Some air streams within closed, pneumatic dust handling systems are essentially free of dust. Such is the case within the clean air duct downstream of dust collectors. In the event of a dust explosion in the equipment connected to the dust-free air ducts, some of the explosion products (dust and flame) will be ejected into the duct. However, because the interior of the duct has no dust to act as fuel prior to the explosion, the distance of explosion propagation (as defined by a flame front) will necessarily be limited.
An experimental project is being conducted to study the distance of flame front propagation into a clean duct for dust explosions occurring in a primary volume which is provided with explosion venting. A near-cubical primary explosion chamber measuring 23 ft3 (0.64 m3) is used. The effect of the most significant process variables on propagation distance is being studied. The process parameters whose effects are being studied are: dust reactivity, vented explosion pressure, explosion vent relief pressure, in-duct air velocity, and dust concentration. Tests conducted to date in an 8 in. (200 mm) diameter, 89 ft (27 m) long duct have resulted in flame propagation distances in the range of 22-89ft (7-27 m).