Process Safety · Lesson 9 of 9 · 12 min read

Toxic and flammable releases

Source terms, dispersion, AEGL and LFL zones, pool fires, and emergency response.

Educational overview. The principles here are general and simplified, and the figures and rules of thumb are typical values, not limits for your plant. Decisions need process-specific data, the applicable codes and standards, and a qualified assessment.

From leak to consequence

Consequence analysis follows the material: source term (how much escapes, how fast) → pool and evaporation (for liquids) → dispersion in the wind → effect on people (toxic dose, flash fire, heat radiation, overpressure).

Liquid through a hole: Q = C_d · A · √(2·ρ·ΔP) (C_d ≈ 0.61)
ΔP = pad / pump pressure + ρ·g·h (liquid head)Gases and liquefied gases (Cl₂, NH₃) escape much faster and partly flash; a 6 mm hole in a chlorine tonner can empty it in well under an hour.

Toxic limits: AEGL

Level	Meaning (60-minute exposure)
AEGL-1	Notable discomfort, irritation; reversible
AEGL-2	Irreversible or serious long-lasting effects, or impaired ability to escape
AEGL-3	Life-threatening effects or death

Example 60-min values: chlorine 0.5 / 2 / 20 ppm; ammonia 30 / 160 / 1,100 ppm. Chlorine’s are tiny, which is why a small chlorine leak needs a far larger emergency zone than a large solvent leak.

Flammable limits and fires

LFL / UFL: a vapour–air mixture burns only between the lower and upper flammable limits (methanol 6–36 %, toluene 1.1–7.1 %). Zones are usually drawn to ½ LFL to allow for pockets of richer mixture.
Flash fire: a cloud ignites and burns back to the source. Deadly inside the cloud, little effect outside.
Vapour cloud explosion: the same cloud in a congested area (pipe racks, buildings) can accelerate into a damaging blast (Flixborough, Buncefield).
Pool fire: harm is by heat radiation: 37.5 kW/m² damages equipment and is fatal; 12.5 kW/m² ignites wood and is lethal within minutes; 4 kW/m² causes pain within about 20 s.

Weather matters

The same leak is far more dangerous on a calm, clear night (Pasquill stability class F, low wind) than on a windy afternoon (class D): the cloud stays concentrated and travels further. Dense gases (chlorine) hug the ground and flow into drains and basements.

Emergency response basics

Detect fast: gas detectors, CCTV, manual call points, siren.
Evacuate crosswind, then upwind, to the assembly point the control room names for the current wind. Head count at the assembly point.
Isolate from a safe place: remote shut-off valves, king valves, emergency kits (e.g. chlorine kit B).
Water curtains knock down soluble clouds (NH₃, HCl, Cl₂); foam blankets cut evaporation from solvent pools.
Never spray water onto a leaking chlorine container: it forms acid and enlarges the hole.

See all of this in 3D: open the virtual plant tour, choose What-If Leak, pick a scenario and press Play the emergency response.

✍️ Check yourself: A chlorine leak and a methanol leak give the same concentration (in ppm) 200 m downwind. Which is more dangerous to people there?

Chlorine AEGL-3 (60 min) is 20 ppm; methanol AEGL-3 is 7,200 ppm. The same concentration means very different harm.

✍️ Check yourself: When should you expect the largest toxic zone from a ground-level leak?

A clear calm night gives stable air (class F) and low wind: little mixing, so the cloud stays concentrated over a long distance.

Key takeaways

Source term → pool / evaporation → dispersion → effect.
AEGL-1/2/3 for toxic effects; LFL / ½ LFL for flammable clouds; 37.5 / 12.5 / 4 kW/m² for fires.
Stable night-time weather gives the largest zones; dense gases hug the ground.
Respond: detect, alarm, evacuate crosswind → upwind, isolate remotely, knock down.