I’ve not heard them called Purge Buttons, but I agree that a button to activate maximum exhaust ventilation, either through the hood alone or including the general exhaust system with the hood systems, is not an uncommon arrangement.  They are intended to provide maximum exhaust when something goes wrong, but here we are talking about a basic beaker spill or the like, more often than not.  They are usually used with VAV systems, where we would otherwise be at less than full system capable exhaust most of the time, and where closing a sash on a spill in the hood, for example, would otherwise result in substantial REDUCTION of exhaust flow.

When I deal with them, my intent is to provide as much assurance as possible that the hood will be negative with respect to the room and the room negative with respect to the hall when there is a spill that the lab occupants cannot deal with properly through the use of an absorbent wipe.  These may or may not be situations calling for building evacuation, but they are more likely to result in an otherwise avoidable building evacuation without that maximum ventilation.

Peter Zavon, CIH
Penfield, NY

PZAVON**At_Symbol_Here**Rochester.rr.com

David,

Thanks for the info about purge systems.   If they are designed to function as you describe I would not disable or override them.  However, my concern is that an unsafe situation (hazardous atmosphere) may exist/persist and local personnel might assume that the purge system will render this “safe” before the atmosphere is actually safe and then find themselves in an unsafe atmosphere.   The scariest situation is that you walk by a lab and observe a victim on the floor and rush in to help.  This is a natural response but emergency responders (firefighters and EMTs) are always taught to first assess scene safety.  If someone appears to be unconscious on the floor, it could be medical (= heart attack), trauma (fell and bonked their head), or asphyxiation due to simple (N2) or chemical (CO) asphyxiant.  Without proper scene assessment, the Good Samaritan might become the second victim.  So, back to the purge system:  How quickly does this work?  My concern is that local personnel might assume the purge system “has worked” before it has worked effectively.  Noses are sometimes crudely reliable but without a gas meter to assess the atmosphere, it may be difficult to assess the safety of the atmosphere.

If I were responding to a major spill as a FF, I’d be happy to learn that  a purge system was functioning since one of the top tactical priorities would be to ventilate.

Dave

Interesting commentary David as you have real world experience as a voluntary firefighter. Most fume hoods, in particular VAV hoods, have some form of fume hood monitor/flow alarm on the hood with an “emergency” purge button of sorts to allow the operator to force the hood to max flow, and thus increase the ventilation at the hood and room.  There are now also systems that actively monitor the room environment (eg. Aircuity) and also increase the ventilation in the room in the advent of a spill.  Essentially what you are recommending is that you don’t enable these purge systems, but allow the responders to assess the situation and respond as they see appropriate?

I’m surely not a ventilation expert, and I’ve never heard of a “purge button” before.  However, in addition to Ralph’s question below about the purpose of using such a device, as an volunteer firefighter for 20 years I can view this scenario of a “significant spill” as a situation that should only be handled by emergency responders.  Any situation that poses an atmosphere that is not human-friendly and/or a fire/explosion hazard should simply call for an building evacuation with appropriate power shut-downs (to eliminate ignition sources) IF POSSIBLE.   

“We found ourselves using the purge buttons a lot to exhaust the room and the hoods  in an emergency.”

I’d re-examine protocols to prevent these episodes rather than re-evaluating response options.

“This is  to allow a  safer environment for emergency personnel to enter the area and do spill clean ups, particularly when the spill happens outside of the hood. “

The firefighters who respond to this will surely be wearing SCBA and have a four-gas meter to check O2/CO/flammables/X  (where is likely H2S).  It is thoughtful of you to consider them but, frankly, they will mostly ignore you and treat any major spill as a worst-case scenario to protect themselves.  Firefighters can’t afford to blindly trust information given to them by dispatch since in some instances it a wrong (even when not intended to be.)   The best thing to is to meet the officer in charge of entering crew at the door, give the best information you have, tell them if you think there is any chance that someone is still in the building, and make sure they have master keys.  They know what to do after that.

Dave

> We found ourselves using the purge buttons a lot to exhaust the room and the hoods  in an emergency.

I'm not clear what hazard is being reduced by this strategy. Are you trying to prevent a fire by keeping the concentration of the spill below the LEL? Or are you trying to control levels below the IDLH? I'm not sure that general ventilation will accomplish these goals, as the spill could be a location in the lab where the ventilation system doesn't effectively clear the air. We are finding significant "dead spots" in many of our lab settings…

My personal opinion is that the ventilation system should not be considered part of the emergency response system, as its value in a specific situation is undeterminable.

- Ralph

Ralph Stuart CIH

Chemical Hygiene Officer

Department of Environmental Health and Safety

Cornell University

rstuart**At_Symbol_Here**cornell.edu