Water mist has been determined to be a preferred alternative to Halon 1301 total flooding to extinguish fires occurring in ship machinery spaces and pump rooms, because it is toxicologically and physiologically inert. Water mist systems produce a drop size distribution with a range of drop sizes under 1000 μm, while the more conventional sprinkler systems produce much coarser particles. The smaller particle sizes have greater cooling efficiencies because evaporation and cooling are controlled by surface area, and the surface area of a large number of small droplets is greater than that of a small number of large droplets of the same total volume.
Coarse droplets from sprinkler systems are efficient at providing boundary cooling to large surfaces such as deck walls and floors, and penetrating flames to get to the seat of a fire, but the large drop sizes that make up these sprays are not as effective on spilled fuel fires or in providing cooling to the regions around a flame. Mist systems also have lower water demands than sprinkler systems, which is beneficial in shipboard applications where prolonged sprinkler discharges may affect stability.
Fire extinguishment through water mist application is controlled by three mechanisms:
- Flame cooling,
- Reduced oxygen concentration by displacement of air by water vapor, and
- Radiant heat attenuation.
Each of these mechanisms is essentially independent, but flame extinguishment generally occurs as a result of a combination of these effects. Three basic parameters that are important in characterizing water mist behavior are flux density, spray momentum, and drop size distribution. Nozzle pressure, flow rate, and obstructions influence these characteristics, but do not themselves characterize the mist. The mist characteristics will also be affected by the presence of a fire, but are initially measured in a non-fire environment.
A number of representative drop-size diameters may be used to characterize sprays. A mist of small-diameter droplets (<50 μm) will exhibit gas-like properties; the droplets following airflow patterns around obstructions and behaving like gaseous fire extinguishants. The main fire extinguishing mechanisms of the smaller water particles are radiant heat attenuation and oxygen displacement. Droplets over 50 μm diameter are projected into the fire zone because of their greater momentum.
The drop-size distribution also varies throughout the protected volume. In a vertically downward spray, the larger particles (particles with greater momentum) will condense as they impact on objects in their path at a faster rate than the smaller particles. As the distance from the nozzle increases, interaction between falling particles will result in particles coalescing and the average particle size increasing. Interaction of sprays from adjoining nozzles will also result in a change in drop-size distribution as will interaction with obstacles.
Entrainment is an important mechanism for directing mist into the flame zone and the region around the flame. The mist particles provide cooling that will slow down the reaction kinetics, which, in the extreme, will cool the liquid fuel to temperatures below the flashpoint. Entrainment of mist into the flame zone may also produce local dilution of the oxygen concentration to a level that will inhibit combustion.
The majority of fire threats in engine rooms, machinery rooms, pump rooms, paint storage rooms, weapons storage rooms, and flammable liquid storage rooms are Class B (flammable and combustible) liquids. Water mist will provide cooling to Class B fires, which will affect ongoing combustion. To achieve extinguishment of liquid fuels by water mist, the particles must cool the flame, cool the fuel or provide a level of oxygen depletion that will alter the fuel/air ratio to one that cannot be ignited. The mist parameters will determine the water particle motion near the flame and throughout the compartment, therefore controlling the cooling and extinguishing behavior. The priority when attempting to achieve flame cooling is to ensure the particles reach the flame. This requires a suitable droplet size distribution to provide the most efficient heat transfer, sufficient particle volume to absorb the heat generated by the fire, and sufficient momentum to drive the particles through the plume.
The system pressure will control the particle velocity and size, and therefore, momentum. However, the benefits of a high supply pressure must be balanced with the weight, power, and volume penalties of these systems. If the pressure from the ship’s fire main is sufficient to produce the appropriate mist parameters for extinguishing, the system can be connected to the fire main without the need to install extra hardware.
This work was done by Ian Burch of the Australian Defence Science and Technology Organisation.
This Brief includes a Technical Support Package (TSP).
Water Mist System for Shipboard Machinery Space Fires
(reference DSTO-0001) is currently available for download from the TSP library.
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