Industrial facilities need proper and effective dust control to keep workers safe.
BY SIMON FRIDLYAND
A huge explosion and fire at the Imperial Sugar refinery northwest of Savannah, GA, on February 7, 2008, caused 14 deaths and injured 38 others, including 14 with serious and life-threatening burns. In the four months following that incident, more than 50 additional combustible dust explosions occurred in the United States alone.
Interestingly, a full quarter of these explosions occurred at food industry facilities, including sugar plants like Imperial’s. (Canadian industrial explosion figures run at about 10% of those in the U.S.)
Combustible dusts are not a new hazard. The first documented explosion occurred in an Italian flour mill in 1785. Since it is not a new problem, why do these incidents continue to occur?
Lack of awareness is likely a primary reason. Do you know the hazards of dust in your workplace? Can you identify combustible dusts, and do you know which safety precautions prevent potentially devastating explosions? Or is your facility another Imperial Sugar refinery just waiting to blow up?
Combustible dust explosions: How do they happen?
Five elements are needed for a combustible dust explosion to occur:
1) combustible dust
4) dispersion and concentration of dust particles, and
5) confinement of the dust cloud.
Two of these elements, oxygen and confinement of the dust cloud, are difficult to control as oxygen is found within the air, and materials are generally processed within confined buildings. The other elements, however, can be controlled through:
* regular cleaning and removal of dust
* use of proper electrical equipment
* control of other ignition sources such as open flames and static electricity
* the isolation of hazards
* the installation of proper sprinkling or extinguishing systems.
Identifying and controlling combustible dusts
If your facility is dusty, that dust is most likely an explosion hazard. Most organic solids, many metals and even some nonmetallic inorganic metals are combustible or explosive if they’re divided finely enough and are present in sufficient concentrations.
Any operation in your facility that creates dust ‑including the transferring of sugar or flour, polishing and grinding, the manufacture of powders, and the handling and processing of solid combustibles such as wood or plastic ‑ puts your facility at risk.
It doesn’t take much, either. Less than 1 mm (1/32 in.) of dust, covering just 5% of a room’s surface area, constitutes a significant hazard.
Part of the solution is to minimize the escape of dust from process equipment or ventilation systems. Dust discharged or leaking from equipment into the atmosphere is acted on by gravity and will settle relatively quickly, depending on the size of the particles, the internal pressure propelling the particles out of the equipment and any air currents in the vicinity. The result is a layer of dust that settles on horizontal surfaces below the leak opening.
Some dusts have particles that are extremely fine and light (i.e., they have a low specific particle density). Such particles could behave more like vapours than like dusts and could remain in suspension for long periods. These particles could travel far from the emitting source and collect on surfaces above the source.
While horizontal surfaces accumulate the largest quantities of dust, vertical surfaces could, in some instances, also accumulate significant quantities. Controlling dust accumulation is critical to preventing explosions.
Dust collection systems
Use cleaning methods that do not generate dust clouds, such as dust collection systems and filters to collect accumulated dust. Be especially aware of buildup areas where dust cannot be seen, or areas that cannot be easily accessed for cleaning. Provide access to all hidden areas to permit inspection; regular maintenance inspections are important in combating dust accumulations. Gasket replacements should be a part of the preventive maintenance process.
How explosions happen
While a dust cloud will ignite and explode readily in the presence of an open ignition source, dust layers, if undisturbed and not in direct contact with the ignition source, will not explode. However, if a small amount of dust is dispersed into the air at the ignition source, a small explosion will occur. The pressure wave from this explosion blows another layer of dust into the air, and a larger explosion then takes place. It is often this secondary explosion that does the most damage.
Around any operations that may generate combustible dusts, all ignition sources, including static electricity, must be eliminated. All equipment and conductive surfaces must be electrically grounded and bonded. Static electricity must be removed from belts using grounded metal combs or similar devices.
Electrical equipment and wiring shall be classified as far as its hazardous location is concerned. Classified electrical equipment ensures that no spark is produced while the equipment is in operation.
Control mechanical sparks and friction by ensuring that bearings are not overheated and that equipment will not produce sparks by implementing a preventive maintenance program and using diagnostic techniques. Enforce and adhere to a non-smoking policy in the plant and control open flames.
The answer to the question of why the industrial explosion incidents continue to occur lies with awareness of combustible dust issues. More education on the subject, as well as enforcement of safety procedures, is probably necessary. In fact, the United States is in the process of adopting new enforceable regulations to minimize combustible dust explosions.
Do we need the same regulations in Canada, or can we apply what we need to do ourselves? Your comments are invited.
Simon Fridlyand, P.Eng., is president of S.A.F.E. Engineering Inc., a Toronto-based company specializing in industrial health and safety issues and PSR compliance. He can be reached 416-447-9757 or firstname.lastname@example.org. For more information, visit www.safeengineering.ca.