What do Kst and Pmax tell you about a dust explosion?
Abstract
Pmax is the highest overpressure a dust cloud reaches when it explodes in a closed vessel; Kst is its normalised maximum rate of pressure rise in bar·m/s and sets the St class. Together they size vents, suppression and isolation; they say nothing about ease of ignition.
Pmax tells you how high the pressure rises when a dust cloud of the worst concentration explodes in a closed vessel. Kst tells you how fast it rises: it is the maximum rate of pressure rise, normalised to a 1 m³ volume, in bar·m/s, and it places the dust in explosion class St 1, St 2 or St 3. Together they describe the severity of an explosion and are the main inputs for sizing explosion vents, suppression, isolation and pressure-resistant equipment. They say nothing about how easily the dust ignites.
Short answer
- Pmax (bar, overpressure): the maximum explosion pressure in a closed vessel at optimum dust concentration. Many organic dusts reach about 7 to 10 bar; fine light-metal powders reach about 12 bar or more.
- Kst (bar·m/s): Kst = (dP/dt)max × V^(1/3), the maximum rate of pressure rise multiplied by the cube root of the vessel volume.
- St classes: St 1 Kst above 0 up to 200, St 2 above 200 up to 300, St 3 above 300 bar·m/s; St 0 means no explosion in the test.
- Higher Kst means a faster explosion, which needs a larger vent area and faster suppression and isolation. Pmax decides what a closed vessel must withstand.
- Both are sample values: they change with particle size, moisture and test conditions. Ignition sensitivity is described by other parameters (MIE, ignition temperatures, MEC).
What the two numbers measure
Both values come from the same test. A weighed amount of dust is blown into a closed sphere, usually the 20 litre sphere or the 1 m³ reference vessel, and ignited at the centre by strong chemical igniters. A pressure sensor records the explosion. The test is repeated over a range of concentrations, and the highest pressure and the steepest pressure rise of the whole series are reported.
| Value | What it is | Unit | Typical use |
|---|---|---|---|
| Pmax | The highest explosion overpressure found over all concentrations tested | bar (gauge) | Strength of explosion-pressure-resistant equipment; input to vent and suppression design |
| (dP/dt)max | The steepest slope of the pressure curve over all concentrations | bar/s | Intermediate value, depends on vessel size |
| Kst | (dP/dt)max × V^(1/3), normalised to 1 m³ | bar·m/s | Vent area, suppression and isolation design; St class |
The German IFA, which runs the GESTIS-DUST-EX database, defines Kst as numerically equal to the maximum rate of pressure rise in the 1 m³ vessel. Because the pressure in a smaller vessel rises faster, results from the 20 litre sphere are converted with the cubic law: for compact, geometrically similar vessels, (dP/dt)max × V^(1/3) is constant. UK guidance HSG103 notes that test work on vessels of up to 600 m³ has supported this relationship.
Worked example: from a test result to the St class
A dust tested in the 20 litre sphere gives Pmax = 8.0 bar and (dP/dt)max = 600 bar/s.
- Cube root of the volume: V = 0.020 m³, so V^(1/3) = 0.271 m.
- Kst = 600 bar/s × 0.271 m = 163 bar·m/s, so the dust is St 1.
- In a 1 m³ vessel the cubic law predicts about 163 bar/s; in a 100 m³ silo, 163 / 100^(1/3) = about 35 bar/s under the same test turbulence.
The silo figure shows why Kst is used as a standardised input to design methods rather than as a prediction of what will happen in a real vessel: turbulence, filling pattern and shape in a plant differ from the test.
How engineers use Kst and Pmax
Explosion venting. Vent areas are calculated with EN 14491 in Europe or NFPA 68 in North America. The inputs include the vessel volume and shape, Kst, Pmax, the opening pressure of the vent and the reduced explosion pressure that the vessel can withstand. For the same vessel, a higher Kst needs a larger vent area.
Suppression and isolation. Suppression systems and isolation devices such as flap valves, rotary valves and chemical barriers are tested for stated ranges of Kst and Pmax. A device qualified for St 1 dusts may not work for an St 2 dust, so the certificate or test report of the device must cover the dust.
Containment. Equipment built to withstand an explosion without venting must be designed for at least the expected maximum pressure, see Dust explosion protection: venting, suppression, isolation and containment.
Simplified rules. Some guidance applies only below a Kst limit. HSG103, for example, says its simple venting arrangement for bucket elevators will usually give adequate protection for dusts with a Kst of 150 bar·m/s or less.
What Kst and Pmax do not tell you
- How easily the dust ignites. That is the job of the minimum ignition energy, the cloud and layer ignition temperatures and the minimum explosible concentration. An St 1 dust can be easier to ignite than an St 3 metal powder.
- Whether the explosion is survivable. "St 1" is often read as "weak", but St 1 dusts such as flour and sugar have destroyed buildings. A pressure of 7 bar is far more than any building can resist.
- The behaviour of your product. The IFA stresses that tabulated values are only guidance; fineness and moisture change both numbers. Small 20 litre results for weak dusts can also be overdriven by the igniters, which ASTM E1226 warns about.
| Class | Kst (bar·m/s) | Typical examples in published data |
|---|---|---|
| St 0 | 0 (no explosion) | silica; very coarse samples such as a 1500 µm sawdust |
| St 1 | above 0 to 200 | flour, sugar, starch, powdered milk, most wood dusts |
| St 2 | above 200 to 300 | cellulose, some fine wood flours |
| St 3 | above 300 | fine aluminium and magnesium powders |
Common mistakes
- Comparing Kst values from different apparatus or igniter energies without checking the method.
- Using the St class to choose Ex equipment. Equipment selection uses the zone, the dust group and the ignition temperatures, not Kst.
- Taking a database Kst for vent design instead of testing the plant's own dust.
Related on excodex
- Dust explosion parameters explained: Kst, Pmax, St classes, MIE and minimum ignition temperatures
- Dust explosion protection: venting, suppression, isolation and containment
- Can I use published Kst values instead of testing my own dust?
- Why can an St 1 dust still cause a devastating explosion?
- Which dust test results do you need to select Ex equipment and size explosion vents?
Standards and further reading
- EN 14034-1 and EN 14034-2, determination of the maximum explosion pressure and the maximum rate of pressure rise of dust clouds.
- ASTM E1226, standard test method for explosibility of dust clouds (E1226-19, reapproved 2025): store.astm.org
- EN 14491 and NFPA 68, dust explosion venting; EN 14373, explosion suppression; EN 15089, explosion isolation.
- IFA, explanation of the GESTIS-DUST-EX characteristics (Pmax, Kst, cubic law, St classes): dguv.de (PDF)
- HSE, HSG103 Safe handling of combustible dusts (2nd edition, 2003): hse.gov.uk (PDF)
- OSHA 3371, Hazard communication guidance for combustible dusts (Kst as the best single-number estimate of explosion behaviour): osha.gov (PDF)
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.