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Smaller dust particles react more quickly with oxygen and require less energy to ignite.
Alot of recent discussion has centered on explosion potential as it relates to dust collectors. Manufacturers are now required by the National Fire Protection Association (NFPA) to minimize explosion potential in their dust collector equipment.
Combustible vs. Non-Combustible
Dust can be separated into two categories: combustible and non-combustible. As its name implies, non-combustible dust will not, under predictable conditions, ignite, burn, release flammable gases, or support combustion when subjected to fire or heat. Combustible dust, on the other hand, will pose a fire or deflagration hazard when suspended in an oxidizing medium, regardless of the size or shape of the dust.
However, particle size does have an effect on the hazard level of dusts. The largest size that is classified as “dust” is any material that can sift through a 0.420 mm screen mesh, as shown in Table 1. Thus, a dust particle is generally smaller than 420 µm. Table 2 shows sizes of common dust.
In general, as the size of the dust decreases, the hazard increases. Finer particles have a larger total surface area, which allows for quicker ignition. To understand this, think of a wood-burning fire. To start a fire, one would use smaller kindling because it ignites more easily than a larger log. The same is true with dust; smaller particles react more quickly with oxygen and require less energy to ignite.
Mechanics of Fires and Explosions
A critical characteristic about a dust explosion or deflagration is that it is a rare event. A process that handles combustible dust could operate for many years without any problems. Then, unexpectedly, all of the conditions that trigger an explosion are present—and it happens.
A general fire or explosion requires three things: fuel, oxygen and an ignition source. These components make up the explosion triangle, as shown in Figure 1.
A dust explosion is a little more complex. It still must have the requirements shown in the explosion triangle: fuel (combustible dust), oxygen and an ignition source. However, two more conditions must be met. First, the dust must be in a state of dispersion in air or another oxidant. Second, the dust cloud must be confined such that its concentration is at or above the minimum explosive concentration (MEC). These five requirements make up the Combustible Dust Explosion Pentagon (see Figure 2).
Primary and Secondary Explosions
Two types of explosions can occur: an initial (or primary) explosion and one or more secondary explosions. Often, these secondary explosions are far more destructive than the primary explosion. Figure 3 illustrates how a primary explosion in a dust collector could occur and level a building with a secondary explosion in less than one second.
In a ventilation system, all equipment is connected by ductwork, which allows for the efficient distribution of air. With enough fuel and oxygen, however, this ductwork can also provide a means to allow a flame front to travel to either end. If a piece of process equipment has an explosive concentration of dust along with an ignition source, a primary explosion could be triggered that would then travel through the ductwork to the dust collector and cause a secondary explosion in the collector. The same is true in the opposite direction: if an explosion were to occur in a dust collector and it was contained within the dust collector, the resulting flame front could travel through the ductwork back to the process equipment, where unsuspecting employees could be hurt.
Protection Against Explosions
The NFPA is an international nonprofit organization that is responsible for creating and maintaining the minimum standards for fire and other hazard prevention. The NFPA was established in 1896 by a group of insurance underwriters focused initially on establishing codes for automated fire sprinklers. Over the years, the organization has created and maintained over 300 consensus codes and standards that influence our lives and safety on a daily basis.
NFPA 70 (National Electrical Code, Articles 501, 502, 503) defines classes, groups, and divisions of hazardous matter and locations. It is important to take note of:
• Class I: Gases, Vapors and Liquids
• Class II: Dusts
• Group E: Metal Dusts (conductive and explosive)
• Group F: Carbon Dusts (some conductive, all explosive)
• Group G: Flour, Starch, Grain, Combustible Plastic or Chemical Dusts (explosive)
• Division I: Ignitable quantities of dust that is normally in suspension or conductive.
• Division II: Dust that is abnormally suspended in an ignitable concentration (dust layers)
• Class III: Ignitable Fibers and Flyings
When dealing with combustible dust, the Occupational Safety and Health Administration (OSHA) organization requires that all necessary safeguards be put in place to collect and handle dust. This includes using electrically powered cleaning devices approved for the hazard classification of the dust. Something as simple as a sweeper or vacuum cleaner used in dusty areas must be approved to operate in a Class II Division 1 Group EFG Hazard Location.
NFPA 68 is known as the Standard on Explosion Protection by Deflagration Venting. It is the standard used to design and use devices that vent the combustion gases and pressures of a resulting deflagration within an enclosure to protect the structural integrity of the structure. NFPA 68 is used to design explosion vents and any supporting systems to not only protect the enclosure but to also guarantee safety for the surrounding area.
Certain parameters must be considered when designing an explosion vent for a specific application. The majority of the variables focus on the attributes of the dust collection system. Two important variables focus entirely on the combustible dust: Pmax and Kst.
Pmax is the maximum pressure in bar, developed in a contained deflagration at an optimum concentration. Kst is the deflagration index, or rate of pressure rise in bar/sec. Based on Kst values, dusts have been classified into three hazard classes: St-1, St-2 and St-3 (see Table 3).
Kstand Pmaxare both determined approximately by a spherical 20 L test chamber that uses a chemical igniter to produce a deflagration. The NFPA 68 provides a list of some common dusts and their respective Kstand Pmaxvalues. However, these values are intended as a reference only; the best way to determine the explosivity of a combustible dust is to have it tested in a lab. Table 4 shows the Kst, Pmaxand dust hazard class values of the dusts from Table 2.
In addition to Kstand Pmax, other variables that depend on the dust collection system must be satisfied. Two notable variables are Pred and Pstat. Pred is the maximum pressure developed in a vented enclosure during a deflagration and is determined by the enclosure strength (Pes). In general, it is acceptable to assume a worst-case scenario and prevent Pred from exceeding two-thirds of Pes. Pstat is the minimum pressure required to activate an explosion vent. The lower the value of Pstat, the less explosion vent area will
One way to protect against explosions is to prevent them from occurring. This method involves focusing on controlling ignition sources. A spark or flaming ember presents the greatest risk of being the ignition source of a deflagration. A spark consists of a dense particle that surrounds itself with an envelope of hot air, which allows the solid particle to be buoyant in the surrounding cooler airstream (see Figure 4).
As the spark travels in the duct, one would assume that the spark would be cooled or drop out and prevent any fire or deflagration. However, this is not often the case because ducts are designed to have as smooth (laminar) a flow as possible. As the spark travels in the duct, the smooth flow continues to provide the spark an envelope of hot air with oxygen to fuel the burning ember.
Manufacturers should develop procedures to improve general housekeeping as it relates to collecting dusts. Documenting these efforts will provide companies with the necessary information for their insurance companies and make management teams aware that this issue is being given the consideration it deserves. c
For additional information, contact Scientific Dust Collectors at 4101 W. 126th St., Alsip IL 60803; call (708) 597-7090; or visit www.scientificdustcollectors.com.