Bill Kurz | Sly
Managing dust within your bulk solids processing facility is easy to do once you know what that entails for your specific material and process. From knowing your material and your dust’s characteristics to identifying dust collection points and determining a dust collector type, this article details what’s involved in establishing a successful dust management system.
Within powder and bulk solids material handling processes, dust management is critical to industrial air pollution control. Operator health, site safety, and regulatory compliance are all dependent on your ability to properly manage any pollutants or particulates present in your airstreams.
Dust is typically managed with industrial air pollution control equipment, such as dry dust collectors and wet scrubbers, along with other ancillary equipment, such as exhaust fans and rotary valves for a dry dust collection system or a recirculation tank for a wet scrubber system. Dust collectors work to separate entrained dust particles from an airstream and properly dispose of the dust. In most facilities, dust management is so important that if the dust collection equipment shuts down, production stops entirely. Since every facility is different, your means of dust management and air pollution control will depend on the specifics of your operation, particularly the characteristics of your dust. The following information outlines the steps to take to define your operation’s dust management needs.
Analyze your operations and unique dust problem
Dust management starts with understanding your specific material application and air pollution control requirements. What type of dust does your material create? How much dust is present in your airstreams? What levels of air pollution control and dust mitigation do you need to reach?
While dust can come from various sources, most dust is categorized as either fugitive dust or process dust. Fugitive dust, also known as nuisance dust, is any dust generated simply from the act of moving material. Equipment that creates dust by moving material includes belt and screw conveyors, bucket elevators, and scale hoppers. Examples of materials known for generating fugitive dust are grains, flours, mined aggregates, salts, spices, and cannabis. Process dust, on the other hand, is created from grinding, screening, or drying materials via processing equipment. These equipment types include pulverizers, vibratory separators, and fluidized-bed dryers. When materials such as barley, coal, or iron go through these equipment types, process dust is often created as a by-product.
Understanding what type of dust you have is important to identifying the threshold limit values (TLVs) for your material or substance. The TLV of a substance is the maximum airborne concentration of that substance to which a worker can be exposed day after day (8 hours each day) for a working lifetime without adverse effects. Established by the American Conference of Governmental Industrial Hygienists (ACGIH), the TLVs for substances are required to be included on materials’ safety data sheets per the Occupational Safety and Health Administration (OSHA) Hazard Communication Standard. You can find more information on TLVs at https://www.acgih.org/science/tlv-bei-guidelines/ or by viewing the different OSHA Z Tables at www.osha.gov/annotated-pels. Governing entities such as OSHA and the Mine Safety and Health Administration (MSHA) set different respirable dust limits for various substances. Regulatory limits often change over time as research evolves, so verifying the current standards for industrial hygiene and safety requirements is best practice. These exposure limits will be a key design factor for your dust management system, as they’ll need to be maintained to ensure site safety and regulatory compliance.
Conduct a particle analysis
To fully define your dust management requirements, you need to better understand your dust’s physical characteristics. Through a detailed particle analysis, you can identify your dust’s characteristics, such as its agglomeration potential, abrasiveness, and particle shape. Two key characteristics of your dust that you want the particle analysis to focus on are bulk density and particle size distribution.
Bulk density, commonly expressed in grams per cubic centimeter (g/cm3) or in pounds per cubic foot (lb/ft3), is a weight-per-unit-volume measurement. Knowing your dust’s bulk density is important because a high bulk density dust requires a more powerful and higher energy dust management system than a low bulk density dust. By quantifying your dust’s bulk or apparent density, you can request that your filter vendor create specialty filters that are optimized for your dust collection needs and allow you to more easily diagnose any dust re-entrainment issues, which is when the dust is mixed back into the original airstream.
Particle size distribution refers to the particle size range present in a dust sample. This is typically expressed in terms of particle size versus the cumulative percentage of total weight or volume. Plotting a sample’s particle size distribution on a log-probability graph allows you to determine the dust particles’ mass-mean diameter, which quantifies the average particle size of the mass that you’re analyzing. For example, limestone typically has a fairly large particle size range, with some fine and some fairly large particles, while carbon black has a very narrow range of fine particles. Particle size distribution and the resulting mass-mean diameter can be used to predict filter performance and thus serve as good theoretical qualification prior to other physical tests such as a bag filter failure analysis. The finer and narrower your particle size distribution, the more efficient your filter media needs to be.
A particle analysis can be done by an independent lab or a filter media supplier if you provide them with an appropriate dust sample. The analysis will result in a particle analysis report, similar to the example shown in Figure 1.
Perform a dust hazards analysis
If you’re handling combustible dust, then you’ll need to perform a dust hazards analysis (DHA) to stay compliant with regulations. A DHA is a systemic review of your facility and processes to identify any potential combustible dust hazards with the goal of improving facility safety. In addition to OSHA-led initiatives, the National Fire Protection Association (NFPA) recently implemented NFPA 652: Standard on the Fundamentals of Combustible Dust, which requires that all applicable facilities that have the potential for a dust explosion perform a DHA.
NFPA 652 creates a unique standard separate from industry- and commodity-specific regulations by building upon other standards, such as NFPA 61: Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities; NFPA 484: Standard for Combustible Metals; and NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids. Not only must a DHA be completed for new processes, but the NFPA also requires DHA updates every 5 years. Using the results from your DHA, a dust collector manufacturer can design your dust management equipment in a way that mitigates the risk of a dust explosion.
Identify dust collection points
Identifying the main places where dust is produced in your process is vital to dust management, as collecting the dust at its source is the most efficient means of air pollution control. Mapping out the areas and processes that are generating dust will allow you to determine where your primary collection points should be.
Dust collection points include obvious dust-generating material handling equipment, such as conveyor belts, bucket elevators, bagging equipment, or pneumatic conveyors, and any grinding, screening, or drying equipment. The specific dust collection point for your equipment type can be found by consulting Industrial Ventilation: A Manual of Recommended Practice for Design by the ACGIH. Taking a proactive approach with your current system setup so that it offers sufficient capacity for any additional collection points that may need to be added in the future is best practice.
Determine your dust collector type
Dust management and air pollution control don’t have a one-size-fits-all equipment solution. Like any good engineering problem, you need to find the right tool for the job if you want to maximize system efficiency and performance.
Dust collectors are categorized into two main categories: dry dust collectors and wet scrubbers. Both equipment types have inherent advantages and disadvantages, depending on your application.
Dry dust collectors have long been the standard dust collector type for enhancing air pollution control in industrial environments. These dust collector types offer high collection efficiencies, enhanced versatility, and require the least amount of energy for standard operation. Dry dust collectors, such as bag and cartridge filter collectors, are often a great choice if you have a dry particulate with practically any range of light to heavy dust loadings, such as dry fertilizers, cosmetics, or minerals. A dry dust collector, as shown in Figure 2, cleans a dust-laden airstream by pushing the airstream through a filter media, which traps the dust and allows the clean airstream to exit the clean filter side and continue downstream in the process. The dust collector then cleans the filters, and the collected dust falls into a hopper at the bottom of the collector. An important note is that not all dry dust collectors use filter media. For example, a cyclone dust collector uses a cyclonic-shaped unit, centrifugal force, and gravity to separate the dust from the airstream. Dry dust collectors are a good choice for controlling combustible dusts, though they require explosion venting and specialty explosion-proof design. Dry dust collectors are typically not a good option when you have a wet airstream, you’re dealing with extremely hot gases, or you need your collector to absorb gas contaminants.
Wet scrubbers, also referred to as wet dust collectors, are great for removing a variety of pollutants or handling difficult wet, sticky, or gummy dusts, including sludge cake, calcium carbonate, and petroleum coke. Instead of using a bag or cartridge filter, wet scrubbers, such as venturi and eductor scrubbers, remove dust and pollutants using a scrubbing fluid, which is usually water. A wet scrubber, as shown in Figure 3, works by introducing a dust-laden airstream into the scrubber unit, which uses a scrubbing fluid to entrap the dust, either absorbing or dissolving the contaminants. Then, the cleaned airstream is vented to the atmosphere and the dust-laden fluid is either disposed of downstream or cleaned before being recycled back into the wet scrubber. This method of dust management makes wet scrubbers inherently well-suited for controlling combustible dust. Wet scrubbers are also suitable for managing wet or sticky particulate as well as mixes of particulate and gases and vapors. Wet scrubbers are generally not a good option when dealing with water-reactive dusts, heavy dust loadings, or when you don’t have access to proper wastewater treatment solutions.
Once you know what type of equipment you need, it’s time to choose your dust collector. Dry dust collectors include baghouse dust collectors, cartridge dust collectors, fluted-media dust collectors, and cyclone dust collectors. Wet scrubber options include impingement scrubbers, venturis, absorption scrubbers, and wet cyclone scrubbers. For optimal results, all of these options can and should be further customized to meet your specific application’s requirements.
Along with the other items presented in this article, many applications will require additional considerations. For example, if your process involves discharging any filtered air back into the atmosphere, then this will most often require you to get a permit from your governing air quality agency. Systems that exhaust air back into the building may require the addition of positive make-up air of equal magnitude to limit excessive negative pressure in the building.
Of course, dust management needs can always change as processes evolve, so you should readdress your systems at these times of change and at regular intervals throughout the year. For any air pollution control equipment that’s used to handle your dust, you’ll also want to perform regular maintenance and part changeouts as necessary to keep everything running smoothly.
For further reading
Bill Kurz (440-274-0390) is the vice president of Sly, where he specializes in field service, troubleshooting, and system design, and has extensive experience in baghouse, cartridge filter, and wet scrubber applications. He has nearly 50 years of experience in the air pollution control industry, working with both wet and dry dust collection equipment.
Sly • Strongsville, OH
866-747-5219 • www.slyinc.com
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