Increasing Danger Due to Particle Size Reduction

As safety professionals we always have to be on our guard against complacency. Unless you have been trained in the hazards of combustible dust, there is a good chance that you are not aware that any object that will burn as a large object will burn with more speed and intensity as the size of the object decreases. The reason for this is the increase in surface area available for reaction. As a particle is reduced in size by a factor of 8 the surface area available for reaction increases by 800 %. The smaller the particle size the more susceptible to ignition and the more violent the reaction. This is why carburetors were used in car engines, to atomize gasoline using the venturi effect to maximize the power generated from the fuel.   If you are struggling with the concept of a 800% increase in surface area by a size reduction of 8, here is a mathematical explanation.

Visualize a solid block of wood 80 cm x 80 cm x 80 cm the surface area would be 80 cm x 80 cm= 6400 cm2 x 6 sides = 38400 cm2 

If you took that same block and cut it with a laser into 10 cm x 10 cm x 10 cm cubes each cube would have a surface area of 10 cm x 10 cm =100 cm2 x 6 sides = 600 cm2 and there would be (8 x 8 x 8) 512 cubes. 600 cm2 x 512 cubes =307,200 cm2 x 100% / 38400 cm2= 800%

The reason that this is important to us in the real world is that as we manufacture wood, metals, food products we create dust particles. The finest dust we produce does not settle to the floor but is transported upward with convection air currents (hot air rising) to settle on overhead structure, pipes and cable trays where it is out of sight. This highly volatile fuel accumulates and is dried by the higher temperature present at the higher level in your building, making it more susceptible to ignition.

The hazardous depth of dust accumulation will be determined by the volume of the room. If the dust accumulation present will allow the Minimum Explosive Concentration (MEC) to be reached if it a dust cloud is created by an upset condition such as a building impact, earthquake or primary explosion in process equipment then all that is required for an explosion is ignition.  Remember that MEC is measured in g/m3 so that is why layer depth is so small. Unfortunately, in industry there are many potential ignition sources present, from pilot lights, to overheated bearings and electrical faults.  

The rule of thumb from the NFPA for dust accumulation is 1/8” for type G products (wood, grain etc.) and 1/32” for type E products (metals). You can refer to NFPA 654 to explain the calculation method for dust layer depth for products with different bulk densities.

This article is not intended to be a definitive explanation on combustibility parameters. It is just a primer to allow people to become more aware of the components of an effective Combustible Dust Management Program, and to understand which questions to ask when seeking assistance with setting up a program.