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Dust particles become airborne during the manufacturing and processing of food, chemicals, pharmaceuticals and other dry products. Particles also become airborne during indoor metalworking processes like welding and plasma cutting.

Some of these particles are toxic or combustible, so it is important to shield workers, products, and expensive equipment from them. When combustible dusts are collected from the air into a dust collection system, the system itself can be a source of combustible dust explosions if not properly protected. Besides being required to do so by OSHA, companies are morally obligated to protect workers from these hazards.

Here are some frequently asked questions about controlling dangerous dusts in order to maintain a safe work environment.

Which industries most often deal with dangerous dusts?

Many industries have combustible dust, but the following are at most risk: metalworking facilities, welding shops, woodworking shops, chemical processors, food manufacturers, and pharmaceutical companies that make solid dose products (tablets).

What are common dust hazards in the food processing industry?

The biggest threats are occupational exposure and combustible dust explosions. Dust can cause dermatitis and allergic reactions. More seriously, dust particles can become embedded in the lungs and can cause respiratory problems like asthma and lung cancer. In addition, many solid food ingredients are combustible, including sugar, starch, spices, proteins and flour. Lastly, food particles can damage other food products. For example, particles that contain gluten or peanuts could cross-contaminate products that are supposed to be gluten free, causing severe allergic reactions for customers who trust those product labels.

OSHA requires companies to control dust emissions in indoor workplaces and to comply with legal limits set for each ingredient and material. If no legal limits are applicable, the company must define in writing, implement, and measure its own environmental safety plan. The FDA’s Food Safety Modernization Act requires food processing facilities to implement measures to prevent or minimize contamination hazards.

What are common dust hazards in the chemical processing industry?

The biggest threats are occupational exposure to toxic dusts and combustible dust explosions. Processes like blending, coating, conveying, crushing, weighing, milling, mixing, and pelletizing all generate dust that will become airborne. If not captured and contained, these dusts expose workers to hazards and can cause combustible dust incidents.

What are common dust hazards in the pharmaceutical manufacturing?

As above, occupational exposure is a common hazard because active pharmaceutical ingredients (APIs) can be toxic and allergenic. In addition, APIs can travel through the air and cross-contaminate other pharmaceutical products. Lastly, many pharmaceutical ingredients are combustible and can cause explosions if not handled correctly.

What are common dust hazards in metalworking facilities?

Metalworking facilities that use processes like welding, thermal cutting, sanding, and polishing are at the most risk because these processes send tiny metal particles into the air that can be toxic. This is especially important if the facility handles iron oxide, lead oxide, manganese, nickel, and chromium. Metalworking facilities must follow the OSHA PEL for these and other metal dusts.

In addition, many metal dusts are highly combustible and can increase the chances of an explosion in the dust collector. Dust collection systems must be sized correctly and have the proper filters and protection devices to mitigate the risk of an explosion. Burnable dusts pose a higher risk for a combustible dust explosion in a dust collector. Even a small amount of dust can have severe consequences.

What equipment is used to capture hazardous dusts?

Industrial dust collectors are used to capture and contain dust and other harmful particles from the air in plants, factories, and other processing facilities. Collectors capture dust by continually cycling the dust-laden airstream through a series of filter cartridges. The dust remains on the cartridges, and the clean air is returned to the work environment. Dust collectors can be placed inside or outside the manufacturing facility.

How does an explosion occur in a dust collector?

A dust collector is a closed vessel, and any closed vessel that is full of dry particles is ripe for an explosion. An explosion usually begins when a suspended cloud of combustible dust is present in high concentration inside the collector. As the fan draws in large volumes of air, an outside spark or ember can be sucked into the collector and collide with the dust cloud under pressure, triggering an explosion. The source of the spark may be a production process, a cigarette butt thrown into a dust capture hood, or a static electricity discharge from improperly grounded nearby equipment.

How do you protect a dust collector from a combustible dust explosion?

First of all, it is important to have all collectors sized properly for the facility they will be handling. Second, it is important to understand that combustible dust explosions can’t always be prevented from occurring in the dust collector. However, employers can put systems in place that ensure that the explosion doesn’t cause harm.

These systems are called explosion protection systems, and there are a variety of options. The most common is explosion venting because it is the most cost-effective, but some facilities may also be required to have explosion isolation valves or integrated safety monitoring filters. All of these mitigate incidents and prevent the flame front and pressure from traveling to process areas. The National Fire Protection Association provides guidelines for designing, locating, installing, and maintaining these explosion protection devices to minimize harm to personnel as well as structural and mechanical damage.

What does explosion venting do?

A well-designed explosion vent functions as a weak element in the dust collector’s pressure envelope. It relieves internal combustion pressure (back pressure) to keep the collector from blowing up into pieces.

Typically, the collector is located outside so that it vents away from buildings and populated areas to a safe location. If the collector is properly equipped and located indoors, standards mandate that a safe area be designated. While explosion venting will usually save the dust collector from being a total loss, the collector can sustain major internal damage. Nonetheless, if personnel remain safe and facility structural damage is minimized, the explosion venting equipment has done its job.

Which facilities are required to have their dust tested?

NFPA standards require a dust hazard analysis (DHA) for any facilities that generate, handle, or store potentially explosive dust. The burden of proof is on manufacturers to demonstrate that their dust is not combustible, so it is important for them to have their process dust tested by a valid third-party testing lab and keep records on file proving that it is not combustible.

If tests show that the facility has combustible dust, NFPA 652 requires employers to complete a DHA of their dust collection systems. They also need to keep this report on file to show when requested by the local fire marshal or other authorities. In addition, explosion venting equipment must be inspected at least annually based on the documented operating experience.

How are vents and discharge ducts sized to make sure they are right for a dust collector?

Chapters 7 through 9 of NFPA 68 provide the calculations to use for properly sizing explosion vents, vent discharge ducts (also called vent ducts), and other components. Reputable dust collector suppliers will follow the vent sizing equations in Chapter 8 (“Venting of Deflagrations of Dusts and Hybrid Mixtures”). They can also supply a calculations sheet that becomes part of the documentation kept on file to demonstrate the plant’s compliance.

Should all dust collectors be installed outdoors?

Obviously, placing dust collectors outdoors is the safest option if they vent away from buildings and populated areas. However, it isn’t always feasible to place them outside. Dust collectors placed indoors must have the appropriate explosion protection system if they will handle any combustible dusts.

Is it safe to recirculate the air from a dust collector back into the work environment?

Recirculating heated or cooled air back into the workspace can provide significant energy savings and eliminate the cost to replace that conditioned air. Containing the air indoors also avoids the time-consuming permitting involved when contaminated air is exhausted outside. This can be safely done even if the facility handles explosive dust by outfitting the dust collector with a safety monitoring filter, which helps isolate the downstream equipment from the progression of a flame front during an explosion.