Awareness and Prevention of Dust Explosions

Awareness and Prevention of Dust Explosions

Introduction

Dust explosions present a serious hazard in many industrial environments. When an explosion occurs, dust particles in the area burn rapidly and create an intense pressure wave. These waves can cause significant loss of life and property. For example, a dust explosion at the Imperial Sugar Factory in the United States in 2008 resulted in 14 deaths and 38 injuries. In the recent past, our country also experienced an explosion beneath a silo at the Soil Products Office in Derince. Identifying locations where dust can be explosive and taking measures against dust explosions is a critical requirement for all industrial facilities. In our country, the obligation of enterprises on this matter falls under the "Regulation on the Protection of Employees from Dangers of Explosive Atmospheres" and the Explosion Protection Document. In the United States, the NFPA 652 standard requires "Dust Hazard Analysis" as of 2020. In Europe and our country, regulatory requirements regarding dust hazard analyses are expected to emerge in the coming years.

Dust Explosion Mechanism

Dust explosion requires three fundamental elements: dust, oxygen, and an energy source. Dust typically consists of particles from organic or metallic materials. Most dust explosions are caused by dust particles suspended in air. In such an environment, if there is an ignition source (such as a spark or open flame), the dust particles in the air burn rapidly and, if this combustion occurs within an enclosed volume, it causes an explosion. When an explosion occurs, the combustion process happens very rapidly and creates an extremely high pressure wave in the air. This pressure wave can destroy everything around it and cause serious damage to people and equipment. Dust explosions typically occur as primary explosions within equipment and enclosed small volumes, while secondary explosions occur in volumes outside the equipment. Dust explosions require a five-parameter set called the explosion pentagon, which consists of the combination of three fundamental elements needed for combustion and two additional elements that convert it to an explosion. Dust explosions can occur in a closed environment—that is, within a confined volume—through the creation of a dust cloud and the ignition of this dust cloud.  

Important factors here:

• Dispersibility/aerosolizability of the explosive dust in air, • Explosibility of the dust, • Mixing of the dust cloud in air with oxygen, • Occurrence with an ignition source. Ventilation effectiveness should be designed to be at a level that prevents the formation of an explosive dust environment in air or maintains it below explosive limits, similar to gases and vapors. In the explosibility of dusts, there is an additional factor compared to liquids, gases and vapors. This is the accumulation of dust in the environment in layers. This is important in terms of the additional risk posed by dust accumulations that could become a dust cloud as a result of any other explosion. For dusts, the primary evaluation is conducted on which dusts will create an explosive atmosphere. Subsequently, the concentrations of these dusts within equipment in the process and the points where they can escape outside the equipment are determined. With the identification of these points, the amount, density and probability of accumulation of dust that could be released from these sources are evaluated. As a result of this evaluation, areas that could cause dust explosions are identified. These areas are called hazardous dust atmospheres.

Assessment of Dust Explosion Potential

Dusts that can cause dust explosions are those that oxidize—that is, dusts of organic and metallic chemicals. In inorganic dusts, since the oxidation reaction does not occur, dust explosions are not possible. Dusts that can cause dust explosions are found in many different industries. The leading industries are as follows, and some examples of these dusts are given in Table 1. • Wood industry, • Food industry, • Metal industry, • Chemical / Pharmaceutical industry, • Paper industry, • Mineral industry, • Plastics industry, • Rubber industry, • Textiles. After the identification of organic and metallic dusts, there are important different criteria. These are listed below: • Dust particle size, • Dust moisture content. Regarding dust particle size, the NFPA 654 standards prior to 2006 contained the following text: "Solid materials separated into particles of 420 microns or smaller in diameter that cause an explosion if dispersed in air and ignited." From 2006 onwards, this text has been replaced by the following: "Particles of combustible solid matter, regardless of particle size and shape, suspended in or capable of being suspended in air or any oxidizing atmosphere, and presenting a risk of flaming or explosion." According to the IEC 60079-10-2 standard: "Solid particles of 500 microns and below can form an explosive mixture with air at atmospheric pressure and normal temperature." The rate of combustion and explosion characteristics are directly related to particle size. These dust particles can exist not only in spherical geometry but also in the following types: • Dusts, • Fibers, • Fine materials, • Chips, • Chunks, • Flake material, • Mixtures of the above. Regarding dust moisture content, it is understood that levels below 30% can create an explosive atmosphere. The accumulation of these dusts in enclosed areas forming layers creates risk. There are three risks created by dust layers. These risks are: 1. Through the impact of primary explosions that may occur within the building, dust layers can form clouds and secondary explosions can occur that may cause greater damage than the primary explosion. 2. Dust layers can ignite from heat present in their environment. 3. Even if a dust layer is thin, it can transform into a cloud and ignite, and this can result in an explosion. These risks are influenced by the cleanliness of the environment. The likelihood of a layer causing fire can be controlled through proper equipment selection and effective cleaning. Changes in the condition of the dust layer can reduce or eliminate the ability of the dust layer to rise to a dust cloud state. In this case, there may be no secondary explosion risk and any fire risk can be similarly reduced. The dust cloud concentration that can form from a dust layer depends on: • The bulk density of the dust layer, • The thickness of the dust layer, • The proportion of the dust layer that is dispersed, • The volume of the resulting dust cloud. According to HSE UK data, when 303 dust explosions were analyzed, ignition sources were determined as follows: 29% unknown causes, 20% friction/mechanical failure, 18% high temperature, 16% open flames, 7% welding/cutting operations, 4% electrical, 3% static electricity and 3% other, and are shown in Figure 1. When the same explosions were examined in terms of which equipment they originated from, the top three were grinders, filters and dryers, as shown in Figure 2.

Conclusion

The first step in preventing dust explosions is to be aware of dust explosions and to recognize the hazards of our process. In many of our projects, we have experienced that enterprises have limited or incomplete knowledge regarding the explosibility of dusts, particularly those in mixed form. In cases of knowledge gaps regarding dusts, dust explosion analyses should be conducted with the assistance of laboratories, and the process should be evaluated for possible explosion prevention and adaptation to explosion conditions. In areas with dust explosion potential, the explosibility-rated selection, design, installation, inspection and proper operation of electrical and non-electrical equipment in accordance with standards reduce the risk of ignition. In conclusion, it is possible to make potentially explosive dust environments safer by identifying under which conditions dust explosions can occur in the process, through dust extraction systems, cleaning and prevention of dust leakage.   References [1] Crowl, D. A., Understanding Explosions, Center for Chemical Process Safety (CCPS). [2] Eckhoff, R., Dust Explosions in the Process Industries. [3] International Electrotechnical Commission (IEC), IEC 60079-10-2. Explosive atmospheres – Part 10-2: Classification of areas – Explosive dust atmospheres.   Yusuf Mert Sönmez Chemical Engineer - Founding Partner ProSCon