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Previous by Date: Subject: Hood certification Date: Sun, 5 Oct 2003 08:03:42 -0500 Author: ginaro**At_Symbol_Here**RACSA.CO.CR |
Next by Date: Subject: Re: Hood certification Date: Mon, 6 Oct 2003 08:39:00 -0400 Author: Wayne Wood |
At 10:27 AM 10/04/2003 -0600, you wrote: >Hello all, > >Does anyone know whether or not a fume hood should be turned off or left >on if fire breaks out in the fume hood? > >We have always assumed that one would turn the fume hood off as not to >provide oxygen to the fire; unless noxious fumes are produced by the fire >in which case the fume hood would be left on until fire personnel get to >the scene. There is, I think, not a single, best answer to this question. Here is the short answer, with explanations below: The fume hood should not be left on during a fire unless all the ductwork for the hood/venting system is fire-rated so as to be able to withstand/contain the heat. If you know that your ductwork is fire-safe, then leaving the hood on makes sense since it will help ventilate the area. A small fire in a fume hood might be able to be extinguished by a trained person using the appropriate fire extinguisher. This should be the first action (after sounding an alarm). If this fire is quickly extinguished then the "ventilation/no ventilation" issue is moot. Let me assume below, that there is a larger fire and the question of what to do with hoods remains. Long answer: As a volunteer firefighter and CHO I have looked into this question before. I believe that one can find the appropriate NFPA standard to support my statement above (but on a Sunday morning in my chemistry office I don't have access to the NFPA standards). The tactical priorities at a fire often involve ventilation before fire suppression (depending upon the unique circumstances of every fire...) since ventilation removes heat and smoke (and other toxic stuff) that will allow for increased chances of victim survival if anyone in trapped in the fire area. Ventilation also provides for a safer environment (less heat, better visibility) for firefighters when they enter the fire area. It is also true that ventilation can provide additional oxygen to feed the fire (which sounds, and is, bad) but depriving the fire of oxygen can also lead to the circumstance where an explosive backdraft can occur. (A backdraft will occur, just as in the movie, when a developing fire gets starved of oxygen, but plenty of heat and combustible materials still exist in the room. When a door or window is opened, a very rapid combustion occurs, explosion.) So, in general it is best to keep a fire ventilated, but still minimize the opportunities for spreading the fire. Closing doors to minimize fire spread is important, and this actions may limit ventilation. (Firefighters are taught to ventilate vertically, which avoids the backdraft explosion even under backdraft conditions.) Thus, my thinking (as a firefighter) was to keep the hoods going during a working fire since this would help with ventilation. The problem with this, as noted above, is that the ductwork for the hood system may not be (and probably is not...) rated to withstand high temperatures. Indeed, if the ductwork is metal, as it probably is, then it will conduct heat to items in proximity to the ductwork and this can start fires elsewhere in the structure. (This same problem exists, potentially, for chimney fires in homes. Firefighters always have to check attics and second floors for fire spread during and after a chimney fire. The use of the infrared "thermal imaging camera" is a wonderful technological addition to the firefighter's toolbox to locate hidden hot spots.) What needs to be done (in my opinion) is 1) to determine (if possible) what kind of ductwork you have, and 2) consult with your local fire department about what they want you to do. (This assumes that you have "mechanical" choice; one respondent to your query already noted that she does not have access to an "off" switch.) Consulting with your local FD is always a nice idea for both safety and PR reasons. After they have considered your particular situation, they can advise of what they want you to do. One final, bottom line is always, of course: buildings can be replaced, but people cannot be replaced. Thus, any steps you take to immediately evacuate people and call 911 should (in my opinion) always take priority over taking steps to save the physical plant. These are always judgment calls that depend highly upon each circumstance of the nature and extent of each fire. In a true emergency, there is only a small amount of time to take action. Shame on anyone who faults you afterwards for not doing something unrelated to life safety! Dave Finster Captain, MTFR (http://www.mtfr.org/index.htm) and David C. Finster Professor and Chair, Department of Chemistry University Chemical Hygiene Officer Wittenberg University 937-327-6441 http://userpages.wittenberg.edu/dfinster/index.html
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