Fire service measures Theoretical fire models
In 2014, the WISH real fire test programme started with a series of laboratory tests on one tonne samples at the FPA’s facilities in Gloucestershire, and concluded in October 2016 in Essex. During the programme, I have developed the following theoretical fire models. The experiments have been repeated with several different materials and at three, six and nine tonne piles to eliminate as far as possible any scaling issues. The test data and the analysis was reported
to the WISH board and is incorporated within the WISH Guide 281
. These firefighting tests not
only serve as a platform with which to develop firefighting tactics; they also serve as validation tests for the theoretical models set out below.
Surface fires
Impermeable fuel bed This is typically a pile of material that has been crushed down so it forms a single homogenous mass, which prevents the movement of convection currents through it.
Semi permeable fuel bed A pile of material with sufficient tensile strength in the individual particles to allow the passage of convection currents to pass through the mass of the pile, but the repose of the individual particles of the mass creates holes that are smaller than the flame height for
the material. This prevents flame passage into the mass of the pile, and results in a surface fire that is ash and char controlled.
Permeable fuel bed A pile of material with sufficient tensile strength to allow the passage of convection currents to pass through the mass of the pile, and the repose of the individual particles creates holes that are greater than the flame height for the material; thus allowing the flame to pass through the mass of the pile.
Piles of material display a tendency
to degrade from permeable fuel beds to impermeable fuel beds, over time or due to degradation as a result of burning. A fire initiated on the surface of a pile of semi permeable or impermeable fuel beds will remain on the surface, and tends to become ash and char controlled. Surface fires do not burrow into the material to become deep seated fires (see Figure 1 below). When the individual particle that makes up the pile has sufficient tensile strength to maintain gaps of sufficient magnitude to allow the flames to pass into the mass of the pile, then the whole of the materials will become involved in fire (see Figures 2 and 3 below).
Deep seated fires This is a fire that initiates within the mass of the pile, will form a hot core in excess of 250°C, and can develop anaerobically, ie without a supply of oxygen. The developing fire will find
FOCUS
Figures 1, 2 and 3 (above, below, right)
www.frmjournal.com NOVEMBER 2018
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