How dust testing can lead to process improvements and reduce explosion risk
A variety of dusts possess explosible properties, and therefore when certain conditions are met, such as the dust being dispersed and enclosed and an ignition source being introduced, combustion is likely to occur. Some combustible dusts include flour, sugar, milk powder, wood, plastics and metal, and countless other variables that must also be considered such as particle sizes and mixtures.
That’s why knowing the combustible properties of the dust handled in your facility is often the first step towards avoiding a devastating dust explosion. Let’s look at three specific types of ‘sensitivity’ dust tests, and how documenting certain explosive properties can be used to leverage process adjustments and facilitate explosion prevention efforts.
Minimum ignition temperature of a layer | MIT (layer) -This measures the lowest surface temperature required to ignite a layer of dust.
- How data may be used: Both monitor and mitigate the hot surfaces within a process to ensure they remain under the dust layer’s MIT.
- Applicable standards: ‘ASTM E2021, Standard Test Method for Hot Surface Ignition Temperature of Dust Layers’ & ‘EN 50281-2-1.’
- How data is documented: Aluminum plate is heated to various temperatures in which smoke or combustion is observed from the dust sample; the lowest temperature resulting in ignition is the MIT (Layer).
- Temperatures of dust layers: Low = <300°C | Med = 300°C – 450 °C | High = >450 °C.
- The lower the MIT, the more susceptible it is to combustion from a hot surface.
Minimum ignition temperature of a cloud | MIT (cloud) – Indicates the lowest thermal ignition required for a dust cloud to self-ignite.
- How data may be used: Ensure conveyed air or volumes within ovens, for example, stay below the dust cloud’s MIT.
- Applicable standards: ‘ASTM E1491, Standard Test Method for Minimum Auto Ignition Temperature of Dust Clouds’ & ‘EN 80079-20-2 and & ‘EN13822’.
- How data is documented: Exposes a dust cloud to varying temperatures until self-ignition is and is not observed. The MIT (cloud) is the lowest ‘boundary’ temperature over a range of dust concentrations.
- Temperatures of dust clouds: Low = <400°C | Med = 400°C – 600 °C | High = >600 °C.
- The lower the MIT (cloud), the more susceptible a dust cloud is to combustion from sources such as hot conveyed air.
Minimum Ignition Energy | MIE – Determines the lowest amount of electrical discharge required to ignite a dust cloud.
- How data may be used: Bonding and grounding may be used in areas identified to have risk of electrical discharge and help prevent or mitigate high energy buildup within the process.
- Applicable standards: ‘ASTM E2019, Standard Test Method for Minimum Ignition Energy of a Dust
- Cloud in Air’ & ‘EN 80079-20-2” & “EN 13822’.
- How data is documented: A dust sample is pneumatically dispersed into a cylinder while a spark is generated across two electrodes. The MIE is the average of the lowest energy at which ignition of the dust cloud is observed and the highest energy at which ignition is not observed.
- Ignition energies: Very low = <10 mJ | Low = 10-30 mJ | Med = 30-100 mJ | High = >100 mJ.
- The lower the MIE, the more susceptible the dust cloud is to ignition from electrostatic discharge.
Once these explosibility values of the unique dust found within your process are documented, a risk assessment specialist will then be able to know where potential combustible dust hazards may or may not exist in the facility. Therefore, certain areas of the process where the dust is determined to be non-combustible, or with a very low severity or likelihood of igniting, may be deprioritised in favor of areas that could result in harm to workers or extensive damage to the process if combustion were to occur.
Furthermore, the risk assessment specialist will use these explosibility characteristics to suggest areas of the process that may require temperature monitoring to ensure a surface doesn’t reach a dust layer’s MIT, methods of minimising dust concentrations or the formations of dust clouds where elevated air is conveyed, and more.
Finally, not only do both NFPA and ATEX require the documentation of the process’s dust explosibility characteristics, but this information is also critical during the design of explosion protection equipment, including explosion venting, suppression and isolation systems.
Whether you need to identify the explosibility of the dust handled in your facility or have already completed dust testing but are unsure what to do next, it’s important to work with a trusted dust testing provider to both perform the dust testing and analyse the results with you to ensure reliable prevention and protection from explosions.
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