The hidden dangers of combustible dust

A build-up of dust may not look dangerous, but combustible dust can be an explosion hazard when it’s suspended in air under certain conditions. The force from such an explosion can cause employee deaths, injuries, and leveling of entire buildings. In 2017, five employees were killed and 14 injured at a Wisconsin milling facility. The explosion also caused the collapse of multiple mill buildings.

Although OSHA doesn’t have a standard on combustible dust, it does have a National Emphasis Program (NEP) (CPL 03-00-008) to focus inspections on facilities in specific industry groups that have experienced either frequent or catastrophic combustible dust incidents, such as agriculture, chemical manufacturing, and recycling operations. OSHA has also published guidance materials on combustible dust.

What’s considered combustible dust?

Combustible dusts can be organic or metal dusts that are ground into very small particles, fibers, fines, chips, chunks, flakes, or a mixture of these. Examples include:

• Metal dust, such as aluminum, magnesium, and some forms of iron dust;
• Wood dust;
• Coal and other carbon dusts, including carbon black;
• Organic dusts, such as sugar, flour, paper, rubber, soap, and dried blood;
• Human food dust;
• Animal food dust; and
• Dusts from certain textiles.

Any combustible material can burn rapidly when in a powdered form. Even materials that do not burn in larger pieces (such as aluminum or iron), given the proper conditions, can be explosible in dust form.

Unlike normal fires, which call for three elements — oxygen, heat, and fuel — combustible dust explosions require five elements:  oxygen (from air), heat (from an ignition source), fuel (the combustible dust), dispersion of dust particles in sufficient quantity and concentration to cause rapid combustion known as a deflagration, and confinement of the dust cloud to an enclosure such as a building, room, vessel, or process equipment. The five elements make up the “dust explosion pentagon.” If one element of the pentagon is missing, a dust explosion cannot occur.

If a dust cloud ignites in an enclosed area such as ductwork or process equipment, it burns very quickly and may explode. Combustible dust incidents range from localized fires to explosions that move through large structures, often following a pattern of “primary” and “secondary” explosions. Primary dust explosions most often occur in process equipment but can also occur in enclosed rooms and structures.

Secondary dust explosions and flash fires follow a primary event or explosion that suspends dust into the air, often by dispersing accumulated dust deposits on equipment and overhead structures and support steel.

Identify and control the hazards

To identify factors that may contribute to an explosion, OSHA recommends a thorough hazard assessment of all:

• Materials handled,
• Operations conducted (including by-products),
• Spaces (including hidden ones), and
• Potential ignition sources.

It’s a best practice to create a written combustible dust safety program that documents dust fire and explosion prevention information at your location and explains how you’ll prevent such incidents and control their damage. The program serves three purposes:

• Establishes organization-specific combustible dust safety procedures to protect both employees and property;
• Ensures uniform policies/procedures to prevent the occurrence of combustible dust explosions are communicated to and understood by employees; and
• Ensures that dust, ignition, and injury/damage control measures are carried out to minimize the possibility of injury to employees or property damage.