What Pmax and KSt measure
Both come from igniting a cloud of the dust inside a closed test vessel, normally a 20-litre sphere or a 1 m³ vessel, over a range of concentrations.
- Pmax is the highest pressure the explosion reaches, in bar g. For most organic dusts it lies between 7 and 10 bar g, far above what ordinary process equipment can withstand.
- (dP/dt)max is the highest rate of pressure rise. It depends on the size of the test vessel, so it is normalised with the cube-root law to give a constant for the dust:
KSt = (dP/dt)max · V1/3 [bar·m/s]
KSt tells you how fast the explosion develops, and therefore how quickly a vent must open and how large it must be.
Dust explosion classes
| Class | KSt (bar·m/s) | Character | Typical dusts |
|---|---|---|---|
| St 0 | 0 | No explosion | Sand, cement, limestone |
| St 1 | 1 to 200 | Weak to moderate | Flour, sugar, milk powder, coal, most food and wood dusts |
| St 2 | 201 to 300 | Strong | Cornstarch, cellulose, some plastics and pharmaceutical powders |
| St 3 | Above 300 | Very strong | Aluminium, magnesium and other fine metal powders |
The class is a convenient label, but vent sizing uses the actual KSt value, not the class.
Typical values
| Dust | KSt (bar·m/s) | Pmax (bar g) | Class |
|---|---|---|---|
| Wheat flour | 87 | 8.3 | St 1 |
| Milk powder | 90 | 8.1 | St 1 |
| Bituminous coal | 129 | 9.2 | St 1 |
| Sugar | 138 | 8.5 | St 1 |
| Polyethylene | 156 | 7.5 | St 1 |
| Cornstarch | 202 | 10.3 | St 2 |
| Aluminium powder | 515 | 11.2 | St 3 |
These are indicative values. They are typical published figures and are the defaults in the Flammer calculator. The same material can give very different results from one plant to the next, so the vents on real equipment should be sized on a test of your own dust, carried out to ASTM E1226 or EN 14034.
Why the same dust gives different numbers
- Particle size. The strongest effect. Finer dust has more surface area and burns faster; KSt can multiply several times as the median size falls.
- Moisture. Damp dust is harder to ignite and explodes less violently. Tests are normally done on dried material to be on the safe side.
- Composition. Fat, sugar and additive content, or a flammable solvent on the powder (a hybrid mixture), can raise the values sharply.
- Where the sample was taken. The fines collected in a filter are finer and more hazardous than the bulk product in the silo below it.
How KSt changes the vent area
In the NFPA 68 dust equation the vent area is directly proportional to KSt, so doubling KSt doubles the vent. For a 25 m³ compact vessel with Pred = 0.5 bar g and Pstat = 0.1 bar g:
| Dust | KSt | Pmax | Vent area (m²) |
|---|---|---|---|
| Wheat flour | 87 | 8.3 | 0.41 |
| Sugar | 138 | 8.5 | 0.66 |
| Cornstarch | 202 | 10.3 | 1.07 |
| Aluminium powder | 515 | 11.2 | 2.85 |
The method behind these figures is explained in NFPA 68 vent area calculation, and you can try your own values on the explosion vent sizing page.