Combustible Dust: More than just a Sawmill Issue

Contribution from Lorne Davies, Specialist Safety Advisor – Combustible Dust: Combustible dust has been at the forefront of prevention efforts since the catastrophic incidents at Babine Forest Products in Burns Lake and Lakeland Mills in Prince George. However, combustible dust has been causing serious incidents, fatalities and property damage since the beginning of manufacturing.

OSHA (Occupational Safety and Health Administration) created the first combustible dust standard in the early 1920s to try and reduce the fatalities caused by combustible dust. Unfortunately, the standard was not compulsory and the accidents and fatalities continued to occur. Singled out by regulators, the grain industry was forced into prescriptive controls after an explosion caused multiple fatalities and in 1987 OSHA developed regulations specific to their industry. OSHA is currently working on a new standard for combustible dust to further understanding of the hazard and update existing regulation.

So why is combustible dust such an insidious hazard? The risk lies in the fact that dust can lie dormant for years waiting for the right conditions to develop to support a fire, a flash or a deflagration event. Serious events are infrequent which leads to complacency by workers and management who are accustomed to dust accumulations in the facility.

For example, areas hard to reach such as overhead cable trays and pipe runs are often overlooked in housekeeping programs. Unfortunately the lack of understanding about the risks can lead to egregious violations, such as the 2008 Imperial Sugar disaster where workplace inspections documented that sugar dust had accumulated to ankle, knee and waist deep levels in the plant.

The catalyst for the catastrophic event is usually some small change to the process material, configuration, equipment or output. In Imperial Sugar’s case, the installation of enclosed conveyors to reduce contamination was the contributing factor to the incident. Lumps of sugar that blocked the conveyor were left unaddressed which resulted in an overheated bearing igniting the sugar dust inside the conveyor. The ignition inside the enclosed conveyor met all the requirements for a dust explosion: ignition, fuel, oxygen, dispersion and confinement.

Had sugar not been allowed to accumulate in the rest of the facility the primary explosion would have been contained in the conveyor. Instead, the shock wave of the primary explosion lifted sugar off the floor and roof structures saturating the air with dust causing a massive secondary explosion that propagated throughout the plant. This avoidable incident cost 14 people their lives, caused 38 more injuries and the complete loss of Imperial Sugar’s Port Wentworth facility.

The chart above llustrates the occurrences of dust explosions in the process industries in the U.S. in 2008.

Most managers and workers are not aware of how combustible the material they are working with is. Materials which are not normally flammable, like iron, can be if reduced to a small particle size. Simple materials such as corn, starch, grain, rubber, flour and sugar have all had explosions causing fatalities. Metals such as aluminum and magnesium have particularly violent explosions. The wood and paper industries have also had explosions that have caused fatalities.

In order to keep our plants safe we all need to understand the principles that allow deflagration and explosion to occur. The first step is educating workers and management to ensure they understand the hazards and how to take the necessary steps to prevent the conditions that could lead to a fire or explosion. Included in this is an understanding of the characteristics of the materials in the process and at which state of the process they become combustible. When the hazards are understood then strategies can be developed to mitigate them. Combustible dust has been viewed by many to be a wood dust problem however the graph above shows that it is every manufacturers concern.