Jet Fire Radiation Fundamentals | Flame Length & Thermal Hazard Guide

1. Overview

A jet fire is a turbulent diffusion flame that results from the ignition of a pressurized gas release. Unlike pool fires, which are controlled by buoyancy, jet fires are momentum-dominated flames that maintain a well-defined shape determined by the release velocity and mass flow rate. Jet fires are among the most common fire scenarios at oil and gas facilities, occurring from flange leaks, pipe ruptures, valve failures, and pressure relief device discharges.

The thermal radiation from a jet fire can cause severe burns to personnel, ignition of nearby combustibles, and structural damage to equipment and piping. At close range, the flame can directly impinge on equipment, delivering heat fluxes of 150-300 kW/m^2 that can cause rapid failure of pressure vessels, piping, and structural steel. Understanding jet fire radiation is essential for facility siting, fireproofing design, and emergency response planning.

Flame Temperature

1,500-2,000°C

Significantly hotter than pool fires (900-1,200°C), producing higher local heat flux.

SEP Range

100-300 kW/m²

Surface emissive power depends on gas type, release rate, and soot formation.

Flame Shape

Conical / Cylindrical

Well-defined shape from momentum. Length proportional to mass rate^0.41.

Duration

Steady State

Persists as long as gas release continues. Can last minutes to hours.

Jet fire vs. flare: A jet fire and a flare are physically the same phenomenon, a momentum-dominated diffusion flame. The distinction is intentionality: a flare is a designed, controlled combustion device with a known release rate and radiation exclusion zone, while a jet fire is an unintended release that may occur at any location and orientation. The same radiation models (API 521, Chamberlain) apply to both.

2. Flame Length Models

Flame length is the most important geometric parameter for jet fire radiation calculations, as it determines the size of the radiating surface and thus the view factor to any receptor. Several validated correlations are available, all relating flame length to the mass release rate and gas properties.

Chamberlain (1987) / Shell FRED Model: Flame Length (meters): L = 18.5 × 𝐎^0.41 Where: 𝐎 = Mass release rate (kg/s) L = Visible flame length (m) This correlation was developed for natural gas flares and has been validated against large-scale test data. For other hydrocarbons, the coefficient is adjusted by the ratio of heats of combustion: L = 18.5 × (H_c / H_c,methane)^0.2 × 𝐎^0.41 Alternative: API 521 Model API 521 uses a point source model for flare radiation: q = F_rad × Q / (4πR²) Where: q = Radiation flux at distance (kW/m²) F_rad = Radiation fraction (0.1-0.3) Q = Total heat release = 𝐎 × H_c (kW) R = Distance from flame center (m) The point source model is simpler but less accurate than the solid flame model at close distances.

Flame Length Data

Release Rate (kg/s)	Flame Length (m)	Flame Length (ft)	Heat Release (MW)
0.1	4.8	16	5
1.0	18.5	61	50
5.0	34	112	250
10	48	157	500
50	88	289	2,500
100	123	404	5,000

3. Radiation Modeling

Thermal radiation from a jet fire is modeled using either the point source method or the solid flame method. The point source model treats the entire flame as a single radiating point and is suitable for distances greater than about five flame lengths. The solid flame model treats the flame as a geometrical shape (cylinder or frustum) with a uniform surface emissive power (SEP), and provides more accurate results at all distances.

Solid Flame Radiation Model: Incident radiation at receptor: q = SEP × F × τ Surface Emissive Power: SEP = (f_rad × 𝐎 × H_c) / A_flame SEP = (f_rad × Q_total) / (π × d × L) Where: f_rad = Radiation fraction 𝐎 = Mass release rate (kg/s) H_c = Heat of combustion (kJ/kg) A_flame = Flame surface area (m²) d = Flame diameter (m) ≈ 0.12 × L L = Flame length (m) Typical SEP values: Methane/Natural gas: 120-180 kW/m² Propane: 150-250 kW/m² Hydrogen: 80-120 kW/m² (low soot) Ethylene: 180-280 kW/m² (high soot) View Factor (cylinder to point): F = (R_equiv / L_dist)² R_equiv = √(L × d / 4) L_dist = distance from flame center to receptor Atmospheric Transmissivity: Wayne (1991) model: τ = 1.006 - 0.01171×log(S_m) - 0.02368×log(S_m)² S_m = partial pressure of H2O × path length (Pa·m)

Radiation Fraction

The radiation fraction (also called F-factor or emissivity factor) represents the proportion of total combustion energy that is emitted as thermal radiation. It is the single most significant parameter affecting radiation predictions and also the most uncertain. For methane and natural gas, the radiation fraction typically ranges from 0.15 to 0.25. Heavier hydrocarbons like propane produce more soot and have higher radiation fractions of 0.25 to 0.35. Hydrogen, which burns with minimal soot, has a radiation fraction of 0.15 to 0.20 despite its very high flame temperature.

4. Wind and Tilt Effects

Wind is one of the most important environmental factors affecting jet fire radiation at a receptor. Wind causes the flame to tilt from its release axis, which changes the geometric relationship between the flame and any receptor point. For a vertical release, wind tilts the flame toward horizontal, dramatically increasing the radiation on the downwind side while decreasing it on the upwind side.

Flame Tilt Model: The tilt angle depends on the momentum ratio: R = u_wind / u_jet Where: u_wind = Wind speed (m/s) u_jet = Jet exit velocity (m/s) For momentum-dominated jets (R < 0.1): Minimal tilt, flame maintains release direction For wind-dominated conditions (R > 0.5): Significant tilt, flame deflected toward horizontal Tilt angle: θ = atan(3.3 × R^0.8) The worst-case receptor location is typically downwind for a vertical release, where wind tilt brings the flame closer to ground level.

The effect of wind on thermal radiation is complex. While tilting the flame closer to a downwind receptor increases the view factor and thus the radiation at that specific location, wind also stretches the flame, reducing the flame diameter and potentially the SEP. Additionally, wind promotes mixing of air into the flame, which can reduce soot formation and lower the radiation fraction. For conservative consequence analysis, the worst-case wind direction should be used, which is typically the direction that tilts the flame toward the receptor of interest.

Crosswind vs. Tailwind Effects

Condition	Effect on Flame	Radiation Impact
No wind	Flame follows release direction	Symmetric radiation pattern
Light crosswind (3-10 mph)	Slight tilt, minor shape change	Modest increase on downwind side
Moderate crosswind (10-25 mph)	Significant tilt and elongation	Major increase downwind, decrease upwind
Strong crosswind (> 25 mph)	Near-horizontal flame	Maximum ground-level radiation downwind

5. API 521 Radiation Limits

API 521 and API RP 521 provide thermal radiation limits for flare and jet fire scenarios that are widely used in the oil and gas industry for facility siting and equipment layout. These limits define the maximum permissible radiation flux at locations where personnel or equipment may be present, based on the exposure duration and the type of activity being performed.

Radiation (kW/m²)	BTU/(hr·ft²)	Condition per API 521
1.58	500	Continuous exposure, areas where people are present continuously
4.73	1,500	Pain threshold; exposure > 60 seconds causes burns
6.31	2,000	Maximum for personnel performing emergency actions with escape routes
9.46	3,000	Maximum for emergency operations lasting several minutes
15.77	5,000	Equipment with fire-resistant coatings or water spray protection
31.55	10,000	Peak heat flux for equipment during short-duration events

Facility siting: API RP 752 uses these radiation thresholds for determining minimum distances between potential fire sources and permanently occupied buildings. The recommended design radiation level for occupied buildings with no fire protection is 4.7 kW/m^2 for steady-state fires and a thermal dose criterion for transient events.

6. Source Term Calculation

The accuracy of a jet fire radiation analysis depends critically on the source term, the mass release rate of gas through the leak or rupture. For pressurized gas systems, the release rate depends on the upstream pressure, gas properties, and the effective orifice area. If the pressure ratio across the orifice exceeds the critical ratio, the flow is choked (sonic) and the release rate depends only on upstream conditions.

Choked (Sonic) Gas Release: 𝐎 = C_d × A × P_1 × √(γ × M / (R × T)) × (2/(γ+1))^((γ+1)/(2(γ-1))) Where: 𝐎 = Mass release rate (kg/s) C_d = Discharge coefficient (0.6-1.0) A = Orifice area (m²) P_1 = Upstream pressure (Pa absolute) γ = Ratio of specific heats (Cp/Cv) M = Molecular weight (kg/kmol) R = Universal gas constant (8314 J/kmol/K) T = Gas temperature (K) Choked flow occurs when: P_2/P_1 < (2/(γ+1))^(γ/(γ-1)) For methane (γ = 1.31): Critical pressure ratio = 0.544 Choked when downstream P < 54.4% of upstream P (i.e., release to atmosphere is choked above ~27 psig)

7. Practical Applications

When to Use Jet Fire Modeling

Facility siting studies and building blast/fire analysis per API RP 752 and API RP 753
Quantitative risk assessment (QRA) for onshore and offshore facilities
Flare system design and radiation exclusion zone determination per API 521
Fireproofing specification for structural steel and pressure vessels
Emergency response planning and safe approach distances
HAZOP and LOPA studies requiring consequence severity estimation
Pipeline routing studies near populated areas

Comparison of Fire Scenarios

Parameter	Jet Fire	Pool Fire	BLEVE Fireball
Duration	Steady state (minutes-hours)	Steady state (minutes-hours)	Transient (5-20 seconds)
Flame temp	1,500-2,000°C	900-1,200°C	1,200-1,500°C
SEP	100-300 kW/m²	30-150 kW/m²	200-350 kW/m²
Control	Momentum dominated	Buoyancy dominated	Expansion driven
Shape	Cylinder/cone	Tilted cylinder	Rising sphere

Conservative assumptions: For safety-critical applications, use worst-case wind direction, upper-bound radiation fraction, and assume no credit for atmospheric transmissivity reduction unless the humidity and distance are well characterized. API 521 recommends a radiation fraction of 0.20-0.30 for most hydrocarbon gases.

Jet Fire Radiation

Flare Radiation

Chamberlain Model

Radiation Model

Contents

Use this guide when: