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Cryogenic and Oxygen Deficiency Hazard Safety
Overview Requirements Standards Definitions    


Cold Burns

The most likely cause of cold burns to the hands and body is through contact with metal surfaces at extremely cold temperatures. Cold burns can occur almost instantaneously, especially when the skin is moist.

The damage from cold burns occurs as the affected tissue thaws. Intense hyperemia (abnormal accumulation of blood) usually takes place in the affected areas. Additionally, a blood clot may form along with an accumulation of body fluids, which decreases the local circulation of blood. If the consequent deficiency of blood supply to the affected cells is extreme, tissue decay may result. Also, cooling of the internal organs of the body can disturb normal functioning, producing a dangerous condition known as hypothermia. It is very dangerous to cool the brain or heart.

Pressure Explosions

A pressure explosion can be caused by cryogenic temperatures either through material degradation or inadequate pressure relief.

Cryogenic temperatures drastically affect the properties of solid materials. Materials can become brittle, or shrink beyond design limits, to cause leaks that are not detectable at room temperature. The ability of materials to withstand long-term exposure to cryogenic temperatures must be carefully investigated to avoid material degradation.

Because material cracks can more easily develop at cryogenic temperatures, any volume cooled externally by a cryogen or any vacuum spaces in contact with cryogen must have the ability to relieve pressure. Cryogen or air may leak into a sealed space through such cracks. Some atmospheric gases will condense under such conditions and exist as a cryogen in the sealed space. Upon warming, these cracks may be too small to vent the gas, and the contained, expanding fluid can shatter the vessel.

Chemical Explosions

The chemical properties and reaction rates of substances are influenced by cryogenic conditions. Condensing a cryogen from a pure gas at room temperature will concentrate the material typically 700 to 800 times its room temperature density. Oxygen-enriched air produced by either liquid helium or nitrogen has many of the same properties as liquid oxygen. Liquid oxygen, for example, can react explosively with materials usually considered to be noncombustible.

Cryogenic fluids with a boiling point below that of liquid oxygen have the ability to condense oxygen out of the air if exposed to the atmosphere. This is particularly troublesome if a stable system is replenished repeatedly to make up for evaporation losses as oxygen may accumulate as an unwanted contaminant. Violent reactions (for example, rapid combustion or explosions) may occur if the material is not compatible with liquid oxygen.

Oxygen enrichment will also occur if liquefied air is permitted to evaporate (oxygen evaporates less rapidly than nitrogen). Oxygen concentrations of 50 percent may be reached near evaporating liquid air. Also, condensed air dripping from the exterior of cryogenic piping will be rich in oxygen. Atmospheric oxygen concentrations above 23.5 percent pose a significant hazard.

Small quantities (less than 100 liters) of liquid hydrogen have been used for experimental targets in End Station A and are supported by a gas storage area behind the end station. This hydrogen is confined to a small cryogenic target volume and cooled with a helium refrigeration unit. Local ventilation in the immediate vicinity of the containment is required to ensure that hydrogen levels do not exceed 10 percent of its lower explosive limit (LEL). Releases of hydrogen gas from the system are unlikely as the containment system is designed to keep the hydrogen at the target cryostat through multiple containment layers. However, if a containment failure occurs, an immediate combustion hazard can develop.

Oxygen Deficiency Hazards

An oxygen deficiency hazard (ODH) exists when the concentration of oxygen is equal to 19.5 percent or less (by volume) at a typical barometric pressure of 760 mm Hg.

Without adequate oxygen, one can lose consciousness in a few seconds and die of asphyxiation in a few minutes. (See Tools, Cryogenic and Oxygen Deficiency Hazard Safety: ODH Risk Assessment Procedures, Table 1, for the biological effects of reduced oxygen concentrations.)

Liquefied gases can easily and quickly create an ODH. When expelled to the atmosphere at room temperature, liquefied gases evaporate and expand 700 to 800 times their liquid volume. Consequently, leaks of even small quantities of liquefied gas can displace large amounts of oxygen, and thereby render an atmosphere lethal.

Dense gases (those with densities significantly greater the atmospheric air at STP) will fill pits and other low areas. Examples of dense gases include various freons, sulfur hexafluoride, and certain cold cryogens. The greater the gas density, the greater the oxygen displacement and the longer the time required for the gas to dissipate.

"Air dense" gases (those with densities approximating atmospheric air at STP) may be used at SLAC in sufficient quantities that they could dilute the available oxygen in a room/enclosed work area. Examples of air dense gas include nitrogen and argon. This hazard is especially prevalent where boil-off nitrogen is used as a vacuum system purge in clean room environments. Failure of the HVAC system normally in use in such locations (due to maintenance shutdown or power outage) can result in accumulation of gas sufficient to create a hazard, even without any failure of any part of the gas handling system.

Light gases (those with densities significantly lesser than atmospheric air at STP) can affect the areas above a cryogen spill. Examples of light gases include helium and hydrogen. Spill experiences at the Fermi and Jefferson laboratories reveal that even with a connecting shaft of only a few square inches, an ODH can arise at a factor of ten in the building above compared with an area three feet horizontally from the spill. Another potential hazard from a cryogen spill can arise with poor outdoor venting, which can produce a more localized ODH.

Users of cryogens and gases that can create ODHs should always be aware of the possibility that localized ODH conditions can exist. An example would be where use of a welding purge gas is exhausted in such a way that oxygen is displaced from between the welder's face shield and his or her body, creating an oxygen deficiency in the breathing space.

Other Atmospheric Hazards

Atmospheric hazards are commonly associated with confined spaces. See Chapter 6, "Confined Space", for more information.

Gases in use at SLAC besides those mentioned previously include isobutane, freons such as R-12 and R-22, H-134a, particle detector gas mixtures, and halons associated with special area fire suppression systems. For more information, contact the SLAC Fire Marshal(ext. 4509) or the SLAC fire technician (ext. 4013). Any of these gases may give rise to atmospheric hazards under certain circumstances. Collectively with the gases that may arise from cryogenic system operation, these gases may be termed non-life supporting gases.

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