There are many materials we come across in our everyday lives that we assume are non-hazardous. Or at least we consider them to be for all intents and purposes. Many of these materials, when found in a fine dust form, are combustible and/or explosible. A dust layer with a thickness of less than 1/8th of an inch can cause an explosion. As the size of the individual pieces, or particles, decreases, the surface area to volume ratio increases. This contributes greatly to the explosive nature of a material. In an explosion, a large amount of energy is released over a very short period of time. As per OSHA, any material posing deflagration or fire hazards with an effective particle diameter of less than 420 microns falls under the criteria of combustible dust. However historical data shown us much larger particles can still cause an explosion due to a multitude of reasons. Many industries — from chemicals, plastics, and pharmaceuticals to food processing, agricultural and mineral recovery — face combustible dust hazards in their facilities. Explosion and fire incidents such as one at the Imperial Sugar refinery in Port Wentworth, GA in February 2008 demonstrate the need to effectively manage these risks.
Imagine starting a fire in the woods out of logs and an accelerant, such as kindling. When you ignite the kindling, it catches fire very quickly and burns itself out very fast. The log is the opposite; it must be subjected to heat for a much longer period of time before it catches fire, and it burns much more slowly than the kindling. Despite being made of the same materials, the log and the kindling behave very differently. The only differences are their size and shape. More specifically, the major variable that causes the difference in their rate of combustion is their surface area to volume ratio. The log is in the form of a single, large piece and only so much of it can be exposed to flame at a given time. By contrast, the kindling has been broken into many pieces and has much more surface area per unit of volume. Even if you took the same mass of kindling as the log, a fire would burn through the kindling much faster. A flame can heat up a much larger portion of it at the same time, burning more of it at once, and releasing its energy all at once. Following this logic, one might assume (correctly) that sawdust would burn even faster than kindling. As we chop the wood into smaller pieces, the same amount of mass, and therefore energy, is present. But the rate at which it releases that energy increases. And as the particle size gets smaller, the minimum amount of energy required causing ignition decreases. This is because more of the material will combust at once, releasing more energy at once. This will cause the surrounding material to combust in a sort of pseudo-chain reaction. The energy released when it combusts is used to heat up more of the surrounding material which then combusts, and so on. At a small enough particle size, many materials can be ignited by static discharges.
In the above discussion, wood was used as an example. Wood is known to be flammable. But even substances that are considered to be non-flammable can be explosive when in a dust form. A fairly counter-intuitive example is aluminum. Metallic materials are generally considered to be non-flammable as they are very unreactive in the presence of heat. At least, they appear to be from a macroscopic point of view. When aluminum is in a single large piece and exposed to heat, it is difficult to observe the slow reactions taking place. When aluminum is in the form of a fine dust and exposed to heat, it is extremely explosive. As a dust, much more of the aluminum mass reacts at once, causing generation of heat and pressure at a much faster rate.
There are many examples of materials that are seemingly harmless on a large scale but can be explosible in the form of a dust. Explosibility screening or testing can determine if a given material is explosible in the form it is being handled.
A typical first step to assessing a hazard level is to review information in the literature. In the case of combustible dust hazards, dust characteristics directly affecting combustibility hazards are a function of process-specific conditions such as concentration, size and shape, moisture content, and dispersion method and duration. Proper characterization of dusts must then be based on testing data of an actual sample from the process.
Chemical characteristics include items such as flammability and explosibility, as well as thermal degradation, instability, sensitivity to ignition and chemical reactivity. Depending on the type of process unit being analyzed and the potential explosion prevention measures to be employed, a testing plan is selected.
In this online course, Kathy Anderson, CCPSC, teaches about everyday dust hazards and how dust becomes combustible as well as basic industry terminology and standards. Best suited for individuals in the process industry that are new to combustible dust or to those looking for a refresher on an introduction to dust hazards. View the agenda now and earn your certification.
ioKinetic has extensive experience with the evaluation and interpretation of dust hazard test data. We can assist with the development of a cost-effective test plan to characterize dust hazards. Let us help you sift through the complicated list of tests to determine which are appropriate for your operations. After the test is conducted, our experts will evaluate and interpret the data and provide a report of the results. We can offer a single package that includes test plan development, testing and data analysis, and interpretation. Our experts will work with you to determine the level of analysis that is required for your particular needs.
To learn more about how we can help you manage risk, contact us today or call us at 1-844-ioKinetic.
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