Dust explosions and precautions to be taken in the milling industry

29 March 20218 min reading

The milling industry is one of the sectors most affected by dust explosions. It is a legal obligation in terms of occupational health and safety to evaluate the ATEX assessment of systems and components in both food and feed sector and to make the designs comply with the relevant legislation.


In this study, dust explosions experienced in various industries and the characteristics of these explosions have been examined and their effects on the milling industry have been evaluated. Measures to be taken within the businesses operating in the milling sector according to the legislation in force in Turkey have been evaluated.

Figure – 1 Flame propagation speed in dust explosion [15]

1. Dust explosions and its characteristics

Dust explosion is the general name for an explosion that takes place when the density of particles in the air exceeds a certain limit and is ignited by a spark or similar effect. Such explosions, which can occur in almost every sector in today's industry, are crucial in terms of facility safety and occupational health.

According to the definition of the American National Fire Protection Association standard; Regardless of particle size and shape, flammable solids suspended in the air or in any oxidizing environment and carrying the danger of ignition or explosion is considered as explosive dust. Dusting is the tendency of dust to form suspended clouds and hung in the air. [1]. Dustiness is also a term used to determine the respirable fraction and assess health risks. It may occur during operations such as transport, unloading, filling, weighing, especially in enterprises where dust-forming components are processed.

Figure – 2: Primary and secondary explosion [16]

About 130 years ago, Holtzwart and VonMeyer (1891) experimentally proved that explosive dust clouds can be ignited by inductive electric sparks. [2]. It has been reported that a dust density of 10 and 1000 grams per cubic meter or cloud is sufficient for dust explosions to occur. [3]. Major explosive dusts in the food industry can be listed as sugar dust, flour, starch, milk powder, cocoa, instant coffee dusts, spice dusts, and custard dust.

Figure – 3: Fire triangle [17]

Dust is generally defined as particles that can hang in the air and sink depending on the conditions. It has been reported that particles with a size of 0.1 micron and smaller hang in the air, while particles with a size of 0.1-10 microns cannot hang in the air. [4]. It should be taken into account that the stagnant air environment can only occur in situations where air circulation is limited, such as in warehouses and silos. Fibers, snips or dusts of various characteristics stemming from solids can create an explosive atmosphere. [5]. Solids with a size of roughly less than 500 μm and that can hang in the air for a certain time are also considered dust. [5].

Figure– 4: Explosion pentagon [18]

It is known that the intensity of the explosion caused by the formation of flammable clouds in the air and the spread of the flame depends on the concentration of oxygen and flammable material in the environment. [6]. It has been experimentally demonstrated that dust explosions do not occur at concentrations below or above the specified density value. [7, 8]. Moreover, it has been proven that all kinds of dust with diameters below 0.5 mm are more or less explosive. [5].

The moisture content of explosive dusts affects the explosion intensity. It is known that the dry dusts increase the intensity of explosion [5].

The maximum and minimum values of dust explosion parameters are uncertain because the probability of explosion depends on certain accidental variables [9]. This situation makes it difficult to clearly determine the time when dust explosions may occur and the appropriate conditions that may cause an explosion.

It has been reported that when a pressure wave hits the flame or the mixture in front of the flame is turbulent, the flame may become turbulent and at this stage of the flame propagation, pressure wave may accumulate and shock wave may occur in front of the flame [3]. In general, the first perceptible shock wave and vibrations occur 25 milliseconds after the first moment of explosion.

Approximately 60 ms after this stage, the dust in the outside environment sticks up and is dispersed. The secondary explosion, which starts with the puncture of the wall of the equipment, such as silos, pipes and etc., spreads to the whole enterprise within approximately 200 ms.

Fire triangle is the combination of combustible material (fuel), caustic material (oxygen) and ignition (ignition source) which are required for a normal combustion event to start and continue.

In dust explosions, five different chains have to come together in order to create an explosive atmosphere [10]. These are; • Explosive atmosphere (confined space) • Explosive materials (Organic substances such as food and agricultural products, synthetic organics such as pesticides, pigments and plastics, -Coal etc.) • Explosive Dust concentration (dust cloud hung in the air) • Spark or energy source that will ignite the explosive atmosphere. • Air (sufficient amount of oxygen)

This is called the explosion pentagon. If any of these five components can be prevented from interacting with each other, the risk of explosion will be eliminated. The first precaution to be taken in protection from dust explosions is to isolate the mentioned environments from each other and avoid formation of explosions. However, if there is still a risk of explosion, countermeasures must be taken. It has been reported that effects such as electric arcs and/or sparks, hot surfaces, sparks caused by friction and static electricity that may come from electrical devices are sufficient for the dust in enterprises to explode [10]. For example, selecting spark-free devices and/or isolating such devices (use of ex-proof units) could be a solution. However, in order to limit the impact of explosion, measures such as establishing explosion-proof structures, fire extinguishing installations and isolating the environments where explosion may occur from other regions have been recommended [10].

Pursuant to the regulations in force in Turkey (Regulation on Equipment and Protective Systems Used in Possible Explosive Atmospheres (2014/34/EU), it is necessary to create a risk matrix considering the probability of the ignition source to be activated, the explosion concentration in the environment, the duration the explosive material is kept and the effect that will occur as shown in Figure 1 and take measures accordingly.

2. Reasons of Dust Explosions

In Tables 1-4, the reasons of dust explosions, the sectors most affected by dust explosions, the facilities where dust explosions are observed and the properties of various explosive dusts in the food sector are presented respectively. As it is seen, there is a risk of dust explosion in almost all industries. In a sector such as the milling industry, where dusts of various characteristics and sizes are widespread, it is a must to evaluate the machinery, systems and components both in the already established facilities and the ones that are still in the process of production and to carry out their design and operation safely within the framework of the legislation in force in Turkey.

3. Applications in the milling industry

The explosion in the WASTERBURN "A" Mill flour mill in Minneapolis, USA in 1878 was the first known mill plant explosion in the world and 22 people died in this incident.

As known, the milling sector forms a whole system consisting of many machinery and components in the process of raw material becoming final product. These can be listed as conveying systems, storage systems, sorting machines, milling machinery and packaging machinery. The sub-components of the systems specified are demonstrated in Table 5. When the specified system and sub-components are examined, it is seen that ATEX assessment can be carried out for all components and the design can be carried out within the framework of the relevant legislation. However, it can be considered that some systems and components have priority over others in terms of both the risk of explosion and the degree of impact. These components are elevators, dust filters, flour mixer, flour finisher, bran finisher, air locks bagging scales and quadro plansifters.

Figure – 6 Wasterburn A Mill mill plant [19]

The components that are essential for assessment both in Turkey and according to the European Union legislation are demonstrated in Table 6. As can be seen in the table, it is understood that almost all components in the sector are subject to ATEX evaluation in one aspect and designs have to be carried out within the framework of the relevant legislation.

4. Results and recommendations

The milling industry is one of the sectors most affected by dust explosions. It is a legal obligation in terms of occupational health and safety to evaluate the ATEX assessment of systems and components in both food and feed sector and to make the designs comply with the relevant legislation. In this respect, it is recommended that the designs of the components and machinery used in the milling sector must comply with the legislation and the end users using such systems should request ATEX certified products from the producers.

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