How to prevent combustible dust explosions?

Whether finely ground organic or metallic particles, combustible dusts can be found in several industries including chemicals, food and beverages, rubber, plastics, and most fossil fuel power plants. Dust explosions can have disastrous consequences for many industries, including fatal injuries to workers and civilians.

However, several proactive measures can be used to significantly reduce the risk of dust explosions. Let's uncover each of them.

The Dust Safety Science reports in their Combustible Dust Incident Reports that dust explosions caused 163 fires and 53 explosions in 2021, killing 69 workers and injuring 215 others. For some industries, the accumulation of combustible dust in a factory is unavoidable given the specifics of their activities and the sheer size of the workplace. And because any solid flammable material can create explosive dust, processing plants in many industries are at risk.

Combustible dust can accumulate inside or escape equipment and settle on any surface in a work area. And the explosion occurs when these accumulations disperse into the air in the presence of an ignition source. This is why dust explosions are serious safety issues affecting process industries that handle materials that generate combustible dust.

What are combustible dusts?

According to the Occupational Safety and Health Administration (OSHA), Combustible dust is defined as: “a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, which presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations.” 

Combustible dusts are often organic or metallic dusts that are finely ground into very small particles such as carbonaceous materials (coal, soot), chemical dusts (sulfur), most solid organic materials (sugar, flour, cereals, wood, etc.), many metals (aluminium, bronze, magnesium, zinc, etc.) and some non-metallic inorganic materials, and textile fibres (cotton).

Generally, some of these materials are not combustible, but under certain conditions (the right size and concentration), they can burn or explode. These particles can accumulate on various surfaces in your work area, such as rafters, roofs, dust collectors, ducts, crevices, drop ceilings and even other equipment.

How do combustible dust explosions happen?

Dust explosion occurs when fine particles are exposed to oxygen and come into contact with an ignition source such as a spark, metallic ember or cigarette butt. This rapid burning process is known as deflagration and results in a high-pressure airwave. Deflagration is a usual fire such as a gas stove, the burning of wood or paper, the combustion of gasoline vapour inside the cylinder of an automobile, etc. 

Deflagration processes happen so rapidly that heated air and gaseous products of fire (carbon dioxide) produce extreme atmospheric pressure that can blow up walls and destroy structures.

• Elements of a Dust Explosion

OSHA states that five elements must be present for a dust explosion to occur. If any one of these elements is missing, the explosion will not occur.

The first three elements are needed for a fire, known as the Fire Triangle:

1. Combustible dust (fuel);
2. Ignition source (heat);
3. Oxygen in air (oxidiser);

The two elements that must be present for a combustible dust explosion to occur:

4. Dispersion of dust particles in sufficient quantity and concentration;
5. Confinement of the dust cloud.

 

The addition of the latter two elements to the fire triangle creates what is known as the “explosion pentagon”. 

• The two waves of dust explosions

Combustible dust explosions often involve two explosions: primary and secondary. The primary explosion is the first to occur when dust suspended in a confined space (duct, tank or collector, processing chamber) subjected to a heat source ignites and explodes. The primary explosion disturbs and shakes up dormant dust which has collected over time on a variety of surfaces. The second wave, or secondary explosion, occurs as this additional dust becomes suspended in the air and also ignites. Secondary explosions are often more destructive than primary ones because of the sheer volume and concentration of additional dust available to fuel them.

How to prevent and control dust explosions?

For some industries, dust accumulation is unavoidable given the nature of their operations and the materials they use. The Combustible Dust Incident Reports indicates that wood processing and wood products (18.5%), agricultural activity (36.6%) and food production (11.6%) represent the largest portion of the overall fire and explosion incidents in 2021.

Several proactive measures can be applied to significantly reduce the risk of dust explosions. The first and most important of these is risk assessment. Many variables must be considered in these assessments, including dust particle size, method of dispersion, air currents, ignition sources, characteristics of the ventilation system, containment of the dust cloud, physical barriers, etc. Fortunately, there is configurable risk assessment software today that can incorporate a comprehensive dust explosion assessment that can be customised to the specifics of each company's operations and infrastructure.

To prevent and control dust explosions, an organisation must deploy aRisk prevention Plan covering the following points:

• The risk assessment process

For dust explosion prevention, a risk assessment process should:

• Determines if the dust present in the work site is combustible,
• Identify all sources of combustible dust (the materials and processes used in the manufacturing methods),
• Locate each potential areas where combustible dust accumulates (visible or hidden),
• Identify processes that cause dust to enter the air or create dust clouds,
• Assess the efficiency of dust accumulation prevention measures,
• Assess the probability and degree of dust to be a flash fire or explosion hazard,
• Identify all potential ignition sources (flames, source of heat, friction points, sparks, electrostatic discharges, etc.),
• Measure the likelihood of combustible dust flash fire or explosion that could cause workers illness, injury or death.

• Processes evaluation

• Analyse manufacturing processes that can generate combustible dust (materials and by-products)  such as abrasive blasting, cutting, grinding, sieving, polishing, cleaning, etc.
• Assess the permit-to-work system to control hot work (welding, grinding wheel work, etc).

• Ignition sources identification

• Determine potential ignition sources (sparks, fire, flames, stoves, heat sources, welding, etc),
• Assess the likelihood of dust entering or accumulating on electrical enclosures or equipment,
• Assess the workplace no-smoking policy efficiency.

• Workers' education and training

Workers are the first line of defence in preventing and mitigating fires and explosions. They should be trained to understand combustible dust and its hazards. The company must administer training on good practices (on safe cleaning methods) for housekeeping rules and measures to reduce dust and eliminate sources of ignition

• Preliminary research and information gathering

Collect documented cases reported in the literature about the materials being associated with a combustible dust explosion in the workplace (ex: USA OSHA Combustible Dust poster). Stating the possibility of a combustible dust hazard is a requirement in some countries (e.g. Canada). 

A critical part of dust explosion prevention that is often overlooked is housekeeping protocol. You must be aware of all open areas and overhead structures where dust can accumulate. Look for hidden places where dust can accumulate, such as behind ceilings, interior ventilation or conveying equipment, ducts, etc. The housekeeping team should be provided with appropriate vacuum cleaners (designed for low-combustibility organic granules and dust).

Take your plant safety to the next level

Following each of the prevention measures mentioned above will reduce the risk of experiencing a dust explosion. But to take worksite safety to another dimension, equip your installation with an integrated management system (IMS). An IMS is the integration of the various management systems of organisations (quality, risk assessment, permit to work, occupational health, and safety, etc.) into a single complete, global and harmonised digital platform. so that it can operate as a single unit with unified goals.

A Digital IMS  is a digital system that organises and manages all documents, reviews, audits, records and data on an accessible platform. It provides a synoptic view of important business processes, creates valuable synergies and consolidates data, a vital asset for modern businesses. A tool that allows organisations to save time, reduce costs, eliminate paperwork and increase incident proactivity/responsiveness.

When dust is disturbed and exposed to certain elements, there is a risk that a serious explosion will occur. The accumulation of even a very small amount of dust can cause serious damage. Therefore, any activity that creates dust should investigate if this is combustible dust. The integration of a dedicated digital tool can help you to put in place state-of-the-art safety measures at each level of your site. However, it is important to ensure that it is the right tool with complementary functions for your specific activities.

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