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 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 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 Ignition Sensitivity Explosion Severity Explosibility Index Weak <0.2 <0.5 <0.1 Moderate Strong Severe >5.0 >2.0 >10.0

EXPLOSIBILITY OF AGRICULTURAL DUSTS MaterialIndexExplosion 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