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

Very large crude and gasoline tanks in refineries (50,000–150,000 m³) mostly use floating roofs to reduce evaporation losses and the risk of a flammable vapor space. Lightning or static discharge ignites rim-seal fires. NFPA 11 defines seal area protection and full surface protection for floating-roof tanks (NFPA 11-2024, 5.3.1.1), and does not permit subsurface or semi-subsurface injection on open-top or covered floating-roof tanks (5.3.3.1). Buncefield 2005 (UK) and Jaipur 2009 (India) are the major reference accidents.

Tank Types

• Open-top floating roof (FRT): roof rides directly on the liquid
• Internal floating roof (IFRT): fixed outer roof plus internal floating roof
• Fixed roof: mostly small tanks and low vapor-pressure products
• Rim seal: primary seal (mechanical shoe or tube seal) plus secondary seal; NFPA 11 Table 5.3.5.3.1 covers each seal arrangement separately
• Vapor retention: low for floating roofs, high for fixed roofs

Rim-Seal Fire

• Cause: lightning, static, welding/grinding sparks
• Area: narrow seal area, typically about 400 mm wide
• Circumference: 80 m diameter tank = 251 m
• Foam application rate: 12.2 L/min/m² (0.3 gpm/ft²) with top-of-seal fixed discharge outlets (NFPA 11-2024 Table 5.3.5.3.1)
• Duration: 20 min (same table)

NFPA 11 Foam Types

• AFFF (aqueous film-forming foam): fast knockdown, weak burnback resistance
• AR-AFFF (alcohol-resistant): required for polar solvents (methanol, ethanol)
• FP (fluoroprotein): stable blanket, preferred for subsurface injection on fixed-roof tanks
• F3 (fluorine-free): the 2026 transition direction due to PFAS restrictions
• High-expansion: for LNG/ammonia

Full-Surface Fire

80 m diameter tank, full surface fire. NFPA 11 Section 5.3 gives seal area criteria for open-top floating-roof tanks; this full-surface example borrows the monitor criteria for fixed-roof tanks (Table 5.2.4.2.2):

• Area: A = π × 40² = 5,027 m²
• Application rate: 6.5 L/min/m² → 32,670 L/min
• Duration: 65 min (Table 5.2.4.2.2: flash point below 38 °C and crude oil)
• Wind and ambient losses: per 5.2.4.2.1.2, a minimum additional factor of 1.5 applies to monitor design; the quantities below exclude this factor
• Foam concentrate: 3% → 980 L/min × 65 = 63.7 m³
• Foam solution: 32,670 × 65 ≈ 2,124 m³ (about 2,060 m³ of it water)

Buncefield 2005 Lesson

• Event: gasoline tank overfill followed by a vapor cloud explosion
• Damage: very severe explosion, 43 injured
• Cause: defective tank level gauge, no independent overfill protection layer
• Lesson: independent, high-integrity high-level alarm
• Process safety: API 2350 overfill prevention

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