Chapter 3
Combustible dusts
Published
- What makes a dust explosive? Particle size, concentration and the dust explosion pentagon
A dust explodes when a fine combustible powder is dispersed in air above its minimum explosible concentration, inside a confined space, and meets an ignition source. How particle size, concentration and moisture decide the risk, with test data for common dusts.
- Dust explosion parameters explained: Kst, Pmax, St classes, MIE and minimum ignition temperatures
Kst, Pmax and St classes describe how violently a dust cloud explodes; MIE, the cloud and 5 mm layer ignition temperatures, MEC and LOC describe how easily it ignites. What each value means, how it is tested in the 20 L sphere and 1 m³ vessel, and how it is used.
- Dust zones 20, 21 and 22 explained with practical examples
Zones 20, 21 and 22 describe how often an explosive dust cloud is present. Definitions, typical zones inside silos, mills, cyclones and filters and around bagging points, the role of housekeeping and dust layers, and the matching EPL Da, Db and Dc.
- Dust layers and housekeeping: how thick is too thick?
Less than 1 mm of settled dust can fill a room with an explosive cloud. How layers form and ignite, why they cause secondary explosions, the NFPA 1/32 inch (0.8 mm) layer depth criterion, safe cleaning methods without compressed air, and how housekeeping affects zoning.
- Dust groups IIIA, IIIB and IIIC: combustible flyings, non-conductive and conductive dusts
Group III Ex equipment is divided by dust type: IIIA combustible flyings, IIIB non-conductive dust, IIIC conductive dust. Definitions, the 500 µm and 10³ Ω·m criteria, examples, marking, and why conductive dust demands dust-tight IP6X enclosures.
- Dust explosion protection: venting, suppression, isolation and containment
When a dust explosion inside plant cannot be ruled out, its effects are limited by containment, venting, flameless venting or suppression, plus isolation of connected ducts. How each works, the design data (Kst, Pmax, Pstat, Pred), a vent sizing example and what to inspect.
- Dust collectors, silos and bucket elevators: where dust explosions start
Dust collectors, silos and bucket elevators hold explosive dust clouds in normal operation, meet ignition sources and connect to the rest of the plant. Why they are high risk, the usual protection packages, what to inspect and the lessons from investigated explosions.
- Surface temperature limits for dust: the two-thirds cloud rule and the 75 K layer margin
In dust areas the maximum surface temperature of equipment must not exceed two thirds of the dust cloud ignition temperature or the 5 mm layer ignition temperature minus 75 K, whichever is lower. How the IEC 60079-14 rules work, thicker layers and worked examples.