Grain silos store 20,000-100,000 t of wheat and use 30-50 m bucket elevators. Grain dust (mostly starch) has a LEL around 40 g/m3 and Kst 100-200 bar.m/s; the US sees 5-10 grain explosions each year. NFPA 61 covers agricultural and food facilities.
Grain Dust Properties
- Wheat dust: Kst 115 bar.m/s (ST1)
- Corn dust: Kst 150
- LEL: 40-60 g/m3
- MIE: 20-50 mJ
- Auto-ignition: 450 C in pile
Bucket Elevator Risks
- Belt slip: 60-100 C friction
- Bearing failure: Spark source
- Tramp metal: Magnetic separator
- Belt alignment: Weekly check
- Speed monitor: 10 percent slip trip
NFPA 61 Deflagration Venting
- Silo top: Vent Av/V = 0.1 m2/m3
- Elevator leg: Vent every 6 m
- Conveyor: Light cover 0.5 kPa open
- Isolation: Rotary valve + chemical barrier
- Ball check valve: Inlet suppression
Dust Collection and Housekeeping
- Aspiration: Every transfer enclosed
- Baghouse: 1 m3/s typical silo
- Housekeeping: 0.5 mm dust on 20 percent area
- Cleaning: Vacuum, NEVER compressed air
- Shift change: End of shift
Turkish and World Incidents
- Polatli TMO 2018: Bucket elevator - 1 dead
- Imperial Sugar GA 2008: 14 dead
- DeBruce Grain KS 1998: 7 dead
- Konya Cumra: Large TMO silo complex
- Thrace: Wheat harvest warehouse plants
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Sources & Further Reading
NFPA 61, NFPA 68, NFPA 654, OSHA 1910.272, FM DS 7-75. NFPA 61 Agricultural and Food Products.