Abstract
Asphyxiation due to the creation of an oxygen depletion atmosphere (ODA) is by far the biggest cause of fatalities when working with cryogenic liquids. Each year, an average of 20 deaths occur in Europe involving people entering an ODA and between 1992 and 2002 in the USA, 85 nitrogen asphyxiation incidents were recorded, in which 80 people were killed. The likely causes of an ODA are confined spaces and poor ventilation, combined with the processing or storage of cryogenic liquids. Prevention can be made by providing well ventilated work areas, supported by the installation of oxygen monitors and alarms. A formal risk assessment document is part of the critical safety provision for personnel working in cryogenic areas. It stipulates how the prevention of an asphyxiation hazard is managed through the introduction of robust control measures and should include oxygen depletion calculations.
Asphyxiation
In general, there is a good perception and awareness of the hazards associated with the use of liquid cryogens in the work place. Areas where liquid cryogens are stored or used are commonly provided with oxygen monitors, appropriate personnel protective equipment (PPE) and safe systems of work. However, asphyxiation due to the creation of an oxygen depletion atmosphere (ODA) is by far the biggest cause of fatalities in these environments. Each year, an average of 20 deaths occur in Europe involving people entering an ODA and between 1992 and 2002 in the USA, 85 nitrogen asphyxiation incidents were recorded, in which 80 people were killed [2].
By definition, asphyxiation is the effect on the body of inadequate oxygen, usually resulting in loss of consciousness and/or death. It is surprising therefore to find reports indicating that in spite of training, the provision of safety information and the correct implementation of risk controls, serious accidents continue to occur resulting in asphyxiation when using cryogenic liquids [1].
Oxygen
Oxygen is the only gas that supports life and there is approximately 21% concentration in the air we breathe. Cognitive skills such as thinking and decision making are impaired when this concentration falls only slightly below this value. Significantly, these effects are not noticeable to the affected individual as the brain cannot detect low oxygen levels. The body’s drive to breathe is solely in response to the carbon dioxide levels in the blood. The working environment therefore becomes hazardous as soon as the oxygen concentration inhaled is less than 18%. With no oxygen present, inhalation of only 1 or 2 breaths of nitrogen or other inert gas will cause sudden loss of consciousness and can cause death.
Causes of ODA
There are three key factors causing an oxygen depletion atmosphere:
Evaporation of liquefied gas, where even a small amount of liquid cryogen on release will eventually convert into a large amount of vapour resulting in a corresponding reduction in the localised oxygen concentration value.
Confined spaces, where the working environment is designed in such a way to prevent sufficient dispersal of any evaporated liquefied gas.
Inadequate ventilation, where the working environment does not have a continuous replenishment of air to both disperse liquefied gases and to maintain the necessary oxygen concentration value.
Ventilation
Adequate cross-ventilation must be provided where cryogenic liquids are used or stored. For rooms above ground level with no special ventilation openings, ventilation will provide typically one air change per hour. Basement rooms only average 0.4 air changes per hour. Consideration must therefore be given to the use of oxygen level sensors and alarm systems in areas where the ventilation is deemed poor.
The correct positioning of fixed sensor is important in order to capture the highest risk situation. Ideally, sensors should be located in positions determined by those who have the knowledge of gas dispersion, the process systems and the equipment being used in the work area. In general, for gases lighter than air the sensors should be above the area where leaks are most likely. For gases heavier than air, the sensors must be at floor level or in inspection pits or ducts into which the heavy gas may flow.
Risk assessment
It is prudent for a risk assessment to be prepared for all areas where the potential for an ODA is possible. This would include confined spaces, areas with insufficient ventilation, places where the storage of cryogenic liquids is made and areas where the processing of cryogenic liquids, such as decanting, are undertaken. These assessments should include oxygen concentration calculations for two possible situations: the normal evaporation of cryogens and the sudden release of a cryogen from a storage vessel.
Normal evaporation of cryogens
where
Sudden release of cryogen from a storage vessel
where
Conclusion
Safety for personnel working in areas where the creation of an ODA is possible is of paramount importance. Risk controls need to be in place based on the findings of an all-encompassing risk assessment.