Fire Safety

Fires are the most common major process accident. Learn the fire triangle, flash point and autoignition, flammability limits (LFL/UFL) and Le Chatelier's rule, fire types (pool, jet, flash fire, fireball), radiation hazards and safe distances, and fire prevention and protection systems.

Flammable liquids and gases are everywhere in the chemical industry. Understanding when they can ignite, how fires behave and how far heat radiation reaches is essential for layout, protection and emergency planning.

An analogy

Fire needs three friends — fuel, oxygen and heat. Remove any one and the fire cannot start or survive.

Fire triangle and properties

Flash point: lowest temperature at which a liquid gives enough vapour to ignite (gasoline −43 °C, diesel > 52 °C). Autoignition: temperature at which it ignites without a spark. A vapour burns only between LFL and UFL (methane 5–15 %).

Fire types

Pool fire: burning liquid spill. Jet fire: ignited pressurised release — intense, can cut through steel. Flash fire: ignited vapour cloud burning without blast. Fireball/BLEVE: vessel rupture of pressurised liquefied gas.

Radiation effects

≈ 1.6 kW/m²: no discomfort for long exposure; 4–5 kW/m²: pain in ≈ 20 s; 12.5 kW/m²: wood ignition, fatal with longer exposure; 37.5 kW/m²: equipment damage. Spacing and firewalls are set from these thresholds.

Prevention and protection

Prevention: control ignition sources (hazardous-area zoning, ATEX/IECEx equipment, bonding and earthing against static), inerting, containment. Protection: fire & gas detection, deluge and foam systems, fireproofing of structural steel, emergency depressurisation, drainage and bunding.

Formulas

• Le Chatelier (mixture LFL): LFL_mix = 1 / Σ(y_i / LFL_i)
• Pool fire heat release: Q = ṁ'' A ΔH_c
• Point-source radiation: q'' = χ_r Q / (4π r²)
• Distance to flux: r = √(χ_r Q / (4π q''))

Worked example: Pool fire radiation

Given: Gasoline pool D = 10 m, ṁ'' = 0.055 kg/m²s, ΔH_c = 43.7 MJ/kg; χ_r = 0.3

Find: Heat release and flux at 30 m and 50 m.

• A = 78.5 m² → Q = 0.055 × 78.5 × 43.7 = 189 MW
• Radiated 56.6 MW
• q''(30 m) = 56.6/(4π × 900) = 5.0 kW/m²; q''(50 m) = 1.8 kW/m²

Answer: 5.0 kW/m² at 30 m; 12.5 kW/m² at 19 m

Worked example: Mixture LFL

Given: 50 % methane (LFL 5.0), 30 % ethane (3.0), 20 % propane (2.1)

Find: LFL.

• 1/(0.5/5 + 0.3/3 + 0.2/2.1) = 1/0.295

Answer: LFL ≈ 3.4 vol %

Common mistakes

• Wrong: Thinking liquids below the flash point are always safe. — Right: Mists and sprays can ignite below the flash point.
• Wrong: Ignoring static electricity. — Right: Bond and earth equipment; limit velocities of low-conductivity liquids.

Applications

• Plant layout: Spacing between tanks, units and control rooms from radiation contours.
• Hazardous-area classification: Zones 0/1/2 define electrical equipment requirements.
• Fire protection design: Deluge rates, foam systems and fireproofing.

Key takeaways

• Fire triangle; flash point, LFL/UFL, AIT.
• Pool, jet, flash fire, fireball.
• q'' = χQ/4πr² sets safe distances.
• Prevent ignition, then detect and protect.

Going further

• Solid-flame model: View factors and flame height (Thomas) give accurate near-field flux.
• Jet fire modelling: Chamberlain model for flame length and radiation.
• Fireproofing: Passive protection rated to UL 1709 hydrocarbon fire curves.