In Powder Explosions and Milling Industry

Precautions to take

Mahmut Bakırhan1*, Ömer Sinan ŞAHİN2, Murat Apakhan1, Hakkı Ekem1

İMAŞ Makina Sanayi A.Ş R&D Center, 4. OSB 407. Sokak No:8 42300 - Konya

m.bakirhan@imas.com.tr

Konya Tek. U., Eng. and Nature Science. F. Department of Machinery, 42075 Campus/ Konya ossahin@ktun.edu.tr

Summary

In this study, dust explosions in various industries and the characteristics of these explosions were examined and their effects on the milling sector were evaluated. According to the current legislation in our country, the measures to be taken in the enterprises operating in the milling sector have been evaluated.

Keywords: Dust explosion, milling industry, ATEX

1. Dust Explosions and their characteristics

Dust explosion is the general name of explosions that occur as a result of igniting with a spark or similar effect after the density of particles in the air exceeds a certain limit. These explosions, which can be experienced in almost every sector in today's industry, are very important in terms of both facility safety and occupational health.

According to the definition of the American National Fire Prevention standard; Regardless of the particle size and shape, any combustible solid dust suspended in the air or in any oxidizing medium and presenting a flashing or explosion hazard is considered particulate dust. Pollination is the tendency of dust to form suspended clouds and remain 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 transportation, unloading, filling, weighing, especially in enterprises where dust-generating components are processed.

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 dust concentrations of 10 and 1000 grams per cubic meter or cloud are sufficient for dust explosions to occur [3]. The main explosive dusts in the food industry are; Sugar powder, flour, starch, milk powder, cocoa, instant coffee powders, spice powders, cream powder can be given as examples.

Dust is generally defined as particles that can hang in the air and settle depending on the conditions. It has been reported that 0.1 micron and smaller diameter particles hang in the air, while 0.1-10 micron particles cannot hang in still air [4]. . Fibers, scraps or dusts of various properties arising from solid materials can create explosive atmospheres [5]. Solid materials with a size of roughly 500 μm and suspended in air for a certain period of time are also considered dust [5].

It is known that the intensity of the explosion, which is caused by the formation of flammable clouds in the air by flammable materials and the spread of the flame, depends on the oxygen and flammable material concentration in the environment [6]. It has been shown by experimental methods that dust explosion will not occur at densities below or above the specified density value [7, 8]. It has also been proven that all types of dust with diameters below 0.5 mm show more or less explosive properties [5].

Moisture content of explosive dusts affects explosion severity. It is known that the dryness of the powders increases the explosion severity [5].

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

It has been reported that when a pressure wave hits the flame or when the mixture in front of the flame is turbulent, the flame may become turbulent and at this stage of flame propagation, pressure wave accumulation and shock wave formation may occur in front of the flame [3]. shock waves and vibrations occur.

3 ms                                   6 ms                                   10 ms                                  15 ms

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

After this stage, when the moment of about 60ms is reached, aeration and scattering of dusts that can be found in the external environment are observed. The silo, pipe, etc. where the explosion started. The secondary explosion, which starts with the penetration of the wall of the equipment, spreads to the entire enterprise within about 200 ms.

Figure – 2: Primary and secondary explosion [16]

Required for a normal combustion event to start and continue; Combustible material (fuel), combustible material (oxygen) and ignition (ignition source) coming together under appropriate conditions is called the combustion triangle.

Figure – 3: Combustion triangle [17]

 In dust explosions; Five different chains must come together in order to form an explosive atmosphere [10]. These;

                    •  explosive atmosphere (limited area)
                    •  Explosives (Organic substances such as food and agricultural products, Synthetic organics such as pesticides, pigments and plastics, -Coal etc.)
                    •  Explosive Dust concentration (airborne dust cloud)
                    •  A spark or energy source to ignite an explosive atmosphere.
                    •  Air (sufficient amount of oxygen)

Figure – 4: Explosion pentagon [18]

This is called the explosion pentagon. If any of these five components is prevented from interacting with the others, the explosion hazard is eliminated. The first precaution to be taken in protection from dust explosions is to isolate the said environments from each other and to prevent the formation of explosions. However, if there is a risk of explosion, countermeasures must be taken. It has been reported that the effects such as arc and/or sparks, hot surfaces, sparks caused by friction effect and static electricity from electrical devices are sufficient for the explosion of dust in businesses [10]. For example, choosing non-sparking devices and/or isolating such devices (use of exproof units) may be a solution. However, in order to limit the explosion effect, measures such as locating the enterprises in explosion-proof structures, fire extinguishing installations and isolating the environments where the explosion may occur from other regions have been proposed [10].

According to the current regulations in our country (Regulation on Equipment and Protective Systems Used in Potentially Explosive Environments (2014/34/EU)), the probability of the ignition source being activated, the duration of the explosive material in the explosion concentration in the environment and the effect to be formed are evaluated in Figure 1. As shown, a risk matrix should be created and measures should be taken accordingly.

Figure – 5: Risk Matrix [14]

2. Causes of Dust Explosion

In Tables 1-4, the causes of dust explosions, the sectors most affected by dust explosions, the facilities where dust explosions are seen, and the characteristics of various explosive dusts in the food sector are presented. As can be seen, there is a risk of dust explosion in almost all industries. As in the milling industry, in a sector where powders of various qualities and sizes exist in our country, it is imperative that the design and operation of the machinery, systems and components be evaluated in a safe way, both in the established facilities and still being produced in our country within the framework of the current legislation.

Table 1. Causes of dust explosions [12]

Table 2. Sectors most affected by dust explosions [12]

Table 3. Facilities with dust explosions [13]

Table 4. Properties of various explosive powders [20]

3. Applications in the milling industry

The explosion in the WASTERBURN “A” Mill flour mill in Minneapolis, USA in 1878 is the first known mill facility explosion in the world and 22 people lost their lives in this event.

Figure – 6 Wasterburn A Mill mill [19] 

As it is known, the milling industry creates a whole system consisting of many machines and components during the process of turning the raw product into the final product. These can be classified as conveying systems, storage systems, sorting machines, grinding machines and packaging machines. The specified systems and components are shown in Table 5, as are the sub-components of the specified systems. When the specified system and sub-components are examined, it is seen that ATEX evaluation can be made for basically all components and that 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, both in terms of explosion risk and degree of impact. These components are elevators, dust filters, flour mixer, flour brush, bran brush, airlocks, bagging scales and square sieves.

Figure - 7 Zone zones in a typical flour mill

Table 6 shows the components that are the basis for evaluation both in our country and according to the European Union legislation. As seen in the table, it is understood that almost all the components in the sector are subject to ATEX evaluation in one aspect and the designs must be carried out within the framework of the relevant legislation.

Table 5. Systems and components for ATEX assessment in the milling industry

Table 6. ATEX elements to be considered in various product groups

4.Conclusions and recommendations

The milling industry is one of the industries most affected by dust explosions. ATEX assessment of the systems and components existing in both the food and feed sector and making the designs compatible with the relevant legislation are also a legal obligation in terms of occupational health and safety. In this respect, it is recommended that the designs of the components and machines used in the milling industry comply with the legislation and that the end users using these systems should request ATEX certified products from the manufacturers.

1.Kaynaklar

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[17]  By User:Gustavb - Own work, CC BY-SA 3.0, March 2006

https://commons.wikimedia.org/w/index.php?curid=618468

[18]  BSI ENGINEER, October 2020

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[19]  Contributor: The Color Archives / Alamy Stock Photo,

https://www.alamy.com/

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