Returning to our jaunt through Unit C, we now come to element C2. This is possibly my favourite subject – fire and explosion. This is largely unchanged from before, mostly clarifying and expanding on what was there. It covers the basic properties of flammables/explosives and the familiar mechanisms of vapour cloud explosions and BLEVEs. The way buildings are affected in fires (structurally) and the prevention of explosions. Of note, is explicit mention of the dust pentagon. It is something that was already effectively included before, in terms of what you need to have in place for a dust explosion to occur.
In order to totally humiliate the fire triangle (or fire tetrahedron) there are five sides to a pentagon – obviously. The first three sides are the same as the fire triangle – fuel (the combustible dust), oxygen (present in sufficient quantities), ignition source (such as a spark). The final two sides of the pentagon are dispersion (the fine dust has to be dispersed in the air as a cloud in sufficient concentration) and confinement (the dispersed cloud has to be enclosed, say, in a container or pipe). In truth there are other factors too – such as the size of the dust particles (big chunks don’t disperse or burn well – which appears to be the same principle for sewage) and moisture content/humidity (wet stuff doesn’t burn well).