What the class means
A mixture of the combustible dust with air is ignited in the laboratory and the rate of pressure rise is measured. The constant that comes out of that test places the dust in one of three classes:
| Class | Explosion behaviour |
|---|---|
| ST1 | Weak |
| ST2 | Moderate |
| ST3 | Violent |
A common misreading
ST1 does not mean “does not explode”. Very common materials — flour, sugar, starch, wood and coal dust — sit in these classes, and most industrial dust explosions involve them. The class describes the severity of the risk, not its existence.
What the class is used for
The constant is used to size protective systems: the area of explosion venting, the response time of a suppression system, the position of an isolation device and the pressure-resistance rating of the equipment are all calculated from it. A venting calculation made without knowing the constant is not a calculation but a guess.
The plant's own dust data is needed
The same material behaves differently at a different particle size and moisture content. So instead of a general assumption — “flour dust is class X” — the data for the dust actually present is used. If no data exists it is obtained; a calculation made without it cannot be defended.