In today’s episode of the Dust Safety Science podcast, we’re talking about the difference between the UN flammable solids test and the ASTM E1226 explosibility test for combustible dust. In particular, we’re talking about a HelpDesk question that came in through our system asking about these two standard testing methods.
The central question posed is whether a powder, classified as non-flammable under the United Nations (UN) flammability solids test, can be conclusively deemed non-explosive. This inquiry is especially pertinent in the context of the UN recommendations on the transportation of dangerous goods and the associated test methods.
The significance of this topic is twofold. Firstly, the UN test method incorporates a preliminary screening process, which offers a relatively simple way to bypass the complex determination of whether a substance is a flammable solid. This has led to questions about the reliability and affordability of this method as a means to ascertain if dust is explosive. The assumption that non-flammable dust cannot explode is a common one, but the podcast aims to dig deeper into the accuracy of this belief.
Dust Can Still Be Explosive Even If It Passes Flammability Tests
Many combustible dusts can pass the United Nations (UN) flammability test but still be explosive in industrial settings. It’s crucial to not wrongly assume that dust is non-combustible just because it passes this test. Making this mistake could mean skipping necessary safety assessments, like a dust hazard analysis, which could lead to dangerous situations, including explosions.
The UN test for flammable solids is detailed in their recommendations for transporting dangerous goods. It aims to identify materials that can ignite and burn quickly or violently. The test has three parts: a preliminary screening, a burn rate test, and a wetted zone test. In the preliminary test for organic materials, you form a strip of powder on a heat-resistant surface and apply a flame for up to two minutes. If the powder doesn’t burn along its length in this time, it’s not considered a flammable solid, and no further tests are needed. If it does ignite, you proceed to the next tests.
The burn rate test uses a V-shaped mold to see how fast the material burns. The wetted zone test checks if burning can continue past a damp area. These tests help classify the burning characteristics of a material. For metals and other materials, the requirements are slightly different.
In summary, the UN test starts with the preliminary screening, where a strip of powder is exposed to a flame. If the material doesn’t burn, it’s not classified as a flammable solid according to the standard.
When a material passes the preliminary UN screening test and is labeled as non-flammable, it raises the question: does this mean the material can’t explode as combustible dust? To answer this, let’s look at the ASTM E1226 test method, which is the standard for testing the explosive potential of dust clouds. This method measures the maximum pressure and rate of pressure rise in a combustible dust sample. It uses a 20-liter sphere, where dust is blown in and ignited to see if it explodes at certain concentrations and under specific conditions. The ASTM standard includes a basic pass/fail screening test, but it warns that just because a material doesn’t explode in this test, it doesn’t mean it’s not flammable.
Dust Can Still Be Explosive Even If It FAILS Flammability Tests
Interestingly, some materials that fail the UN flammability test can still be explosive as dust. For example, sugar might not catch fire in a pile and would fail the UN test, but sugar dust is known to be explosive, as seen in several sugar refinery explosions. Other materials like Cornstarch and Lycopodium also might fail the UN test but are still explosible.
We contacted several testing labs to confirm this. They shared examples and test data showing that some materials classified as non-explosive in the ASTM screening test could still be flammable. This shows that the UN test for flammability might not always be a reliable indicator of a material’s explosive potential as dust.
In a test using the ASTM E1226 standard, fermented soybean meal showed significant explosive potential. The test results indicated a maximum pressure of 7-8 bar and a maximum explosion severity (KST) of around 130 bar meters per second, classifying it as a moderately explosive material. This means it could cause injuries or equipment damage in a flash fire or explosion. However, the same soybean meal failed the UN flammability test. It didn’t propagate a flame in the preliminary strip or V mold tests. During the burn rate test, it glowed but didn’t burn enough to be considered flammable.
This example illustrates how a material can be non-flammable under one test method but still highly explosive under another. It’s easy to assume that if something doesn’t burn, it won’t explode. But the science behind this is more complex.
The flammability test is based on long-term reaction kinetics and heat transfer mainly through conduction. This involves how a material burns over a longer period and how heat moves through a pile of dust. The explosibility test, however, works differently. It involves dust particles suspended in air with significant space between them, leading to different reaction dynamics. This means that the conditions and mechanisms in explosibility tests are entirely different from flammability tests, explaining why a material might fail to burn as a solid but still explode as dispersed dust.
When discussing an optically thick medium like a dust cloud, we refer to its opacity to light. Imagine a simple experiment where a light bulb is obscured by a dust cloud; the light doesn’t pass through, indicating the cloud’s density. However, this cloud is largely composed of air interspersed among the particles. The combustion process within such a cloud is unique and complex. It begins with a single particle igniting and reacting at an incredibly fast rate, in a matter of microseconds to milliseconds. This rapid reaction is influenced by various factors, such as the rate at which the particle releases volatile compounds or its heating rate, as well as the kinetics of the reaction itself.
Once a particle ignites, it heats the surrounding air and particles through convection or radiation. If the initial reaction is intense enough to heat the neighboring particle sufficiently for it to ignite before the first burns out, a chain reaction ensues. This propagation of reaction and flame through the dust cloud happens at a microscale – far quicker than seconds or minutes.
This process contrasts starkly with heat conduction in a solid mass. In the dust cloud scenario, it’s about the heating rate across the spaces between particles. This fundamental difference in physical mechanisms explains why some materials burn or react in one situation but not in another. It’s not just a matter of whether the material reacts, but also how the energy is released and dispersed during the reaction.
Conclusion
It’s clear that the interaction of particles in a dust cloud, their rapid ignition, and the subsequent heat transfer processes present unique challenges that differ significantly from traditional conduction-based fire scenarios. As we continue to explore these vital safety topics, it becomes increasingly apparent that a comprehensive approach to testing and safety is essential for effective dust hazard management in any industry dealing with combustible materials.
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources mentioned
The resources mentioned in this episode are listed below.
Dust Safety Science
Combustible Dust Incident Database
Dust Safety Science Podcast
Questions from the Community
Dust Safety Academy
Dust Safety Professionals
Standards
ASTM E1226
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