Page 38 - OHS, May 2020

COMBUSTIBLE DUST
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, con- veying, crushing, weighing, milling, mixing and pelletizing all gen- erate dust that will become airborne. If not captured and contained, these dusts expose workers to hazards and can cause combustible dust incidents. OSHA requires chemical companies to comply with permissible exposure limits (PEL) for workers. The PEL is the max- imum air concentration to which a worker can be safely exposed for an eight-hour shift without potentially suffering adverse health effects. For example, the PEL of zinc oxide is 15 micrograms per cubic meter of air.
What are common dust hazards in the
pharmaceutical manufacturing?
As above, occupational exposure is a common hazard because ac- tive pharmaceutical ingredients (APIs) can be toxic and allergenic. It is critical to understand the toxicological properties of this dust to determine the PEL of each API. In addition, APIs can travel through the air and cross-contaminate other pharmaceutical prod- ucts. 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 you work with iron oxide, lead oxide, manganese, nickel, and chromium. Metalworking facilities must follow OSHA permissible exposure limit (PEL) for these and other metal dusts. In addition, many metal dusts are highly combustible and can increase the chances of an explosion in your dust collec- tor. Dust collection systems must be sized correctly and have the proper filters and protection devices to mitigate the risk of an ex- plosion. 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. Much of this airborne dust is too small to be seen with the naked eye. Collectors capture dust by con- tinually 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 are generally
large pieces of equipment that 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 be- gins when a suspended cloud of combustible dust is present in high concentration inside the collector. As the fan draws in large vol- umes of air, an outside spark or ember can be sucked into the col- lector 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 under- stand that combustible dust explosions can’t always be prevented from occurring in the dust collector. However, they can put sys- tems 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 system 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 fil- ters. All of these mitigate incidents and prevent the flame front and pressure to travel to process areas. The NFPA provides guidelines to design, locate, install and maintain these explosion protection devices to minimize harm to personnel as well as structural and mechanical damage. It is important to note that if the dust collector is protected properly, inside the safe place for an explosion to hap- pen rather than in an open facility or around employees.
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. The vent’s function is illustrated in the series of photos be- low that show a staged deflagration in a cartridge dust collector equipped with an explosion vent.
Typically, the collector is located outside so that it vents away from buildings and populated area to a safe location. If it is prop- erly equipped and located indoors, standards mandate that you designate a safe area. 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 fa- cilities 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.
34 Occupational Health & Safety | MAY 2020
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