Wednesday April 30, 2008 - Vol. VII Issue 3[Download PDF for Printing]
COMBUSTIBLE DUST EXPLOSIONS
Most people would not think that dust made up of ordinary
materials such as food could explode, but under certain circumstances it
can. On 7 February 2008, a violent
explosion occurred in a silo where refined sugar was stored at the Imperial
Sugar Company Plant on the Savannah River at Port Wentworth, Georgia, killing
12 people and injuring many others.
This dust explosion was not a solitary incident as many industrial dust
explosion accidents have occurred in the past.
In this newsletter we will examine the five essential circumstances that
must come together before a dust explosion can occur. But first, let us examine some real-world accidents.
Imperial Sugar Co. 7 February 2008 Explosion
Earth image of general location before blast, plant entrance at location A,
Savannah River at right
after blast, from Chemical Safety Board website
The sugar refinery plant is known in Port Wentworth,
Georgia, as the Dixie Crystals plant.
In 1997, Imperial Sugar Co. acquired Savannah Foods & Industries
Inc., which makes Dixie Crystals and where the explosion and fire
occurred. The refinery accounts for
about 9% of the total U.S. capacity, or 58% of Imperial Sugar Company’s capacity. There were about 100 workers in the plant
at the time of the accident.
See Savannah Morning News at http://savannahnow.com/node/444690
for additional photos.
At about 7 PM on 7 February 2008, in what was described by
the CEO of Imperial Sugar as “a sugar dust explosion” occurred in a silo where
refined sugar was being stored before repackaging. The CEO was at the plant at the time of the explosion. A fire quickly took place causing a partial
collapse of the nearby four-story building. The Savannah Fire Department and others from the surrounding area
responded at about 7:30 PM. Nine people
were killed by the explosion or resulting fire, 62 people were reported injured
and subsequently treated including 22 people who were hospitalized. As of February 19, 16 people remained
hospitalized in critical or serious condition from burn injury. It took fire crews a week to extinguish the
fire. Some of the dead were not located
in the rubble for several days, and some of the burn victims later died raising
the total dead to 12 (which may increase).
The U.S. Chemical Safety Board (CSB), the government agency charged with
investigating the cause of chemical accidents, arrived at the site on 8
February. The CSB confirmed that
finely-divided sugar dust was involved in the explosion, but at the writing of
this newsletter the CBS has not issued a statement as to the ignition source.
U.S. Chemical Safety and Hazard Investigative Report on
Combustible Dust Explosions
The U.S Chemical Safety Board issued a 108-page
investigative report dated November 2006 (report no. 2006-H-1) titled
Combustible Dust Hazards Study. The
entire report is available at http://www.chemsafety.gov/index.cfm?folder=completed_investigations&page=info&INV_ID=53
. (or go to the CSB home page and link to
The report researched the history of 281 major combustible
dust explosions and fires in the United States from 1980 and 2005. The 281 incidents resulted in 119 worker
deaths, 718 injuries, and destroyed many industrial facilities. Excluded from the study were (1) grain
handling facilities, (2) coal mines, (3) transportation incidents, and (4) non-manufacturing
facilities. Grain handling facilities
were excluded from the study because these facilities are already under the
OSHA Grain Handling Facilities Standard.
One of the reasons why the study was done was to determine whether OSHA
needed to beef up standards to protect workers against dust explosions in other
industries (the answer was yes). The
percentage breakdown for the 281 incidents by dust material is listed below:
The percentage breakdown for the incidents by industry is
listed below (from CSB report):
The CSB investigation also reviewed the Material Safety Data
Sheets (MSDSs) of 140 known combustible dusts as to how effectively they
communicated the explosion potential.
Although 59 percent of the MSDSs included some language referring to the
explosive nature of the dust, most of the information was not specific, and
only seven of the 140 MSDSs referenced the applicable National Fire Protection
Association (NFPA) standard for managing dust hazards. Partly because of lack of information,
employees in these facilities and to some extent even management were unaware
in many situations that the materials could explode violently under certain
conditions. Sometimes the MSDSs used
vaguely worded instructions such as “Avoid dusty conditions” as opposed to
“This material in its finely divided form presents an explosion hazard”. Newer MSDSs were generally better at
communicating hazards than those 20 or 30 years old.
The CSB investigation found many dust explosion incidents
resulted from accumulated dust being disturbed by some incident that resulted
in the dust becoming airborne. The
first incident may be a minor explosion or other disturbance in some ductwork
or process equipment that lifted the dust into the air. The disturbance caused dust on the floor and
other locations to become airborne causing secondary explosions. The CSB found that although MSDSs often
contained a dust warning hazard saying dusty conditions should be avoided, none
explicitly stated that dust accumulations should be avoided to prevent
secondary dust explosions.
Fires and Dust Explosions
Firemen and First Responders are familiar with the classic
For a fire to occur, all three elements (a fuel, oxygen, and
an ignition source) must be present. The
oxygen can come from the air, which is composed of 21% oxygen. In a few situations, the oxygen can come
from a chemical which is in contact with the fuel, for example nitrates and
ethers. The ignition source could be
static electricity, lightning, a lighted cigarette, another fire, sparks from
equipment, etc. The fuel is anything
that can burn and could include metal fines.
This fire triangle is applicable to dust fires.
A dust explosion
requires the simultaneous presence
of two additional factors, dust suspension and confinement. If any of the five elements depicted below
are removed, a dust explosion will not occur, although a fire can still occur
with oxygen, an ignition source, and the combustible dust serving as a fuel.
The combustible dust is the fuel. The dispersion is any event that causes settled dust to become
airborne. The confinement could be the
process equipment, ductwork, a storage vessel, or silo. The ignition source could be static
electricity or anything that causes a spark.
The suspended dust if ignited burns rapidly, and confinement allows for
rapid buildup of pressure resulting in an explosion.
Often an initial dust explosion might occur in some process
equipment or ductwork causing dust which has accumulated on floors or other
areas to become lofted resulting in secondary explosions. The initiating event for a secondary
explosion is not necessarily another dust explosion but could be some other
event that cause the accumulated dust to become airborne.
The CSB report examined incidents at two facilities
involving dust explosions that spread through pipes and vents, from one piece
of equipment to another and to other areas of the facility as the explosions
caused the settled dust to become airborne.
The pressure increased as the explosion moved from one location to the
next increasing the damage. It is this
disturbance of previously accumulated dust which causes the greatest damage and
is not communicated very well in industry.
Physical Properties of Combustible Dusts
The NFPA defines a combustible dust as any finely divided
solid material that is 420 microns (0.42 millimeters) or smaller in diameter
and that presents a fire or explosion hazard when dispersed and ignited in
air. Materials 420 microns and smaller
pass through a U.S. No 40 Standard Sieve, and is about the size of fairly
coarse sand. The dust also must be
combustible. Some dusts, such as quartz
sand or table salt no matter how finely ground will not burn because they are
not combustible. Many metal powders
will burn and could form explosive mixtures if suspended in air.
There are several physical properties that can be measured
to determine the explosiveness of dusts.
Particle size is a major factor, the smaller the particle size the
larger the surface area relative to weight, which allows the particles to
rapidly react with oxygen in the air.
Table 1 Measured Properties of Combustible Dusts
ASTM Test Method
MEC (Minimum explosive concentration)
ASTM E 1515
Analogous to lower flammability limit in vapor cloud
explosions; measures the minimum amount of dust dispersed in air required to
spread an explosion
Kst (Dust deflageration index)
ASTM E 1226
Measures the relative explosive severity compared with
Pmax (Maximum explosion overpressure generated
in the test chamber)
ASTM E 1226
Used to design enclosures and predict the severity of the
(Maximum rate of pressure rise)
ASTM E 1226
Predicts the violence of an explosion (related to Kst
MIE (Minimum Ignition Energy)
ASTM E 2019
Predicts the ease and likelihood of ignition of a
dispersed dust cloud
AIT (Auto Ignition Temperature)
Minimum AIT of dust clouds
National Fire Protection Association (NFPA) Standards
The CSB and regulatory agencies recognize that the voluntary
NFPA standards provide effective technical guidance to prevent industrial dust
explosions. Portions of the voluntary
standards have been codified into many state fire systems in the United States,
but have not been adopted by other states and local jurisdictions. Enforcement
is often lacking, and many state inspectors do not have adequate training to
recognize the potential for dust explosions according to the CSB report. OSHA has issued Grain Facility Standards 20
years ago for addressing the risk of dust explosions in the grain industry, but
has not set up dust standards for other industries. However since the recent Imperial Sugar accident, several U.S.
Senators (Enzi, Isakson, Chambliss, Kennedy, and Murray) are working with OSHA
and CSB as they investigate this accident, to affect change in the interest of
Table 2. List of
Voluntary NFPA Standards Relating to Industrial Dust
Standard for the Prevention of Fire and Dust Explosions
from the Manufacturing, Processing, and Handling of Combustible Particulate
Standard for Combustible Metals -2006
Standard for the Prevention of Fires and Dust Explosions
in Agriculture and Food Processing Industries - 2008
Guidelines for Deflagration Venting - 2007
Standard on Explosive Prevention Systems -2008
The National Electric Code - 2008
Recommended Practice for the Classification of Combustible
Dusts and Hazardous (Classified) Locations for Electrical Installations in
Chemical Process Areas - 2004
Standards for the Prevention of Sulfur Fires and
Explosions - 2007
Standards for the Prevention of Fires and Explosions in
Wood Processing and Woodworking Facilities - 2007
The NFPA Standards are updated about every five years; the
latest edition is listed. They can be
purchased by typing in the name of the standard, e.g. “NFPA 654”, in a computer
search engine such as Google, which provides a link to a site where the document
may be ordered.
AristaTek in researching websites on the subject of
industrial dust explosions noted that there was considerable motivation by
parties to prompt industry to make work places as safe as possible. The motivation came from several fronts:
and injuries to workers, and loss to their families, as noted in the news
of economic loss in terms of bankrupted and out-of-business facilities
which were destroyed by dust explosions; loss in stock value for Imperial
firms encouraging injured workers to file claims
the CSB reports are not bashful as to laying blame. For example, the CSB report 2006-H-1
reviewing dust explosions cited earlier contained actual examples
including pictures of destroyed facilities with captions such as “Figure
14. Mr. XXXX did not follow NFPA
484 guidance on locating and maintaining the dust collector, which
exploded on October 29, 3003” [AristaTek withheld the name XXXX in this
Newsletter]. That organization
also had plenty of blame for OSHA for failure to codify standards.