Chapter 2

The Basics of Dust Explosions

2.1 Conditions for Dust Fires and Explosions

2.1.1 Explosion Limits

Eckhoff1 notes that most solid organic materials, as well as many metals and some nonmetallic inorganic materials, will burn or explode if finely divided and dispersed in sufficient concentrations.

Combustible dusts can be intentionally manufactured powders. Examples include corn starch or aluminum powder coatings. They may also be generated by handling and processing solid combustible materials such as wood and plastic pellets. Work activities such as polishing, grinding, transporting, and shaping many of these materials can produce very small particulates, which become airborne and settle on surfaces, crevices, dust collectors, and various equipment surfaces. When these dusts are disturbed, they can generate potentially explosive dust clouds.

Relatively small amounts of accumulated dust can cause explosions and fires. The Chemical Safety Board2 has reported that the explosion that devastated a pharmaceutical plant in 2003 and killed six employees was caused by dust accumulations mainly under 0.25 inches deep. The NFPA states in its standard3 that more than 1/32 of an inch of dust over 5 percent of a room’s surface area poses a significant explosion hazard.

Like all fires, a dust fire occurs when fuel (i.e., the combustible dust) is exposed to an ignition source in the presence of an oxidizer such as oxygen in air. Most people are familiar with the fire triangle, depicted in figure 2.1. The removal of any one of the legs of the triangle eliminates the possibility of a fire and explosion. As an aside, it may be impossible to eliminate all sources of ignition in an industrial setting, and as such, removing or controlling the fuel and the oxidizer become critical to risk mitigation.

Figure 2.1 The Fire Triangle.

Another important concept for all fires is illustrated by the slide in figure 2.2. From a plot of vapor pressure (or concentration since concentration is proportional to vapor pressure through Raoult’s Law), the reader may recall from basic chemistry that the vapor pressure of an ideal solution is directly dependent on the vapor pressure of each chemical component and the mole fraction of the component present in the solution.

Figure 2.2 Illustration of the concept of flammability.

Before a fire or explosion can occur, three conditions must be met simultaneously. A fuel (ie. combustible gas) and oxygen (air) must exist in certain proportions, along with an ignition source, such as a spark or flame (i.e., the fire triangle, figure 2.1). The ratio of fuel and oxygen that is required varies with each combustible gas or vapor.

The minimum concentration of a particular combustible gas or vapor necessary to support its combustion in air is defined as the lower explosive limit (LEL) for that gas. Below this level, the mixture is too “lean” to burn. The maximum concentration of a gas or vapor that will burn in air is defined as the upper explosive limit (UEL). Above this level, the mixture is too “rich” to burn. The range between the LEL and UEL is known as the flammable range for that gas or vapor.

The term, flash point, refers to the lowest temperature at which a volatile material can vaporize, forming an ignitable mixture in air. Measuring a flash point requires an ignition source. At the flash point, the vapor may cease to burn when the source of ignition is removed.

The flash point is not to be confused with the autoignition temperature, which does not require an ignition source, or the fire point, the temperature at which the vapor continues to burn after being ignited. Neither the flash point nor the fire point is dependent on the temperature of the ignition source, which is much higher.

The flash point is often used as a descriptive characteristic of liquid fuels, and it is also used to help characterize the fire hazards of liquids. “Flash point” refers to both flammable liquids and combustible liquids. There are various standards for defining each term. Liquids with a flash point less than 60.5 or 37.8°C (141 or 100°F), depending upon the standard being applied, are considered flammable, while liquids with a flash point above those temperatures are considered combustible.

These are the standard definitions used to describe fuels but for combustible dusts the explanation of the conditions responsible for fires is a little more complex. A dust explosion requires the simultaneous presence of two additional elements to the ones shown in figure 2.1; namely both dust suspension and confinement are the two additional elements required – thus giving rise to the Dust Explosion Pentagon (figure 1.4 in chapter 1). Suspended dust burns more rapidly and confinement allows for pressure buildup. The removal of either the suspension or the confinement elements prevents an explosion, although a fire may still occur.

In an analogous way of the flammability range commonly used for vapors, the concentration of suspended dust must be within an explosible range in order for an explosion to occur. Dust explosions can be very energetic, creating powerful waves of pressure that can destroy buildings.

When all of the elements of the dust explosion pentagon are in place, rapid combustion known as deflagration can occur. Deflagration is defined as a rapid burning slower than the speed of sound. When this occurs within a confined space such as an enclosure (e.g., building, room, vessel or process equipment) the resulting pressure rise can cause an explosion (a rapid burning faster than the speed of sound). People caught in dust explosions are often either burned by the intense heat within the burning dust cloud or injured by flying objects or falling structures, or may be thrown great distances.

Important points to remember are the 5 basic ingredients that are required for a dust explosion to occur:

Some important NFPA definitions to bear in mind:

NFPA 654 – “A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.”

“Any finely divided solid material that is 420 microns or smaller in diameter (material passing a U.S. No. 40 Standard Sieve) and presents a fire or explosion hazard when dispersed in air.”

Many combustible fiber segments, flat platelets, and agglomerates do not readily pass through a No. 40 sieve, but they can be dispersed to form a combustible dust cloud.

2.1.1.1 Minimum Explosible Concentration

MEC values are determined in the U.S. per the ASTM E1515 test procedure involving tests with various dust concentrations and a pyrotechnic igniter in a 20-liter sphere. The MEC corresponds to the smallest concentration that produces a pressure at least twice as large as the initial pressure at ignition. MEC values are not very sensitive to particle diameter for diameters less than about 60 um, but increase significantly with increasing diameter above this approximate threshold.

The majority of the materials known to cause explosions have MEC values in the range 30 to 125 g/m3. These concentrations are sufficiently high that a 2 m thick cloud can prevent seeing a 25 watt bulb on the other side of the cloud.

2.1.1.2 Confinement

The confinement needed for a dust explosion is usually from the process equipment or storage vessel for the powder or dust. In the case of fugitive dust released from equipment and containers, the room or building itself can represent the confinement.

Often, the dust cloud occupies only a fraction of the equipment or building volume. The resulting explosion hazard is called a partial volume deflagration hazard.

Pressures produced from partial volume deflagrations and the corresponding deflagration venting design bases are described in NFPA 68. Example applications include dust collectors and spray driers.

2.1.2 Ignition Sources

The ignition criteria for a dust explosion can take several forms: e.g., hot temperatures, burning embers and agglomerates, self-heating, impact/friction, electrical equipment and electrostatic discharges.

2.1.2.1 Hot Surfaces

An example of a hot surface is a dust cloud accidentally entering a hot oven or furnace. In a plastics plant a phenolic resin dust explosion incident occurred because a resin dust cloud was generated during the cleaning of fugitive dust from the area around the oven. In an application like this it is important to know the minimum dust cloud oven ignition temperature, which can be determined by oven tests described in ASTM E1491.

2.1.2.2 Burning Embers and Agglomerates

Burning embers and agglomerates are common ignition sources in dust explosions. Smoldering or flaming particulate embers or agglomerates (also called smoldering nests) are produced by frictional heating, e.g. during sanding or cutting, local...