What is thermal radiation?
Thermal radiation is the heat flux received at a distance from a fire, expressed in kilowatts per square metre. It is the effect measure used to set separation distances, assess escalation to neighbouring equipment, and check that escape routes and manned locations remain survivable.
Radiation levels and what they mean
- 1.6 kW/m2 — no discomfort for prolonged exposure; commonly used for continuously manned areas.
- 4.7 to 5 kW/m2 — pain in about 15 to 20 seconds; used as an escape-route and emergency-action criterion.
- 12.5 kW/m2 — piloted ignition of wood and damage to plastic; often used for escalation to unprotected equipment.
- 37.5 kW/m2 — damage to process equipment and structural steel over sustained exposure.
How the flux is calculated
Two approaches are common. The point-source method treats the fire as a single emitting point releasing a fraction of the combustion heat as radiation, with flux falling as the inverse square of distance and reduced by atmospheric transmissivity. It is simple, robust, and reasonable in the far field.
The solid-flame method represents the fire as a geometric body — a cylinder for a pool fire, a cone or tilted cylinder for a jet flame, a sphere for a fireball — radiating at a surface emissive power, with a view factor between that surface and the receiver. It is more accurate close to the flame, where the point-source method under-predicts.
Dose, not just intensity
Harm depends on how long a person is exposed, not only on the peak flux. Fatality probits use a thermal dose in units of (kW/m2)^(4/3) times seconds, so a brief exposure to a high flux may be less harmful than a long exposure to a moderate one. Escape modelling — where people are, how quickly they can move out of the radiation field, and what shielding exists — therefore materially changes the result.
Factors that change the answer
- Atmospheric transmissivity — humid air absorbs a meaningful fraction of the radiation over long distances.
- Flame tilt from wind, which moves the hot surface towards downwind receptors.
- Fraction of heat radiated, which depends on fuel sootiness — a sooty flame radiates a larger fraction.
- View factor geometry, including whether the receiver is vertical, horizontal or oriented for maximum flux.
Frequently asked questions
What units are used for thermal radiation?
Incident radiant heat flux in kW/m2. Dose for harm calculations is expressed as (kW/m2)^(4/3) times exposure seconds, sometimes called thermal dose units.
Which criterion should be used for a flare?
API 521 gives design radiation levels for flare systems, distinguishing between continuous manned areas, areas where personnel can escape within a short time, and equipment-only areas. The applicable value should come from that hierarchy and the project design basis.
Does clothing protect against thermal radiation?
Ordinary work clothing gives some protection and is often credited in vulnerability models, but the crediting assumption should be stated explicitly because it can change fatality estimates significantly.
Calculators for this topic
- Flare Radiation — Flare thermal radiation calculator
- Gas Discharge — Gas discharge rate calculator for pressurised releases