Addressing the Hidden Hazard of Combustible Dust

As the CEO of a large manufacturing company your safety committee co-chair has raised some concerns around possible combustible dust accumulations in the facility.  Despite being incident-free for many years, you’re uncertain about the validity of these concerns. How should you proceed?

Firstly, it’s crucial to assess the materials used in the manufacturing process to determine if they are known to produce combustible dust. Following this, a thorough inspection of the facility and see where and which dust is accumulating. It is important to note that not all dusts are combustible, for example companies that sand their parking lot during winter months may find deposits of non-combustible road sand dust inside large overhead doors.

Once you have determined the types of dust present and the sources of the dusts, you have two options.  The first is to have the dust tested at a dust laboratory for combustibility, usually a GO-No-Go test will confirm or deny the flammability of a dust sample. The second choice is to assume it is combustible. If you choose to test your dust you can also have additional tests done to determine combustibility characteristics such as Minium ignition temperature (MIT) or Minimum ignition energy (MIE) to assist you with selecting the appropriate control measures and developing safe work procedures for safe handling and cleanup activities.

Once you have identified all the types of dusts present in your facility, the next step is to review your processing equipment to evaluate and control the credible ignition sources that under the right conditions could ignite the dust present if it was dispersed into an ignitable cloud.  Depending on the complexity of this task you might decide to retain some additional expertise to assist with the review.

When the hazards and controls have been reviewed and deemed adequate, then the focus shifts to developing procedures and policies to clearly communicate the hazard to all employees and contractors.

At this point, you might realize that this project is more substantial than initially anticipated, prompting you to question its priority. Should you start immediately or postpone until you have more resources available next year?

These are common questions faced by manufacturing companies in all sectors. Our human risk assessment logic tends to rely on short term experiences to evaluate the risk and consequences of tasks and processes.  For instance, if we stand on the top step of a ladder to complete a task without incident, we may assume it is safe to continue using this behaviour. However, in reality, if the probability of falling off a ladder might be 1 in 1000. This means we could engage in this at-risk behaviour for many years before experiencing a fall.  Unfortunately, when a fall does occur, it could result in life changing injuries.  Falls from ladders are one of the leading causes of fatalities in the construction industry. 

Combustible dust incidents are characterized by their low frequency but high consequence nature.  While they don’t occur often, when they do, they create major consequences for the people and companies involved. Fatalities and the total destruction of facilities from combustible dust incidents are reported every year globally.  

Despite our understanding of the hazards associated with combustible dust for over a century, why do we continue to have these catastrophic events? The answer lies in the intricate nature of dust combustion.  Various factors must align to ignite a dust cloud. Even a slight change in moisture content or particle size can transform a dust that would not ignite yesterday to being ignitable today.  Additionally, upset conditions such as the presence of flammable vapours from solvents could lower ignition levels of a dust cloud. A minor event such as a small fire created by an overheated bearing could be significantly escalated by improper use of a fire extinguisher, which would disperse the fuel into a large fireball.

The extent to which the fires propagate will depend on two main factors: the amount of fuel present in the area dispersed by the pressure wave created by the initial deflagration and your emergency response protective systems. The pressure wave is responsible for dislodging accumulated dust and dispersing it into an explosible concentration. This cloud will be ignited by the primary event ignition source causing a domino effect, this allows the fire to propagate throughout the facility. Emergency response planning, and protective systems can contain fire to the area of the primary event, and limit damage due to the primary event.

As we are all aware of the explosion pentagon elements and we know that removing the fuel will prevent a fire, deflagration or explosion from occurring, why don’t we simply just remove the fuel?

While it may sound straightforward, in reality, removing the fuel to prevent fires, deflagrations or explosions is very challenging. One of the constant factors in business is change. As the business evolves the original process id updated or modified to adapt to changing market conditions. However the original safety controls such as dust collection systems, electrical system hazard classification ratings are not usually reviewed for possible redesign with every change in process.

When protective systems like dust collectors begin to lose effectiveness and allow fugitive dust to escape the process, the responsibility for keeping dust levels within regulatory compliance levels falls to the maintenance/housekeeping team. During economic difficulties when cost cutting measures are necessary, non-productive labour such as housekeeping personnel are targeted for possible reduction. Consequently, cleaning efforts may be focused primarily on hi-visibility areas, such as tour routes, walkways, floors and handrails, where dust accumulation is easily observed by supervisors, regulators and regulators.

Unfortunately, the most hazardous dust accumulations tend to occur on elevated surfaces such as beams, roof structures, ductwork and cable trays. This dust is typically composed of very fine particles transported to these areas by rising warm air convection currents. Once deposited, these particles remain in place and continue to dry until they are removed. The depth of dust becomes critical when the volume of dust present will meet the requirements for the minimum explosible concentration (MEC) in the volume of air present in the space. This is approximately 1/32” for metals, 1/8” for wood dust depending on bulk density of the dust. Cleaning these areas of the facility is often challenging and time consuming. These areas are often out of sight so they are left to accumulate more dust layers until they are discovered and addressed, or a serious incident occurs.

Once enough dust has accumulated to reach an explosible concentration, it is only a matter of time before the remaining variables align. Typically, the trigger for this type of event is an upset condition, such as a primary explosion in a piece of process equipment.  This explosion creates the pressure wave that dislodges accumulated dust, creating what is often referred to as the “snow globe effect”.

This effect disperses the fuel into the air creating an ignitable oxygen-fuel mixture (Minimum explosive concentration (MEC)). Other possible triggers could be an earthquake or a piece of mobile equipment striking the building.

If a dust cloud is present, then a competent ignition source is all that is required to ignite the mixture. Most facilities have numerous ignition sources present ranging from pilot lights for heating appliances to hot work activities such as welding and grinding, overheated bearings, mechanical or electric sparks.

The extent of the incident and damage to the structure will depend on the accumulated fuel present. Damage to the structure will depend on the level of confinement, deflagrations in a confined area will lead to explosions (rupture of the structure).

So, what can be done? The most effective measure to address combustible dust accumulation is to eliminate the production of fugitive dust from the process, by capturing the dust at source. This involves monitoring dust collection systems so they are effectively capturing the dust. Having an effective management of change process is crucial. This ensures that safety systems evolve with the process when changes are made. Simply shifting the responsibility of dust control onto the maintenance or cleanup crew is not cost-conscious nor effective in the long term. It is far more effective to capture the dust at source than to allocate resources to manually remove the dust from difficult to reach areas using a vacuum cleaner and a manlift on a monthly basis.

If combustible dust is produced at a facility, it is not enough to have a good dust collection system.  In order to be diligent a regular inspection process is required to confirm that systems are working effectively, and dust levels are not accumulating beyond safe threshold levels. All systems are susceptible to wear and tear, so implementing a preventative maintenance program will reduce upset conditions and related fires caused by equipment failure minimizing downtime.

When combustible dust is present in a facility it is crucial to ensure that employees and contractors are aware of the hazards present and the procedures to follow to complete their tasks safely.  Providing training in safe work practices and emergency procedures is paramount to ensuring that people are not injured as a result of improper use of compressed air, fire extinguishers or straight stream fire hose nozzles.

We collectively need to shift our perception of combustible dust from being merely a nuisance dust to being a fuel that is capable of endangering our friends and livelihood. Just as you would not be comfortable leaving a 100-litre spill of gasoline on the floor, it is equally concerning to leave an eighth of  an inch of combustible dust on your overhead beams and ducting. This shift of mindset is required to address the risks associated with combustible dust hazards in our workplaces before incidents occur.