A lot of folks recognize the need for mechanical exhaust ventilation in areas where there is a flammable and toxic exposure hazard. The general rule of thumb for this mechanical exhaust ventilation is 1 CFM per square foot of floor space with at least six air changes per hour. But there are two (2) other requirements that come into play that many systems do not have:
- Failure alarm
- Emergency Shutdown Switch
In this article, I want to explain why the codes/standards require these mechanical exhaust systems to have an EMERGENCY SHUTDOWN SWITCH.
But first, let’s review the code language: (emphasis by me)
| CHAPTER 50 HAZARDOUS MATERIALS—GENERAL PROVISIONS … 5004.3.1 System requirements. Exhaust ventilation systems shall comply with all of the following: 1. Installation shall be in accordance with the International Mechanical Code. 2. Mechanical ventilation shall be at a rate of not less than 1 cubic foot per minute per square foot [0.00508 m3/(s × m2)] of floor area over the storage area. 3. Systems shall operate continuously unless alternative designs are approved. 4. A manual shutoff control shall be provided outside of the room in a position adjacent to the access door to the room or in an approved location. The switch shall be a break-glass or other approved type and shall be labeled: “VENTILATION SYSTEM EMERGENCY SHUTOFF.” 5. Exhaust ventilation shall be designed to consider the density of the potential fumes or vapors released. For fumes or vapors that are heavier than air, exhaust shall be taken from a point within 12 inches (305 mm) of the floor. For fumes or vapors that are lighter than air, exhaust shall be taken from a point within 12 inches (305 mm) of the highest point of the room. 6. The location of both the exhaust and inlet air openings shall be designed to provide air movement across all portions of the floor or room to prevent the accumulation of vapors. 7. Exhaust air shall not be recirculated to occupied areas if the materials stored are capable of emitting hazardous vapors and contaminants have not been removed. Air contaminated with explosive or flammable vapors, fumes, or dust; flammable, highly toxic, or toxic gases; or radioactive materials shall not be recirculated. |
Think about this: what if a fire occurs within the ventilated space? Do we want to “fan the flames” with this ventilation system?
Of course not, hence the code requires:
- a manual shutoff control shall be provided OUTSIDE of the room
- in a position adjacent to the access door to the room or in an approved location*
- switch shall be a break-glass or other approved type
- switch shall be labeled: “VENTILATION SYSTEM EMERGENCY SHUTOFF”
* An “approved location” should be based on personnel being able to adequately monitor the necessary alarms and signals and on the fire department being able to gain access
quickly when responding to emergency situations. Additionally, the room must be occupied by persons who are trained to respond to the various alarms and signals.