Hazards of Combustible Dust

Hazards of Combustible Dust

Combustible Dust Explosion (Source)

Combustible dust explosions pose a real danger in a wide variety of factories and workplaces. The U.S. Chemical Safety Board conducted a study in 2006 that found there were at least 281 dust explosions from 1980 to 2005 resulting in 119 fatalities and 718 injuries (Ref. 1). After a particularly deadly explosion in 2008 at an Imperial Sugar refinery, OSHA stepped up on their combustible dust enforcement and awareness program. Antiquated refineries or factories, such as the Imperial Sugar refinery in this disaster, generate safety hazards, particularly combustible dust risks, due to the lack of renovations to the original equipment (Ref. 1). Managers’ and employees’ awareness of combustible dust hazards protects the safety of the facility and helps prevent combustible dust explosions. Inspectors or knowledgeable safety leaders must conduct a thorough analysis of workplaces to ensure the reduction of combustible dust explosions and implement plans to control risk factors.

What is combustible dust?

The National Fire Protection Association defines combustible dust in the NFPA 654 as “any finely divided solid material that is 420 microns or smaller and presents a fire or explosion hazard when dispersed and ignited in the air” (Ref. 2). Most of the time, this refers to combustible material, but some non-combustible solid materials may become combustible once formed into dust (Ref. 2).

Combustible dust affects a wide variety of industries including agriculture, chemicals, food, grain, fertilizer, tobacco, plastics, 3D printing, welding, recycling, coal power generation, and more (Ref. 3). Any facility that transports, handles, processes, polishes, grinds, shapes, abrasively blasts, cuts, crushes, mixes, sifts, and screens materials may create combustible dust (Ref. 4). If your facility works with any of the following materials, safety leaders should have a thorough Dust Hazard Assessment conducted on site to prevent combustible dust incidents (Ref. 5).

Materials or Products with Potential Combustible Dust

Agricultural Products

Egg white
Milk, powdered
Milk, nonfat, dry
Soy flour
Starch, corn
Starch, rice
Starch, wheat
Sugar
Sugar, milk
Sugar, beet
Tapioca
Whey
Wood flour

Agricultural Dusts

Alfalfa
Apple
Beet root
Carrageen
Carrot
Cocoa bean dust
Cocoa powder
Coconut shell dust
Coffee dust
Corn meal
Cornstarch
Cotton
Cottonseed
Garlic powder
Gluten
Grass dust
Green coffee
Hops (malted)
Lemon peel dust
Lemon pulp
Linseed
Locust bean gum
Malt
Oat flour
Oat grain dust
Olive pellets
Onion powder
Parsley (dehydrated)
Peach
Peanut meal and
skins
Peat
Potato
Potato flour
Potato starch
Raw yucca seed dust
Rice dust
Rice flour
Rice starch
Rye flour
Semolina
Soybean dust
Spice dust
Spice powder
Sugar (10x)
Sunflower
Sunflower seed dust
Tea
Tobacco blend
Tomato
Walnut dust
Wheat flour
Wheat grain dust
Wheat starch
Xanthan gum

Carbonaceous Dusts

Charcoal, activated
Charcoal, wood
Coal, bituminous
Coke, petroleum
Lampblack
Lignite
Peat, 22%H20
Soot, pine
Cellulose
Cellulose pulp
Cork
Corn

Chemical Dusts

Adipic acid
Anthraquinone
Ascorbic acid
Calcium acetate
Calcium stearate
Carboxy
methylcellulose
Dextrin
Lactose
Lead stearate
Methyl-cellulose
Paraformaldehyde
Sodium ascorbate
Sodium stearate
Sulfur

Metal Dusts

Aluminum
Bronze
Iron carbonyl
Magnesium
Zinc

Plastic Dusts

(poly) Acrylamide
(poly) Acrylonitrile
(poly) Ethylene (low-pressure process)
Epoxy resin
Melamine resin
Melamine, molded (phenol-cellulose)
Melamine, molded (wood flour and mineral
filled phenolformaldehyde)
(poly) Methyl acrylate
(poly) Methyl acrylate, emulsion polymer
Phenolic resin
(poly) Propylene Terpene-phenol resin Urea-formaldehyde/ cellulose, molded
(poly) Vinyl acetate/ ethylene copolymer
(poly) Vinyl alcohol
(poly) Vinyl butyral
(poly) Vinyl chloride/ ethylene/vinyl acetylene suspension copolymer
(poly) Vinyl chloride/ vinyl acetylene emulsion copolymer

How do combustible dust explosions occur?

A combustible dust explosion develops when a large and concentrated dust cloud ignites within a confined space such as a vessel or building (Ref. 2). The fire created burns rapidly and threatens the safety of nearby employees due to flames, additional explosions, and flying debris (Ref. 2). Deflagration, or rapid combustion, of the combustible dust, produces extreme air pressure resulting in the destruction of walls or structures (Ref. 4).

Creation of a combustible dust explosion can only occur in the presence of all five elements of the Dust Fire and Explosion Pentagon (Ref. 2). The five elements needed to create a combustible dust fire explosion include the confinement of a dust cloud, ignition source, combustible dust, oxygen in the air, and dispersion of dust particulates. Please see the graphic below illustrating the Dust Fire and Explosion Pentagon (Ref. 2).

Dust Fire and Explosion Pentagon

Dust Fire & Explosion Pentagon
1
Confinement of Dust — within an enclosure or structure
2
Ignition Source — open flame, heat source, spark, etc
3
Combustible Dust — fuel to burn and start the fire
4
Oxygen in the Air — to keep the fire building
5
Dispersion of Dust Particulates — high concentration and spread

To prevent combustible dust explosions, facilities must control at least one of the five elements. Oxygen and confinement prove difficult to control, but safety leaders can easily control the ignition source, combustible dust build-up, and the dispersion of combustible dust (Ref. 2).

Secondary Explosions

A combustible dust explosion can cause one or more secondary explosions (Ref. 2). The primary explosion might loosen accumulated dust or damage a combustible dust storage device leading to ignition then explosion (Ref. 2). Secondary combustible dust explosions prove to be more devastating than the initial explosion because of the increased quantity and concentration of the combustible dust cloud. See the graphic below for an illustration of how a primary explosion can spread causing additional explosions (Ref. 2).

Secondary Combustible Dust Explosion Mechanism

Secondary Combustible Dust Explosion Mechanism
1
Explosion breaks through enclosure and sends blast wave outward.
2
Accumulation of dust loosens from blast wave and forms dust cloud.
3
Heat from primary explosion ignites dust cloud causing a secondary explosion.

Combustible Dust Hazard Assessment

Any facility involved with combustible dust must conduct a complete Dust Hazard Assessment. The Dust Hazard Assessment closely examines the facility, questions the combustibility of the product’s dust, and inspects electrical components capability (Ref. 2). Awareness of hazards will help safety teams understand areas to improve safety (Ref. 3).

First, in the facility analysis, safety managers must identify the processes and materials involving combustible dust. In addition, they must closely examine any hidden areas or surfaces for dust build-up. Also, the facility analysis includes the identification of ignition sources such as open flames or heat (Ref. 2).

Next, safety leaders must test the materials for the ability to ignite and combust for dust combustibility analysis (Ref. 2). Any material that will burn in air in solid form could become explosive as dust. However, some stable solid materials, such as metals, can easily become combustible as dust (Ref. 2). The Material Safety Data Sheets detail the combustibility of material, but sometimes safety leaders may need to consult the chemical manufacturers for a more detailed analysis (Ref. 2). Likewise, attributes of the material determine its ability to ignite and explode, such as the particle size, shape, and moisture content (Ref. 2). In some cases, particles larger than 420 microns may combust requiring safety leaders to consider and test all factors and materials to ensure facility safety (Ref. 2).

Lastly, the facility electrical component analysis entails clarifying the special electrical classification required for use with combustible dust (Ref. 2). Similarly, this report incorporates consideration of the electrical properties of the combustible dust to select the appropriate electrical classification (Ref. 2).

All new facilities, old facilities needing upgrades, and whenever a facility changes a part of the procedure or process must conduct a formal Dust Hazard Assessment. To ensure ongoing safety and protection, safety leaders and employees should conduct daily informal dust hazard assessments (Ref. 6).


Control Methods

After the Dust Hazard Assessment, the safety managers must implement control measures and best practices for dust, ignition, and damage control to optimize workplace safety.

Dust Control

Proper dust control helps prevent combustible dust explosions. Facilities must utilize specialized dust collection systems to capture dust at the source (Ref. 4). Source capture particulate extraction coupled with ambient air or airborne dust collection systems provide an extra layer of protection against dust build-up (Ref. 1).

Equally important, safety leaders must instill employees with the best dust control practices for everyday prevention efforts. Accordingly, safety managers need to create and implement an inspection, testing, cleaning, and control program to ensure the safety of the facility (Ref. 2). The NFA 654 recommends correctly sealing off process and ventilation systems to prohibit or minimize the escape of dust (Ref. 2). Also, employees must complete appropriate housekeeping on a regular basis and protect or eliminate flat surfaces to inhibit the build-up of combustible dust (Ref. 4).

The OSHA standard 29 CFR 1910.14 requires certain abrasive blasting, grinding, or buffing operations with dust to operate ventilation systems to keep the workspace clean and free of dust especially for combustible dust (Ref. 2). As a rule of thumb, employees and safety leaders should maintain and keep the workspace as clear of combustible dust as much as possible.

Ignition Control

To avoid combustible dust explosions, safety leaders must control, minimize or eliminate if possible all ignition sources (Ref. 6). Ignition sources consist of flames, hot surfaces, smoldering, mechanical heat sources, electrical discharge, static electricity, and smoking (Ref. 6).

Some processes, such as welding and smoldering, rely heavily on the ignition source, making removal impossible. However, in these situations, safety leaders should focus on dust control practices to help thwart combustible dust hazards. Similarly, workplaces with hot surfaces such as dryers and heaters should target dust removal precautions and isolation techniques, if possible.

On the other hand, to limit additional friction and unneeded sources of heat, employees or managers must ensure the upkeep and repair for mechanical equipment, particularly wearable parts such as bearings (Ref. 4). Not to mention, closing off electrical equipment stops combustible dust accumulation or dust from entering equipment crevices (Ref. 2). As well as, bonding equipment to the floor helps hinder static electricity build-up and the likelihood of creating a spark (Ref. 2). Correspondingly, management should designate smoking areas for employees who work around combustible dust or ban it entirely from the work zone. The open flame and burning embers from smoking create an ignition source. Additionally, safety leaders must maintain and test all ignition sources to ensure proper working condition (Ref. 2). Lastly, utilizing the proper electrical equipment suitable for use around combustible dust provides a crucial element for ignition control.

Damage Control

At the same time, damage control tactics try to isolate the danger of the explosion or direct it away from employees. First, isolating the hazard of dust collection or storage device with distance or a barrier protects the workplace from a potential combustible dust explosion. Also, fire vents or explosion vents in the buildings and equipment can direct the exploding force out of an exit away from employees. Similarly, other ways to minimize damage or impede explosions include fire prevention systems such as sprinklers or chemical fire suppression systems (Ref. 2).


Combustible Dust Prevention

Proper training and housekeeping practices contribute to the prevention of combustible dust explosions.

Training

For the first line of defense, employees that work around the combustible dust hazard must undergo proper training for prevention tactics. Training encompasses the identification of hazards and unsafe conditions in order to take preventative action and inform management (Ref. 2). Likewise, training incorporates a thorough overview of the formal plant wide combustible dust control program in order to make all affected employees very knowledgeable and familiar with procedures. Training must take place before starting the job, periodically to refresh understanding, and when processes change or after the introduction of new hazards (Ref. 2).

Similarly, proper training involves management to provide vital knowledge for conducting the initial Dust Hazard Assessment and developing the combustible dust control program. Also, managers should encourage employees to notify them about unsafe practices and working conditions to instill a climate of workplace safety.

Housekeeping

Moreover, facilities must implement best practices for cleaning areas with combustible dust to reduce explosion hazards. Proper housekeeping requires regular cleaning to restrict the build-up of dust. Regular housekeeping includes cleaning any hidden areas, where dust may accumulate to properly protect facility safety (Ref. 2). OSHA warns that a combustible dust layer as small as 1/32” (the thickness of a paperclip) on 5% of the floor can cause a combustible dust hazard if spread out over a sufficient area (Ref. 1). In contrast, dry sweeping or cleaning with compressed air creates a combustible dust cloud resulting in a combustible dust hazard (Ref. 2). OSHA recommends cleaning with only approved methods such as an approved floor mounted dust vacuum to restrict spark hazards. Even plants with stringent dust collection devices should have a housekeeping backup plan in case of equipment failure.

Please note: This post is for informative purposes only. Sentry Air Systems products are not suitable for combustible dust applications. Certified dust collection systems must be utilized for proper workplace safety.


Sources

1. Kennedy, David. “Handling Combustible Dusts.” Occupational Health & Safety, 1 May 2018, https://ohsonline.com/Articles/2018/05/01/Handling-Combustible-Dusts.aspx.

2. “Combustible Dust in Industry: Preventing and Mitigating the Effects the Effects of Fire and Explosions.” Occupational Safety and Health Administration, 31 July 2014. https://www.osha.gov/dts/shib/shib073105.html.

3. “Hazard Alert: Combustible Dust Explosions.” OSHA Fact Sheet, Dec 2014. https://www.osha.gov/OshDoc/data_General_Facts/OSHAcombustibledust.pdf.

4. “Combustible Dust.” Canadian Centre for Occupational Health and Safety, 19 Feb 2015. https://www.ccohs.ca/oshanswers/chemicals/combustible_dust.html.

5. “Combustible Dust.” Occupational Safety and Health Administration. https://www.osha.gov/Publications/combustibledustposter.pdf.

6. Lawton, Paul. “What You Can Do to Prevent Combustible Dust Explosions.” EHS Daily Advisor, 9 Jun 2014. https://ehsdailyadvisor.blr.com/2014/06/what-you-can-do-to-prevent-combustible-dust-explosions/.