DUST EXPLOSION PREVENTION

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DUST EXPLOSION
PREVENTION
RONALD C. JESTER
• University of Delaware,
Extension Safety Specialist
STATS OVERVIEW
• First documented dust explosion – Turin,
Italy in 1785
• Major modern-day explosion – Corpus
Christi, Texas in 1981. It resulted in 9
fatalities, 30 injuries and $30 million in
damages
• Ten year average – 13 explosions/year
FIRE TRIANGLE
• Fuel
• Heat
• Oxygen
DUST EXPLOSION
• Fuel
• Heat
• Oxygen
• Confined space
Prevention: Disconnecting the
Triangle
Or Simply
Removing a Leg of the Triangle
THE FUEL
• DUST
• DUST CLOUDS
• COMBUSTIBLES
• FLAMMABLES
THE HEAT SOURCE
•
•
•
•
•
•
Welding and cutting operations
Overheated bearings
Metallic objects in system
Slipping belts
Smoking
Improper electrical fixtures in a potentially
explosive atmosphere
• Static electricity
• And many more
OXYGEN
• It is everywhere
• Not a factor that can be eliminated
TYPES OF DUST EXPLOSIONS
• Primary – initiated by an ignition source
• Secondary – occurs when the blast wave
(shock wave) from a primary explosion
propagates and causes layers of dust in
other areas to become suspended in air.
Dust suspension by the primary explosion is
extremely flammable
HAZARDS OF DUSTS
•
Ignition sensitivity - ease of ignition
[ ] ignition temperature
[ ] minimum energy of ignition
•
Explosion severity – severity of the ensuing
explosion
[ ] maximum explosion pressure
[ ] maximum rate of pressure rise
EXPLOSIBILITY INDEX
• A product of the ignition sensitivity and the
explosion severity
EXPLOSIBILITY INDEX
Type of
Explosion
Weak
Ignition
Sensitivity
<0.2
Explosion
Severity
<0.5
Explosibility
Index
<0.1
Moderate
0.2-1.0
0.5-1.0
0.1-1.0
Strong
1.0-5.0
1.0-2.0
1.0-10
Severe
>5.0
>2.0
>10.0
EXPLOSIBILITY OF
AGRICULTURAL DUSTS
Material
Index
Explosion
Corn
8.4
Strong
Cornstarch
35.6
Severe
Mixed Grain
9.2
Strong
Soybean meal
7.5
Strong
Wheat flour
3.8
Strong
Wheat starch
49.8
Severe
MOISTURE
• Moisture in dust particles raises the ignition
temperature of the dust
• The moisture in the air surrounding a dust
particle has no significant effect on the
course of deflagration once ignition has
occurred
DUST CONCENTRATION
• LEL – lower explosive limit
Sample purity, oxygen concentration, strength of
ignition source, turbulence of dust cloud and
uniformity of dispersion all effect LEL
• UEL – upper explosive limit
There is a question of whether a clear cut UEL
exists; and the information is of questionable use
PARTICLE SIZE
• The smaller the size of dust particle the
easier it is to ignite the cloud
• The smaller the size of dust particle the
higher the rate of pressure rise
• As dust particle size decreases, so does the
lower explosive limit concentration, ignition
temperature and energy necessary for
ignition.
“DEADLOCK OVER
EXPLOSIVE DUST”
Grain dust is more explosive than
coal dust or gunpowder, but for most
of this century the grain-handling
industry has treated it as though it
were not much worse than cobwebs.
MAXIMUM PRESSURES
• 10 Atmospheres
• Occur in thousandths of a second
• Impact of various pressure magnitudes:
– 2-3 psi will level a concrete structure
– Less than 8 psi can cause failure of a steel bin
FORCES IN EXPLOSIONS
•
•
•
•
Debris hurled 2 miles
Heard at distances of 100 miles
Railroad ties twisted like taffy
Steel reinforced concrete structures leveled
HISTORICAL PERSPECTIVE
• Corn is being handled in 40% of explosions
• Primary explosion is not as serious as what
follows
• Hot work identified as the probable ignition
source
• 70% of explosions in grain elevators occur in
bucket elevators
• April is the high risk month
• Tuesday is the day not to work
PREVENTION TECHNIQUES
• The best method of prevention is good old
housekeeping.
• Dust controlling measures will help prevent
a secondary explosion, but not a primary
explosion.
• The best way to prevent dust explosions is
to eliminate ignition sources.
SAFETY PHILOSOPHY
•
•
•
•
•
Every incident can be avoided
No job is worth getting hurt for
Every job will be done safely
Incidents can be managed
Safety is everyone’s responsibility
COMMITMENT
To be successful, safety must be more
than a program or procedures. It
must be a company philosophy – an
attitude that is unquestioned.
Less McGraw, Fluor Daniel
CONCLUSIONS
• Grain dust is explosive
• Prevention primarily includes:
- elimination of heat sources
- good housekeeping
- preventive maintenance
• Fires and explosions put each employee’s life and
job at stake
• Employees are the single most important factor in
hazard reduction
• Make safety a core value
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