Combustible Dust

by Mar 26, 2024Hot Work

The presence of combustible dust in a work environment is synonymous with a potential explosive atmosphere. NFPA 70 or the National Electrical Code classifies explosive work environments according to the type of hazard present:

  • Class I: possible presence of flammable gases or vapours;
  • Class II: possible presence of combustible dust; and,
  • Class III: possible presence of easily flammable fibres or loose particles.

In addition, each class is subdivided to distinguish between areas where the substance is present under normal operating conditions and areas where the substance may occur under abnormal operating conditions. This classification allows for the selection of appropriate electrical equipment and is the first step to be taken. It will enable the risk areas to be identified and the measures to be put in place.

Explosion Hexagon

The explosion hexagon is an extension of the fire triangle concept: for a fire to occur, three elements must be present in adequate concentrations: a fuel, e.g., wood; an oxidizer, e.g., oxygen; and a source of energy, e.g., spark.

In addition to the elements mentioned above, the explosion hexagon includes:

  • The presence of particles suspended in the atmosphere;
  • The concentration of the particles with regard to the lower and upper explosive limits; and,
  • The containment of the elements, e.g., in a tank of ventilation duct.

A fuel suspended in the air therefore forms an explosive atmosphere. In the presence of concentrations between the lower and upper explosive or flammable limits, a single flame, spark or hot surface can cause a dust cloud to explode.

In the case of combustible dusts and in the absence of containment, an explosion may not generate a blast, but a fireball may still be generated from the rapid ignition of the fuel.

Safety Measures and Prevention

To reduce the risks associated with an explosion resulting from an explosive atmosphere, three types of measures must be put in place.

Prevent the Creation of an Explosive Atmosphere

By controlling the presence of the fuel or oxidiser

For the control of fuel (dust), a dust collection system must be installed at each source of dust emission and be coupled with a rigorous housekeeping policy (preventive cleaning) to prevent dust accumulation. This is a requirement of Section 54 of the Regulation respecting occupational health and safety (RSST). In addition, according to Section 58 of the RSST, systems for the extraction, conveying, transfer or treatment of combustible dust must comply with the applicable NFPA standards (NFPA 61, NFPA 484, NFPA 664, NFPA 654) depending on the field (agri-food, wood industry, metal dust, etc.).

For preventive cleaning of combustible dusts, the equipment and work methods should evidently not generate dust clouds or be a source of energy such as static electricity; e.g., use a vacuum cleaner or clean with water rather than broom or use compressed air.

The concentration of oxidant, e.g., oxygen, can also be reduced to prevent the formation of an explosive atmosphere. This involves replacing the air in an enclosed area, e.g., inside process equipment, with an inert gas such as nitrogen. This method requires additional safety measures because of the risk of hypoxia or even anoxia.

Prevent the Ignition of an Explosive Atmosphere

Each task in a location where explosive dust is, or may be, present should be considered as a potential source of energy:

Of course, hot work tasks are sources of ignition.

Tools used, such as cleaning tools, can generate sparks:

  • When a steel tool is rubbed against a stainless-steel wall to loosen dirt;
    • Common non-sparking tools are made of plastic, brass, bronze, copper-aluminum alloys, and others such as wood and leather.
  • Because of static electric charges:
    • Section 52 of the RSST states that “any metallic equipment and machine must be bonded together and commonly grounded or be grounded separately to a grounding network with equivalent conductivity so as to prevent the accumulation of static electricity”.

Workers themselves can bring static electric charges. Necessary precautions are to be taken if the energy level is high enough to ignite airborne dust present in the environment.

Mitigate the Effects of an Explosion

Even when all necessary measures have been taken in the work environment, the possibility of an explosive atmosphere inside of an equipment remains during normal operation. As we cannot guarantee that no failure will occur during the lifetime of an equipment, it is essential to put in place fire and explosion protection systems, as to reduce the impacts of any failure that may occur.

Several systems exist, including:

  • Containment: The equipment itself is designed to withstand the maximum possible pressure resulting from an internal deflagration;
  • Deflagration venting: A designated weak point in the structure opens under the force of deflagration, allowing the expanding gases and flames to escape and dissipate.
  • Deflagration suppression systems: The objective of deflagration suppression systems is to detect and stop a combustion in a confined space while it is still in its early stage. They are also known as spark extinguishing systems.

Such a system involves high-speed flame-extinguishing systems that detect and extinguish a deflagration before destructive pressures are generated.

  • Explosion isolation systems: The goal of explosion isolation systems is to prevent the progression of the explosion from one equipment to another.
    • Mechanical isolation can be provided by fast-acting valves and rotary airlock valves.
    • Chemical isolation is achieved by the rapid discharge of a chemical extinguishing agent into the interconnecting pipe or duct.

Isolation devices or systems are most often used in combination with deflagration suppression systems.

In summary, the management of combustible dust hazards is based on the identification of risk areas, the implementation of measures to prevent the creation of an explosive atmosphere, of measures to prevent the introduction of an energy (heat) source and finally of fire and explosion protection systems.

The implementation of emergency measures and worker training are also required to complete the risk reduction measures.

References

Regulation respecting occupation health and safety (RSST)

Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, NFPA 61

Standard on Explosion Protection by Deflagration Venting, NFPA 68

Standard on Explosion Prevention Systems, NFPA 69

Recommended Practice on Static Electricity, NFPA 77

Standard on the Fundamentals of Combustible Dust, NFPA 652

Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, NFPA 654

Abonnez-vous à notre infolettre

Veuillez vous inscrire pour recevoir les nouveautés, les mises à jour et nos formations disponibles.

  • This field is for validation purposes and should be left unchanged.

Questions or comments?

Intervention Prévention Inc.

Intervention Prévention œuvre dans le domaine de la sécurité au travail en offrant des services spécialisés répondant aux normes CSA Z462 – Sécurité électrique en milieu de travail, CSA Z460 – Maîtrise des énergies dangereuses : Cadenassage et autres méthodes, et CSA Z432 – Protection des machines. Nos domaines d’expertise sont donc reliés au cadenassage, à la sécurité électrique et à la sécurité machine. La satisfaction de nos clients est le gage de notre compétence

COORDONNÉES

137-2020 rue André-Labadie, Beloeil (Québec) J3G 0W6