1) What construction and installation techniques must be observed while selecting and installing a fume hood that will reduce the chances of a fume hood fire spreading to the remaining lab space and possibly the entire building?
Any fume hood fire will interact with the room it is in. The suspended ceiling in such a lab is a key feature in determining how such a fire will spread. Flames may travel up through the fume hood into the duct system in this scenario. Let’s look at how such a ceiling is installed and what it is supposed to do during a fire.
Suspended ceilings made from fire-rated rectangular tiles are often used in lab construction as a way of retarding fire progression by preventing fires in a room from quickly spreading upward and affecting the room ceiling, which would accelerate the spread of fire to the floor above.
It has been found that such a ceiling must have all tiles in place to perform this function. Open rectangular spaces left during maintenance or replacement destroy the entire protective strategy. Once through such an opening, fire and heat can rapidly spread horizontally and upward thereby propagating the flame to the immediate floor above.
If the fume hood liner material is breached with either cracks due to heat or dislodged plumbing access panels, very bad things happen! The hood fire can channel up the “chimneys” formed by the space between the outer steel shell of the fume hood and the hood liner material into the suspended ceiling cavity.
An actual dislodged access panel (Figure 5) is shown above. The panel is smaller than its circled opening and is fit into the cutout with a gasket. In this photo, the gasket has been dislodged by explosive force, causing the panel to fall through the opening, allowing a fire pathway upward. In figure 6, a screwed-in panel larger than the covered opening resists explosive displacement.
In no case should interior fume hood plumbing access panels be smaller than the opening they cover. Gasketed material used to “seal” such openings should be avoided. More generally, access panels and gaskets should never be manufactured from a material which melts or can be distorted into a shape that can fall or be pushed through the hole they are covering.
Once a fire breaks through the hood wall liner material or access openings, it is free to move upward. Figure 7 shows two hoods soffeted into a suspended ceiling; an open gap above this hood will allow flames to enter the suspended ceiling cavity, ruining its effectiveness. A finished suspended ceiling to the wall is preferred construction! (figure 8) This fume hood installation has a clear separation between ceiling and hood superstructure, preserving the fire retardant characteristics of the finished ceiling.
If a hood fire spreads into this suspended ceiling cavity, it will move horizontally relatively undeterred. Since many labs have “walls” that terminate at the suspended ceiling, these areas are often vast and untidy fire throughways.
NFPA 45 Section 188.8.131.52 requires sprinkler systems in all NEW labs in accordance with NFPA 13. NFPA 13 states complete suspended ceilings should have sprinklers below such a ceiling. Incomplete ceilings, sometimes called “cloud ceilings”, may require sprinklers above and below the suspended ceiling line. Differentiation here is a bit unclear. Obviously, the area above a ceiling should be properly protected by sprinklers, in a manner consistent with NFPA 13 and local code.
The general vulnerability of suspended ceiling design was summarized by Francis L. Brannagan 5 as follows:
“In the first edition of Building Construction for the Fire Service (1971), I pointed out the basic deficiency of this system: the loss or failure of one tile exposes the entire floor to the fury of the fire below. I was told by a U.S. General Services Administration (GSA) fire protection engineer that the GSA never built a building for its own account with this type of construction.”
To summarize, laboratory suspended ceiling design should not include an opening for the fume hood superstructure to “fit through”. A complete suspended ceiling should always be placed above the entire lab, including the fume hood superstructure. Such a complete suspended ceiling will resist flames traveling from the inner containment area of a fume hood into the painted steel outer shell where they can easily “chimney up” past the fume hood superstructure.
For this reason, contractors should never use the fume hood superstructure as an anchoring point for a suspended ceiling in a lab. A gaping hole above the hood in the suspended ceiling results. This condition obviously exacerbates the spread of any fume hood fire through the suspended ceiling space.
There is at least one very notable example of how suspended ceiling spaces can become involved in quickly spreading lab fires. It happened in 2012 in Tulsa Oklahoma:
“The historic Barnard School Building went up in flames about 5 a.m. on September 5. When firefighters arrived, the building exploded, rocking midtown Tulsa and sending eight firefighters to the hospital.
After a week-long investigation, officials said the fire and explosion resulted because of “construction related to the installation of an exhaust vent in the lab area,” according to a news release.
The explosion occurred because the fire had been smoldering in the void between the chemistry lab ceiling and the floor of the room and hallway above, the release said.
Investigators said the fire migrated north under the hallway floor into the classroom, and the crawl space below where it vented from the classroom window. The resulting smoke explosion or “backdraft” occurred when oxygen was introduced into the area by the firefighters entering the room to extinguish the fire.”
News 6 Tulsa 14 September 2012.