1. Why Does Dust Also Explode? (Dust + Oxygen + Ignition Source)
When combustible dust exists asa suspended cloudoror a layered accumulationit will also deflagrate on contact with an ignition source — historical accidents at flour mills, woodworking plants, polishing shops and coal pulverizing facilities all trace back to this. Dust explosions differ from gas explosions in ignition energy and initiation conditions, soa gas explosion-proof box (Ex d) cannot be used directly as a dust explosion-proof box. The two certification systems are independent.
2. Dust Zone Classification: 20/21/22 (Corresponding to Gas Zones 0/1/2)
| Zone | Dust Presence State | Typical Location | EPL |
|---|---|---|---|
| Zone 20 | Dust cloud present continuously or frequently | Inside silos, interior of dust collectors | Da |
| Zone 21 | Dust cloud occasionally present during normal operation | Dust loading points, bagging stations | Db |
| Zone 22 | Dust cloud present briefly under abnormal conditions | Floor area around bag filters | Dc |
3. Ex tD: How Enclosure Protection Works (Dust Ingress Prevention + Temperature Limiting)
The mainstream form of dust explosion protection isEx tD (enclosure protection "t"): the enclosure keeps dust out + surface temperature is controlled. Two types:
- Type A(European system, e.g. Ex tD A21): assessed by enclosure ingress protection rating —Zone 20/21 requires IP6X (dust-tight),Zone 22 requires IP5X (dust-protected);
- Type B(North American system, e.g. Ex tD B21): assessed by dust-tight joints, overpressure leakage test.
- The new standard uses the unified designation Ex t IIIC T80°C Db; confirm the system with the design team during selection.
4. How is surface temperature determined? (assessed with a 5 mm dust layer)
Temperature assessment in dust environments differs from gas: it must be below theignition temperature of the dust cloud, and also below theignition temperature of the dust layer (5 mm thick) minus 75K. Flour dust clouds ignite at about 380–420°C, but a 5 mm flour layer can smolder at 200°C — so surface temperature limits for dust-rated equipment are often tighter, and the nameplate states the specific temperature value directly (e.g. T80°C).
5. Typical industry selection comparison (match your site)
| Industry | Dust type | Dust group | Typical configuration |
|---|---|---|---|
| Flour / feed mills | Flour dust (non-conductive) | IIIB (partly IIIA) | Ex tD ⅢB T135℃ Db + IP6X |
| Wood processing | Wood dust (non-conductive) | ⅢB | Ex tD ⅢB T135℃ Db |
| Aluminum and magnesium polishing shops | Metal dust (conductive) | ⅢC | Ex tD ⅢC T135℃ Db + IP6X |
| Plastics / resin plants | Plastic dust (conductivity varies) | ⅢB/ⅢC | Determine from the dust test report |
FAQ FAQ
Can a gas explosion-proof box be used in a dust environment?
Not directly interchangeable. Ex d is certified under the gas system; dust environments require Ex tD / Ex t dust certification, and the two use different temperature assessment methods.
Must Zone 20 use IP6X?
Yes. Equipment enclosures in Zone 20/21 require IP6X (dust-tight); Zone 22 requires IP5X.
What does IIIC mean?
IIIC isconductive dust(such as metal powder), the most stringent requirement; IIIB non-conductive dust; IIIA combustible flyings (such as fiber, cotton lint).
What routine maintenance does dust explosion protection require?
Regularlyclean accumulated dust from the cabinet(the thicker the dust layer, the worse the heat dissipation and the higher the risk of smoldering), keep enclosure seals intact, and check the sealing of cable entries.
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