Sprinkler protection of ductwork with combustible contents
NFPA Journal®, May/June 2011
By their nature, ducts present not only the direct fire hazard of their contents, but the potential for transmitting a fire along their paths. Rules for installing sprinklers in ducts are found in two sections in NFPA 13, Installation of Sprinkler Systems. One deals with the protection of commercial-type cooking equipment and its associated ventilation. The other reference, in a smaller but more general section, calls for sprinklers to be located on 10-foot (3-meter) centers in horizontal ducts, at the top of each vertical duct, and at the midpoint of each offset or jog in a vertical duct.
Sprinklers are rarely installed in typical cooking equipment ducts, due to an exception that allows omission of sprinklers and spray nozzles from ducts, duct collars, and plenum chambers where all cooking equipment is served by listed grease extractors. Since NFPA 96, Ventilation Control and Fire Protection of Commercial Cooking Operations, is widely adopted and requires listed grease removal devices for all commercial cooking equipment, the exception is far more common than the rule.
NFPA 96 also requires an automatic fire extinguishing system to protect the grease removal devices, hood exhaust plenums, and exhaust duct systems, as well as for cooking equipment that produces grease-laden vapors, but requires that the systems meet ANSI/UL 300 or equivalent standards. The requirements of ANSI/UL 300 are generally met by wet-chemical extinguishing systems. NFPA 13 acknowledges that cooking equipment below hoods that contain automatic fire extinguishing equipment does not require protection from the overhead sprinkler system. The protection concept offered by the wet-chemical systems involves extinguishing the fire before it can enter the exhaust ducts, so there are commonly no sprinklers or other extinguishing nozzles within the downstream ductwork.
This does not mean that sprinklers are not required in other types of ducts. The Uniform Mechanical Code, for example, requires an approved fire suppression system in all ducts with a cross-sectional area exceeding 10 square inches (65 square centimeters)conveying materials, fumes, mists, and vapors, unless they are nonflammable and noncombustible under all conditions and concentrations. An exception is made for laboratory hoods and exhaust systems. NFPA codes and standards, such as NFPA 33, Spray Application Using Flammable or Combustible Materials, also require sprinklers or other automatic fire extinguishing systems in ducts.
One of the best documented fires involving a nonsprinklered duct took place in July 1998 on the cruise ship Ecstasy that sailed out of Miami, Florida. The National Transportation Safety Board concluded that the fire began with a welding spark in the main laundry room on a lower deck. While the small amount of lint on the floor in that room was insufficient to activate the ceiling sprinklers, the high lint build-up in the laundry exhaust ducts carried the fire up two decks and to the exhaust plenum on the unsprinklered open mooring deck at the rear of the ship. There, it ignited 10,000 pounds (4,536 kilograms) of polypropylene mooring lines and other combustibles. The ship lost its propulsion power and steering, and required six tugboats to fight the fire and tow it back to port.
The fire was brought under control after four hours, but 14 crew members and eight passengers suffered minor injuries. The ship’s sprinkler system was credited with limiting the spread of the fire to adjacent decks, preventing much greater damage.
Incidents such as the Ecstasy fire are the specific instances that support the need for sprinkler protection in all ducts with combustible contents.
Russell P. Fleming, P.E., is the executive vice-president of the National Fire Sprinkler Association.