By Taylor Morgan of Camfil Air Pollution Control
OSHA has issued a final rule that provides stronger protection for workers against the harmful effects of respirable crystalline silica dust. Silica is a common mineral found in products and materials used in many industries. If your processes produce dust, you may have a respirable silica dust hazard, and you need to determine if your facility is at risk. This article summarizes the impact of the new OSHA rule, how to achieve compliance, and the specific role of cartridge dust collection as a proven and reliable engineering control strategy.
This article will focus on the standard for general industry and will highlight the most significant aspects of the new standard; the detrimental health effects and how to determine if your workers are at risk; and the proper design and use of high efficiency dust collection as a recognized engineering control to achieve compliance.
The New OSHA Standard: What you need to know
The percentage of silica in the product will affect the OSHA exposure limit that you need to maintain. Key Provisions of the Industrial Silica Rule Significant aspects of the new rule are: Reduced exposure limit: The new OSHA Permissible Exposure Limit (PEL) for respirable crystalline silica has been reduced to 50 micrograms per cubic meter of air, averaged over an 8-hour time-weighted average (TWA) work shift. This limit is two to five times stricter than the previous threshold limits of 100 micrograms per cubic meter of air for general industry and 250 micrograms for construction. The new more stringent PEL is expected to enhance worker protection by sharply reducing both short-term and long-term exposure to respirable silica dust. Engineering controls: OSHA requires employers to use engineering controls such as water to keep the dust down, and/or dust collection (ventilation) to capture airborne particulate and keep worker exposure below the 50 microgram PEL. While engineering controls are the preferred approach, employers are required to provide personal respiratory protection when engineering controls are not able to limit exposures to the permissible level. Exposure control plan: Employers are required to develop a written exposure control plan (hazard plan) to show how compliance will be achieved. The plan should also limit access to high-exposure areas and incorporate training of workers on silica risks and basic safety practices so they can recognize how to limit their own exposure. Medical surveillance: Medical exams, lung health monitoring and recordkeeping are required for employees who have been identified as “highly exposed workers”. Exposures above 25 micrograms per cubic meter in an 8-hour TWA over 30 days per year represents an action level where the medical surveillance is required. Effective engineering controls are often capable of maintaining silica dust concentrations below this action level.
How to determine if you are in compliance Are your workers exposed to harmful levels of silica?
Wherever a process generates crystalline silica dust, OSHA states that air monitoring must be performed to determine a worker’s 8-hour TWA exposure. Several different accepted methods of monitoring are listed in Appendix A of the ruling. The role of cartridge dust collection OSHA has stated in its general provisions that “the first and best strategy is to control the hazard at its source. Engineering controls do this, unlike other controls that generally focus on the employee exposed to the hazard.” OSHA goes on to say that when a hazard cannot be removed or enclosed completely to isolate it from the workplace; the solution is to “establish barriers or local ventilation to reduce exposure to the hazard in normal operations”. These principles apply not only to crystalline silica but to all hazardous dusts. A well-designed dust collector is an accepted and proven engineering control that will filter hazardous contaminants to make indoor environments safer and healthier. Dry media dust collectors containing high efficiency cartridge filters along with HEPA secondary filters are the best control for respirable particulate, ensuring that it will not spread and be inhaled by workers in other areas of the plant.
What technologies or engineering controls are available?
The new OSHA crystalline silica PEL of 50 micrograms per cubic meter is achievable using this technology. With the new silica rule 0.05 mg/m3 cartridge filters alone cannot meet this requirement. Cartridge filters in dust collectors with secondary HEPA filtration are effective in controlling hazardous dusts that have PEL limits of 5 micrograms per cubic meter, or 10 times lower than this limit. The importance of dust testing: The collection and lab testing of dust samples is a long established practice to help plant engineers and safety managers make informed dust collection decisions, especially where hazardous dusts are involved. The first step of lab testing silica dust is particle size analysis. This allows proper selection of filter media with regard to the efficiency required at various particle sizes. Testing also determines moisture content of the dust, which can have an impact on the performance of a dust collector. Silica is an inert mineral and therefore does not pose fire or explosion risks. As a result, in most applications it will not be necessary to test the dust for explosibility properties. However, if there is anything in the MSDS to indicate a mixed dust that may contain other combustible ingredients, you may need to request explosivity testing as stated in NFPA 652: Standard on the Fundamentals of Combustible Dust. Dust collection system design considerations. There are many factors that impact the proper design of the dust collection system. Lab testing of dust samples, as noted above can play an important role in guiding this design process by identifying the properties of the dust. Environmental factors also have an impact on equipment decisions. Here are the main points to consider as you set out to design a dust collection system for crystalline silica dust control: Type of capture system: Source capture is the most effective control for dust emissions from any manufacturing process, whether the dust is hazardous like silica or just a nuisance. With source capture, some form of hood or enclosure is used to control the dust at the point of generation so it never has the chance to become airborne into the factory. Negative air pressure is maintained on the enclosures to help ensure containment of the dust. Ambient air cleaning systems, by contrast, work much like HVAC filtration where all the air in the room is cleaned from remote pick-up points. It is not considered a viable option for silica control because it does not prevent the dust from becoming airborne and dispersed throughout the work space. Workers would be required to wear personal protective equipment (PPE) at all times.
What are some of the considerations when choosing a dust collector?
A reputable filtration manufacturer should provide you with a written guarantee of emissions performance for the dust collection system you purchase, so it’s a good idea to obtain that documentation. However, a manufacturer’s guarantee does not constitute proof of compliance to OSHA: Air sampling must be performed, as above, to ensure that factory air is below the required threshold for silica exposure.
Another design consideration involves the mounting position of the filter cartridges. Depending on the collector brand and type, filters may be installed horizontally or vertically. With horizontally-mounted systems, a heavy dust such as silica, a mineral with the density of rock, builds up on top of the filters and is not dislodged by pulse cleaning. To address this problem, manufacturers of dust collectors with horizontally-mounted cartridges recommend that you rotate the filters periodically. This unnecessarily increases employee exposure to silica dust. Vertical mounting allows the high density silica dust to release uniformly from the filter pleats, since it doesn’t have to fight gravity. This reduces the load on the filters, helps extend filter life, and reduces exposure since the filter compartment only has to be opened when it is time to replace the filters. Maintenance Practices Even the longest-lasting filters need to be replaced eventually: Change-out is required when differential pressure through the system reaches the maximum level specified by the filter manufacturer. This is very important to ensure that filters are effectively controlling dust.
Taylor Morgan is a sales engineer for Camfil Air Pollution Control (APC) email: Taylor.Morgan@camfil.com or 1-870-933-7658. Camfil APC is a global manufacturer of dust, mist and fume collection equipment and is part of Camfil, the largest air filter manufacturer in the world. For further information, contact 1-800-479-6801 or 1-870-933-8048; email firstname.lastname@example.org; website www.camfilapc.com.
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