FOCUS
Fire service measures
Figures 4 and 5 (top two), 6 and 7.1 (second two), and 7.2 and 7.3
a passage to the surface of the material and, ultimately, breach the surface. This breach will provide an air supply to the hot core gases, resulting in a fuel controlled fi re burning at the peak heat release rate for the material. This fi re behaviour has been observed in coal heaps and documented2
(see Figure 4 above).
Stacked bale fi res When baled material is stacked above 1m in height, a surface fi re that affects the vertical space between the individual pillars will generate vortices driven by the convection currents. The fl uid dynamic fl ows produced by the vortices can be suffi cient to strip any ash and char away from combustible material, and accelerate phase change in low grade plastics to a point where the process is almost instantaneous. This can result in an unusually high heat release rate for any given material within this region (see Figures 5, 6 and 7 above).
26 NOVEMBER 2018
www.frmjournal.com As the stack bindings fail, together with partial
thermal expansion of the material at the top of the stack, a section of the stack will fall and bury burning material. Stacked bales tend towards degeneration into piles in this way, and as a result, the pile of material will generally develop several buried deep seated fi res. This behaviour is occasionally mistaken for
a surface fi re burrowing into a pile. When the bales have a high plastic content by volume the fi res tend to vaporise the fuel bed, which results in a signifi cant reduction in the calving behaviours. During a test with high grade plastic HDPE bottles, the stack was observed to become unstable, which resulted in a burning bale falling off of the top of the stack. This is obviously an additional hazard for fi refi ghting. Note: the RDF tested generally had a plastics content of 30 to 40%, and this was suffi cient to suppress the fi re growth. The other materials tested contained 85 to 95% plastic,
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