Avoiding dust explosions: preventive measures
Estellito Rangel Junior
Petrobras
Rio de Janeiro, Brazil
estellito@petrobras.com.br
ABSTRACT
In recent years it has been noticed about fires and
explosions in grain storage installations that resulted in losses of
millions of dollars and also with fatal victims.
This paper discusses some occurrences of dust explosions,
describes the dust characteristics and suggests preventive
measures.
1. Introduction
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. Examples of such occurrences are:



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
In February 1999, a deadly fire and explosion occurred in Jahn Foundry in
Massachusetts, with 3 killed and 9 injured. The Occupational Safety Health
Administration (OSHA) and state and local officials conducted a joint
investigation of this incident. The joint investigation report indicated that a fire
initiated in a shell molding machine from an unknown source and then
extended into the ventilation system ducts by feeding on heavy deposits of
phenol formaldehyde resin dust.
In January 2003, devastating fires and explosions destroyed the North
Carolina’s West Pharmaceutical plant that manufactured rubber drug-delivery

components. Six employees were killed and 38 people, including two
firefighters, were injured.
Imperial Sugar refinery, in Port Wentworth, GA, USA, in February 2008
resulted in 14 people died and 38 people injured. The Figure 1 shows the
results of this explosion.
Figure 1: Explosion at Imperial Sugar, USA, February 2008
2. 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.
The amount of dust accumulation necessary to cause an explosive concentration can
vary greatly. This is because there are so many variables – the particle size of the dust,
the method of dispersion, ventilation system modes, air currents, physical barriers, and
the volume of the area in which the dust cloud exists or may exist. As a result, simple
rules of thumb regarding accumulation (such as writing in the dust or visibility in a dust
cloud) can be subjective and misleading. The hazard analysis should be tailored to the
specific circumstances in each facility and the full range of variables affecting the hazard.
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.
The Figure 2 shows a dusty environment.
Figure 2: An industrial plant with grain transportation
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, during cleaning operations using brooms).
If a dust explosive cloud comes into contact with a source of ignition of just few
millijoules, 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.
3. Statistic data
The Health and Safety Executive in UK, stated that on average there are 70 to 100
incidents each year in the UK that involve explosive atmospheres. Companies affected
by potential dust explosions include manufacturers of sugar, coal, cocoa, flour-based
products, milk powder, tea, grain, fodder and tobacco, as well as wood and metal
processing companies. When it comes to sourcing electrical equipment for areas where
there is a risk of dust explosions, many companies in these sectors are not fully aware of
the recommendations of latest relevant standards.
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. In the late 1970s a series of devastating grain dust explosions in grain elevators
in USA left 59 people dead and 49 injured.
4. 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.
Combustible dust-air mixtures have different ignition temperatures. The surface
temperature of the equipment in areas with a risk of dust explosion may reach 2/3 of the
minimum ignition temperature of the surrounding dust-air mixture at the maximum and
with a 5 mm thick layer of dust must be at least 75 K below the minimum ignition
temperature of the dust (glow temperature).
It is therefore the owner‘s job to ensure that the cleaning and maintenance intervals of
installations with a dust explosion risk are scheduled so that no dust layers thicker than 5
mm are allowed to form. At higher dust deposits the minimum ignition temperature
(glow temperature) of the dusts are reduced drastically. Examples for the ignition and
glow temperatures can be taken from Table 1.
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. These special electrical equipment are called as “Ex equipment”.
Considering that an electrical installation poorly maintained 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.
Table 1 – Ignition temperatures of some dusts
Dust type
(name of the solid)
Minimum ignition
temperature of a dust layer
(glow temperature) in acc.
with IEC 61241-2-1
method A
o
Cellulose
Cotton
Grain
Sawdust
C
370
350
290
300
Minimum ignition
temperature of a dust cloud
in acc. with IEC 61241-2-1
method B
o
C
500
560
420
400
5. Preventive measures
The following measures are recommended to avoid explosions:
 Use appropriate electrical equipment and wiring methods;
 Identify and control static electricity risks;
 Control smoking, open flames, and sparks;
 Control mechanical sparks and friction;
 Use separator devices to remove foreign materials capable of igniting
combustibles from process materials;
 Separate heated surfaces from dusts;
An inspection plan is very important to know if the safety in your plant is affected.
The following steps are necessary to perform an inspection of the electrical installation in
a hazardous location:
 Keep the area classification documentation updated;
 Form an inspection team with skilled persons;
 Get the installed Ex equipment data;
 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 60079-10-2
standard as basis. 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 is needed to keep
industrial plants safe and to guarantee the integrity of Ex equipment.
Many non-conformities' causes 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 trained personnel are available in the facility, it will be necessary to hire a
specialized company to perform the inspection.
3) Getting equipment data
It is necessary to know the inventory of the electrical Ex equipment installed,
including their types, gas groups, T class and categories.
The conformity certificates of each electrical and electronic equipment shall be kept
available to the maintenance personnel.
4) Listing all defective items
All equipment found with non-conformities under the relevant standards, 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.
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 kept. To keep the conformity certificate valid after a
maintenance service, only original spare parts recommended by the manufacturer can be
used. The maintenance service quality is one of the factors affecting safety and
operational costs.
6. Conclusions
Despite their deceptive “harmless appearance”, combustible dusts have a huge
destructive power. Design, installation and maintenance on electrical installations at
locations where explosive atmospheres can occur, need to be performed strictly under the
technical and legal requirements, by skilled personnel.
Workers are the "first line of defense" in preventing and mitigating fires and
explosions. If the people closest to the source of the hazard are trained to recognize and
prevent hazards associated with combustible dust in the plant, they can recognize unsafe
conditions in a quick way, and take the appropriate measures.
A regular inspection program applied to electrical installations is an important tool to
prevent undesirable sources of ignitions that can lead to dust explosions.
7. References
1.
Murdoch, Peter - Inspections of installations in hazardous areas adds value to safety and
maintenance strategies. In: II Hazardex, Coventry, UK, 2003.
2.
Rangel Jr., Estellito, Nägle, Rainer - Dust explosion protection in Brazil. Ex Magazine,
Waldenburg, Germany. R. Stahl Schaltgeräte GmbH, August 2006.
3.
NFPA 654: Standard for the Prevention of Fire and Dust Explosions from the Manufacturing,
Processing, and Handling of Combustible Particulate Solids. NFPA, Quincy, MA, USA.
4.
Rangel Jr., Estellito - The importance of inspections in classified areas, Vector Magazine,
Johannesburg, South Africa. EE Publishers, October 2005.
5.
Eckoff, R.K. - Dust explosions in the process industries. Butterworth Heinemann, 1997.
6.
Rangel Jr., Estellito - Handling changes from Divisions to Zones, IEEE IAS Applications
Magazine, Piscataway, USA. IEEE, May 2004.
7.
Schoeff, R.W. - Case study of dust explosion at DeBruce Grain Co. terminal elevator. In: III
Worldwide Seminar on the Explosion Phenomenon and on the Application of Explosion Protection
Techniques in Practice. Ghent, Belgium, 1999.
8.
IEC 61241-17 - Electrical apparatus for use in the presence of combustible dust Part 17:
Inspection and maintenance of electrical installations in hazardous areas (other than mines). IEC, Geneva,
Switzerland.
9.
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. Guararema, Brazil, 2003.