How is jet fire flame length calculated?

Jet fire flame length is calculated using the Chamberlain model or API 521 correlations. The Chamberlain (Shell FRED) model relates flame length to the mass release rate and effective source diameter: L = 18.5 * (m_dot)^0.41 for natural gas. API 521 provides similar correlations. Flame length depends primarily on the mass flow rate, gas composition, and release velocity.

What is the safe distance from a jet fire?

Safe distance from a jet fire depends on the release rate and gas composition. For personnel, the thermal radiation threshold is typically 4.7 kW/m2 (pain threshold at 60 seconds) or 6.3 kW/m2 (API RP 521 permissible for personnel with escape). For equipment, 15.8 kW/m2 or 31.6 kW/m2 are used depending on the exposure duration and type of equipment.

How does wind affect jet fire radiation?

Wind causes the jet flame to tilt in the downwind direction, which changes the radiation profile. For a vertical release, wind tilts the flame, increasing radiation on the downwind side and decreasing it on the upwind side. The tilt angle depends on the ratio of wind velocity to jet exit velocity. Crosswind conditions are typically the worst case for a receptor located downwind of a vertical release.

Jet Fire Radiation Calculator | Pressurized Gas Release Flame Model

Release Parameters

Mass Release Rate

kg/s

Release Pressure

psig

Orifice / Hole Diameter

inches

Release Orientation

Release Height Above Grade

Gas Properties & Environment

Gas Composition

Molecular Weight

g/mol

Heat of Combustion

kJ/kg

Wind Speed

mph

Ambient Temperature

°F

Relative Humidity

Distance to Receptor

Understanding Jet Fire Modeling

What is a Jet Fire?
A jet fire occurs when a pressurized gas leaks through a hole or rupture and ignites. The resulting turbulent diffusion flame can extend tens or hundreds of feet, producing intense thermal radiation. Jet fires are one of the most common fire scenarios in oil and gas facilities.

Key Parameters:
Flame Length: L = f(mass rate, composition)
Flame Tilt: function of wind speed ratio
SEP: 100-300 kW/m² (depending on gas)
Radiation: q = SEP × F × τ

Key Standards:
API 521 (flare radiation), API RP 521 (guide for pressure-relieving systems), Shell FRED (fire radiation evaluative device), API RP 752 (facility siting).

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Results

Flame Length

feet

Flame Tilt Angle -

Effective Flame Tip Height -

Surface Emissive Power -

Total Heat Release -

View Factor at Distance -

Atmospheric Transmissivity -

Radiation at Distance -

Safe Distance (4.7 kW/m²) -

API 521 Limit (6.3 kW/m²) -

Equipment Limit (15.8 kW/m²) -

Formulas

L = 18.5 × 𝐎^0.41 (Chamberlain model)

SEP: Q_rad / A_flame

Radiation: q = SEP × F × τ

Tilt: cos(θ) = f(u_w/u_j)

Q_total: 𝐎 × ΔH_c

Standards & References

API 521
Pressure-Relieving & Depressuring Systems — Flare Radiation
API RP 521
Guide for Pressure-Relieving and Depressuring Systems
Shell FRED
Fire Radiation Evaluative Device
API RP 752
Permanent Occupied Buildings in Petrochemical Plants
Chamberlain (1987)
Developments in Design Methods for Predicting Thermal Radiation from Flares

Engineering Notes

Jet fire vs. pool fire: Jet fires have higher flame temperatures (1,500-2,000°C) and higher SEP than pool fires, but are more directional
Momentum dominated: Jet fires are controlled by release momentum, not buoyancy, producing a well-defined flame shape
Flame engulfment: Objects within 1-2 flame lengths can receive 100-300 kW/m², causing rapid equipment failure
Wind effects: Crosswind can dramatically increase radiation on the downwind side by tilting the flame toward the receptor
Sonic release: When upstream pressure exceeds ~27 psig for methane, the release is choked (sonic), and mass rate depends only on upstream conditions

API 521 Radiation Limits

1.6 kW/m² — Continuous exposure, public areas
4.7 kW/m² — Pain threshold at 60 seconds
6.3 kW/m² — Personnel with escape (API 521)
9.5 kW/m² — Emergency actions (API 521)
15.8 kW/m² — Equipment with fire protection
31.6 kW/m² — Equipment, short exposure

Jet Fire Radiation Calculator

Release Parameters

Gas Properties & Environment

Understanding Jet Fire Modeling

📚 Learn the Theory

Results

Formulas

Standards & References

Engineering Notes

API 521 Radiation Limits

Related Calculators

BLEVE Fireball

Pool Fire Modeling

Flare Radiation

Gas Loss from Hole