Jet fire flame length and radiation calculator
A jet fire is the ignited, momentum-driven release of a pressurised flammable fluid. Its hazard is concentrated: a long, high-emissive-power flame that can impinge directly on adjacent equipment and drive escalation far faster than a pool fire. The calculation starts from the same source term as an unignited release.
Open the Jet Fire moduleWhat this calculator returns
- Flame length and lift-off distance
- Flame tilt under wind
- Surface emissive power and radiated fraction
- Radiation intensity versus distance and impingement reach
Required inputs
- Mass release rate from the discharge calculation
- Fuel heat of combustion and composition
- Release direction and elevation
- Wind speed and target location
Calculation method
The flame length is correlated with the release rate through the fuel heat release rate; the flame is anchored a short lift-off distance from the orifice.
The flame is represented as a tilted cone or cylinder radiating at a surface emissive power derived from the radiative fraction of the heat release.
Received radiation at a target combines the view factor for the tilted flame shape with atmospheric transmissivity.
Governing equations
Q = m dHcTotal heat release rate from the ignited release.
L = 0.00326 ( m dHc ) ^ 0.478Flame length correlation from the heat release rate.
q = SEP F tauReceived radiation from the solid-flame representation.
Nomenclature
- m
- mass release rate, kg/s
- dHc
- heat of combustion, J/kg
- Q
- heat release rate, W
- L
- flame length, m
- SEP
- surface emissive power, kW/m2, typically 200 to 350 for a hydrocarbon jet
Assumptions and limitations
- The release is single-phase gas with a steady rate over the exposure of interest.
- The flame is fully developed and attached after the lift-off distance.
- Flame impingement heat flux is treated separately from radiative flux to a distant target.
Reference practice
- Aligned with API 521 radiation criteria and the jet fire treatment in the CCPS consequence guidelines.
Worked example
A 2 kg/s natural gas jet fire from a 50 barg line, still air, target at grade.
| Step | Value | Basis |
|---|---|---|
| Heat release rate | about 100 MW | Q = m dHc with dHc near 50 MJ/kg |
| Flame length | about 25 m | Correlation on the heat release rate |
| Surface emissive power | about 250 kW/m2 | Typical for a clean natural gas jet flame |
| Distance to 37.5 kW/m2 | roughly one flame length beyond the flame tip | Escalation contour for adjacent equipment |
Flame impingement, not distant radiation, is what usually drives escalation for a jet fire — the layout check is whether the flame length reaches an adjacent vessel or structural member.
Illustrative numbers only — rerun the module with the project basis of design before using any result.
Common questions
How is jet fire flame length calculated?
From the heat release rate, which is the mass release rate multiplied by the heat of combustion. Flame length scales with roughly the 0.48 power of the heat release, so it grows slower than the release rate.
Is a jet fire worse than a pool fire?
For escalation, usually yes: the emissive power is several times higher and the flame reaches horizontally to impinge on equipment that a pool fire only heats radiatively.
Does blowdown limit the jet fire?
Yes. Emergency depressurisation reduces the source term with time, shortening the flame and limiting the duration of impingement to below the failure time of the exposed steel.
Related calculators
- Gas Discharge — Gas discharge rate calculator for pressurised releases
- Flare Radiation — Flare thermal radiation calculator
- Pool Fire — Pool fire radiation calculator for liquid spill fires