Can I use published Kst values instead of testing my own dust?
Abstract
For a first assessment, yes; for designing vents, suppression or isolation, usually not. Published Kst values describe one sample and vary widely with particle size and moisture, so design should use tests on the finest, driest dust representative of your process.
For a first assessment, yes; for the design of explosion vents, suppression or isolation, usually not. A published Kst describes one tested sample, and samples of the "same" material differ widely with particle size, moisture and composition. UK guidance recommends designing process equipment with test results on samples representative of the finest and driest material likely to occur in your process, and not just with data from other sources.
Short answer
- Screening and orientation: published values (for example from the GESTIS-DUST-EX database) are fine for deciding that a well-known material such as flour or wood dust is explosible and for a first estimate of its class.
- Design: vent areas, suppression and isolation depend directly on Kst and Pmax. A value that is too low gives an undersized vent, and the reduced explosion pressure can then exceed the strength of the vessel.
- Spread is large. Wheat flour entries in GESTIS-DUST-EX range from 31 to 125 bar·m/s; aluminium dusts from 12 to well over 500 bar·m/s.
- Exception: HSE guidance accepts that for some natural products whose results scatter, cautious assumptions based on a set of published tests may be as reliable as a single new test. "Cautious" means choosing from the upper end and documenting why.
- Whatever you use, the value must be traceable (source, sample description, test method) and the protection device must be qualified for it.
Why published values can mislead
The German IFA, which runs GESTIS-DUST-EX, states that its tabulated values can only serve as a guideline for the design of protective measures, because explosion behaviour depends to a large extent on particle size distribution and also on moisture. Each database entry shows the fineness and moisture of the tested sample for that reason. Some examples of the spread:
| Material | Samples in GESTIS-DUST-EX | Kst range seen |
|---|---|---|
| Wheat flour | coarse and fine grades, different moisture | 31 to 125 bar·m/s (all St 1) |
| Aluminium | fine powders to sanding dust | 12 bar·m/s (sanding dust fraction) to 577 bar·m/s (fine powder) |
| Wood | coarse sawdust to fine wood flour | no explosion (1500 µm sawdust) to 211 bar·m/s |
Three further reasons make literature data risky for design:
- Your dust is not the product. The dust in a filter, cyclone or silo head space is finer and often drier than the material on the specification sheet. Fines raise Kst.
- Methods differ. Older data may come from other apparatus, ignition energies or dispersion systems. HSG103 warns that care is needed in interpreting data from old sources, and ASTM E1226 notes that a 20 litre sphere can overdrive weak dusts and underdrive some metal dusts compared with a 1 m³ vessel.
- Additives and mixtures. Small amounts of a finer or more reactive component, solvents or a flammable gas (a hybrid mixture) can raise the severity above any published value for the main material.
When a published value can be acceptable
| Purpose | Published data acceptable? |
|---|---|
| Deciding whether a well-known material is explosible | Yes, as orientation; testing still needed to claim "not explosible" |
| First budget estimate of vent or suppression cost | Yes, with a conservative value |
| Area classification and Ex equipment selection | Kst is not used for this; ignition temperatures and dust group are, and they should also be representative |
| Final vent, suppression or isolation design | Normally test the actual dust; published data only with a documented conservative basis and the agreement of the designer and supplier |
| Unusual or new product, mixtures, metal dusts | Test |
If you do use literature, choose values from samples at least as fine and as dry as your worst-case dust, take the upper end of the range, and record the source and sample details in the explosion protection document or dust hazard analysis. The Irish Health and Safety Authority (HSA) notes that the database can serve for orientation, but where the properties of a powder are not known, testing in a laboratory will normally be needed.
How to get a useful test result
- Sample the worst case: fines from filters, ledges and transfer points, plus the product. Record where each sample came from.
- Agree the preparation with the laboratory: sieving and drying make the result conservative, and the test report must state the particle size distribution and moisture as tested.
- Order the tests the designer needs: usually Kst and Pmax for vents and suppression; minimum ignition energy and ignition temperatures if equipment selection and static control are also open.
- Repeat the test when the product, raw material supplier, grinding or drying changes.
Common mistakes
- Taking the first value found online without checking particle size, moisture and method.
- Using an average of database values. Design needs a conservative value, not a typical one.
- Testing the bagged product instead of the fines in the equipment being protected.
- Fitting a protection device qualified for St 1 to a dust that has never been tested.
Related on excodex
- Dust explosion parameters explained: Kst, Pmax, St classes, MIE and minimum ignition temperatures
- Dust explosion protection: venting, suppression, isolation and containment
- What do Kst and Pmax tell you about a dust explosion?
- How do I know if my dust is combustible?
- 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 Pmax and Kst 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.
- HSE, HSG103 Safe handling of combustible dusts (2nd edition, 2003), appendix on the use of test data: hse.gov.uk (PDF)
- IFA, explanation of the GESTIS-DUST-EX characteristics and their limits: dguv.de (PDF); example entry, wheat flour type 405: staubex.ifa.dguv.de
- Health and Safety Authority (Ireland), ATEX guide on combustible dust: hsa.ie
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.