Oxygen Deficiency Hazards (ODH) are a significant safety issue in cryogenic facilities. ODH occurs when inert gases such as nitrogen, helium or argon displace room air and thus lower the percentage of oxygen in the space below that required for human life. OSHA defines an area as oxygen deficient if the percentage of oxygen is less than 19.5% by volume.
ODH is a particular hazard in cryogenics due to the very large volume change between a cryogenic liquid (or cryogen) at its normal boiling point and the resultant gas at room temperature and pressure. For example, 1 liter of liquid helium at 4.2K and 1 Bar results in roughly 700 liters of helium gas at 300K and 1 Bar. Nitrogen and argon have similar volumetric ratios and neon is even worse, with a volumetric ratio of 1341 between the cryogenic liquid and the gas at room temperature and pressure. The result is that small amounts of cryogenic liquids, if converted to gas, can produce inert atmospheres that are unable to support life.
It’s very important to understand that this is not a hazard associated only with large cryogenic facilities or applications. Even small amounts of cryogenic liquids can create lethal conditions in small areas and rooms.
Adding to the hazard is that at sufficiently low concentrations of oxygen, the first indication of a problem is sudden unconsciousness, followed rapidly by coma and death.
Oxygen deficiency hazards can be safely mitigated. The key is to first be aware of the problem and then to determine the extent of the hazard. Once this is done, appropriate mitigations may be taken to reduce the hazard.
Anyone dealing with cryogenics should calculate the effect of a release of the cryogens on the oxygen concentration in the room or space. Among the questions to consider are: “What happens if the release of cryogens occurs during non-working hours? Will the area be rendered unsafe for staff who reenter it at the start of the next working day?” Scenarios such as equipment failure or the opening of a relief valve should also be examined.
Typical ODH mitigations include the use of fixed or personal oxygen monitors that alarm if the oxygen concentration becomes unsafe and enhanced ventilation systems (which would have to be treated as safety systems with appropriate reviews and backups). The best mitigation is to use engineering design to reduce the problem by techniques such as minimizing the amount of cryogens in use, maximizing the size of the space in which they are used and ensuring that relief systems vent outside the building.
It is crucial that all staff involved with cryogenics and visitors to areas containing cryogenics receive training about oxygen deficiency hazards and the proper response to events or alarms.
There are a large number of resources for the determination and mitigation of oxygen deficiency hazards. US Department of Energy Laboratories such as Fermilab, Jefferson Lab and SLAC all have formal ODH policies and accepted techniques for determining ODH risks. These are available to the general public via the following links: http://2csa.us/3q ( Fermilab) and http://2csa.us/3s (SLAC).
The Compressed Gas Association also has developed a number of valuable documents on ODH safety. These may be found on the CGA website.
Recent papers on oxygen deficiency hazards include: “Analysis for Liquid Cryogen Spillage in the Superconducting Cyclotron Building at VECC,” Roy, et al., Proc. ICEC22-ICMC2008 (2009), “Diffusion of Gases in Air and its Affect on Oxygen Deficiency Hazard Abatement,” Theilacker and White, Adv. Cryo. Engr. Vol 51. (2006), “Summary of the Experimental Studies of Cold Helium Propagation along a Scale Model of the LHC Tunnel,” Chorowski et al., Adv. Cryo.Engr. Vol. 49 (2004) and “Investigation of Personal and Fixed Head Oxygen Deficiency Hazard Monitor Performance for Helium Gas,” Arenius et al., Adv. Cryo. Engr. Vol 47. (2002).
