Cryogenic substances are liquefied gases that are kept in their liquid state at very low temperatures e.g. nitrogen, oxygen, carbon dioxide, helium and argon.
- extreme cold
- oxygen depletion (nitrogen, carbon dioxide, helium and argon)
- aids combustion (liquid oxygen only)
- skin burns
- embrittlement of materials
- asphyxiation (small amounts of liquid can expand into very large volumes of gas).
The following are some of the risks associated with the handling of cryogenic materials that can lead to oxygen depletion:
- over filling of the receiving vessel from a pressurised container will result in a spill. This can occur due to lack of concentration, distraction or leaving unattended;
- pouring from a large vessel into another vessel (particularly if the receiving vessel is small) can result in a spill;
- vessels which are not designed for liquid nitrogen may break and cause a spill;
- damage to a vessel due to an impact or blow will result in a rapid release of gas and/or liquid;
- boiling or splashing due to the receiving vessel being "hot" in comparison to the liquid will release gas and or liquid.
These risks must be controlled by the development of a safe system of work, which can be defined as the set of controls necessary to minimise the risks associated with the work.
Cryogenic substances should be used and stored in well ventilated areas
Personal Protective Equipment (PPE)
PPE is intended to protect the wearer from accidental contact with cryogenic substances and must never be immersed for this reason it should ALWAYS be inspected for damage before it's worn.
Protection should be provided as follows:
- faceshield should be worn to protect the face
- eye protection
- gloves with cuffs (EN511. Gauntletts are not recommended as liquid can splash into wide cuff of the gauntlett.
Areas where cryogenic substances are used and stored must have adequate ventilation. Adequate means that oxygen levels are maintained at 20.8% concentration during normal storage and handling.
Gas release can occur during storage of liquid nitrogen:
- from open non-pressurised vessels during routine operations;
- due to the warming of liquid nitrogen vessels. In the case of non-pressurised vessels this occurs naturally through the insulation. In pressurised vessels when the pressure increases the valve opens to relieve the build up and then resets;
- due to the failure of the pressure relief valve which will rupture the burst disc on pressurised vessels. This will cause a rapid release of gas in a short space of time.
The volume of the storage area in relation to the quantity of gas released will determine the extent to which oxygen will be depleted. You should calculate the oxygen concentration that would result should the full contents of the largest vessel be accidentally released in a short space of time i.e. worst case scenario.
If the resultant O2 concentration is greater than 18% then it may not be necessary to install an O2 depletion monitor, however if it is less than this a monitor is essential.
N.B. An alarm is not an alternative to the provision of a safe system of work but is an additional control measure, which gives an early warning that something has gone wrong, i.e. a spill or rapid release of liquid nitrogen. Guidance on monitors and detectors can be found at the following link:
If lift transportation is unavoidable, the lift must have a security control system to enable staff to maintain independent control of the lift during the transport operation. The container should be placed in the lift by a member of staff at the outset of the operation. Then, by means of the security control system, the lift must be sent directly to its destination floor with no intermediary stops. Members of staff must not travel in the lift with the container and no other person must be allowed to travel in the lift with the container. A second person should meet the lift at the destination floor and remove the container. The lift can then be released from the security control system and returned to normal use.
The following must be considered when preparing procedures to deal with emergencies:
Ensure that staff are aware of the action to take if:
- the oxygen depletion alarm is heard i.e. not to enter the area
- the oxygen depletion alarm sounds while working in the area i.e. evacuate
- the fire alarm sounds while working with cryogenic substances i.e. a procedure to make safe. If it has not been possible to make safe before evacuating the building then the fire brigade must be informed of the risk of oxygen depletion
Spills and Accidental Release
In the event of a large spillage or accidental release or whenever the detection monitor sounds, the following procedures should be followed:
- Evacuate the area. Deploy warning signs where necessary
- DO NOT ENTER ANY AREA WHEN A MONITOR IS SOUNDING
- Do not re-enter area unless it is safe to do so. The presence of oxygen deficiency monitors will indicate the oxygen levels in the vicinity
- Ventilate the area. Open doors and windows or activate forced ventilation to allow any spilt liquid to evaporate and the resultant gas to disperse.
- Try to stop the release if at all possible e.g. turn off valves, but only if it is safe to do so.