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 OH2 (Ordinary Hazard Group 2) is often enough, but some installations require EH1 (Extra Hazard Group 1) design density. Note: NFPA 13 has no OH3 class; OH3 is an EN 12845 classification.
Design Density Selection
- OH2: 0.20 gpm/ft2 (8.1 mm/min) / 1500 ft2 (140 m2) — enclosed chip storage, under conveyor
- EH1: 0.30 gpm/ft2 (12.2 mm/min) / 2500 ft2 (230 m2) — pile height >3 m, damp chips
- EH2: 0.40 gpm/ft2 (16.3 mm/min) / 2500 ft2 (230 m2) — pallet wrap, MDF panel storage
Values from NFPA 13-2025 Table 19.2.3.1.1 (cases where a 3000 ft2 / 280 m2 design area is not required). The table gives density and area by hazard class only; it does not define which class a chip pile falls into.
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
- Layout: in enclosed conveyor tunnels, transverse sprinkler spacing
- Spacing: every 6 m along the conveyor
- Type: quick-response (QR), 93 °C (intermediate temperature class, NFPA 13-2025 Table 7.2.4.1(a))
- Waterflow switch: interlocked with conveyor stop
Paper Mill Specifics
- Paper machine drying cylinder at 150 °C - high (121-149 °C) or higher temperature class sprinkler
- Water spray in pulper for early activation
- Deluge sprinkler on dust collection silo
- FM Global DS 7-2 paper manufacturing guideline
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Download SprinkCalc on the App StoreNFPA 13, FM Global DS 7-19, FM Global DS 7-2, ASTM E119. NFPA 13 Sprinkler standard.