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Secondary Dust Explosions: How to Prevent them
or Mitigate their Effects?
Je´roˆme Taveau
Institut de Radioprotection et de Suˆrete´ Nucle´aire, IRSN/DSU/SERIC/BEXI, 31, avenue de la Division Leclerc,
92 260 FONTENAY AUX ROSES cedex, France; [email protected] (for correspondence)
Published online 8 August 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/prs.10478
Dust explosions are frequent and particularly devastating
in the process industries. Secondary dust explosions are the
most severe ones, and occur when the blast wave from a primary explosion entrains dust layers already present in the
plant, creating a large dust-air flammable mixture ignited by
the first explosion. As the blast wave propagates through the
plant, dust fuels the emerging flame, leading to extensive
damage owing to the large quantity of dusts involved and the
consequent strong pressure wave.
Several cases of secondary dust explosions have been analyzed by Eckhoff. Major accidents have also occurred in the
US in recent years, causing the Chemical Safety Board (CSB)
to produce a specific report highlighting the increasing number of dust explosions. An illustration of this point is the massive explosion that occurred on 7 February 2008 at the Imperial Sugar Company in Port Wentworth (Georgia), causing
14 fatalities and injuring 36 people.
The consequences of secondary dust explosions can be disastrous. However, only a minor initiating ‘‘primary’’ explosion
can quickly develop into a major secondary explosion if
appropriate measures have not been taken in advance.
This article describes several secondary dust explosion accidents that occurred in France and in the US and present some
practical solutions to prevent or mitigate these accidents. Ó 2011
American Institute of Chemical Engineers Process Saf Prog 31:
36–50, 2012
Keywords: dust; explosion; secondary; Imperial Sugar; prevention; protection; best practices
INTRODUCTION
Dust explosions are frequent in the process industries, if
we compare to other major accidents, and particularly devastating. Various authors [1,2] have listed many cases in developed countries. In 2006, the Chemical Safety Board [3] produced a specific report that highlighted the increasing number
of dust explosions in US facilities. Indeed, Chemical Safety
Board (CSB) identified an average of 10 dust explosion incidents per year from 1980 to 2005, corresponding to five fatalities and 29 injuries per year (Figure 1). Year 2003 was particularly catastrophic since three massive dust explosions occurred
(West Pharmaceutical [4], CTA Acoustics [5], and Hayes Lemmerz [6]), resulting in a total of 14 deaths and 81 injuries.
Many dusts are combustible and therefore represent a
potential explosion hazard if they are both airborne and
Ó 2011 American Institute of Chemical Engineers
36
March 2012
exposed to an ignition source. A damaging dust explosion
can occur if the following conditions are fulfilled (Figure 2):
• An oxidant (oxygen of the air) must be present;
• The dust must be combustible and airborne, and its concentration in the air must be within the explosible range;
• An ignition source must be present (ignition source must
be efficient, i.e., of sufficient energy and duration to ignite
a dust cloud);
• Finally, the environment must be sufficiently confined (or
congested) to produce significant explosion overpressures.
According to Zalosh et al. [8], secondary dust explosions are the most severe ones: ‘‘Perhaps the most devastating
dust explosion scenario is the generation of a secondary dust
explosion in the building surrounding the equipment in which
a primary explosion takes place. The secondary explosion
occurs when the blast wave emanating from the ruptured
equipment or conveyor lifts the accumulated dust into suspension, and the flame from the primary explosion subsequently ignites the suspended dust cloud. The resulting devastation and casualties are associated both with the burning of
building occupants and with the structural damage to the
building.’’ Figure 3 from Eckhoff [1] shows the main mechanisms involved in a secondary dust explosion.
Numerous secondary dust explosions have occurred
over more than 150 years. This article reviews selected
cases which occurred in France and USA in recent years
and attempts to focus on the recurring factors. Also, a
review of practical prevention and mitigation solutions is
given, based on recognized and generally accepted good
engineering practices, coming from standards and professional guidelines.
FOCUS ON TWO FRENCH PAST ACCIDENTS INVOLVING
SECONDARY DUST EXPLOSIONS
Metz, France, 1982 (12 Killed, two Injured)
Site Description
The malt house silo was located on the Moselle River, near
Metz (east of France). Built in 1973–1974, this facility was
dedicated to the reception of barley shipments by rail, road,
and boat. This process was designed to transform barley into
malt for breweries.
Built in reinforced concrete, the silo was composed of 14
vertical cylinders (7 m in diameter, 43 m in height) clustered
into three rows and supplied by conveyor belts from a 62-m
high handling tower. The handling tower contained cleaning
machines, scales, presses, vacuum pumps, and bucket elevators. Only a portion of the installation had been set up with a
Process Safety Progress (Vol.31, No.1)
Figure 1. Dust incidents, injuries, and fatalities in the USA
(1980–2005) [3]. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
Figure 2. Dust explosion pentagon [7]. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
dust removal system leading to a single plenum chamber. The
facility did not have proper explosion venting to release excess
pressure in the event of an explosion.
The Accident
The explosion occurred on October 18, 1982 at 2:15 pm.
At this moment, several employees and subcontractors were
working in the facility: seven operators in the handling tower
(four operators installing dust removal ducts and three operators repairing slabs), two employees in the elevator pit
(cleaning), one employee in the control room and three drivers [9]. Witnesses reported two successive explosions a ‘‘few
seconds’’ apart, the second more powerful than the first.
Given the type of operations in progress just before the
accident, the investigation concluded that an initial explosion
occurred in the handling tower, generated by the combination
of an ignition source introduced during the works, or a
smoker, with an explosive atmosphere. This primary event
caused dust to spread inside the facility, leading to the second
explosion throughout the tower, the upper gallery and spaces
between cells.
Process Safety Progress (Vol.31, No.1)
Figure 3. Mechanisms involved in a secondary dust explosion [1].
Figure 4. View of the damage caused by the Metz explosion
[9].
Consequences
Twelve people were killed by the blast and the resulting
projectiles. Two other people were slightly injured: one in an
adjacent workshop and the other inside the malt house enclosure.
Damage was bordered on the facility and its surroundings.
The tower fell on top of the rail spur leading to the malt
house, and eight of the 14 cells were completely destroyed
(Figures 4 and 5). An estimate of damage caused by the accident amounted to 16 million dollars.
Lessons Learned
Beyond the inadequacy of technical devices (insufficient
dust removal system and lack of explosion venting), this disaster highlights the fundamental importance of human and
organizational factors relative to the execution of hot works.
Hot work is a dangerous operation and needs to be performed safely according to written procedures, especially in
facilities handling powders and bulk solids. Communication
and coordination between operators is crucial when working
in hazardous processes.
Published on behalf of the AIChE
DOI 10.1002/prs
March 2012 37
Figure 5. View of the malt house silo before and after the explosion [9]. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
Figure 6. Schematic depiction of the supposed explosion
path [10].
The lack of written safety procedures, hot work permits,
and, more generally, the absence of a risk analysis and subsequent prevention/monitoring measures all constitute anomalies
that induced this accident.
This accident led to the first revision of the silo regulation
in France in 1983.
Figure 7. View of the damage caused by the Blaye explosion
[10]. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
conveyor to provide a connection between the vertical silo
and an adjoining hangar. The aboveground under-cell space
contained 10 chain reclaim conveyors and an air-blowing
unit. Silo dust removal was performed by a centralized air
suction network set up at several points along the cereal circuit, using a fan positioned in the upper part of the northern
handling tower.
Blaye, France, 1997 (11 Killed, one Injured)
Site Description
The Accident
The cereal silo was located at the Blaye port complex, on the
right bank of the Gironde Estuary, near Bordeaux (south-west of
France). It offered a total capacity of 130,000 tons of cereals. The
facility was dedicated to handling and storing cereals for maritime
export.
The silo, built in two segments in 1970 and 1974, was
composed of 44 circular reinforced concrete cells, arranged in
three rows for a total capacity of 47,240 m3. Two handling
towers, one containing bucket elevators and a centralized
dust removal circuit (northern tower), and another housing a
grading machine and two grain cleaners-separators (southern
tower), were connected by an 80-m long, concrete-walled gallery running over the cells and housing conveyor belts. The
northern tower was connected directly to both the over-cell
gallery and the under-cell space. The over-cell gallery primarily contained three conveyor belts and a material-handling
The explosion occurred on August 20, 1997 around 10:15
am, while a dumper truck was unloading corn into a delivery
pit. Witnesses reported that the first explosion occurred in the
northern handling tower before propagating into the over-cell
gallery up to the southern end of this gallery.
As no component parts of the centralized dust removal
system were found, it is plausible that the explosion began
inside.
The investigation also led to point out sources internal to
the dust removal circuit (mechanical impacts, friction in the fan,
or fire caused by self-heating in the dust chamber) as plausible
ignition sources. The break downstream of the dust suction fan
could have been responsible for the spreading of a large quantity of dust in the northern handling tower. Then the explosion
spread to the under-cell part (Figure 6), most likely via the hoppers positioned at the intersection of the two building seg-
38
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Published on behalf of the AIChE
Process Safety Progress (Vol.31, No.1)
Figure 8. Propagation of the explosion in the DeBruce grain elevator [17]. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
ments, causing a more violent explosion because of their elongated shape.
Consequences
Eleven people were killed by the explosion (seven
employees, three subcontractors, and a fisherman) and one
was seriously injured. Ten of the victims were found in the
administrative and technical premises, apparently unable to
react. The eleventh victim, the fisherman, was found buried
underneath rubble on the Gironde riverbank side [10,11].
The vertical silo collapsed over its central and northern
parts. Only 16 of the 44 cells were still intact after the accident (Figure 7). The northern handling tower, as well as the
immediately adjacent cells, was almost entirely destroyed. The
over-cell gallery was totally destroyed; an air extraction fan
was found 30 m farther away from the silo.
Many projectiles hit nearby storage tanks. Damage to residences was noticed within a 500 m radius away from the silo.
Large projectiles (metal, concrete, or glass) were observed at
distances of up to a hundred meters farther away from the
silo.
Lessons Learned
This accident shows how much large concrete silos without explosion venting and separation walls represent a serious
hazard in case of a dust explosion, as excess of pressure cannot be released in the environment and the resulting secondary dust explosion can easily propagate through the entire
plant, causing widespread damage and also fatalities and injuries outside the facility.
For this type of old confined facilities, strong prevention
measures must be taken (see Best practices section): in particular, an effective housekeeping program is absolutely crucial
in order not to fuel the ‘‘primary’’ explosion and lead to a disastrous secondary explosion.
The Blaye accident also emphases hazards posed by dust
removal systems: as dusts are concentrated at one location,
the risk of dust explosion is high and strong safety measures
Process Safety Progress (Vol.31, No.1)
must be taken, such as prevention of ignition sources (e.g.,
grounding and bonding to avoid build-up of electrostatic
charges) and explosion mitigation (isolation, suppression,
and/or venting).
This accident resulted in the updating of the regulations dating from 1983 (after the Metz accident) and the proposal of
many safety improvements [11], among them the reinforced
monitoring of centralized dust removal systems, and also the
increase of separation distances between silos and administrative premises.
Evolution of the French Regulatory Framework for
Facilities Handling Powders and Bulk Solids
A new prescriptive regulation framework was set up in
France just after the Blaye accident [12]. Nevertheless, these
new safety prescriptions were quite strict and often inapplicable to small and medium-scale agricultural facilities.
Regarding the difficulties encountered for the application of
this regulation framework, a working group was set up by the
French Ministry of the Environment, with the strong involvement
of companies and technical experts. It led to a new decree asking
for a performance-based approach1 [15] and a best practices guide
[16].
The best practices guide was designed to give practical
advice to deal with fire and explosion hazards in facilities
handling powders and bulk solids (such as: design of buildings and equipments, layout, prevention of explosive dust
clouds and ignition sources, housekeeping, detection of
abnormal conditions, means of protection, and safety organization). This guide is continuously updated by the working
group regarding the state of the art.
Also high-risk facilities handling powders and bulk solids
are regularly inspected and safety reports may be peerreviewed by independent bodies to ensure that the level of
protection is appropriate.
1
This decree was slightly modified in February 2007 [13] to take into
account the changes introduced by the new approaches of land-use planning
and risk analysis in safety reports after the Toulouse accident [14].
Published on behalf of the AIChE
DOI 10.1002/prs
March 2012 39
Figure 9. Postulated ignition source of the DeBruce grain elevator explosion [17]. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
east side, and used to store maintenance equipment and supplies. Eight warehouse structures were located on the east
side of the northern portion of the structure. Dust collection
in the facility consisted of two cyclone-type collectors at
ground level and a bag-type collector located on top of the
truck trailer loading station. A collection bin was located adjacent to the bag collector near the truck station. A dust collector was located on the roof of the headhouse.
The Accident
Figure 10. Dust accumulation in the DeBruce grain elevator
[17]. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FOCUS ON SOME US PAST ACCIDENTS INVOLVING
SECONDARY DUST EXPLOSIONS
Haysville, United States of America,
1998 (seven Killed, 10 Injured)
Site Description
Reported in Guinness Book of Records as the world’s largest grain elevator, the DeBruce Grain elevator was located at
Haysville, 4 miles southwest of Wichita, Kansas. It was constructed in 1953 and consisted of a total of 246 concrete silos,
each measuring 9.1 m in diameter and 36.6 m in height, for a
capacity of 2,464 m3 of grain. The total capacity of the facility
was 739,200 m3, for a total length of 823 m.
The reinforced concrete headhouse structure located in the
center of the facility stood 65.2 m high, and contained four
bucket elevators for transporting product to the upper levels
of the facility. The conveyor system at this facility consisted of
four independent belts (two north and two south). The loading areas consisted of a rail siding on the west side of the facility and a truck loading area on the east side. A building
was located alongside the southern half of the facility on the
40
March 2012
Published on behalf of the AIChE
The explosion occurred on June 8, 1998, at approximately
9:20 a.m. At the time of the incident, 27 employees, contractors, and drivers were on-site. Witnesses reported hearing several small explosions and then the large blast that seemed to
come from the headhouse.
The investigation concluded that the initial explosion
occurred when dust was ignited in the east tunnel of the
south array of silos. Figure 8 shows the propagation of the initial explosion to the overall facility.
The most probable ignition source was created when a
concentrator roller bearing, which had seized due to no lubrication, caused the roller to lock into a static position as the
conveyor belt continued to roll over it, wearing it, and leading
to a high temperature rise (Figure 9). Dust accumulation was
also pointed out (Figure 10) [17–19].
Consequences
Three employees and four subcontractors were killed,
whereas three other employees, five contractors and two visitors were injured [19] (Figure 11). The headhouse suffered
severe structural damage on all levels (Figure 12). Bucket elevators suffered extensive damage. Several silos suffered major
structural damage but did not collapse. Six silos in the north
section of the facility had large portions blown out in the
blasts, causing the contents of the silos to spill outside the
structure. Many silo tops were blown off or displaced during
the blast (Figure 13).
Lessons Learned
This accident is again an example of potential hazards
posed by large concrete silos, particularly with underground
galleries that cannot be equipped with venting panels: when a
dust explosion propagates inside this kind of elongated enclosure, resulting overpressures are quite severe and may lead to
extensive damage both inside and outside the facility. In this
DOI 10.1002/prs
Process Safety Progress (Vol.31, No.1)
Figure 11. View of the DeBruce grain elevator after the explosion [17]. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
Figure 13. View of the northern gallery after the explosion
[17]. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
ards were not sufficiently taken into account by the management of the DeBruce elevator, whereas the facility was daily
invaded by dust leaking from the process conveyors2.
Port Wentworth, United States of America,
2008 (14 Killed, 36 Injured)
A short description of the facility and the accident is given
here [20]. More details are available in a recent article published in the Process Safety Progress journal [21].
Figure 12. View of the damage on East side of headhouse
[17]. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
case, the use of blast proof walls could be advised to avoid
explosion propagation to the overall facility, especially between
handling towers and galleries.
There were also ineffective housekeeping and no detection
of conveyor malfunction (hot bearings).
The investigation performed by the Grain Elevator Explosion Investigation Team showed that the dust explosion hazProcess Safety Progress (Vol.31, No.1)
Site Description
The Imperial Sugar Port Wentworth facility was built in the
early 1900’s. At the beginning, the facility produced granulated
sugar; over the years, the facility added refining and packaging
capacity, raw sugar, and product warehouses. It was one of
the largest sugar refining and packaging facilities in the United
States.
Refined sugar was stored in three concrete silos (12 m in diameter, 30 m in height) fed by two belt conveyors and then
transferred from silos 1 and 2 by a steel conveyor belt
(enclosed by a stainless steel frame but not equipped with dust
removal system or explosion vents) to different process areas:
2
‘‘Some workers testified that on some occasions you could not see your
hand in front of your face at arm length’’ [17].
Published on behalf of the AIChE
DOI 10.1002/prs
March 2012 41
Figure 14. Dust accumulation in Imperial Sugar Port Wentworth facility before the accident [20]. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
The explosion was fueled by massive accumulations of
combustible sugar dust throughout the packaging building
(Figure 14).
Consequences
This accident caused 14 deaths and injured 38 others,
including 14 with serious burns. Buildings were heavily damaged (Figure 15).
Lessons Learned
Figure 15. View of the damage to the Imperial Sugar Port
Wentworth facility [20]. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
bulk sugar truck and train loading area, south packing and
‘‘Bosch’’ buildings, and powdered sugar production equipment.
A complex system of screw conveyors, bucket elevators, and
horizontal conveyor belts transported granulated sugar throughout the packing buildings. Packaged products were palletized
and transferred to a warehouse for distribution to customers.
The Accident
Shortly before 7:15 p.m. on February 7, 2008, a massive
explosion shattered the packing buildings and silos. The
explosion propagated through packing and palletizer buildings and ignited fires in the refinery and bulk sugar building,
tens of meters from the packing buildings where the incident
begun. Fireballs erupted from the facility for more than 15
min [20].
CSB concluded that the primary dust explosion most likely
occurred in the middle of the silo tunnel: a failure caused the
granulated sugar to block the outlet under silos 1 and 2, and
then to spill off the moving steel conveyor. The sugar dust
accumulated above the minimum explosible concentration
and was ignited, probably by a hot bearing inside the
enclosed steel belt conveyor.
42
March 2012
Published on behalf of the AIChE
Comparing to the other accidents, this one did not involve
a large concrete silo. Nevertheless, strong secondary dust
explosion still occurred, leading to the death of 14 people
and also to widespread damage. The enclosure installed on
the steel conveyor created a hazardous confined area and led
to this disaster.
This accident points out the need of an effective operating
feedback and hazards awareness: indeed, although the facility
experienced several small fires and explosions in the past and
managers were aware of sugar dust explosion hazards, no
action was taken to minimize and control these hazards; operators worked in unsafe conditions but did realize, as it was
the case for years: it underlines the issue of a very high tolerance to the risks or so-called ‘‘normalization of deviance’’:
when you work in a dangerous environment everyday, you
may loose your ability to detect hazardous conditions and
hence take actions to mitigate the risks.
This accident also addresses the difficulties associated with
the management of change, as the installation of an enclosure
on the steel conveyor belt was not identified as a potential
hazard that could lead to an explosion. Once again, housekeeping was not done properly: CSB found that the explosion
would likely not have occurred if routine housekeeping was
enforced.
Other US Accidents Involving Secondary
Dust Explosions
Plenty of other cases are unfortunately available in the
open literature: they show that secondary dust explosions do
not only occur in the grain and sugar industries, as could be
deducted from the examples developed in this article. As
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Process Safety Progress (Vol.31, No.1)
Figure 16. West Pharmaceutical facility destroyed by polyethylene dust explosion [3]. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
Figure 17. CTA Acoustics’ production area after resin dust
explosions [3]. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
reported by CSB [3] and Frank [22], many cases were also
noticed in the manufacturing (paper, plastic, rubber, textile,
and metal), energy (coal, peat, and wood), and chemical (acetate, pharmaceuticals, dyes, and pesticides) industries:
• Malden Mills Industries, Methuen, Massachusetts, Decem-
•
•
•
•
ber 11, 1995 (37 injured): the initial event was likely a dust
explosion involving nylon flock fibers. Managers and
employees did not understand that the fibers were an
explosion hazard before the disaster. This accident shows
that dust explosion hazards identification is a critical step,
as you cannot protect your facility from a risk you do not
know. It is the starting point of a risk analysis that enables
to define appropriate prevention and mitigation measures
to reduce the risk;
Ford Motor Company, Dearborn, Michigan, February 1,
1999 (six killed, 36 injured): although the initial event was
a natural gas explosion, strong evidence was found of a
secondary coal dust explosion, as coal dust accumulated
on horizontal surfaces in the powerhouse. Facility design
(presence of flat surfaces) and housekeeping program
were found to be inadequate by the Michigan Occupational Safety and Health Administration (MIOSHA);
Jahn Foundry, Springfield, Massachusetts, February 25,
1999 (three killed, nine injured): the incident likely
involved a resin dust explosion in the shell mold area,
spreading through the building due to heavy resin deposits. The underlying causes of the explosion included inadequate design and maintenance of the gas burner system
and inadequate housekeeping again;
Rouse Polymerics International, Vicksburg, Mississippi, May
16, 2002 (five killed, seven injured): investigation found that
hot rubber entrained in the exhaust from product dryers fell
onto the building roof, igniting a fire that was pulled into a
product bagging bin. A primary fire occurred inside the bagging bin and spread to a screw conveyer, igniting a secondary dust explosion. Occupational Safety and Health Administration (OSHA) pointed out the inadequacy of gas-fired dryers and dust collector design and procedures, housekeeping,
the lack of awareness of dust explosion hazards and the ineffective management of change;
West Pharmaceuticals (Figure 16), Kinston, North Carolina,
January 29, 2003 (six killed, dozens injured): polyethylene
dust above a suspended ceiling was suspended in air and
ignited, causing a severe secondary dust explosion. CSB
pointed out that dust explosion hazard was not addressed
during the reviews and electrical equipment was inad-
Process Safety Progress (Vol.31, No.1)
Figure 18. Fire following aluminum dust explosion at Hayes
Lemmerz International [3]. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
equate. Once again, this accident highlights the lack of
dust explosion hazards recognition and the lack of proper
risk analysis, as operators and managers were not aware
that fine polyethylene dust could be formed as the rubber
dried. Also, it points out the practical difficulties to perform
an efficient housekeeping, as the dust accumulated above
a drop ceiling, so it was hidden from view. New facilities
should be designed for easy effective cleaning and avoidance of elevated flat surfaces (see NFPA 654), where it is
difficult for operators to capture dust or even see it;
• CTA Acoustics (Figure 17), Corbin, Kentucky, February 20,
2003 (7 killed, 37 injured): curing oven left open likely
caused ignition of combustible resin dust stirred up by
workers cleaning the area near the oven, causing multiple
explosions. This accident hightlights the lack of communication and coordination between operators working at the
same hazardous place. Facility design was not adequate
(presence of flat surfaces, no firewalls between process
units), housekeeping was not sufficient and inadequately
done using compressed air;
Published on behalf of the AIChE
DOI 10.1002/prs
March 2012 43
Table 1. Measures for the prevention of explosive dust clouds.
Functional Requirements
Measures
Minimize dust cloud formation
Minimize dust explosibility
Minimize dust layers formation
Grain cleaning before storage
Oil adding systems
Use of lower mass flow rates [16]
Prevention of process leakages: sealing and dust removal (NFPA 654)
Reduction of dust emission at chutes (Holbrow et al. [26], Wheeler et al. [27])
Collect of dust leaks (capture hoods, ‘‘elephant trunks’’)
Use of bigger particle size
Control that dust concentration is below the Minimum Explosive Concentration (NFPA
654 [24])
Inerting (NFPA 654, EN 15281 [25])
Avoidance of elevated flat surfaces (NFPA 654, [16])
Implementation of a good housekeeping program (NFPA 654)
Figure 19. Enclosed conveyor [28]. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
• Hayes Lemmerz International (Figure 18), Huntington, Indiana, October 29, 2003 (one killed, several injured): CSB
found that explosion likely originated in a dust collector and
spread through ducting, causing a large fireball to emerge
from the furnace. Dust collector was not adequately vented
or isolated from other parts of the process. Also housekeeping was not implemented and no lessons were learned from
previous accidents.
BEST PRACTICES TO PREVENT SECONDARY DUST
EXPLOSIONS OR MITIGATE THEIR EFFECTS
Safety measures can be classified in three main classes
[23]:
• prevention of explosive dust cloud;
• prevention of ignition sources;
• explosion mitigation.
Figure 20. Dust removal system [28]. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Prevention of Explosive Dust Clouds
Table 1 presents some practical solutions to prevent explosive dust clouds; as examples:
reduction of mass flow rates of conveyor systems and
use of special devices at chutes can greatly minimize
dust cloud formation;
use of bigger particle size can sharply minimize dust
explosibility and explosion strength (also big particles
are less easily airbone and remain a shorter time in the
air than small ones). Decrease of oxygen concentration
(i.e. inerting) is effective as well theoretically, but
remains difficult to achieve practically (only for particular processes and small volumes).
The following paragraphs give an overview of current prevention and protection techniques that could be used against
dust explosions, based on recognized and generally accepted
good engineering practices, coming from standards and professional guidelines, and develop those which were particularly highlighted by the reviewed accidents.
Also, as secondary dust explosions cannot develop if there
is no fuel (i.e., dust layers), important prevention measures
consist of prevention of leakages and a good housekeeping
program.
Equipments should be dust-tight (Figure 19) and equipped
with a dust removal system (Figure 20) when an explosive
dust cloud can form inside (it is particularly true for bucket
elevators, conveyor systems and at transfer/discharge points),
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Process Safety Progress (Vol.31, No.1)
Figure 21. The potential hazards of thin dust layers [1].
Figure 23. When disappearing, white crosses indicates significant dust accumulation. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
Figure 22. Large-scale gallery used by Tamanini [29].
• to limit the extent of dust migration, and hence the size of
the room that must be cleaned;
to avoid a dust explosion inside the equipment, like what
occurred in the steel conveyor belt of the Imperial Sugar Port
Wentworth facility. Also, operators must check as frequently
as possible if there are no fugitive sources in the facility and
take actions to avoid them.
As leakages cannot be completely eradicated, housekeeping is very important. According to Eckhoff [1], even a
1-mm layer of a dust of bulk density 500 kg/m3 on the
floor of a 5-m high room may generate a cloud of average
concentration 100 g/m3 if dispersed all over the room, or a
cloud of average concentration 500 g/m3 if partially dispersed up to 1 m above the floor (Figure 21).
Moreover, Tamanini [29] carried out a series of cornstarch
explosion tests in a full-scale gallery (2.4 m height, 2.4 m
width, and 24.4 m long) and showed that a flame only needs
a very small amount of dust (77 g/m3 for a smooth, unobstructed gallery, corresponding to a layer of cornstarch 1/100
inch thick) to propagate, as the dust can be dispersed only in
the lower part of a volume and therefore gives higher explosive dust concentrations (Figure 22).
These experiments highlight the need of good housekeeping practices.
Frank and Holcomb [30] provided some general guidance
to deal with dust leakages and dust accumulation, which are
reproduced here:
• to design, maintenance, and operate equipments to minimize dust emissions;
• to capture dust at the release point;
Process Safety Progress (Vol.31, No.1)
• to design facilities for easy effective cleaning (no flat elevated surfaces, see for example Ford Motor Company,
West Pharmaceutical and lastly CTA Acoustics accidents);
• to establish and enforce housekeeping (see the Imperial
Sugar Port Wentworth example), by defining schedules
and responsibilities;
• to ensure that housekeeping programs comprehensively
address all areas where combustible dust may accumulate;
• to ensure that housekeeping is safely conducted.
Very simple indicators, such as white crosses on the floor
(Figure 23), can be used to detect dust accumulation. Also if
you can write your name in the accumulated dust, a combustible dust hazard is likely to be present.
Prevention of Ignition Sources
Table 2 presents some practical solutions to prevent ignition sources.
PR EN 1127-1 [38] distinguishes 13 different types of ignition sources. According to Figure 24, mechanically generated
sparks (25%), smoldering fires (11%), mechanical heating and
friction (9%), and electrostatic charges (9%) are the most
likely in the process industries.
By reviewing the four major accidents presented in the first
sections, conveyors (hot bearings, i.e. mechanical heating)
were implied in two of them (DeBruce grain elevator and Imperial Sugar Port Wentworth facility explosions), whereas dust
removal systems (mechanically generated sparks) were
involved in the two others (Metz and Blaye explosions). This
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Table 2. Measures for the prevention of ignition sources.
Functional Requirements
Avoid/control
ignition sources
Measures
Mechanical and electrical equipments adapted to hazardous locations (NFPA 499 [31], NFPA
70 [32], ATEX European Directives [33,34])
Limitation of the use of low resistivity materials; bonding and grounding
(NFPA 77 [35])
Maintenance of equipments to avoid friction, hot surfaces (NFPA 654, [16])
Detection of conveying equipments and dust removal systems malfunction [16]
Control of hot surfaces (engine protection, thermal insulation) [16]
Delivery pits covered with grates with small openings [16]
Use of magnetic separators [16]
Detection of hot particles [16]
Lightning protection (NFPA 780 [36])
Fire prevention and protection (fireproof belts, fire detectors, extinguishers) [16]
Hot work permit for welding, cutting, hot tapping (NFPA 654, NFPA 51B [37])
No smoking [16]
Figure 24. Type of ignition sources involved in dust explosions in the Federal Republic of Germany (1965-1985, 426 dust explosions in total) [1]. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Figure 25. Type of plant/equipment involved in dust explosions in the Federal Republic of Germany (1965-1985, 426 dust
explosions in total) [1]. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
tendency is also well illustrated by Figure 25, which shows that
dust collecting systems are involved in 17% of dust explosions
(10% for conveying systems).
It emphasizes the need to correctly monitor the operation of critical equipments (Figures 26 and 27 give examples of control devices that can be used for a belt con-
46
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March 2012
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Process Safety Progress (Vol.31, No.1)
Figure 26. Belt displacement control device [27]. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Figure 27. Rotation control device [27]. [Color figure can be
viewed in the online issue, which is available at
wileyonlinelibrary.com.]
Table 3. Measures for explosion mitigation.
Functional Requirements
Limit the consequences of
a primary explosion
Avoid the propagation of the
primary explosion
Limit the consequences of a
secondary explosion
Measures
Venting (NFPA 68 [39], EN 14491 [40], EN 14797 [41]): bursting disks, venting
panels, vent ducts, explosion doors, light construction [16]
Explosion extinguishing systems: powder, water (NFPA 69 [42], EN 14373 [43])
Explosion resistant equipment (NFPA 69, EN 14460 [44])
Explosion isolation between vessels (NFPA 69, EN 15089 [45]): acting valve (PR EN
16009 [46]), diverter (PR EN 16020 [47]), rotary lock (Siwek [48]), screw
conveyor, triggered barriers (Lebecki et al. [49])
Flame arresters (PR EN ISO 16852 [50])
Layout/unit segregation (NFPA 654, [16])
Explosion isolation between parts of a building (blast proof walls) [16]
Venting [28]
veyor). Process interlock should be used to forbid
running operations when safety devices are not operational.
Also attention must be paid to preventative maintenance (lubrification, infrared thermography) and hot work
permit.
Explosion Mitigation
Table 3 presents some practical solutions to mitigate explosions, such as:
using venting techniques (release of the excess of pressure in the environment), extinguishing systems or
explosion resistant equipments to limit the consequences
of a primary explosion;
installing explosion isolation systems (valves, rotary
locks. . .) and flame arresters to avoid the propagation
of the primary explosion.
Also, since severe dust explosions are often characterized by
flame propagation through elongated volumes, the use of blast
proof walls could be advised to avoid explosion propagation to
the overall facility, especially between handling towers and galleries (see Metz, Blaye, and DeBruce explosion cases), as
shown on Figure 28.
Process Safety Progress (Vol.31, No.1)
The use of open or light constructions (i.e. the avoidance
of reinforced concrete and underground structures), as well as
the siting of equipments outside the buildings (particularly for
bucket elevators), can limit the resulting damage in case of
explosion.
Also, Tamanini [28] showed that venting a primary explosion in a gallery could be effective at reducing explosion
overpressure and sometimes preventing flame propagation.
Nevertheless, as it is not always true, this technique must be
associated with blast proof walls or triggered barriers to be
effective.
Finally, it is indispensable to move away administrative
and technical premises from hazardous locations: indeed, in
the Blaye accident, most of the victims were in the administrative premises.
Example of Prevention and Mitigation Principles
Applied to a Dust Collector
Review of accidents, and also statistical data (Figure 25),
showed that dust collectors are among the most hazardous
equipments in a facility handling powders or bulk solids. So
strong prevention and protection measures must be taken to
avoid a primary dust explosion in this kind of equiment, and/
or to avoid its propagation to the overall plant.
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Bonding and grounding should be considered to reduce
the hazards of static electricity accumulation. Also IR detectors
(1) should be installed so that hot particles cannot be sucked
up by the dust collector and then provoke a fire and/or an
explosion.
Regarding explosion protection, several measures could be
applied to a dust collector inside a building (Table 4, Figure
29):
venting panel(s) (2) and/or extinguishing system(s) (3)
should be installed to limit the maximum pressure inside
the equipment in case of explosion;
also, dust collector must be isolated from other process
equipments in order to avoid propagation of a primary
dust explosion. This could be achieved by installing a
rotary lock (4) on ducting between the collector and the
associated dust storage chamber, and an acting valve (5)
on ducting between the collector and the process equipment (activated by pressure sensors for example).
Figure 28. Blast proof walls between a gallery and a handling
tower [27]. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
Dust collector should also be put in an area apart from the
rest of the facility or separated by fire/blast walls to avoid
escalation events in case of an accident.
After that attention must be paid on the protection of the
dust collector itself with an extinguishing system and a venting panel.
Of course, prevention is also highly important and all prevention measures that are applicable must be considered: the
most important are detection of hot particles by IR detectors
(1), bonding and grounding.
CONCLUSIONS
Secondary dust explosions can have dramatic consequences, as shown from past accidents reviewed in this article.
Numerous techniques exist to prevent secondary dust explosions or mitigate their effects; the most critical ones have
been presented which are, from the author’s point of view:
prevention of leakages, housekeeping, design and monitoring
of equipments, separation of large volumes using blast proof
walls, venting, and appropriate layout.
Nevertheless, technical measures are not sufficient as major
accidents are always a combination of technical and organizational failures. As a proof, reviewed accidents showed the
preponderant rule of operators and managers in dust explosion occurrence.
The development of a company’s safety culture is a key
issue: operators must understand the risks related to the handling of powders and bulk solids to adapt their behavior at
workplace and be involved in early detection of process deviances. They must as well be trained to properly react in case of
an incident (see Imperial Sugar example). The reviewed accidents also emphasize the crucial importance of a serious
involvement of the management to take action.
When working in a hazardous plant, maybe the highest
risk is the ‘‘normalization of deviance’’: when you work in a
dangerous environment everyday, you may loose your ability
to detect hazardous conditions and hence take actions to mitigate the risks. Dust explosions that occured in the past
remind us that disasters can quickly occur if companies do
not pay enough attention and accept abnormal and unsafe
work conditions.
As an example, in France, a performance-based regulation
framework has been set up after the Blaye accident in 1997,
to give more flexibility to companies to adapt safety measures
to their own type of activities. This framework was accompanied by a best practices guide written with the help of technical experts to provide practical advice to deal with fire and
explosion hazards in facilities handling powders and bulk solids. Also high-risk facilities handling powders and bulk solids
are regularly inspected and safety reports may be peerreviewed by independent bodies to ensure that the level of
protection is appropriate. This process seems to give fruitful
results, as from this time, the number of serious incidents has
significantly decreased.
Table 4. Example of main prevention and mitigation measures for a dust collector
Prevention Measures
Mitigation Measures
Bonding
Grounding
Preventive maintenance
Detection of hot particles by IR detectors (1)
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Venting panel with a venting duct and a flame arrester (2)
Extinguishing system with water or chemical powder (3)
Isolation with a rotary lock (4)
Isolation with an acting valve (5)
Unit segregation from other process area
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Figure 29. Illustration of prevention and mitigation measures
for a dust collector. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
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