1. travel
2. transports
3. road travel
4. road accident

GCPS 2011 __________________________________________________________________________
Secondary Dust Explosions:
How to Prevent them or Mitigate their Effects?
Jérôme TAVEAU
Process safety engineer, Fire and explosion specialist
Institute for Radiological Protection and Nuclear Safety
Plants, Laboratories, Transports and Waste Safety Division
Industrial Risks, Fire and Containment Assessment and Study Department
Fire and Explosion Study and Assessment Section
31 avenue de la Division Leclerc
92 260 FONTENAY AUX ROSES cedex, FRANCE
[email protected]
Prepared for Presentation at
American Institute of Chemical Engineers
2011 Spring Meeting
7th Global Congress on Process Safety
Chicago, Illinois
March 13-16, 2011
UNPUBLISHED
AIChE shall not be responsible for statements or opinions contained
in papers or printed in its publications
GCPS 2011 __________________________________________________________________________
Secondary Dust Explosions:
How to Prevent them or Mitigate their Effects?
Jérôme TAVEAU
Process safety engineer, Fire and explosion specialist
Institute for Radiological Protection and Nuclear Safety
Plants, Laboratories, Transports and Waste Safety Division
Industrial Risks, Fire and Containment Assessment and Study Department
Fire and Explosion Study and Assessment Section
31 avenue de la Division Leclerc
92 260 FONTENAY AUX ROSES cedex, FRANCE
[email protected]
Keywords: dust, explosion, secondary, Imperial Sugar, prevention, protection, best practices.
Abstract
Dust explosions are frequent and particularly devastating in the process industries, and secondary
dust explosions are the most severe ones.
A secondary dust explosion can occur when the blast wave from a primary explosion lifts dust
layers 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 explosions because of the large quantity of dusts involved and the very high energy
of ignition.
Several cases of secondary dust explosions were noticed and analyzed by Eckhoff [1]. Major
accidents have occurred in the US in recent years, conducting the Chemical Safety Board (CSB)
to produce a specific report that highlighted the increasing risk 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.
As consequences of secondary dust explosions could be dramatic, only a minor explosion
(beginning in an equipment for example) can quickly develop into a major secondary explosion
if some appropriate precautions are not taken.
This article aims to give an overview of several secondary dust explosion accidents and present
some practical solutions to prevent or mitigate these disasters.
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1. Introduction
Dust explosions are frequent and particularly devastating in the process industries: some 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 risk of dust explosions in US facilities.
CSB identified an average of ten dust explosion incidents per year from 1980 to 2005,
corresponding to five fatalities and twenty-nine injuries per year (Figure 1). Year 2003 was
particularly catastrophic since three massive dust explosions occurred (West Pharmaceutical [4],
CTA Acoustics [5], Hayes Lemmerz [6]), resulting in 14 deaths and 81 injuries.
Figure 1: Dust incidents, injuries, and fatalities in the USA (1980-2005) [3]
Many dusts are combustible and therefore can represent a potential explosion hazard if they are
airborne. A dust explosion can occur if the following conditions are fulfilled (Figure 2):
•
a comburant (oxygen of the air) must be present;
•
the dust must be combustible, 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 confined (or congested) to produce significant explosion
pressures.
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Figure 2: Dust explosion pentagon [7]
Secondary dust explosions are the most severe ones. According to Zalosh et al. [8]: “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 shows the mechanisms involved in a secondary dust explosion [1].
Figure 3: Mechanisms involved in a secondary dust explosion [1]
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Several secondary dust explosions occurred in the past: after reviewing some of them that
occurred in France and USA, a review of practical prevention and mitigation solutions will be
given, based on the current state of the art.
2. Focus on some past accidents involving secondary dust explosions
2.1 Metz, France, 1982 (12 killed, 1 injured) [9]
Site description
The malt house silo was located on the Moselle River. 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 3 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. A portion of the installation had been set up with a dust removal system leading
to a single plenum chamber. The facility did not have explosion venting area to release excess
pressure in the event of an explosion.
The accident (Figure 4)
The explosion occurred on October 18, 1982 at 2:15 pm. At this moment, several employees and
subcontractors were working in the facility: 7 operators in the tower (4 operators installing dust
removal ducts and 3 operators repairing slabs), 2 employees in the elevator pit (cleaning),
1 employee in the control room and 3 drivers.
Witnesses reported two successive explosions a “few seconds” apart, the second more powerful
than the first.
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.
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Figure 4: Dust explosion pentagon for the Metz accident
Consequences (Figures 5 and 6)
Twelve people were killed by the blast and the resulting projectiles. The last victim was finally
removed on October 23 at 5:45 pm. 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 8 of the 14 cells were completely destroyed. An estimate of
damage caused by the accident amounted to 11 million euros.
Lessons learned (Table 1)
The investigation concluded about the inadequacy of the technical devices (dust removal system,
no explosion venting) and the lack of company’s safety culture (ineffective maintenance, lack of
written safety procedures, no hot work permit, absence of a risk analysis, etc.).
This accident led to the first revision of the silo regulation in France.
Table 1. Main failures having conducted to the Metz accident
Technical failures
Ineffective dust removal system
No separation of large volumes (blast proof walls)
No explosion venting
Human and organizational failures
No managerial involvement
No adequate information to subcontractors
No good understanding of fire and explosion hazards
No risk analysis
No written safety procedures
No hot work permit system
Ineffective housekeeping
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Figure 5: View of the damage caused by the Metz explosion [9]
Figure 6: View of the malt house silo before and after the explosion [9]
2.2 Blaye, France, 1997 (11 killed, 1 injured) [10, 11]
Site description
The cereal silo was located at the Blaye port complex, on the right bank of the Gironde Estuary.
It offered a total capacity of 130,000 tons of cereals. The facility was dedicated to handle and
store cereals for maritime export.
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The silo, built in two segments in 1970 and 1974, was composed of 44 circular reinforced
concrete cells, arranged in 3 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 3 conveyor belts and a material handling conveyor to
provide a connection between the vertical silo and an adjoining hangar. The aboveground undercell 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.
The accident (Figures 7 and 8)
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.
The investigation led to point out sources internal to the dust removal circuit as plausible ignition
sources (Figure 7). 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 8), most likely via the hoppers positioned at the intersection
of the two building segments, causing a more violent explosion because of their elongated shape
(H/D > 10).
Figure 7: Dust explosion pentagon for the Blaye accident
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Figure 8: Schematic depiction of the supposed explosion path [10]
Consequences (Figure 9)
Eleven people were killed by the explosion (7 employees, 3 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 14 days after
the accident, buried underneath rubble on the Gironde riverbank side.
ҏThe vertical silo collapsed over its central and northern parts. Only 16 of the 44 cells were still
intact after the accident (Figure 9). The northern handling tower, as well as the immediately
adjacent cells, were 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.
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Figure 9: View of the damage caused by the Blaye explosion [10]
Lessons learned (Table 2)
The independent expert, mandated by the French Ministry of the Environment to investigate this
accident, concluded about the necessity to update the regulations dating from 1983 (after the
Metz accident) and proposed many safety improvements [11]:
•
prevention of explosive dust clouds:
¾ continuously check the suction efficiency of the centralized dust removal systems;
¾ isolate the various parts of the silo (tower, galleries, cells).
•
prevention of ignition sources:
¾ install stone and metallic objects removers;
¾ install spark detectors;
¾ install temperature detection on equipments.
•
explosion mitigation:
¾ separate the various parts of the silo (tower, galleries, cells) so as to limit
explosion propagation;
¾ install centralized dust collection systems in open air;
¾ lay out explosion vents;
¾ move away administrative and technical premises from silos.
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Table 2. Main failures having conducted to the Blaye accident
Technical failures
No control of the efficiency of dust removal
systems
No device to detect and collect foreign bodies
No fire/spark detectors
No separation of large volumes (blast proof walls)
No explosion venting
Human and organizational failures
Ineffective safety organization
No good understanding of dust hazards
Procedural shortfalls
Inappropriate maintenance of equipments
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 the writing of a new decree asking for
a performance-based approach1 [13] and a best practices guide [16].
This new decree defined functional requirements to fulfil, but also reinforced the rule of risk
analysis in safety reports and asked to record and to analyze all near misses and incidents that
occur in the facilities in order to take appropriate actions.
The best practice guide was designed to give practical advice to deal with fire
hazards in facilities handling powders and bulk solids (such as: design of
equipments, layout, prevention of explosive dust clouds and ignition sources,
detection of abnormal conditions, means of protection, safety organization).
continuously updated by the working group regarding the state of the art.
and explosion
buildings and
housekeeping,
This guide is
2.3 Haysville, United States of America, 1998 (7 killed, 10 injured) [17, 18, 19]
Site description
Reported in Guinness Book of Records as the world's largest grain elevator, the DeBruce Grain
elevator was located at Haysville, approximately 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 approximately 739,200 m3, for a total length of 823 m.
The reinforced concrete headhouse structure located in the centre 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 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
1
This decree was slightly modified in February 2007 [14] 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 [15].
GCPS 2011 __________________________________________________________________________
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 (Figures 10, 11, 12 and 13)
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 10 shows the propagation of the initial explosion to the
overall facility.
Figure 10: Propagation of the explosion in the DeBruce grain elevator [17]
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 11). Dust
accumulation was also pointed out (Figure 12).
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Figure 11: Postulated ignition source of the DeBruce grain elevator explosion [17]
Figure 12: Dust accumulation in the DeBruce grain elevator [17]
Figure 13: Dust explosion pentagon for the Haysville accident
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Consequences (Figures 14 to 18)
3 employees and 4 subcontractors were killed; 3 employees, 5 contractors and 2 visitors were
injured [19].
The headhouse suffered severe structural damage on all levels (Figures 16 and 17). 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 18).
Figure 14: View of the DeBruce grain elevator after the explosion (1) [17]
Figure 15: View of the DeBruce grain elevator after the explosion (2) [17]
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Figure 16: View of the damage on West side of headhouse [17]
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Figure 17: View of the damage on East side of headhouse [17]
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Figure 18: View of the northern gallery after the explosion [17]
Lessons learned (Table 3)
The investigation performed by the Grain Elevator Explosion Investigation Team (GEEIT)
mainly showed that the dust explosion hazards 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.
Several technical failures were also noticed (Table 3).
Table 3. Main failures having conducted to the Blaye accident
Technical failures
No control of the efficiency of dust removal
systems
No device to detect and collect foreign bodies
No detection of hot bearings
No fire detectors
No separation of large volumes (blast proof walls)
Not enough explosion vents
Human and organizational failures
Ineffective safety organization
No good understanding of dust hazards
Procedural shortfalls
Inappropriate maintenance of equipments
Ineffective housekeeping
2.4 Port Wentworth, United States of America, 2008 (14 killed, 36 injured) [20]
Site description
The Imperial Sugar Port Wentworth facility was built in the early 1900s. At the beginning, the
facility produced granulated sugar; over the years, the facility added refining and packaging
2
“Some workers testified that on some occasions you could not see your hand in front of your face at arm length”
[17].
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capacity, raw sugar and product warehouses. It was one of the largest sugar refining and
packaging facilities in the US.
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 area: bulk sugar truck and train loading area, south packing and “Bosch” buildings,
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
(Figure 19).
Figure 19: General view of the Imperial Sugar Port Wentworth facility [20]
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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 minutes [20].
CSB concluded that the primary dust explosion most likely occurred in the middle of the silo
tunnel: granulated sugar spilled off the moving steel conveyor at the blocked outlet under silos 1
and 2, so sugar dust concentration was above the Minimum Explosible Concentration (MEC)
inside the enclosed3 conveyor, and then was ignited probably by a hot bearing inside the
enclosed steel belt conveyor (Figure 20).
Figure 20: Dust explosion pentagon for the Port Wentworth accident
The explosion was fueled by massive accumulations of combustible sugar dust throughout the
packaging building (Figure 21).
Figure 21: Dust accumulation in Imperial Sugar Port Wentworth facility before the accident [20]
3
In 2007, some belt conveyors were enclosed to address sugar contamination concerns, but not equipped with a
dust removal system nor explosion vents.
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Consequences (Figure 22, 23 and 24)
This accident caused 14 deaths and injuring 38 others, including 14 with serious burns.
Buildings were heavily damaged (Figures 22, 23 and 24).
Figure 22: View of the damage to the Imperial Sugar Port Wentworth facility (1) [20]
Figure 23: View of the damage to the Imperial Sugar Port Wentworth facility (2) [20]
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Figure 24: View of the damage to the Imperial Sugar Port Wentworth facility (3) [20]
Lessons learned (Table 4)
The investigation showed that managers were aware of sugar dust explosion hazards but did not
take action to minimize and control sugar dust hazards. The facility experienced several small
fires and explosions, but no lessons were learned. This accident also addresses the difficulties
associated with the management of change, as the enclosure installed on the steel conveyor belt
created a hazardous area, but this kind of change in the process was not supposed to result in a
dangerous situation. CSB found as well that the explosion would likely not have occurred if
routine housekeeping was enforced.
Table 4. Main failures having conducted to the Imperial Sugar accident
Technical failures
No adequate equipment sealing (bucket elevators,
screw conveyors)
No dust removal systems in open working area and
closed conveying equipments
Ineffective dust removal system on process
equipment (air flow in ducts significantly below the
minimum dust conveying velocity, undersized fans)
No hazardous area classification and corresponding
equipment sitting
No audible or visual alarm devices in the working
areas
No separation of large volumes (blast proof walls)
Human and organizational failures
Ineffective safety organization
No communication about dust explosion hazard
between management and operators
No learning from past incidents (several small fires
and explosions)
Improper risk analysis
Inappropriate maintenance of equipments
Written housekeeping not implemented
No adequate safety and evacuation trainings
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3. Best practices to prevent secondary dust explosions or mitigate their effects
Safety measures can be classified in three main classes [21]:
•
prevention of explosive dust cloud;
•
prevention of ignition sources;
•
explosion mitigation.
The following paragraphs give an overview of current prevention and protection techniques that
could be used against dust explosions.
3.1 Prevention of explosive dust clouds
Table 5 presents some practical solutions to prevent explosive dust clouds.
Table 5. Measures for the prevention of explosive dust clouds
Functional requirements
Minimize dust formation
Minimize dust explosibility
Minimize dust cloud formation
Minimize dust layers formation
Measures
Grain cleaning before storage (NFPA 654 [22])
Use of bigger particle size
Control that dust concentration is below the
Minimum Explosive Concentration (NFPA 654)
Inerting (NFPA 654, EN 15281 [23])
Use of bigger particle size
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.
[24], Wheeler et al. [25])
Prevention of process leakages: sealing + dust
removal (NFPA 654)
Collect of dust leaks (capture hoods, “elephant
trunks”)
Avoidance of elevated flat surfaces (NFPA 654, [16])
Implementation of a good housekeeping program
(NFPA 654)
Secondary dust explosions cannot develop if there is no fuel (i.e. dust layers), so important
measures for the prevention of secondary dust explosions are the prevention of leakages and a
good housekeeping program.
Equipments should be dust-tight and equipped with a dust removal system (Figures 25 and 26),
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 if there are no fugitive
sources in the facility and take actions to avoid them.
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Figure 25: Enclosed conveyor [26]
Figure 26: Dust removal system [26]
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
500 g/m3 if partially dispersed up to 1 m above the floor (Figure 27).
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Figure 27: The potential hazards of thin dust layers [1]
Also, Tamanini [27] 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, since the dust can be dispersed only in the lower part
of a volume and therefore gives higher explosive dust concentrations (Figure 28).
Figure 28: Large-scale gallery used by Tamanini [27]
It highlights the need of good housekeeping practices.
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Frank and Holcomb [28] provided some general guidance to deal with dust leakages and dust
accumulation, such as:
•
to design, maintenance, and operate equipments to minimize dust emissions;
•
to capture dust at the release point;
•
to limit the extent of dust migration and size of the room that must be cleaned;
•
to design facilities for easy effective cleaning (no flat elevated surfaces);
•
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 29), can be used to detect dust
accumulation. Also, flat elevated surfaces must be avoided in new facilities.
Figure 29: When disappearing, white crosses indicates dust accumulation
Accidents presented in paragraph 2 illustrate that dust accumulation has been responsible of
several secondary explosions in agricultural facilities. Frank [29] also reviewed other accidents
that occurred in the chemical industry (West Pharmaceutical, CTA Acoustics, Hayes Lemmerz)
and which could have been avoided by an effective housekeeping.
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3.2 Prevention of ignition sources
Table 6 presents some practical solutions to prevent ignition sources.
Table 6. Measures for the prevention of ignition sources
Functional requirements
Avoid/control ignition sources
Measures
Mechanical and electrical equipments adapted to
hazardous locations (NFPA 499 [30], NFPA 70 [31],
ATEX European Directives [32, 33])
Limitation of the use of low resistivity materials;
bonding and grounding (NFPA 77 [34])
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]
Magnetic separators [16]
Detection of hot particles [16]
Lightning protection (NFPA 780 [35])
Fire prevention and protection (fireproof belts, fire
detectors, extinguishers) [16]
Hot work permit for welding, cutting, hot tapping
(NFPA 654, NFPA 51B [36])
No smoking [16]
PR EN 1127-1 [37] distinguishes 13 different types of ignition sources; among them
mechanically generated sparks, smoldering fires, mechanical heating and friction and
electrostatic charges are the most likely in process industries according to Figure 30, which give
types of ignition sources involved in dust explosions in the Federal Republic of Germany,
between 1965-1985, for a total of 426 dust explosions.
4%
16%
25%
3%
5%
5%
11%
5%
8%
9%
9%
Mechanical sparks
Smoldering nests
Mechanical heating and friction
Electrostatic charges
Fire
Spontaneous ignition (self-ignition)
Hot surfaces
Welding and cutting
Electrical machinery
Unknown or not reported
Others
Figure 30: Type of ignition sources involved in dust explosions [1]
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14%
20%
3%
5%
5%
5%
17%
8%
10%
13%
Silos and bunkers
Dust collecting systems
Milling and crushing plants
Conveying systems
Dryers
Furnaces
Mixing plants
Grinding and polishing plants
Sieves and classifiers
Unknown and others
Figure 31: Type of plant involved in dust explosions [1]
As shown on Figure 30, mechanical heating and friction represents almost 10% of the ignition
sources involved in dust explosions. This tendency is illustrated by the review of the past
accidents presented in paragraph 2, as hot bearing was implied in two accidents (DeBruce grain
elevator and Imperial Sugar Port Wentworth facility explosions). It emphasizes the need to
correctly monitor the operation of conveyors and dust removal systems, which were implied in
the two other accidents (Metz and Blaye explosions).
Figures 32 and 33 give examples of control devices for a belt conveyor that can be used.
Figure 32: Belt displacement control device [26]
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Figure 33: Rotation control device [26]
3.3 Explosion mitigation
Table 7 presents some practical solutions to mitigate explosions.
Table 7. Measures for explosion mitigation
Functional requirements
Limit the consequences of a
primary explosion
Avoid the propagation of the
primary/a secondary explosion
Limit the consequences of a
secondary explosion
Measures
Venting (NFPA 68 [38], EN 14491 [39], EN 14797
[40]) : bursting disks, venting panels, vent ducts,
explosion doors, light construction [16]
Explosion extinguishing systems: powder, water
(NFPA 69 [41], EN 14373 [42],
Explosion resistant equipment (NFPA 69, EN 14460
[43])
Explosion isolation between vessels (NFPA 69, EN
15089 [44]): acting valve (PR EN 16009 [45]),
diverter (PR EN 16020 [46]), rotary lock (Siwek
[47]), screw conveyor, triggered barriers (Lebecki et
al. [48])
Flame arresters (PR EN ISO 16852 [49])
Layout/unit segregation (NFPA 654, [16])
Explosion isolation between parts of a building (blast
proof walls) [16]
Venting [27]
Dust explosions leading to widespread damage are often characterized by flame propagation
through galleries and handling towers, i.e. 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 34.
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Figure 34: Blast proof walls between a gallery and a handling tower [26]
The use of open or light constructions (i.e. the avoidance of reinforced concrete and underground
structures) can as well limit the pressure rise inside the facility, and therefore limit the resulting
damage and the propagation of the explosion.
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Also, Tamanini [27] showed that venting a primary explosion in a gallery could be effective at
reducing explosion overpressure and sometimes preventing flame propagation. Nevertheless, it is
not always true, so this technique must be associated with blast proof walls or triggered barriers
to be effective.
Finally, it is really important to move away administrative and technical premises from
hazardous locations. In the Blaye accident, most of the victims were in the administrative
premises.
4. Conclusions
Secondary dust explosions can have dramatic consequences, as shown from past accidents
reviewed in this paper.
Several techniques exist to prevent secondary dust explosions or mitigate their effects; the most
critical ones have been presented: 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. 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 fire
and explosion risks related to the handling of powders and bulk solids to adapt their behavior at
workstation 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:
managers must be aware of the risks, communicate and take action to minimize and control
them.
During operations in a hazardous plant, maybe the highest risk is the normalization of deviance;
dust explosions that occurred in the past remind us that disasters can quickly occur if companies
do not pay enough attention and accept abnormal and unsafe work conditions.
In France, a performance-based regulation framework has been set up after the Blaye accident in
1997, to give more flexibility to companies in order to adapt safety measures to their own type of
activities. A best practices guide was also 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. This process seems to give fruitful results, as from this time, the number of serious
incidents has significantly decreased.
GCPS 2011 __________________________________________________________________________
5.
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GCPS 2011 __________________________________________________________________________
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GCPS 2011 __________________________________________________________________________
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GCPS 2011 __________________________________________________________________________
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