Which dust test results do you need to select Ex equipment and size explosion vents?
Abstract
For Ex equipment: the dust's cloud and 5 mm layer minimum ignition temperatures and its volume resistivity (for group IIIB or IIIC). For vents, suppression and isolation: Kst and Pmax. Add MIE for static control, LOC for inerting and MEC for conveying design.
To select Ex equipment you need the minimum ignition temperature of the dust cloud, the minimum ignition temperature of a 5 mm layer and the volume resistivity of the dust; to size explosion vents (and choose suppression or isolation) you need Kst and Pmax. Before either, a screening test should show that the dust is explosible at all. The minimum ignition energy (MIE), limiting oxygen concentration (LOC) and minimum explosible concentration (MEC) answer other design questions.
Short answer
- Step 1, is it explosible? A go/no-go screening test classifies the dust as explosible or not. If not, no Ex equipment or venting is needed for that dust.
- Ex equipment: cloud ignition temperature and 5 mm layer ignition temperature set the permitted maximum surface temperature; volume resistivity decides between group IIIB and IIIC; particle size separates dust from combustible flyings (IIIA).
- Venting, suppression, isolation, containment: Kst (rate of pressure rise, which gives the St class) and Pmax (maximum explosion pressure).
- Static electricity: MIE. Inerting: LOC. Keeping ducts below the explosible range: MEC.
- Test the finest and driest form of the dust that occurs in the plant, including fines from filters and deposits: usually the fraction below 63 µm for cloud tests (the GESTIS-DUST-EX explanation uses below 250 µm for tests on deposited dust).
What each decision needs
| Decision | Test result | How it is used |
|---|---|---|
| Is there a dust explosion hazard at all? | Explosibility screening (go/no-go) | Area classification needed or not |
| Maximum surface temperature of equipment | Cloud minimum ignition temperature | No more than two thirds of it |
| Maximum surface temperature of equipment | 5 mm layer ignition temperature | No more than this value minus 75 K; thicker layers need more margin or a test at that thickness |
| Equipment group IIIA, IIIB or IIIC | Volume resistivity, particle size | 10³ Ω·m or less is conductive (IIIC); above is non-conductive (IIIB); particles above 500 µm are flyings (IIIA) |
| Vent area, suppression, isolation devices | Kst and Pmax | Inputs to EN 14491 or NFPA 68 vent sizing; suppression and isolation are certified for ranges of Kst and Pmax |
| Containment (pressure-resistant equipment) | Pmax | Design pressure of the vessel |
| Earthing, bonding, FIBC type, personnel earthing | MIE | Static control measures become stricter as MIE falls |
| Inerting | LOC | Oxygen set point, kept below the LOC with a margin |
| Conveying and extraction design | MEC | Keeping concentrations in ducts below the explosible range |
| Fire spread in deposits and filters | Burning behaviour (combustion class) | Whether a fire spreads in settled dust |
The surface temperature rules are explained in Surface temperature limits for dust, and every parameter in Dust explosion parameters explained.
Why equipment selection and vent sizing need different tests
The two questions are about different things. Equipment selection is about sensitivity: how hot a surface can be before the dust ignites, and whether conductive dust could get into an enclosure and cause shorts or sparks. Vent sizing is about severity: how fast and how high the pressure rises once the dust is burning. A dust can be hard to ignite and still very violent, or the reverse, so one family of results cannot replace the other.
Test methods also come in families. In Europe the explosion pressure tests (Pmax, Kst, MEC, LOC) are in the EN 14034 series; the ignition tests (cloud and layer temperatures, MIE) and resistivity are in ISO/IEC 80079-20-2; North America uses the corresponding ASTM methods. Values from different methods can differ, so a vent calculation or a temperature check should use values from the method the design standard expects.
Planning a test programme
- Take the right samples. The Irish Health and Safety Authority advises basing the assessment on the finest fraction that may be present, usually obtained by sieving through 63 µm. Include fines from filters, cyclones and ledges, not only the product.
- Screen first. A non-explosible result for a representative, worst-case sample can end the dust explosion question for that material. A "no ignition" result in the open modified Hartmann tube alone is not enough: the GESTIS-DUST-EX explanation calls for further tests in a closed vessel, such as the 20 litre sphere, before a dust is called non-explosible.
- Order by decision. For a new dust collector, Kst and Pmax of the collected fines come first; for electrical equipment in a room, cloud and layer temperatures and resistivity; for powder handling and big bags, MIE.
- Check the conditions. Moisture, temperature, solvent residues and hybrid mixtures change results. If the process runs hot or with solvent vapour, say so to the laboratory so that it can test the relevant condition.
- Record and reuse. Put the results in the explosion protection document or dust hazard analysis, and retest when the product, supplier, particle size or process changes.
Example: A plant installs a filter for a new spice blend and new lighting in the packing room. The filter supplier needs Kst and Pmax of the filter fines (sieved below 63 µm and dried) to size the vents and choose the inlet isolation device. The electrical designer needs the cloud and 5 mm layer ignition temperatures and the resistivity of the dust settling in the room. MIE is added because operators empty bags by hand. Database values for similar spices are useful for orientation but are not used for the design.
Can you use published data instead?
Databases such as GESTIS-DUST-EX hold results for thousands of tested samples. The institute that runs it stresses that tabulated values only serve as a guideline for designing protective measures and that the dust actually handled should be tested when conditions are unclear. Published values are a sensible first step and a plausibility check, not a substitute for testing the dust that will be in the vent calculation.
Common mistakes
- Testing the product, not the fines. Filter fines are more sensitive and often more violent.
- Using only the cloud temperature. The layer value minus 75 K often governs.
- Sizing vents from an St class alone. Vent calculations need the Kst and Pmax values themselves.
- Forgetting resistivity. Without it the choice between IIIB and IIIC equipment is a guess.
Related on excodex
- Dust explosion parameters explained: Kst, Pmax, St classes, MIE and minimum ignition temperatures
- Surface temperature limits for dust: the two-thirds cloud rule and the 75 K layer margin
- Dust explosion protection: venting, suppression, isolation and containment
- Can I use published Kst values instead of testing my own dust?
- How do I know if my dust is combustible?
Standards and further reading
- ISO/IEC 80079-20-2:2016 (under revision), material characteristics, combustible dusts test methods (combustibility screening, ignition temperatures, MIE, resistivity).
- EN 14034 parts 1 to 4, explosion characteristics of dust clouds (Pmax, Kst, lower explosion limit, LOC).
- EN 14491 and NFPA 68, explosion venting; IEC 60079-14, selection of equipment (surface temperature limits).
- Health and Safety Authority (Ireland), ATEX delivery guide on dust: hsa.ie
- GESTIS-DUST-EX explanation of the characteristics: dguv.de (PDF), database: staubex.ifa.dguv.de
- INERIS poster with dust groups by resistivity and the cloud and layer rules: ineris.fr (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.