Cement, mortar, clinker dust and fuel silos need special protection against dust explosion. Clinker itself does not burn but coal/petcoke, bypass dust and fly ash can. NFPA 68 deflagration venting and NFPA 69 explosion prevention (inerting) offer silo protection options.
Silo Types and Risk
- Coal dust silo: St1 (Kst 50-150)
- Petcoke silo: St1-St2 (Kst 150-200)
- Clinker bypass dust: alkali salts, dust - self-heating with moisture
- Fly ash silo: St1 (from thermal plant)
- Cement silo: non-combustible (CaCO3-CaO-CaSO4)
NFPA 68 Vent Calculation
500 m3 coal silo, Kst 150, Pmax 8 bar, Pred 0.5 bar:
- Av = (Kst x V^0.75) / (Pred x 1000)
- Av = (150 x 500^0.75) / (0.5 x 1000)
- Av = (150 x 105.6) / 500
- Av = 31.7 m2 (panel area)
- Placement: upper half of silo, straight vent duct
St Class and Kst
- St0: Kst = 0 (non-combustible)
- St1: Kst 1-200 (most common - coal, flour, sugar)
- St2: Kst 200-300 (petcoke, alumina)
- St3: Kst >300 (metal dust - Al, Mg)
- Kst measurement: 1 m3 KSEP test (VDI 2263)
Panel Design
- Bursting pressure: Pstat = 0.1-0.2 bar
- Panel efficiency: Ef = 0.5 (with vent duct)
- Flame vent direction: no personnel, minimal debris
- Isolation valve: NFPA 69 isolation - prevent secondary explosion
- Recoil: silo opposite side loaded - support required
NFPA 69 Inerting Alternative
- N2 inerting: O2 <8 percent for coal
- CO2 inerting: alternative, lower cost
- LOC: coal 14 percent, petcoke 12 percent
- HRD suppression: 25 kg monoammonium phosphate at 50 bar
Silo venting with MEP Calc
MEP Calc sizes cement/coal silo NFPA 68 Av panel area, St class, Kst and NFPA 69 inerting calculations.
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Sources & Further Reading
NFPA 68, NFPA 69, NFPA 652, NFPA 664, VDI 2263, FM DS 7-76. NFPA 68 Explosion Venting standard.