Combustible Dust Testing: A Three-Page Comprehensive Guide
- May 23, 2025
- 5 min read
Combustible dust presents significant safety challenges across many industrial environments. These finely divided solids, when suspended in air, can create conditions ripe for flash fires or explosions (deflagrations, or deflagrations turned to detonations, depending if the flame front is moving at subsonic or supersonic speeds). Understanding the explosibility of your facility's materials is not optional—it is the first and most critical step in establishing a defensible risk management plan. As outlined in NFPA 652, the facility owner or operator bears the responsibility for determining whether the dust being handled poses such risks.
Combustible dust is defined as a finely divided solid (generally under 500 microns in diameter) capable of remaining suspended in air and igniting when exposed to an ignition source. Examples include flour, powdered sugar, coffee dust, metal shavings, plastic particles, and even agricultural products. These materials are common in industrial settings, and their presence often goes unnoticed due to familiarity. This can lead to complacency, a serious concern given the magnitude of hazard combustible dust poses in enclosed environments.
NFPA 660 is a newly developed standard that incorporates many combustible dust standards. NFPA 652 also serves as the overarching standard for combustible dust safety in the United States. Chapter 5 requires users of combustible dusts to determine whether the materials they handle are explosible. This can be achieved either through referencing published industry data or by conducting laboratory testing. The standard explicitly prohibits the assumption of safety based on the absence of previous incidents.
Industry-published data may include values from NFPA 652 Annex A, Rolf Eckhoff’s Dust Explosions in the Process Industries, or the German IFA database. These sources provide generalized information such as KST and Pmax values for commonly handled materials. While this data can be useful for preliminary analysis and design calculations, it may not accurately represent your specific process conditions—especially regarding particle size, moisture content, or purity.
Many Safety Data Sheets (SDS) lack key explosibility parameters such as minimum ignition energy (MIE), minimum explosive concentration (MEC), or limiting oxygen concentration (LOC). These omissions make SDS an unreliable source for determining combustibility.
Laboratory testing is strongly recommended for proprietary, unique, or poorly documented materials. Testing provides facility-specific insights and supports accurate hazard analysis and engineering control decisions. A conservative approach may also involve assuming combustibility in the absence of testing and applying appropriate safety measures.
The accuracy of explosibility data depends significantly on how the sample is collected and prepared. Common sample sources include dust collectors, overhead beams, duct interiors, and process equipment surfaces. Floor sweeps may not represent airborne material, leading to false-negative or skewed results.
Samples can be submitted “as-received” or prepared according to the ASTM E1226 protocol. As-received testing reflects current process conditions, including moisture and particle size. ASTM protocol preparation, on the other hand, calls for drying the dust below 5% moisture and sieving to under 75 microns. This worst-case preparation ensures conservative results, often required for regulatory compliance and engineering design.
Proper packaging includes sealed, double-bagged plastic containers or hard jars to prevent contamination and moisture absorption. Sample quantity should be sufficient for repeated testing—typically several hundred grams to a few pounds.
It should be emphasized that any possible hybrid mixture of different dusts must be tested as so, to represent the worst-case explosion metrics.
The go/no-go test is a preliminary screening using devices like the Hartmann tube or 20-liter sphere. These tests determine whether a material is capable of sustaining ignition. If the result is positive, further quantitative testing is conducted. A successful ignition in the 20-liter sphere, for example, confirms the dust’s explosibility and the need for mitigation.
Quantitative Explosion Testing Metrics
KST (Deflagration Index): Indicates the rate of pressure rise during an explosion. It is used to classify dust into four categories:
ST 0: No explosibility
ST 1: Weak (KST up to 200 bar·m/s)
ST 2: Strong (200–300 bar·m/s)
ST 3: Very Strong (above 300 bar·m/s)
Pmax: Maximum pressure developed during deflagration inside a test chamber. This value helps size explosion vents and containment systems.
MEC (Minimum Explosive Concentration): The lowest dust concentration in air that can sustain combustion. Important for evaluating risk in dust collectors and airborne dispersion zones.
LOC (Limiting Oxygen Concentration): The maximum oxygen concentration under which a dust cloud cannot ignite. Often used to design inerting systems using nitrogen or CO₂.
MIE (Minimum Ignition Energy): The smallest energy input (e.g., electrostatic discharge) that can ignite a dust cloud. Low MIE values indicate higher ignition risk and require enhanced static control measures.
Real-world data shows how particle size drastically influences explosibility. Powdered sugar (32 microns) exhibits higher KST and lower MEC than table sugar (290 microns), meaning smaller particles ignite more readily and burn more violently. Moisture content also affects ignition—dry dust is more combustible. This highlights the importance of testing the material as it exists in your operation. However, even though table sugar may be regarded as less explosible than powdered sugar through the lens of KST, it still poses an incredibly dangerous explosion hazard. For example, the tragic explosions at the Imperial Sugar refinery in 2008 took 14 lives, injured 36 people, and cost hundreds of millions of dollars in damage.
Ignitability parameters include:
Cloud ignition temperature (ASTM E1491): The lowest temperature that ignites a dispersed dust cloud.
Layer ignition temperature: Measures the risk of a settled dust layer igniting on a hot surface.
Other testing may assess electrostatic discharge sensitivity, flame propagation through settled layers, and conductivity. These insights guide housekeeping, ventilation, and bonding/grounding strategies.
Explosibility of Metal Dusts
Metal dusts—aluminum, magnesium, titanium, etc.—require special consideration. NFPA 68 mandates testing these materials in a 1-cubic-meter chamber or doubling 20-liter sphere values due to their high reactivity. Overdriving from excessive ignition energy can also create misleading results because the intense initial ignition burns the majority of the particulate with no flame propagation or pressure wave produced, which is why test labs often recommend reduced energy levels to simulate realistic conditions. Additionally, legacy metals chemically react with water, therefore care must be taken during facility design and operations.
Conclusion
Combustible dust testing is foundational to safe facility design and regulatory compliance. It provides critical data for developing suppression strategies, designing vent areas, and implementing controls to reduce ignition and explosion risk. Whether you rely on published data or laboratory testing, understanding the behavior of your specific dust is the key to informed decision-making.
With the guidance of NFPA Standards and robust testing protocols like ASTM E1226, facility managers, engineers, and safety professionals can ensure they are not only compliant but also effectively safeguarding life and property from combustible dust hazards. For any further inquiries regarding this topic, as well as for code consulting and fire engineering design support related to your project, please don’t hesitate to contact us via email at contact@engineeringfireprotection.com.
Application of any information provided, for any use, is at the reader’s risk and without liability to Engineering Fire Protection (EFP). EFP does not warrant the accuracy of any information contained in this blog as applicable codes and standards change over time. The application, enforcement and interpretation of codes and standards may vary between Authorities Having Jurisdiction and for this reason, registered design professionals should be consulted to determine the appropriate application of codes and standards to a specific scope of work.
