Rapid combustion, such as a deflagration, can occur with many kinds of equipment, including dryers, mixers, vibrating screens, mechanical conveyors, bucket elevators, silos, cyclones, multicyclones, dust collectors, or other equipment in which a problematic quantity of combustible fine dust may build up.
When there is an explosive concentration of dust1 and an ignition source2, it takes only a short period of time for a deflagration to occur and lead to:
- Explosion of the equipment
- Propagation of a pressure wave or a flame front through piping and conveyor systems toward upstream or downstream equipment, which can make them, in turn, explode
- A fireball in the immediate area that can injure workers and trigger secondary combustion (fire, second explosion)
Every scenario must be considered to properly control risks! Plant managers have many wide-ranging obligations and responsibilities, particularly in occupational health and safety (OHS).
What should be done if a hazard has been identified and workers are under threat of an imminent explosion? Even if the probability of such an explosion is low, the situation must be corrected—there is no other option. However, it is unreasonable to shut down the plant’s operations until the issue has been resolved completely. The challenge is a considerable one, especially when the authorities (fire departments or agencies responsible for enforcing OHS legislation) are involved.
Below are a few suggestions for more effective management of explosion hazards.
Risk mitigation plan
Developing a risk mitigation plan must be a priority when risks have been identified at a plant, unless simple solutions can be implemented rapidly.
Once an explosion hazard is probable or confirmed, the company must show due diligence and take prompt action to protect both the workers and the facility. In the very near term, plant managers and those in charge of OHS will need to mitigate these risks by establishing simple, yet restrictive, means to reduce the probability that such an event will occur and to minimize its effects.
With risk control, the priority is to manage foreseeable ignition sources like electrostatic discharge, mechanical sparks and hot surfaces. It’s important to distinguish between permanent approaches (ignition source avoidance or performance-based design) and temporary ones (pending the implementation of a permanent explosion protection solution).
For example, an unprotected silo is an explosion hazard because it is used to store wheat flour for instance exposes nearby workers to the threat of burns, falls and projectile injuries. The low probability of an explosion does not lessen its devastating effects, which considerably increase the risk3. Mitigation measures are thus required in addition to controlling ignition sources and restricting access beyond a safety perimeter around the silo and around the equipment at direct or indirect risk of explosion or explosion propagation. Special attention must also be given to the equipment inside buildings to control the risk of a second deflagration and structural damage.
Mitigation measures are critical and can be maintained for a long time. In fact, it can take several weeks to design and engineer an explosion protection solution. It also takes time for budgets to be approved and for the safety devices to be made, delivered and installed. A period of 6 to 12 months is not unusual. Unless these mitigation measures are introduced, the inspectors and representatives of the regulatory body may require that operations be shut down, even if the design process is under way.
Conducting the risk assessments (e.g., combustible dust) before the authorities (e.g., insurers, fire department, etc.) get involved positions the plant advantageously and averts undue outside pressure. Decision-making will be all the more relevant and the costs, both direct and indirect, will be lower.
To determine which actions are priorities for risk management, hazards must be categorized based on their probability of occurrence, their effects and worker exposure to these hazards. One good practice is to prepare an action plan. Moreover, immediate risks must be identified since action must be taken quickly if there are hazards to workers’ health and safety. For example, if a work station is exposed to flashback or projectiles, it will require temporary protection solutions like screens, delocalization or a safety perimeter. Also, simple measures like eliminating any dust that has accumulated on high surfaces reduces the risk of secondary explosions.
Appropriate workforce training
Plant workers and any subcontractors who may be exposed to combustible dust should be given appropriate training. The training should cover the basic principles of how deflagration works (theory), how to identify hazardous situations, what actions to take in an emergency, and the safety measures and devices that should be applied.
The right advisors
Few specialized services for combustible dust and explosion protection are available in the market. Most manufacturers and consulting firms know too little about these issues to provide the relevant assistance. As a result, the wrong solutions are often introduced in plant settings. For instance, a chemical suppression system is sometimes installed on equipment that is unlikely to explode instead of more appropriate passive solutions, like deflagration venting.
Those who manage industrial installations are obligated to ensure that the facilities are equipped and laid out so as to protect workers4. However, standards and regulations stop short of identifying how and what measures to implement for explosion protection.