Water that thinks it's a Gas

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In Europe, fires are divided into six classes using alphabetical references as follows;

  • Class A: These are fires involving flammable solids, e.g. wood, cloth, rubber, paper, and some types of plastics. An example of this type of fire would be a campsite fire.
  • Class B: These are fires involving flammable liquids or liquefiable solids, e.g. petrol, oil, paint and also some waxes & plastics, but not cooking fats or oils.
  • Class C: These are fires involving flammable gases, e.g. natural gas, hydrogen, propane, butane.
  • Class D: These are fires involving combustible metals, e.g. sodium, magnesium, and potassium.
  • Class E: These are fires involving any of the materials found in Class A and B fires, but including electrical appliances, wiring, or other electrically energized objects in the vicinity of the fire, with a resultant electrical shock risk if a conductive agent is used to control the fire.
  • Class F: These are fires involving cooking fats and oils. The high temperature of these types of fats and oil when on fire far exceeds that of other flammable liquids which means that normal fire extinguishers should not be used.
  • Class ‘A’ fires require the largest droplets because there is the possibility of deep seated fires with solid combustibles and deep seated fires need larger droplets to cool and wet the fire. It is recommended that Class A fires have a droplet size of Dv0.9 ≥200 microns. This basically means that the droplets have an average size of 200 microns. Class B fires require the smallest droplets because liquid fuel fires create large amounts of heat. Small droplets have a larger surface area and can therefore absorb more heat from the fire than larger droplets.[1]

Note: Mists having a droplet size of Dv0.9 ≥200 microns were once classified by NFPA 750 as Class 1 mists but that classification has long been dropped.

[1] NFPA 750 2010 – Properly designed water mist systems can be effective on both liquid fuel (Class B) and solid fuel (Class A) fires. Research indicates that fine (i.e. smaller than 400 microns) droplets are essential for extinguishment of Class B fires, although larger drop sizes are effective for Class A combustibles, which benefit from extinguishment by fuel wetting. For this reason, the definition of water mist in this standard includes sprays with Dv0.99 of up to 1000 microns.

The relationship between drop size distribution and extinguishing capacity of a water mist is complex. In general, very fine particles enhance heat absorption and generation of water vapour. With liquid (Class B) fuels, too many "large" drops could agitate the surface of the fuel and increase burning intensity. On the other hand, larger drops could assist the spray to penetrate and wet charred, smouldering Class A fuels. Larger drops could also entrain finer drops in their wake and improve the transport of much smaller drop sizes into the seat of the fire.