The food industry has a high risk of explosion because they work with raw materials of vegetable origin. These materials, such as flours, starches, cakes, etc., generate dust clouds where they are contained, either for processing, packaging or transporting them. The equipment where dust explosions can occur are silos, hoppers, bag filters and cyclones.
A DUST EXPLOSION is defined as a combustion of a dust cloud that propagates itself, and develops fast enough, so that the gases and steam, product of this combustion, generate in a sufficiently confined environment, a sufficiently high pressure elevation and at a sufficiently fast speed to produce the rupture of the confinement and generate the effects of an explosion.
Pentágono de explosión
Ignition sources, according to their origin, can be classified as: a) Chemical origin, caused by some chemical reaction (decomposition, polymerization, self-heating, etc.) b) Thermal origin, contact with hot elements or surfaces. c) Mechanical origin, caused by friction, shock or fracture of hard materials, with the possibility of presence in mechanical equipment. d) Electrical origin, caused by sparks or electric arcs, both by active conductors and by eventual accumulations of static electricity or by local heating that exceeds the ignition temperature of the dust.
What is an explosive or combustible dust?
There are several definitions but all agree that it is a very small particulate that when dispersed in the air, has the ability to ignite under certain conditions. NFPA654 (Ed. 2006) defines combustible dust as a solid particulate that presents a fire or deflagration hazard, regardless of size or shape, when suspended in air (or other oxidizing medium) at various concentrations.
It is considered as particulate matter:
- Husks (flakes).
- Mixture of any of the above.
Particle size of common materials (microns):
- Salt: 100.
- White sugar: 450-600.
- Talc: 10.
- Spores: 10-30.
- Flour: 1-100.
Foods that apply as combustible powders
Garlic, alphalfa, cotton, rice starch, corn starch, wheat starch, sugar, cocoa powder, coffee, coconut shell, lemon peel, onion powder, spice powder, potato starch, sunflower, gluten, xanthan gum, rice flour, oat flour, peanut flour, rye flour, corn flour, potato flour, wheat flour, lemon, malt, apple, peach, tobacco blend, potato, parsley, rice powder, wheat grain powder, walnut powder, soybean powder, beet, flaxseed, cassava seed, sunflower seed, semolina, tomato and carrot.
What should we know about the powders we work with?
Dusts used as raw materials in an industry must be characterized, i.e. analyzed and verified whether or not they are combustible and therefore explosive. To start the analysis of dusts, it is important to consult the NFPA 68, 69 and 654 standards; in these documents you can find everything you need to understand the risk generated by combustible dusts in the industry.
In a characterization of a dust, the following information can be obtained: particle size, dust concentration, minimum ignition energy, explosive limits, deflagration index Kṣt, explosive index Pmax. In the NFPA standards, theoretical values of Kst and Pmax can be obtained, which is the minimum information required to locate each dust vs. risk.
Illustration 1. Graph of the behavior of an explosion of combustible dusts
How to work the risk when working with combustible dusts?
- Prevention: Mechanical or pneumatic cleaning, information to personnel, training, etc.
- Mitigation: Apply some of the existing solutions in the market. Fike offers Venting and Isolation.
- Suppression: Elimination of the risk; Fike offers systems with electronic sensors that activate fire suppression agents that control the fire and eliminate the explosion.
- Containment: Mounting equipment capable of containing the explosion, e.g. confining the equipment in a bunker. This method is expensive and unfeasible in the food industry.
Illustration 2. Vent Panels
Passive mitigation with Fike
Fike specializes in solutions that mitigate the risk of explosion. The first option is venting, which consists of calculating a minimum venting area of an equipment at risk, this area is replaced in the equipment with vent panels or vents, which act as a fuse and in case of a sudden pressure rise, they break and allow the fireball that is created in an explosion to dislodge the equipment. This solution protects the equipment and prevents it from being destroyed and affecting people and/or neighboring equipment. The vent panels are easy to assemble and do not require any subsequent maintenance, only a visual inspection.
Illustration 3. Flame Arrester
When the equipment is in a place of the plant where people circulate, Fike suggests the installation of an additional equipment called flame arrester. This is responsible for circulating the fireball, which is generated in the explosion by a labyrinth system responsible for converting the fireball into hot air simply; This makes that in case a person passes at the right time of the explosion, only receives the effect of a hot air and not a fireball.
Illustration 4. Isolation valve
In case of having equipment interconnected by ducts and handling combustible dusts, Fike also offers an isolation solution. It should be noted that the venting does not extinguish the fire and it travels upstream feeding with air saturated with dust; Therefore, the use of a ValvEx valve by its construction, makes a quick closure avoiding the passage of fire to other parts of the plant.
Flexilatina of Colombia, as a technical and commercial representative of Fike, is at your disposal to design this type of mitigation and provide the necessary solutions.
Illustration 4. Block valve