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Assessing risks of superconductive
magnet quenches

A. Excerpts from:

http://www.osha-slc.gov/Preamble/ConSpaces_data/CON_SPACE2.html (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

If you keep your head out of the cloud as you dash for the door, you should be ok. If you are up on a ladder and suddenly can't see anything, you are in a bit more trouble. Of course, this volume of gas does not appear instantaneously, allowing time to escape. But if you are wrestling with a transfer line and are reluctant to just let it go and flee, you increase your risk substantially.
I think it would be extremely unwise to attempt a fill by yourself without the presence of someone who knows what might happen in the event of a quench and what to do provide assistance, particularly in a small room.
It should be noted that the OSHA limit (see 29CFR1915.12) is 19.5% oxygen, and anything less is defined as an oxygen deficient atmosphere. According to the Compressed Gas Association (http://cganet.com), the following physiological effects of low oxygen availability (percentages are by volume) are observed:
- 15-19% Decreased ability to perform tasks. May impair coordination and induce early symptoms in persons with head, lung or circulatory problems.
- 12-15% Breathing increases, especially in exertion. Pulse rate up. Impaired coordination, perception, and judgement.
- 10-12% Breathing further increases in rate and depth, poor coordination and judgement, lips slightly blue.
- 8-10% Mental failure, fainting, unconsciousness, ashen face, blueness of lips, nausea and vomiting.
- 6-8% 8 minute exposure may be fatal in 50-100% of cases; 6 minute exposure may be fatal in 25 to 50% of cases; 4-5 minutes, recovery with treatment.
- 4-6% Coma in 40 seconds, followed by convulsions, breathing failure and death.
This site also contains the following warning:
Exposure to atmospheres containing 8-10% or less oxygen will bring about unconsciousness without warning and so quickly that the individuals cannot help or protect themselves....
The University of Cambridge also has a nice page of Safety Information for their NMR Facility.

For more information, contact

William C. Stevens, Director
NMR Facility, Southern Illinois University
Carbondale, IL 62901-4405
wstevens@siu.edu
618-453-6498 voice, 618-453-6408 fax

NMR Facility Home Page

Assessing risks of superconductive magnet quenches

A. Excerpts from:

1. Atmospheric Hazards.

B. Calculations of risk

C. Comments from NMR professionals

Assessing risks of superconductive
magnet quenches