Of all the substances that are flammable, one that is often overlooked or forgotten about is dust. However, dust is incredibly flammable and dust explosions are a very serious risk that employees and employers in various industries need to be aware of and have measures in place to prevent them from occurring.
What Exactly Are Dust Explosions?
Dust explosions involve fine particles of specific materials that are suspended in the atmosphere combusting quickly. In most cases, dust explosions tend to happen in enclosed spaces but can happen elsewhere if the circumstances are correct and there isn’t sufficient air ventilation or dust extraction in place.
Anywhere where there are high concentration levels of a powdered form of combustible material present in the atmosphere or another gaseous medium such as oxygen that oxidizes, there is a chance that dust explosions can occur.
As such, some of the locations that dust explosions is a common hazard includes grain elevators, coal mines and various other industrial environments such as factories and warehouses.
Extremely high over-pressures begin building up when a material quickly combusts, causing shrapnel to be scattered and often major damages to the actual structure of the building. A shock wave often follows a detonation and if the speed of the resulting flames spreading is at a subsonic level. it is normally referred to as explosions or deflagration.
Different Classifications of Dust Explosions
There are two main classifications of dust explosions and these are:
- Primary Dust Explosions – These normally happen internally in process equipment and other similar locations, and are usually managed with pressure relief provided by ductwork leading to the air on the exterior.
- Secondary Dust Explosions – These are caused by the disruption of accumulated dust on the interior of a building or structure that is then ignited by an initial explosion. Secondary dust explosions, therefore, constitute a more serious threat as they produce an uncontrolled and considerably more dangerous explosion inside a working area. It is unsurprising really that secondary dust explosions account for a larger percentage of fatalities than primary dust explosions.
Conditions That Are Required For Dust Explosions to Occur
There are actually 5 distinctive conditions required for dust explosions to occur. These conditions are:
- Presence of combustible dust particles
- High concentrations of the dust present is suspended in the atmosphere
- There is the presence of atmospheric oxygen or another type of oxidant in the air
- There is a source of ignition presence
- They occur in confined space. Buildings and even the interiors of machines can be considered enclosed spaces.
Dust Sources
Most materials that burn can create dust explosions. Including the likes of sawdust and coal. There are many less obvious materials that are seemingly harmless that can create a highly dangerous cloud of dust that can become explosive, including:
- Coffee
- Cocoa
- Powdered milk
- Sugar
- Starch
- Flour
- Grain
Dust can become explosive as a result of activities such as the transportation of grain, which is why explosions in grain silos occur. Mining for coal creates coal dust and milling wheat to make flour generates large volumes of flour dust. A serious example of the possibilities of flour, a substance that most people would consider to be fairly harmless, creating dust explosions is the fact that a mill was destroyed and killed a total of 14 employees at the mill and four neighbouring buildings. The accident took place at the Washburn A Mill on May 2nd, 1978 in Minnesota.
Sufficient Concentration
There is a specific value, that if the concentration of dust in the air is lower than it, it is impossible for an explosion to occur. This is known as the LEL or lower explosive limit. The safe level is 25% or lower than the LEL. Interestingly, if air to fuel ratio increases about the UEL or upper explosive limit, there is not enough oxidant present to make an explosion possible.
Necessary Oxidants
Generally, the normal level of oxygen found in the atmosphere is enough for a dust explosion to occur, if all other required conditions are present too. Environments that have atmospheres comprised purely of oxygen or higher than normal levels of oxygen are considered to be particularly dangerous. Likewise, so too are gases with strong oxidisation such as fluorine and chlorine. Risk of a dust explosion occurring is also increased if there is the presence of chemical and gaseous compounds that have potentially high oxidative properties, including dichromates, perchlorates, nitrates, chlorates and peroxides.
Ignition Sources
Although a naked flame is usually the form of ignition that most are familiar with responsible for causing explosions, it is not the only one capable.
In fact, in Germany more than half were caused by non-flame related sources in 2005. Other sources of ignition responsible can be therefore:
- Self-ignition
- Fire
- Hot surfaces
- Machinery or equipment causing electrical arcing
- Friction
- Electrostatic discharges
As the effects of dust explosions can be not only costly but even fatal, it is important for companies and employees working in industrial and other environments where the risk of these accidents occurring are aware of proper safety measures.
Prevention
The ATEX regulations were introduced to help reduce the risk from explosive atmospheres. This involves altering and improving working practices as well as the use of specially-designed equipment.
Dust extraction equipment is useful for helping prevent dust explosions by improving cleanliness; however due to the high concentrations of dust inside the systems it can itself be an explosion risk. Our dust extraction systems are designed to minimise these risks; for more information see our ATEX blog.