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Materials such as silica, limestone, sand, cement, fly-ash, etc., are inert materials in their pure form (i.e., these materials will neither burn nor support combustion and do not pose a risk of fire or dust explosion). However, during the ceramic manufacturing process, other ingredients (such as organic materials and metallic powders) are often added to create the final product. These additional ingredients are generally in powder/dust form and may be combustible.
In general, some 70% of dusts are explosible and, given an adequate ignition source and appropriate dust/air concentration, can cause a dust explosion. Most finely divided metal dusts are also explosible. Some materials will not present a dust explosion hazard due to their granular nature. However, it is possible during the processing of these materials to create finely divided dust that could be explosible. This applies to the operations where fine dust is intentionally generated (e.g., during milling) and/or unintentionally generated (e.g., during grinding, sawing, polishing or due to material attrition).
Non-explosible materials such as sand or silica could become explosible if mixed with other explosible material (such as organic and/or metal dust) in adequate quantities. Hence, it is important to screen representative samples to determine and document if a mixture of material is explosible or not.
Case in PointIn a foundry operation, powdered sand was being mixed with a water-based liquid and plasti-flakes to create resin-coated sand, which was then used in a core-making process. Sand is a known inert material; however, information on the explosibility of plasti-flakes was not available.
During the operation, the plasti-flakes generated fine particulate dust due to material attrition. The dust generated from plasti-flakes was tested, and it was determined that the fine particulate dust was explosible. The discovery led to more questions regarding a safe operation and the coated fine sand was also tested.
The test results showed that the fine coated sand was also explosible. A dust collection system that was used to collect dust from various locations of the sand coating process was also at risk of dust explosion and required an explosion-protection system to be installed. Deciding to test the representative samples of fine dust led to the discovery that the overall process was inherently unsafe, and certain improvements were implemented to abate the risks.
Dust Explosion ConditionsA number of conditions must exist simultaneously for a dust explosion to occur (see Figure 1):
- Dust must be combustible
- Dust must be airborne
- Dust concentrations must be within explosible range
- Dust must have particle size distribution capable of propagating a flame
- The atmosphere in which the dust cloud is present must be capable of supporting combustion
- An ignition source with sufficient energy to initiate flame propagation must be present
Testing ProceduresThe key to establishing whether a process is at risk is to have an understanding of the explosion and thermal properties of powder materials and mixtures. It's important to consider some guidelines to ensure that meaningful results are produced:
- Don't assume that your powder is non-explosible
- Don't use old test results (some early test methods are no longer valid)
- Don't rely solely on published results
- Don't use results whose sample origins and test methods aren't defined or don't precisely match yours
Explosion Screening Test
This is the first test generally performed for powders whose explosibility is not readily known. The test is conducted in general accordance with American Society for Testing and Materials (ASTM) E1226. The objective is to determine the combustion of a dust cloud in the presence of a suitable ignition source. Trials are performed at varying dust concentration, and a sample is labeled as either "explosible" or "non-explosible" at the end of the testing. In other words, this is a "go/no-go" test; no quantification of the explosion is made.
If the powder is non-explosible ("no-go"), it might not be necessary to run further tests. However, some "no-go" powders can be explosible at high temperatures. If it is understood that a suspended powder will be exposed to higher-than-ambient temperatures during processing, it is essential to conduct the explosion screening test at the processing temperature. It is also important to remember that some dusts pose a fire hazard even though they do not pose a dust cloud explosion hazard, and different testing may be required to assess this hazard.
Dust Explosion Severity
In order to assess dust explosion risks, both the severity of a possible explosion and the likelihood of ignition must be determined. To determine the dust explosion severity, the tester suspends a powder sample in a 20-liter spherical explosion chamber and a high-energy ignition source is introduced to cause an explosion. The sample size is varied to determine the optimal dust cloud concentration.
The maximum pressure and rate of pressure rise are measured and used to determine the deflagration index (Kst) value of the material, which indicates the level of the explosion violence. These data are used to design dust explosion protection measures such as relief venting, suppression or containment. The Kst test is performed in accordance with ASTM E1266.
Minimum Explosible Concentration
The minimum explosible concentration (MEC) test determines the lowest concentration of a dust/air suspension (cloud) that can give rise to flame propagation upon ignition. The MEC test is performed in accordance with ASTM E1515 or International Standards Organization (ISO) method 6184-1. This ignition sensitivity test allows an understanding of the ease of formation of an explosible dust cloud within equipment. The control of dust/air concentration is a significant hazard management method.
Minimum Ignition Energy
The minimum ignition energy (MIE) test determines the lowest spark energy that is capable of igniting a dust cloud at its optimum concentration for ignition. The MIE test is performed in accordance with ASTM E2019, British Standard 5958, and International Standard: IEC 61241-2-3, and indicates the type of ignition source that is of concern for a material.
Minimum Ignition Temperature of a Dust Cloud
The minimum ignition temperature of a dust cloud (MITCloud) test determines the lowest temperature capable of igniting a dust dispersed in the form of a cloud. The MIT is an important factor in evaluating the ignition sensitivity of dusts to ignition sources such as heated environments, hot surfaces and friction sparks. The MIT test is performed in accordance with ASTM E1491 and European Standard 61241-2-1.
Minimum Ignition Temperature of a Dust Layer
The minimum ignition temperature of a dust layer (MITLayer) test determines the lowest temperature capable of igniting a dust layer of standard thickness (5-12.7 mm). The MIT is an important factor in evaluating the ignition sensitivity of a dust layer to ignition sources such as heated environments, hot surfaces and friction sparks. The lower value of MITCloud or MITLayer is also used to specify the maximum surface temperature of electrical devices in hazardous areas, per the National Electric Code.
Secondary Dust ExplosionsThe majority of serious dust explosions over the years have not been caused by the initial/primary explosion inside the plant, but from a secondary explosion or a series of secondary explosions within the building. A small initial event causes a pressure wave to propagate into the workplace, and dust deposits around the workroom are dispersed into a cloud, which ignites. This can happen in a series of connected rooms and areas that have dust accumulations due to poor housekeeping.
Housekeeping activities must ensure that secondary fuel sources are not available. Of key importance is an evaluation of dust release points and exhaust ventilation needs. It is also important to understand the characteristics of dust to ensure that correct tools, equipment and methods are applied during housekeeping efforts. If in doubt, it is highly recommended to seek the advice of a dust explosion hazard safety expert.
For more information, contact Chilworth Global at (609) 799-4449, email email@example.com or visit www.chilworth.com.