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Pneumatic Conveying and the Mitigation of Safety Issues for Respirable Crystalline Silica and Combustible Dust

February 16, 2017
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Enclosed conveying lines prevent release of dust into plant work areas.
Enclosed conveying lines prevent release of dust into plant work areas.
A pneumatic conveying transporter allows for gravity-driven separation of material from motive air and dust.
A pneumatic conveying transporter allows for gravity-driven separation of material from motive air and dust.
Proper material containment allows for effective dust collection.
Proper material containment allows for effective dust collection.

In recent months, the critical concern about both combustible dust and respirable crystalline silica have been increasing. The safety of workers and neighborhoods surrounding industry sites where dust and silica are generated is paramount. OSHA (the Occupational Safety and Health Administration) is taking an active role in rulemaking and enforcement, regarding the mitigation of both materials. This article reviews the issues with each area of concern and how pneumatic conveying can be part of your mitigation solution.

Pneumatic Conveying in Brief
A pneumatic conveying system transfers powders, granules, and other dry bulk materials through an enclosed horizontal or vertical conveying line. The motive force for this transfer comes from a combination of pressure differential and the flow of air (or another gas) supplied by an air mover, such as a blower or compressor.
    
There are two basic types of pneumatic conveying: dilute phase and dense phase. Either can run under pressure or vacuum. In dilute phase conveying, particles are fully suspended in the conveying air and transported at low pressure and high velocity. It is most often used with non-abrasive, non-fragile materials that have a light bulk density (typically less than 62 lb/cu ft). Common examples are flour, sugar, corn starch, plastic granules, sodium bicarbonate, hydrated lime, and activated carbon.
    
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In dense phase conveying, particles aren’t suspended in the conveying air and are transported at high pressure and low velocity. Material is moved in slugs, which makes it a better choice for conveying fragile or abrasive materials. Commonly handled materials include silica sand, feldspar, fly ash, glass cullet, alumina, glass batch mix, and carbon black.

Respirable Crystalline Silica

Safety issues related to crystalline silica

Crystalline silica is a common mineral that is one of the building blocks of materials such as rock/stone, sand, concrete, brick, and mortar. Exposure to this substance is common in workplace operations that involve the cutting, drilling, crushing, and handling of these materials. This includes those found in construction, glass manufacturing, sand blasting, concrete production, and other industrial processes, as well as other, less obvious applications, such as dental laboratories and jewelry production.
    
In their official overview of the final rule, OSHA states that “Workers who inhale very small crystalline silica particles are at increased risk of developing serious—and often deadly—silica-related diseases” including silicosis (an incurable and sometimes fatal lung disease), lung cancer, COPD (chronic obstructive pulmonary disease), and kidney disease.
    
OSHA had not updated its PELs (permissible exposure limits) since the 1960s. Its now updated rule takes into account this health concern information that has been gathered over the past several decades.

What industries are affected?
Respirable crystalline silica is an issue in a broad range of industries. Construction businesses (i.e. cut stone and stone product, abrasive blasting) will need to comply with OHSA’s new standards by June 23, 2017. Solutions here will primarily be personal respirators, to protect each individual as they cut or blast or etch materials that can emit respirable crystalline silica. General industry (i.e. glass manufacturing, concrete production) must comply by June 23, 2018. These industries generally utilize mechanical conveying systems. It is in this general industry category where pneumatic conveying can offer solutions to minimize exposure to silica dust.

Compliance solutions pneumatic conveying can offer
Minimize dust generation. Respirable crystalline silica is dust that comes from handling silica and sand. Conventional mechanical method for handling sand often release a lot of dust from transfer points or removed access covers. Dense phase pneumatic conveying’s systems do not have these types of release points.

Contain the dust. Pneumatic conveying is completely enclosed, with materials moving through pipelines. This offers your plant and workers the benefit of little to no exposure to silica dust during material conveying. Properly designed pneumatic conveying systems also include dust collection equipment, which filters the silica dust from the motive air, so that air can be released safely back into the atmosphere.

Combustible Dust
Safety issues with combustible dust
Dust-related fires and/or explosions can have many possible causes, making it difficult to determine what is required for responsible regulation. OSHA’s own definition of combustible dust reads, in part, “all combustible particulate solids of any size, shape or chemical composition that could present a fire or deflagration hazard when suspended in air or other oxidizing medium.” The solids in question include wood, fertilizer, sugar, textiles, and many other substances. In general, nearly all manufacturing deals with some amount of potential combustible dust.
    
Combustible dust is a critical concern for any industry that processes and conveys bulk solid materials. Changes in regulations have brought this issue into even greater focus. The NFPA has issued stricter safety standards to prevent and mitigate explosions and fires in plants where combustible dust is present. Several NFPA standards pertain to combustible dust in various individual industries, but the NFPA’s intent is to have consistent standards across industries and materials.

What makes dust combustible?
While nearly any manufacturing process can generate dust that can be combustible, it doesn’t automatically become combustible. There are five elements that must all be present in order for dust to combust and explode. They are:
•    Fuel
•    Ignition
•    Oxygen
•    Distribution
•    Containment

The first three elements – fuel, ignition source, and oxygen – will drive a dust fire. The last two elements – dispersion or distribution of the dust in the air and improper confinement of dust – will cause that fire to turn into an explosion. Eliminate or suppress any one of these elements and you have eliminated the combustible dust danger.

How pneumatic conveying addresses these elements
Avoid adding more fuel. Dry bulk materials contain dust, which, unfortunately, can’t be eliminated entirely. However, mechanical conveying, which is a frequent conveying choice, facilitates greater release of that dust into the air at each transfer point along the conveying design. It also adds to accumulation with insufficient containment methods. Dense phase pneumatic conveying uses high-pressure, low-volume air to convey material at a low, gentler velocity. Hence, less dust is released, which minimizes increasing the fuel supply. And the fully contained system prevents collection of the dust on building surfaces.

Remove ignition sources. Ignition sources come in many forms. According to OSHA, the top source is mechanical sparks (30%). These sparks can be generated by such things as overheating bearings or belts or an out-of-alignment system component. Pneumatic conveying systems have very few moving parts to come in contact with the conveyed materials. And that means fewer possible contacts with sparks and heat sources.

Reduce oxygen. Oxygen becomes an issue in combustible dust explosions when the concentration of oxygen to air to material reaches a very specific level limiting oxygen concentration point (LOC). So your conveying system doesn’t need to remove all oxygen. It just needs to reduce the oxygen concentration so it’s below your material’s LOC. While each material varies greatly, the required volumetric percentage of oxygen to conveying gas is typically less than 15 percent.
    
With pneumatic conveying systems, which are completely enclosed, this can be done by using an inert gas, such as nitrogen, rather than air for conveying. This will remove the dust explosion threat for the vast majority of dry bulk materials, and is particularly useful with very volatile materials.

Manage dust dispersion. Very specific concentrations of dust in the atmosphere are needed in order pose an explosion risk. A fully enclosed pneumatic conveying system allows you to control the air-to-material ratio, to achieve a safe dust dispersion within the convey line. Through testing, safe concentration levels (as set by the NFPA) can be determined for your material and application.
    
Fully enclosed pneumatic conveying also means dust is contained and can’t accumulate in the larger work area. Dust accumulation on plant surfaces and floors provide fuel sources throughout the building and contribute to secondary explosions and fires. Pneumatic conveying helps minimize this kind of surface build-up.
    
Contain the cloud. This fifth element can seem like a contradiction to managing dust distribution. Why would pneumatic conveying be recommended if containment of the dust can cause combustible dust explosions? It’s vital to remember that proper containment of the dust cloud isn’t dangerous – but improper containment can be. Pneumatic conveying does contain the dust, but also allows you to utilize dust collection and disposal equipment to eliminate incorrect containment. It’s also important to understand that containment is only an issue if all the other elements are in place. A properly designed pneumatic conveying system will prevent the other conditions from existing – so containment can be a non-issue.

Pneumatic Conveying – Not only for Mitigation
Throughout this article, we have discussed the many benefits that dense phase pneumatic conveying can offer for mitigation of safety issues with both respirable crystalline silica and combustible dust. While safety is a major benefit of pneumatic conveying, it’s not the only advantage a properly designed pneumatic conveying system can offer. Pneumatic conveying can lower your cost of ownership through:

•    fewer moving parts, lowering maintenance costs
•    layout flexibility, allowing a design that meshes with your plant footprint and process needs
•    integration with current process equipment, lowering costs to upgrade your systems

Working with an experienced pneumatic conveying system supplier can ensure that your system efficiently and effectively conveys your material, giving you lower costs and higher productivity, while keeping your plant and workers safe.

References
See www.osha.org for detailed information on respirable crystalline silica and combustible dust standards.
    
Mitch Lund is a chemical engineer and product manager for Nol-Tec Systems Inc. He has four years of experience in field testing for pneumatic conveying system design and new product development. Dianne Novak is a marketing communications specialist with over 20 years of experience in business-to-business marketing communications. For more information, visit www.nol-tec.com.

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NOL-TEC Systems Inc.