An Efficacy Evaluation of Water Mist Protection Against Solid Combustible Fires in Open Environment

Abstract

CEN and NFPA standards have accepted water mist for protecting solid combustible fires more challenging than light hazard fires in open environment, but with little data in the public domain to support such applications. Therefore, a series of exploratory experiments was conducted to evaluate the efficacy of fire suppression in open environment by water mist sprays for a rack storage of corrugated cardboard cartons on wood pallets (Fire A), and for a palletized storage of cartoned plastics stacked on wood pallets (Fire B). Both are in the accepted hazard categories by both standards. Representative water mist sprays were employed to provide water application densities of 4.1 mm/min and 6.1 mm/min for Fire A, and 8.1 mm/min for Fire B. These sprays had downward thrust forces ranging from 18 N to 73 N per spray, and volume median droplet diameters from 75 µm to 345 µm. A standard ½-in. pendent sprinkler was also employed to provide the suppression performance references for these two fire hazards. In each experiment, four open sprinklers or nozzles were centered 1.7 m above the fuel array with nozzle spacing ranging from 2.6 × 2.6 m to 3 × 3 m. All experiments were conducted under a 20-MW calorimeter with no ceiling presence. Water discharge in each experiment was started at a designated fire convective heat release rate of 1000 kW. The experiments indicated that, for the same application density, not all water mist sprays having different characteristics would lead to a comparable fire suppression result. Therefore, each water mist system should be tested individually for the same intended protection. Furthermore, the results showed that when water mist protection was effective, it required application densities comparable to those of sprinkler protection to provide comparable fire suppression results in open environment under the present experimental conditions.