Combustible dust testing is performed to assess risks related to the handling of fine powders that may theoretically create an explosive atmosphere but are not known explosives. Examples include powder-form chemicals, metal dust, sawdust, and food powders managed in large quantities in settings such as industrial plants and warehouses.
The initial identification of combustible dust is typically based on the ISO/IEC 80079-20-2 standard, which outlines a step-by-step procedure to determine whether powders have characteristics that may cause them to ignite or explode. Further tests, some described in ISO/IEC 80079-20-2 and others based on additional ASTM and EN standards, can then be performed to identify ignition conditions and explosion intensity.
The results from these tests are used to categorize materials based on their combustibility potential. They will also dictate whether materials fall under the ATEX Directive or other hazardous material regulations and classification systems, which specify the appropriate handling procedure and equipment design required to minimize the risk of combustion.
Selecting the sample and testing conditions
Combustible dust samples are often collected directly from the production line and tested in “as-is” condition. To ensure that risks are not underestimated, samples should be representative of the material in the worst-case form it is found in the real-life process. This includes the smallest particle size and lowest humidity level that may realistically occur. As a full combustibility and explosivity assessment consists of several tests, each of which will typically require at least 0.5 kg of sample material, several kilograms or liters of powder should be sent to the laboratory.
By default, testing is conducted in standard atmospheric conditions, where the temperature is 20°C ± 10 °C, the pressure is 80 kPa to 110 kPa, and the air oxygen content is 21%. However, temperatures from -20 to +60 °C can be used if a lower or higher temperature is more representative of the real-life process conditions.
Definition of combustible dust in ISO 80079-20-2
Particle size distribution analysis is one part of the initial evaluation of dust samples’ combustibility potential. This is because, according to ISO/IEC 80079-20-2, dust can only be combustible if it contains some particles with the following size characteristics:
Nominal particle size less than 500 µm (“combustible dust”)
Elongated particles or fibers with one dimension greater than 500 µm and a length-to-width ratio of 3 or more (“combustible flyings”)
If the sample contains sufficiently small particles, it is potentially combustible, and the assessment continues with ignition tests.
Ignition testing
Ignition tests are performed next to determine whether fine dust or flyings ignite and how much energy ignition requires. This step is also known as the “go/no go test” or initial explosivity screening.
The procedure outlined in ISO/IEC 80079-20-2 is three-fold, but if ignition occurs during any of the tests, the material is considered combustible, and the subsequent steps can be skipped:
First, a test is performed in a Hartmann tube with a spark. If the sample does not ignite, its minimum ignition energy is greater than 1 J, and the material is considered hard to ignite.
Second, a test with a hot coil ignition source is performed, also in the Hartmann tube. If the sample does not ignite, its minimum ignition energy is at least 10 J.
Third, a test is performed in a 20-liter sphere. If the material still fails to ignite, it is not combustible dust.
The Godbert-Greenwald (GG) oven test can be used as an alternative if there is insufficient material for the sphere test. If the material does not ignite at 1,000 °C during the test, it is not combustible. If it does, more material should be sourced to confirm the results with the 20-liter sphere test.
Standard methods for further characterization of combustible dust
After dust has been identified as combustible, further tests are required to determine the conditions under which it may ignite or explode. Commonly applied standard methods to evaluate the combustibility and explosibility of dust samples are listed in Table 1.
Table 1: Tests to characterize combustible dust
Test | Standard(s) | Description |
Minimum ignition temperature (MIT) of dust clouds | ISO/IEC 80079-20-2 ASTM E1491 | The lowest temperature of hot air at which the most easily ignitable mixture of dust and air will ignite. |
MIT of dust layers | ISO/IEC 80079-20-2 ASTM E2021 | The lowest hot surface temperature at which the dust layer will ignite. |
Electrical resistivity of dust | ISO/IEC 80079-20-2 | Classification of dust as conductive (resistivity ≤ 1 × 103 Ω⋅m) or non-conductive (resistivity > 1 × 103 Ω⋅m). |
Minimum ignition energy (MIE) | ASTM E2019 EN 13821 | The lowest electrical energy sufficient to cause ignition of the most easily ignitable dust mixture. |
Low explosion limit (LEL) | EN 14034-3+A1 ASTM E1515 | The lowest concentration of the air-dust mixture that leads to an explosion. |
Maximum explosion pressure | EN 14034-1+A1 ASTM E1226 | The maximum pressure that may arise during the explosion of the combustible atmosphere in a closed vessel. |
Explosion constant | EN 14034-2+A1 ASTM E1226 | Volume-dependent parameter indicating explosion intensity, used to categorize dusts into explosion classes St1, St2, and St3. |
Limited oxygen concentration (LOC) | EN 14034-4+A1 ASTM E2931 | The maximum concentration of oxygen in the dust-gas mixture that does not lead to an explosion. |
Burning behavior of dust layers | EN 17077 | Classification of dust into classes BZ 1 to BZ 6, based on how easily fire spreads in settled dust. |
Spontaneous ignition behavior, self-ignition temperature (TSI) | EN 15188 | The ability of the material to combust spontaneously, indicating how large quantities can be stored safely. |
Choosing the tests
Measurlabs provides a comprehensive selection of combustible dust and explosivity testing, including the standardized tests listed above. The most appropriate testing procedure will be chosen based on multiple variables, such as substance properties, process environment, operational specifications, transportation conditions, and local laws and regulations.
In addition to combustible dust and explosivity determinations, additional testing according to the UN Manual of Tests and Criteria or the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) may be required to determine the appropriate transportation conditions and packaging for the material.
Our experts can help you plan a suitable testing procedure. Do not hesitate to contact us through the form below to start the discussion on how we can best support your company in this field.