The Ceiling Height Limits for Water Mist Protection of Two Solid Combustible Fire Hazards in Open Environment

CEN and NFPA have expanded the application of water mist in open environments from light solid combustible fire hazards to several in the higher hazard categories. A previous analysis and test evaluation showed that such application expansion is feasible, provided that the water mist spray meets droplet size and application density requirements for the intended fire challenge. However, this evaluation was performed exclusively for water mist sprays discharging at 1.65 m above the burning combustibles. This led to the question of whether the water mist protection could still be effective for larger clearances of nozzle discharge above the combustibles. Therefore, a series of fire tests was conducted subsequently to determine the ceiling height limits for both deluge and automatic nozzle protections. As for the previous evaluation, the two tested fire challenges were a 2-tier (2.74 m) high rack-storage of corrugated cardboard cartons on wood pallets, and a palletized storage of cartoned expanded polystyrene up to 1.68 m high. Both fire challenges were in the CEN OH-3 category, and respectively represented fire hazards in the FM Global HC-2 and HC-3 categories, which in turn corresponded to the NFPA OH-1 and OH-2 fire hazards. Each water mist spray discharged 76 l/min at 16.5 bar, with a volume median droplet diameter of around 220 µm. The nozzles were spaced 3.05 × 3.05 m to provide a designated application density of 8.1 mm/min. Quick-response thermal sensors, with an activation temperature of 74°C and a response index of 27.6 (m s)^1/2, were employed to initiate the nozzle discharge. The tests showed that the deluge protection is applicable to both fire challenges for ceiling heights up to 9.1 m. However, for the automatic nozzle protection, the ceiling height limit is reduced to 6.1 m, with a water supply requirement of providing 17 nozzle operations for a safety factor of about 50% over the maximum number of nozzle operations observed in the tests.