Grain dust explosions: risks, standards and protection
29 August 2014
This article from Italian safety equipment manufacturer Cortem looks into the explosion risks inherent in storing grain and other fine particle food products, the safety standards that apply to the sector and the types of equipment suitable for different zones.
Grain silos store food industry products such as wheat, corn, rice or other similar products. All loading operations are usually carried out using mechanical means such as pipes and conveyor belts through loading inlets placed in the upper part of the silo tower. Once stored, the products are taken from unloading doors in the lower part or they can be collected and transported by means of pneumatic or manual conveying systems.
All these food industry products contain elements such as carbon, hydrogen and oxygen and are therefore combustible. If the grains undergo grinding before storage the resulting particle size may fall below 500 µmm, in which case they can mix with air to create an explosive atmosphere.
Even if the majority of particles are larger, finer dusts may well be present and therefore may still constitute a danger. The presence of combustible dusts always involves explosion risk if an ignition source is present.
Dust cloud explosions
Within silos there is always air and the stored grain deposits layers of dust. During loading and unloading operations, these layers can generate a dust cloud as particles are sucked up by the circulating air.
The dispersed combustible dust clouds in the air form an explosive atmosphere. These clouds, if triggered, are able to oxidise fast enough to generate an explosion. The reactivity of the dust cloud is as great or as small as the particles within it.
For a dust cloud to be considered explosive, the concentration of dust within will be between the Lower Explosive Limit (LEL) and the Upper explosive Limit(UEL), expressed in grammes of powder per air volume, g/m3.
The classification of hazardous areas in a silo can be carried out using IEC 60079-10-2 (CEI 31-90), which replaced the previous EN 61241-10 standard.
Zoning within the containment system
Inside the silo, the presence of grain dust with emissions continuously released into the atmosphere gives an Atex Zone 20 classification for the entire internal volume of the silo.
If loading and unloading operations are carried out through automatic conveying systems, within the conveying system dust and air may create a mixture in concentrations that may fall within the explosive field. In this case, the interior of the transport system is also classified as Zone 20.
Zoning outside the containment system
During the phases of loading and unloading, a certain amount of dust is normally emitted from the doors that communicate with the outside. In case of frequent but temporary emissions, this area would be classified Zone 21, or possibly Zone 22 if only on an infrequent basis.
If there are open automatic transport systems, such as conveyors and open bucket elevators, they can be considered first or second degree emission sources depending on individual circumstances, thus respectively gaining a Zone 21 or 22 classification.
Any electrical control systems for loading and unloading equipment must be manufactured and certified to be used in the appropriate zone, with the appropriate marking.
· Zone 20: CE EX II 1D
· Zone 21: CE EX II 2D
· Zone 22: CE EX II 3D
Methods of protection
Dusts in suspension can create potentially explosive clouds, so all electrical and mechanical equipment must be designed, manufactured and maintained in accordance with the specific standards for explosion protection to avoid providing an ignition source..
Recently, the EN 60079-0 standard introduced the Group III for the classification of dusts that may form potentially explosive places. The list below shows the categories and the groups of equipment that can be used.
· Group III A – Substance: Fibres – Categories: III A, III B, III C
· Group III B – Substance: Non-conductive dusts Categories: III B, III C
· Group III C – Substance: Conductive dusts – Category: III C
Similar to what happens for gases, the equipment manufactured for the most dangerous groups, can also be used for the inferior groups. Equipment suitable for the Group III C can, therefore, be used for all applications, while a device built for the Group III A, can never be used for a Group III B and III C.
Methods of protection for dusts
Originally, there was no division between protection methods for gases and dusts, and the same methods of protection for gases were also used for dusts. Today, with the evolution of regulations and standards, equipment is available designed specifically for dust explosion risk environments.
Equipment for gases is designed on the principle that it is impossible to prevent their penetration inside the enclosure, while ingress prevention is a common feature of equipment for dust explosion-risk environments.
Methods of protection
The two basic principles of protection are segregation and prevention.
In the following table you can find the types of protection that can be used.
Choice of the method of protection
To select the right method of protection there are no fixed rules. Manufacturers build equipment to suit every situation. The parameters of choice concern: the respect of rules for the choice of material depending on the type of zone, where the equipment will be installed, the facility of installation, and, especially, the ease of maintenance. Equipment should be chosen not only based on the initial cost, but considering the total cost for the entire period of its useful life. A piece of equipment that requires less maintenance will usually be preferable.
Ex t explosion protection will be adequate for most electrical equipment applications used near a silo. Pressurisation has high conduction costs (the maintenance of internal overpressure provides a continuous use of energy for the pumping of air or inert gas), while the encapsulation is applied only to equipment that, once its life cycle is over, must be replaced.
With regard to intrinsic safety, the power involved is so low that the use is appropriate for process instrumentation and control.
Nowadays, with all the existing regulations, there is a plethora of different markings to be aware of. Equipment suitable for the use in dust explosion-risk areas such as a silo, will have the following marks:
· CE marking
· Number of the Notified Body. Eg: CESI
· EX marking
· Group II (for surface)
· ATEX Category (1, 2 or 3)
· Letter "D" for dust (Dust)
· Protection. Eg: Ex ta III A
· Temperature class. Eg: T6
· Certificate number: Eg: CESI 13 ATEX 123 X
Obviously, the equipment must be provided with the certificate of conformity issued by the certification body. Today, unfortunately, some products are available on the market that do not conform, while constantly evolving regulations can make it hardtop be sure products conform to current standards.
The more serious manufacturers are therefore adopting identification systems (such as holograms) that can be checked in real time by visiting the manufacturer's website.
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