cold vapour or gas can cause similar damage to the skin to heat burns. Unprotected parts of the skin coming into contact with uninsulated items of cold equipment may also become stuck to them and the flesh may be torn on removal.

Liquid air condensation — While nitrogen and helium appear to be safe from the risk of combustion because they are inert, these liquids are cold enough at normal boiling points to condense oxygen from the atmosphere. This produces higher oxygen content than normal air, increasing the risk of combustion.

Asphyxiation and hypothermia — The evaporation of inert cryogenic liquids or solids may, in evaporating, produce oxygen-deficient atmospheres, which will result in asphyxiation if breathed. Atmospheres containing less than 10% oxygen can produce brain damage and perhaps death. Low air temperatures arising from the proximity of liquefied atmospheric gases can cause hypothermia and all people at risk should wear warm clothing.

Effect of cold on lungs — Transient exposure to very cold gas produces discomfort in breathing and can provoke an asthma attack in susceptible people.

Exposure avoidance and safety

• Contact with cold surfaces — where possible insulate all exposed cold surfaces using suitable materials.
• Splashes and spillages — use suitable PPE; use appropriate manual handling equipment when moving vessels containing cryogenic liquids.
• Report all leaks immediately to site emergency response, emergency services and your supplier.
• Prolonged exposure to low temperature environments — use suitable insulating PPE; minimise time of exposure.
• Inadequate design/incorrect choice of materials — only use competent system designers; only use approved materials; conduct regular planned preventive maintenance; do not exceed the flow rate specified for the equipment; comply with relevant design standards.

Information and training

All people who work with low-temperature liquefied gases or systems using such gases should be given adequate training on the risks of asphyxiation, fire hazards, cold burns, frostbite and hypothermia. Special attention should be drawn to the insidious nature of the risks due to the rapidity of the effects, coupled with the fact than an operator may be completely unaware that a hazardous condition has developed. Fire response procedures, including locations of shut-off points, must be in place and training conducted.

Protective clothing

Protective clothing is only intended to protect the wearer handling cold equipment from accidental contact with liquefied atmospheric gases or parts in contact with it. Non-absorbent leather or insulated gloves should always be worn when handling anything that is, or has been recently, in contact with cryogenic liquids. The gloves should be a loose fit so that they can easily be removed if liquid should splash onto or into them. Gauntlet gloves are not recommended because liquid can easily splash into the wide cuff.

It is essential that clothing is kept free of oil and grease where oxygen is in use. Goggles or a facemask should be used to protect the eyes and face when carrying out operations where spraying or splashing of liquid may occur. Long-sleeved clothing should be worn. These clothes should be without open pockets or turn-ups where liquid could collect. Trousers should be worn outside boots for the same reason.

Warning signs

Warning signs should be displayed as necessary and barriers should be placed indicating the extent of the hazard. Any pictogram used should comply with Australian regulation AS 1319.

Image courtesy of BOC.