Paper mills, MDF plants and biomass power plants store wood chips in high fuel-load piles. Moisture raises interior temperature — posing a spontaneous combustion risk. NFPA 13 OH3 (Ordinary Hazard 3) may not suffice; some installations require EH1 (Extra Hazard 1) design density.
Design Density Selection
- OH3: 0.20 gpm/ft2 (8 L/min/m2) / 1500 ft2 — enclosed chip storage, under conveyor
- EH1: 0.30 gpm/ft2 (12 L/min/m2) / 2500 ft2 — pile height >3 m, damp chips
- EH2: 0.40 gpm/ft2 (16 L/min/m2) — pallet wrap, MDF panel storage
FM Global DS 7-19 adds moisture and pile height to the required density.
Spontaneous Combustion
Interior temperature in a wood chip pile:
- Moisture triggers microbial decomposition (mesophilic bacteria)
- Temperature 40-60 C (thermophilic bacteria)
- 60-80 C — chemical oxidation begins
- >80 C — chemical chain reaction, ignition
Prevention: pile height <6 m, automatic temperature monitoring, rotation every three weeks.
Conveyor Sprinklers
- NFPA 13 20.4: in enclosed conveyor tunnels, transverse sprinkler spacing
- Spacing: every 6 m along the conveyor
- Type: quick-response (QR) high-temperature 93 C
- Waterflow switch: interlocked with conveyor stop
Paper Mill Specifics
- Paper machine drying cylinder at 150 C - high-temp sprinkler
- Water spray in pulper for early activation
- Deluge sprinkler on dust collection silo
- FM Global DS 7-2 paper manufacturing guideline
OH3/EH1 design with SprinkCalc
SprinkCalc drives OH3 vs EH1 decisions, operating area, flow and K-factor selection with hydraulic calculations for wood chip storage.
Download SprinkCalc on the App Store
Sources & Further Reading
NFPA 13, FM Global DS 7-19, FM Global DS 7-2, ASTM E119. NFPA 13 Sprinkler standard.