What is Combustible Dust & How to Prevent It
What is Combustible Dust?
Combustible dust is any finely divided solid material (wood, plastics, paper, rubber, combustible metals, etc.) that can present an explosion hazard when suspended in air or when accumulated in a confined space and subject to pressure build-up. Examples of combustible dust include aluminum and magnesium: they do not easily burn as larger pieces; however, they can be explosible in dust form when not properly managed.
The life science industry is not immune to the risk posed by combustible dust. Many pharmaceutical production processes, such as batching, compression, dispensing, granulating, and mixing, create dust hazards. Common airborne combustible dust particles which are a by-product of these operations, and may present an explosion risk if not controlled, include calcium, lactose, lithium, magnesium, potassium, sulfur, and sodium.
What Causes a Combustible Dust Explosion
For a fire to occur, three distinct characteristics within the “fire triangle” must be present:
- Combustible dust (fuel)
- Ignition source (heat)
- Oxygen in the air
Where combustible dust is the fuel, the uncontrolled dispersion of dust particles, or the accumulation of dust in a confined area, are contributing factors to the risk of fire.
The surface area of the particulate dispersion can have a tremendous impact on the explosion. For example, in a small room (200 ft2), dust accumulations would present a severe secondary explosion hazard; in a large room (20,000 ft2), a covering of dust in a 200 ft2 area would not present as significant of a hazard. As a general good practice measure, 1/16 in. of dust, which is close to the thickness of a quarter, is cause for cleanup to minimize the explosive risk. A more practical technique to determine the plausibility of a dust explosion would be to run your finger through the accumulation. If your finger leaves a mark, this indicates that the dust is subject to dispersion, presenting an explosive hazard.
A prime example of a combustible dust explosion in the life science industry occurred in 2003 at a West Pharmaceutical Services’ rubber-manufacturing plant in Kinston, North Carolina. The facility produced rubber stoppers and other products for medical use. According to the U.S. Chemical Safety and Hazard Investigation Board, the fuel for the explosion was a fine plastic powder, which accumulated above a suspended ceiling over a manufacturing area at the plant and ignited. The vast explosion not only destroyed the manufacturing facility, but caused six deaths, dozens of injuries, and hundreds of job losses.
Prevention of Combustible Dust
Several prevention measures can be followed to prevent combustible dust explosions. The three “Cs” for controlling and avoiding incidents include:
Contain the dust within process equipment engineered to handle combustible dust safely. If dust cannot be contained within the process equipment, collect the dust at the point of release before it escapes into a work area.
Install dust collection systems and use vacuum systems designed for combustible dust collection. When capturing and containing dust, it is recommended that companies use high-efficiency particulate air (HEPA) vacuum systems. Air-operated industrial strength vacuum systems do not have electric motors, like shop-type vacuums, and therefore allow dust to be contained without the risk of a spark hazard from the hot motor while operating.
Combustible dust must be cleaned regularly with soft natural brooms or brushed before being collected in an approved or rated vacuum cleaner. Areas to be cleaned regularly to minimize dust accumulation include walls, floors, equipment, ledges, and rafters, and the area above suspended ceilings and other concealed areas. If ignition sources (bearings, belts, buckets, milling machinery, etc.) are present, practice the above cleaning methods only if they do not generate dust clouds.
Other Control Methods include:
- Control of Ignition Sources. Periodic inspections, lubrication of machinery parts, and vibration detection methods (heat-sensitive tape) are ways to control ignition sources. Installing appropriately rated electrical equipment, equipment grounding and bonding, and limiting exposed flames also control ignition sources.
- Damage Control. Isolate the dust hazard by using distance or barrier. Explosion venting serves as a relief panel to safely vent the explosion away from work areas and property structures, thus minimizing damage to equipment or injury to workers. Explosion suppression is another control measure that will allow systems to enact a pressure sensor when ignition occurs, thus sending a release signal to stop the explosion before it becomes dangerous.
NFPA Prevention of Combustible Dust Standards
When uncertain if dust particles are combustible or non-combustible, it is advisable to perform a Dust Hazard Analysis (DHA). This involves employing trained professionals to conduct sampling and test for dust combustibility hazards. Laboratory testing can be performed to determine if powders are combustible or explosive, ignition sensitivity (minimum energy of electrostatic needed to ignite dispersed dust), dust flammability (minimum oxygen concentrations needed for fire) limits, and dust explosion severity. The DHA will also include an analysis of processes and equipment where dust may be present, a review of existing controls and suitability, and, if necessary, additional recommendations to manage hazards.
The NFPA (National Fire Protection Association) 652 publication, Standards on the Fundamentals of Combustible Dust, describes a technical standard outlining the basic requirements for identifying and managing fire, flash fire, and explosion hazards of combustible dust. The NFPA 652 standard applies to operations that manufacture, process, mix, repackage, or handle combustible dust or particulate solids. If properly adopted and utilized, the NFPA 652 and the NFPA 654 (Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids) publications will help protect the industrial facility, the workers in and around the facility, and, ultimately, the community.
As evidenced by the West Pharmaceutical Services tragedy, combustible dust can have catastrophic impacts on life science operations if not addressed and controlled appropriately. There are several different types of materials and processes within the life science industry that are susceptible to combustible dust explosions. Identifying the hazards, along with the materials that may be subject to ignition, are critical first steps to controlling combustible dust explosions. Practicing safe-housekeeping practices and completing Dust Hazard Analyses of materials and processes can ensure the facility, the workers, and the surrounding community remain unharmed from deadly dust explosions.
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