Flare thermal radiation calculator
Flare radiation governs the sterile area around a flare stack and the exposure of nearby structures. This module sizes the flame from the relieving load, accounts for wind tilt, and then evaluates the incident radiation at any point at grade so exclusion radii can be drawn against the usual design criteria.
Open the Flare Radiation moduleWhat this calculator returns
- Flame length and flame tilt under the design wind
- Radiant heat flux at grade as a function of distance
- Exclusion distances for selected radiation criteria
- Radiation contours for the plot-plan
Required inputs
- Relieving mass flow and gas heating value
- Flare tip diameter and stack height
- Wind speed and fraction of heat radiated
- Radiation criteria to be checked
Calculation method
The total heat release is the product of the relieving rate and the net heating value; a fraction of that energy is radiated from the flame.
The flame is represented as a point source located at the flame centre, displaced by the wind-induced tilt.
Incident flux at a receptor combines the inverse-square distance decay with an atmospheric transmissivity term for humidity absorption.
Governing equations
Q = m HcTotal heat release from the relieving stream.
I = tau F Q / ( 4 pi D^2 )Incident radiant flux from an equivalent point-source flame.
D = sqrt( tau F Q / ( 4 pi I_allow ) )Exclusion distance for an allowable radiation level.
Nomenclature
- Q
- heat release rate, W
- m
- relieving mass flow, kg/s
- Hc
- net heating value, J/kg
- F
- fraction of heat radiated, dimensionless
- tau
- atmospheric transmissivity, dimensionless
- D
- distance from the flame centre to the receptor, m
- I
- incident radiant flux, W/m2
Assumptions and limitations
- Point-source flame representation, which is conservative close to the flame.
- Solar radiation is not added to the calculated flux unless the criterion states it.
- Steady relieving rate and wind speed.
Reference practice
- Radiation criteria and the point-source method follow API 521 flare system design practice.
Worked example
A flare burning 50 MW of heat release with a radiant fraction of 0.30, a view factor of 0.2 and an atmospheric transmissivity of 0.8, assessed at 200 m from the flame centre.
| Step | Value | Basis |
|---|---|---|
| Radiant heat | 15 MW | Q_rad = 0.30 x 50 MW |
| Geometry | Point source at 200 m | Spherical spreading over 4 pi r^2 |
| Received flux | about 4.8 kW/m2 | q = tau F Q_rad / (4 pi r^2) |
| Acceptance | Below the 6.3 kW/m2 criterion | API 521 limit for areas where personnel can take shelter within a few seconds |
Radiation falls with the square of distance, so the sterile radius around a flare is set almost entirely by flame height and the radiant fraction assumed for the fuel.
Illustrative numbers only — rerun the module with the project basis of design before using any result.
Common questions
What radiation levels does API 521 use?
1.58 kW/m2 for continuous personnel exposure, 4.73 kW/m2 for limited exposure with escape, and 6.31 kW/m2 where escape takes only a few seconds; equipment criteria are higher.
What radiant fraction is typical?
Roughly 0.15 to 0.20 for methane and 0.25 to 0.35 for heavier hydrocarbons; the received flux scales directly with it.
Does solar radiation need to be added?
Yes for personnel exposure criteria, where about 1 kW/m2 of solar load is normally added to the flare contribution.
Related calculators
- Gas Dispersion — Atmospheric gas dispersion calculator
- Gas Discharge — Gas discharge rate calculator for pressurised releases