Dust explosions and how to avoid them

What are the elements of a combustible dust explosion?

The Occupational Safety & Health Administration (OSHA) defines combustible dust as fine particles suspended in the air that are a fire hazard in certain conditions. They explain that:

…..“the familiar fire triangle of oxygen, heat, and fuel (the dust), dispersion of dust particles in sufficient quantity and concentration can cause rapid combustion known as a deflagration. If the event is confined by an enclosure such as a building, room, vessel, or process equipment, the resulting pressure rise may cause an explosion. These five factors (oxygen, heat, fuel, dispersion, and confinement) are known as the “Dust Explosion Pentagon”.”

Creating secondary dust explosions

In the most destructive cases, an initial explosion can reach a second site where combustible conditions in a larger dust cloud exist, except for an ignition factor that has now been supplied by the primary event. These secondary dust explosions, due to different oxygen concentrations or forms of containment such as ducts, silos or even warehouses, tend to cause the largest loss of life. Recent examples are the explosion in Beirut in 2020 and West, Texas in 2013 both involved this phenomenon with ammonium nitrate as the fuel, which is commonly used in the agricultural and mining industries.

What is a combustible dust?

Other examples of well-known combustible dust are coal and flour wherein the carbon molecule chains in these clouds allow deflagration to escalate to an explosion. In every case where a combustible dust hazard is possible, reference to the Manufacturers Safety Data Sheet (MSDS) underlines avoidance of ignition sources.

Zirconium dust explosions due to the high flammability of the compound that is used in corrosion-resistant alloys and the nuclear industry are not well known publicly. However, the high flammability of this and many other metal powders are identified on their safety data sheet.

Certain materials in their pure chemical state will not form combustible dust, including cement, gypsum, and limestone. Although in appearance they may have similar particle sizes to the combustible dust the datasheet will not specify the same hazard classification. A simple check using the safety data sheet for any powder or compound being handled in a process plant or shipment for the following two hazard classification types should be mandatory:

• “Combustible Dust” e.g., flour
• “Highly flammable” e.g., Zirconium or other reactive metal powders

Controlling contributing factors

The key to avoiding a dust explosion is removing one of the five contributors so that an explosion cannot occur. Due to the nature of storing, transporting, or using a huge range of industrial powders in large vessels where oxygen is present, removal of most of the 5 factors in the “Dust Explosion Pentagon” is not possible. For instance, very few industries would operate in a vacuum to remove oxygen whilst quantity and dispersion concentrations of the powders in the air during industrial processes cannot always be tightly controlled. Consequently, the actual oxygen concentration in any dust cloud, which is critical to the likelihood of its ignition, is difficult to manage throughout a manufacturing process where different levels of confinement and dispersion will occur.

Key knowledge for safe handling

The two factors that can be most easily monitored to keep risks as low as reasonably practicable (ALARP) are knowing the characteristics of the powder itself. This can be achieved by referring to the data sheet and by removing any likely ignition source, such as sparks, electrostatic discharge or friction hot spots due to poor plant maintenance. For reference, the European Chemicals Agency (ECHA) has a database of over 235,000 safety data sheets available online for which are compliant with the UN’s Globally Harmonised System (GHS).

Control of ignition sources

Removal of ignition sources is the single most controllable factor in avoiding dust explosions. National Fire Protection Association regulations, regarded as mandatory in many US industries, regulate the metal/non-metal mining and mineral processing industry. This industry has a high prevalence of confined combustible dust where sulfide explosions due to processing copper ore are a known risk.  NFPA 122 in Scope A.1.1 specifically states:

“Ignition sources for these hazards are present and cannot always be controlled. The most common ignition source in this industry is uncontrolled hot work.”

In the United Kingdom, the Health and Safety Executive also concur on the removal of ignition sources as being critical. Their document HSG103 on safe handling of combustible dusts states:

“The great majority of dust explosions start inside the process plant, and most of the control measures concern conditions inside the dust handling system. They can be grouped under the headings of:

• controls over dust cloud formation
• preventing the explosive atmosphere by inerting
• avoiding ignition sources
• plant controls”

Control of the size of a dust cloud in a plant and oxygen concentration in that cloud is not precise and inerting, using a gas such as nitrogen also carries a risk of asphyxiation which has happened on occasion. This leads us back to avoidance of ignition sources as being the primary control method to prevent combustion as “plant controls” refers to mitigation of fire or explosion by ventilation, sprinklers, or blast diversion after the combustion event, to avoid secondary dust explosions.

The HSE also goes on to state in paragraph 31 of the same publication that:

“Sparks from hot work may travel a considerable distance, particularly if you carry out the work at a high level. You can greatly reduce the risk of ignition by adopting cold cutting methods.“

The risk assessment required for work permit generation in process plants handling a highly flammable powder or combustible dust will necessarily focus on risk reduction or prevention. In these situations where prevention of the ignition factor is paramount, the use of cold work or non-sparking tools in these environments is both cost-effective and safer as it avoids the requirement for “hot work” habitats. This safety tool policy adoption, along with regular plant maintenance to prevent friction hotspots, ensures the lowest possible risk to personnel and property.