Refinery Floating-Roof Tank: NFPA 11 Rim-Seal Foam and Buncefield Lesson

Very large crude/gasoline refinery tanks (50,000-150,000 m3) use floating roofs to reduce air exposure. Lightning or static ignites rim-seal fires. NFPA 11 specifies rim-seal foam, semi-subsurface and full-surface foam methods. Buncefield 2005 (UK) and Jaipur 2009 (India) are major accidents.

Tank Types

• Floating roof (FRT): roof rides on liquid
• Internal FRT (IFRT): fixed outer + internal floater
• Fixed roof: stationary - small tanks
• Rim seal: primary (shoe) + secondary (fin)
• Vapor retention: low for floating, high for fixed

Rim-Seal Fire

• Cause: lightning (50-70 percent), static, welding
• Area: narrow rim - 400 mm width
• Circumference: 80 m dia tank = 251 m
• Foam rate: 12.2 L/min/m2 (0.3 gpm/sqft)
• Duration: 20 min

NFPA 11 Foam Types

• AFFF (aqueous film): fast knockdown, weak burnback
• AR-AFFF (alcohol-resistant): polar solvent (methanol)
• FP (fluoroprotein): long blanket, ideal for subsurface
• F3 (fluorine-free): 2026 transition - PFAS ban
• High-expansion: for LNG/ammonia

Full-Surface Fire

80 m dia tank, full surface:

• Area: A = pi x 40^2 = 5,027 m2
• Flow: 6.5 L/min/m2 → 32,670 L/min
• Duration: 65 min (including 0.35 mobilization)
• Foam concentrate: 3 percent → 980 L/min x 65 = 63.7 m3
• Water: 32,670 x 65 = 2,124 m3

Buncefield 2005 Lesson

• Event: petrol tank overfill, vapor-cloud explosion
• Damage: 2 Mt equivalent blast, 43 injured
• Cause: defective tank level gauge
• Lesson: independent high-level alarm, high-integrity
• Process safety: API 2350 overfill prevention

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