Prevention of Dust Explosions


Estellito Rangel Junior

The risk of dusts explosions is present during all stages of the grain processing industry, starting from the reception of the raw material, passing through the conveyor belts, until its storage in silos. The main component of this risky formula is the possible high concentration of organic dust in the typically confined spaces of locations such as underground tunnels and elevators.

In recent years we heard about fires and explosions in grain storage installations that resulted in losses of millions of dollars in different countries. A combustible dust hazard study conducted by the U.S. Chemical Safety and Hazard Investigation Board (CSB) found that nearly 280 dust fires and explosions have occurred in U.S. industrial facilities over the past 25 years, resulting in 119 fatalities and over 700 injuries. Examples of such occurrences are:

a) Imperial Sugar refinery, in February 2008, USA, where 14 people died and 38 people injured.

b) De Bruce grain elevator in Wichita, KS, USA, exploded in 1998, killing seven and injuring several others. The ground shook was so hard that people believed that McConnell Air Force Base, close to the facility, was under missile attack. Windows were shattered for blocks in every direction. Flames shot hundreds of feet into the air and the smoke plume shocked city habitants.

Fig. 1 shows an explosion occurred in a warehouse.

The dust behavior

With the majority of grains the development of a fast combustion process is possible when particles’ size are small enough. Under confinement, this combustion will reach explosive conditions that produce hot gases, which in turn will increase the pressure inside the vessel. As in the case of gas explosions, a dust explosion is caused by the simultaneous presence of a source of ignition and an explosive atmosphere. Sparks produced by electrical equipment (motors, control stations, switches, etc.), are one of the most common sources of ignition found in industrial installations with explosive atmospheres.

It is important to note that the behavior of combustible dusts atmospheres are quite different from gas atmospheres. While flammable gases spread out in air seek to reach a homogeneous concentration, dust particles tend to settle and to accumulate in layers. Dust particles can also remain in suspension for a given period of time, depending on their density and the particles' diameters. Dust particles can also travel from the point where they were released to other points farther away. Leakage from one piece of process equipment to another piece of equipment or component, such as into an electrical terminal panel, is therefore possible.

Dust can also accumulate on the floor, on piping, on equipment surfaces, on cable trays, and on electrical motors, for example.

A demonstration of a risky dust accumulation is illustrated on Fig. 2

The dust particles can come into contact with sources of ignition when accumulated in layers, when forming a cloud, when in suspension, and also during "normal" operation (for example, at cleaning operations using brooms).

If a dust explosive cloud comes into contact with a source of ignition (only few millijoules are enough), an initial ignition will be produced. This would be the primary explosion, generally with subsonic speed (also known as “deflagration”), which generates a considerable volume of hot gases that in turn generates a pressure wave. After this, dust that has settled nearby easily becomes suspended, forming a new dust cloud in front of the first flash. The initial flame now becomes a new source of ignition for the newly formed cloud (flammable mixture). The process thus repeats itself, over and over, quickly producing a sequence of new secondary explosions, with increased energies that potentially cause the destruction of the entire plant.

Statistic data

Statistics show that in 129 agricultural dust explosions between 1988 and 1997 in the United States, 70 % of the probable causes of ignition could be identified. These causes of ignition are shown in Table 1. Table 2 shows the location of the primary accident, which was identified in 91 % of the 129 cases studied. (please see Tables as thumbnail below).

Characteristics of dust explosions

To produce a dust explosion, the following conditions must happen simultaneously:

-Combustible dust in suspension;

-Dust concentration in suspension must be above its lower explosive limit (LEL);

-Dust must have particles of adequate size;

-Air (oxygen) present;

-Source of ignition with adequate power.

In general, it can be said that it is more difficult to initiate a dust explosion than it is to initiate an explosion of flammable gas or vapor atmosphere, because the necessary energy to ignite dust is higher (magnitude of mJ) than that which is necessary to ignite gases (magnitude of µJ).

Electrical equipment to be installed at these areas need to comply with relevant standards. They are also called as “Ex equipment”.

Considering that if badly maintained, electrical installations can act as a source of ignition in hazardous atmospheres, an inspection plan is highly recommended as a tool to monitor the integrity of the installation, keeping the safety of goods and people.

Establishing an inspection plan

The following steps constitute an inspection plan in a facility:

1) Know the area classification documentation;

2) Select the inspection team;

3) Get the data of all installed Ex equipment;

4) List all equipment in inadequate condition.

Let’s explain each item in a more detailed way:

1) Area classification documentation

An area classification study is required to be developed using the IEC 60079-10 standard. Considering its relevancy, all data taken into consideration to define the extent of the classified areas have to be recorded and kept available to the plant employees.

2) The inspection team

Adequate training of personnel in topics related to hazardous areas are needed to keep industrial plants safe and to guarantee the integrity of Ex equipment.

Many related non-conformities are attributed to the low qualification of maintenance professionals. It is worth to say that good workmanship is the result of competent people working within good systems of work.

If no adequately trained personnel is available in the facility, it will be necessary to contract a specialized company to perform the inspection.

3) Getting equipment data

It is necessary to know the inventory of the electrical equipment installed, their types, gas groups, T class and categories.

The conformity certificates of each electrical and electronic equipment, shall be kept available to maintenance personnel. Care is necessary if a letter suffix ("X" or "U") appears following the certificate number, as it means that specials conditions for use are applied.

4) Listing all defective items

All equipment found with non-conformities regarding its Ex integrity need to be identified and the necessary repairs shall be carried out without delay.

It is important to note that if the integrity of the Ex equipment is affected, the safety of the plant is at risk.

The repair work on Ex equipment should only be carried out by the manufacturer or authorized repair shops.

The following items are the most common non-conformities found at inspections on Ex equipment:

1) Inadequate equipment for the classification of the area;

2) Inadequate cable entry devices;

3) Corrosion of enclosures.

Maintenance resources shall be directed at eliminating these failures that affect the safety of the plant.

When maintaining Ex equipment, is absolutely necessary that the certified components´ characteristics are respected. To keep the conformity certificate valid after a maintenance service, only spare parts recommended by the original manufacturer can replace defective parts. The quality of maintenance is one of the factors affecting safety and operational costs.


Despite their deceptive “harmless appearance”, combustible dusts have a huge destructive power. Design, installation and maintenance on electrical installations at these places, need to be performed strictly under the technical and legal requirements, with trained personnel.

A regular inspection program is an important tool to prevent undesirable sources of ignitions regarding dust explosions.


[1] IEC 60079-17 – Explosive atmospheres Part 17: Electrical installations inspection and maintenance.

[2] Peter Murdoch, "Inspections of installations in hazardous areas adds value to safety and maintenance strategies". In: II HAZARDEX, 2003, Coventry, UK.

[3] RANGEL Jr., Estellito. - Electrical installations in hazardous locations: points to consider. In: V INDUSCON - INTERNATIONAL CONFERENCE OF INDUSTRIAL APPLICATIONS, Salvador, Brazil, IEEE, 2002.

[4] ECKHOFF, R.K. - Dust explosions in the process industries. 2nd Edition, Butterworth Heinemann, 1997, ISBN: 0 75063 270 4.

[5] SCHOEFF, R.W. - Case study of dust explosion at DeBruce Grain Co. terminal elevator, Haesville, Kansas. In: 3rd WORLDWIDE SEMINAR ON THE EXPLOSION PHENOMENON AND ON THE APPLICATION OF EXPLOSION PROTECTION TECHNIQUES IN PRACTICE, 1999, Ghent, Belgium.

[6] RANGEL Jr, Estellito. - Electrical installations in explosive atmospheres: Inspection and maintenance are basic activities for safety. In: I IEEE ESW Brazil - INTERNATIONAL SEMINAR OF ELECTRICAL SAFETY IN THE WORKPLACE, 2003, Guararema, Brazil.

About the author:

Estellito Rangel Junior has been working in electrical installations in hazardous locations and standardization activities for twenty years. Member of the Brazilian Standards Technical Association – ABNT, he is also a Brazilian representative at IEC Technical Committee 31, coordinates area classification studies, and has presented technical papers about this subject in specialized magazines and international seminars.

Fig. 1: Explosion at Coinbra warehouse, Brazil, in November 2001. The structure on the right was identical to that shown on the left (Credit: Aedata).

Fig. 2: Excessive dust accumulation on an electric motor. (Credit: LOM)


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