The dynamics of dust explosions

Usually explosions in civilian settings, such as homes, or industrial settings, such as factories, are due to gas leaks.

However, there are also explosions due to the presence of potentially explosive Powders, such as flour, sugar, or even metal powders.

While in the case of gases the situation is quite intuitive, that is, for an explosion to occur the three elements represented in the triangle article will be needed, in the case of powders the explosion needs different requirements.

As for dust, its presence is usually within the containment system, such as vacuum systems.

In other words, when there is dust generation in a work environment, there are often vacuum systems that make the environment suitable for the presence of people. A vacuum system consists of:

• A vacuum cleaner
• A Filter
• Channels that reach to where dust is generated or needs to be vacuumed (fume hoods for example).

Within such a vacuum system, it often happens that the dust concentration is above the MEC (or minimum explosible concentration), making the atmosphere potentially explosive.

If triggered, an explosion would then occur. This is called a primary explosion. Thus, the primary explosion normally occurs within a vacuum system or, in general, machinery, such as an auger or bucket elevator, and may remain internal to the containment system.

In the event that such an explosion exits due to a failure of the containment system, a pressure wave will form that can disperse potentially flammable dust present in the environment and deposited on surfaces. This causes a repeated series of secondary explosions, fueled by flames from the primary or any other ignition sources in the environment.

This phenomenon just described has been the cause of many accidents over the past decades, especially in food establishments, which have to deal with micron-sized dust that is extremely dangerous. Suffice it to recall the case of the Port Wentworth sugar mill in Georgia.

The accident was caused by the explosion of sugar dust accumulated between the hollow spaces of the distribution system, generating an explosion that spread throughout the plant within seconds due to the explosive power of the dust present.

When talking about explosions, it is good not to confuse and especially to make differences based on their behavior. This is why a distinction is made between detonation and deflagration.

Deflagration: is a type of explosion characterized by rapid combustion that spreads at susbonic speed; this is often associated with the combustions of gas or dust mixtures, in detail this phenomenon occurs when a chemical reaction, which in the case previously considered was combustion, propagates through heat transfer and the chemical reaction occurring in the flame front, which as it expands pushes surrounding materials, creating a pressure wave that propagates through the medium at a speed slower than the speed of sound.

Detonation: is a type of explosion, or better said a type of explosive chemical combustion reaction. This occurs by propagation of the flame front at a supersonic speed, i.e., above the speed of sound, with an eventual formation of a shock wave with differences on the chemical-physical front that differentiate it from deflagration. depending on the speed downstream of the flame front, there are three different types of detonations:

• Strong detonation (downstream subsonic vel.)
• weak detonation (downstream supersonic vel.)
• Chamapn-Jouguet detonation (downstream sonic vel.)

For a potentially explosive environment to be created, unlike gases, the three basic elements for an explosion will not be enough, namely:

• fuel
• oxidizer
• ignition

but it is important that the dust be diffused in the ‘air to form a cloud; Therefore, 4 elements are needed which, graphically speaking, are placed at the vertices of a square which are:

• combustible (dust)
• oxidizer (the oxygen in the air)
• dispersion
• ignition

This set of elements results in the creation of ideal conditions for the manifestation of the flash fire phenomenon.

Flash fire: a phenomenon consisting of a rapid and widespread fire of a flammable dust cloud, blown up in the open.