Silos, cyclones, filter units, mills, conveying lines — any equipment handling fine combustible dust carries an ignition risk. When ignition occurs, the pressure in an enclosed volume rises very rapidly.
Even where the aim is to prevent an explosion entirely, the design must account for what happens if one occurs.
Three strategies
Venting. A weak panel opens at a defined pressure and discharges the flame in a safe direction. The most common and most economical route. The vent area is sized from the volume, the explosion severity of the dust and the strength of the equipment.
Suppression. The pressure rise is detected and a suppressant is injected. Used where venting is impossible — inside buildings, with toxic products.
Isolation. Prevents the explosion propagating to other equipment through connected lines. This is the main protection against a chain explosion between silos.
What the analysis contributes
The explosion pressure itself is usually determined by standard calculation methods. CFD contributes concretely elsewhere:
- The vent jet and fireball. Where does the flame and pressure wave from the vent panel go, and how far? Safe distances and personnel positions depend on this — a compressible, transient problem.
- Dust accumulation. Where the dust settles shows where the risk is. This is a particle transport problem, and dead zones are the critical points.
- Collection and ventilation effectiveness. Keeping the suspended dust concentration below the lower explosive limit is the primary control; that is a capture efficiency problem.
A caution
Explosion modelling is a sensitive area and the results should not be used as absolute values. Competent standards and experimental dust characterisation (explosion severity, minimum ignition energy) are the basis; the analysis supports them, it does not replace them.
References
- Magnussen, B. F. & Hjertager, B. H. (1977). On mathematical modeling of turbulent combustion with special emphasis on soot formation and combustion. Symposium (International) on Combustion, 16(1), 719–729. DOI: 10.1016/S0082-0784(77)80366-4
- Morsi, S. A. & Alexander, A. J. (1972). An investigation of particle trajectories in two-phase flow systems. Journal of Fluid Mechanics, 55(2), 193–208. DOI: 10.1017/S0022112072001806