Physical scaling of water mist protection for ignitable liquid cut-off rooms:

Abstract

Based on the Froude-modeling scaling approach, water mist protection for a 7.47 × 7.47 × 7.47-m cut-off room where ignitable liquids are used was projected from the previously determined extinguishing requirement for heptane pool fires in a ½-scale enclosure and then verified with full-scale testing. The full-scale building occupancy was required to accommodate cut-off room door openings ranging from 1.86 × 3.73 m high up to 3.73 × 3.73 m. The fire challenge was a heptane spill fire cascading from the top of a 1.83-m dia. by 2.74-m high steel tank at a spill rate of 37.9 L/min. Two off-the-shelf nozzles were selected as candidates for the full-scale building to approximate the scaled-up water mist spray requirement. At a discharge pressure of 90 bar, one nozzle discharged 28 L/min, with a spray angle of 100° and a volume-median droplet dia. of 115 µm; the other discharged 24.4 L/min, with a 180° spray angle and median dia. of 96 µm. Nine nozzles were arranged at the ceiling level in a 3 × 3 matrix with a spacing of 1.86 × 1.86 m. Before the two candidate protection schemes were challenged with the spill fire, scaled-up heptane pool fires were used to ensure that the fire extinguishing propensity was consistent with that observed in the ½-scale enclosure. The pool fire tests showed that the nozzle with the smaller spray angle provided better fire extinguishing performance, which was later confirmed in the spill fire tests. The protection scheme with the smaller spray angle could extinguish the spill fire for door openings up to 3.05 × 3.05 m. For larger openings, two additional downward water mist sprays were required in the door opening to expedite fire extinguishment by reducing ventilation through the door opening. Water mist protection could provide adequate cooling to fuel tanks as long as such protection could extinguish the fire. Overall, the investigation demonstrated that physical scaling is a useful tool to provide an engineering estimate of water mist protection requirements. Language: en