Assessing risks of superconductive
magnet quenches

A. Excerpts from: (Occupational Safety and Health Administration)

OSHA has determined that a variety of confined space hazards have caused deaths and injuries. The following discussion describes the hazards identified by OSHA. Where the Agency has obtained incident data subsequent to the publication of the NPRM, the circumstances of some of those incidents are summarized as "examples". The discussion also references the portions of the NPRM where pertinent incidents were described.

1. Atmospheric Hazards.

OSHA's review of accident data indicates that most confined space deaths and injuries are caused by atmospheric hazards. OSHA has classified those hazards into three categories: toxic; asphyxiating; and flammable or explosive atmospheres, in order to account for their differing effects.

a. Fatalities in asphyxiating atmospheres. In its analysis of these confined space incidents, OSHA uses the term "asphyxiating atmosphere" when referring to an atmosphere which contains less than 19.5 percent oxygen. Oxygen levels under 19.5 percent are inadequate for an entrant's respiratory needs when performing physical work, even if the space contains no toxic materials.the original atmosphere in the space may intentionally have been wholly or partly inerted using such gases as helium, nitrogen, argon, or carbon dioxide. Victims of asphyxiation often are unaware of their predicament until they are incapable of saving themselves or even calling for help. (emphasis mine)

B. Calculations of risk

The NMR Laboratory at SIUC is approximately 7m x 8m x 3m to the suspended ceiling, thus having an approximate volume of 168,000 liters. To reduce the oxygen content of the room from 21% to 19.5% would require displacing 2520 L oxygen with gaseous helium. This would require vaporizing 17.1 L of liquid helium to displace 12,000 L air. The helium dewar of my Oxford 500/51 magnet contains approximately 70 L liquid helium, enough to lower oxygen content to 14.9%, but likely would produce an "asphyxiation gradient" unsuitable to sustain consciousness at head level but probably breatheable near the floor.

C. Comments from NMR professionals

For more information, contact
William C. Stevens, Director
NMR Facility, Southern Illinois University
Carbondale, IL 62901-4405
618-453-6498 voice, 618-453-6408 fax

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