What is flare radiation analysis?

Flare radiation analysis calculates thermal radiation intensity from flare flames to determine safe separation distances for personnel and equipment per API RP 521.

What model is used for flare radiation calculations?

The point source model is commonly used to estimate flare radiation levels, treating the flame as a single radiant point to calculate heat flux at various distances.

What standard defines flare radiation limits?

API RP 521 defines allowable thermal radiation limits for personnel exposure and equipment protection, used to establish safe facility layout distances from flare stacks.

Flare Radiation Analysis Fundamentals

1. Thermal Radiation Limits

API RP 521 Table 12 specifies maximum thermal radiation for personnel and equipment.

API RP 521 Table 12 Summary

Exposure Type	Btu/hr-ft²	kW/m²	Duration	Application
Continuous	500	1.58	8 hours	Manned areas (includes solar)
Emergency	1,500	4.73	2-3 min	Shutdown actions, valve ops
Escape	2,000	6.31	~30 sec	Evacuation routes only
Equipment	3,000	9.46	-	Cable trays (with fireproofing)
Steel	4,000	12.6	-	Uninsulated structures

Design practice: Use 1,500 Btu/hr-ft² for grade-level equipment access. Reserve 500 Btu/hr-ft² for permanently manned areas (control rooms).

2. Point Source Model

Treats flame as a point radiating uniformly in all directions. Conservative for preliminary design.

Radiation Intensity: I = (F × Q × τ) / (4π × R²) Safe Distance: R = √[(F × Q × τ) / (4π × I_limit)] Where: I = Radiation intensity [Btu/hr-ft²] F = Radiation fraction (see table) Q = Heat release [Btu/hr] = ṁ × LHV τ = Atmospheric transmissivity (0.7–1.0) R = Distance from flame center [ft]

F-Factor (Radiation Fraction)

Fuel Type	F-Factor	Range
Hydrogen	0.15	0.10–0.18
Natural Gas (lean)	0.20	0.18–0.23
Natural Gas (rich)	0.25	0.23–0.28
Propane, Butane	0.30	0.28–0.32
Crude Oil, Diesel	0.35	0.33–0.40

Atmospheric Transmissivity (τ)

Conditions	τ
Arid / Desert	0.95–1.0
Temperate	0.85–0.90
Humid / Coastal	0.70–0.80

Example Calculation

Given: Q = 100 MMBtu/hr, F = 0.25, τ = 0.85
       I_limit = 1,500 Btu/hr-ft² (emergency)

R = √[(0.25 × 100×10⁶ × 0.85) / (4π × 1,500)]
  = √[21.25×10⁶ / 18,850]
  = √1,127,320
  = 1,062 ft from flame center

If flame center is 226 ft above grade:
D = √(R² - H²) = √(1062² - 226²) = 1,038 ft horizontal
                

3. Flame Geometry

Flame length sets the location of the radiating source — a longer flame puts the flame center higher and farther downwind, which changes every receiver's intensity. API STD 521 determines flame length primarily from net heat release (read from its flame length vs. heat-release chart, Annex F), not from tip diameter or jet momentum alone. The calculator uses the published curve fit of that chart:

API 521 flame length (heat-release basis): L = 0.00604 × Q^0.4776 (subsonic / buoyancy-dominated) High-momentum (sonic) flames burn shorter: L = 0.004503 × Q^0.459882 (sonic basis) Blended by tip Mach number: L = (1 − Ma) × L_subsonic + Ma × L_sonic Where: L = Flame length [ft] Q = Net heat release [Btu/hr] = ṁ × LHV Ma = Mach number at the flare tip

Why heat release, not a jet Froude/density correlation? A buoyant-jet form such as L/D = 5.3·Fr⁰·⁴·[(ρ_a−ρ_g)/ρ_a]⁰·² is only valid for gas lighter than air; for heavy hydrocarbon relief (MW > ~29) the density term goes negative and the correlation breaks down. The API 521 heat-release method is valid for gases both lighter and heavier than air, which is why it is the basis used here.

Flare-type adjustments: sonic/staged tips use the sonic length; steam- and air-assisted tips entrain air and burn shorter; multipoint flares split the duty across N burners (each sees ≈ Q/N), giving shorter individual flames.

Typical Flame Lengths (API 521 basis)

Net Heat Release	Flame Length (subsonic)	Flame Length (sonic)
100 MMBtu/hr	~40 ft	~22 ft
500 MMBtu/hr	~86 ft	~45 ft
1,000 MMBtu/hr	~120 ft	~62 ft
2,000 MMBtu/hr	~167 ft	~85 ft
5,000 MMBtu/hr	~259 ft	~130 ft

Computed from L = 0.00604·Q⁰·⁴⁷⁷⁶ (subsonic) and L = 0.004503·Q⁰·⁴⁵⁹⁹ (sonic), Q in Btu/hr. A subsonic 2,444 MMBtu/hr flare ≈ 184 ft.

4. Flare Selection

Assisted flares reduce radiation by improving combustion. Steam/air assist can reduce F-factor by 40-55%.

F-Factor by Flare Type

Flare Type	Multiplier	Effective F	Radiation Reduction
Single Point (reference)	1.00	0.25	-
Sonic Flare	0.75	0.19	25%
Multipoint Ground	0.70	0.18	30%
Air Assisted	0.60	0.15	40%
Steam Assisted	0.55	0.14	45%

Technology Comparison

Factor	Single Point	Steam Assist	Air Assist
Capital cost	Lowest	Moderate	Highest
Operating cost	None	High (steam)	Moderate (power)
Smoke suppression	Poor	Excellent	Good
Reliability	Excellent	Good (steam req'd)	Good (power req'd)
Best for	Emergency only	High capacity	Remote sites

Common Design Errors

Using 500 Btu/hr-ft² for equipment: That's for continuous human exposure. Use 1,500 for grade-level design.
τ = 1.0 everywhere: Humidity reduces transmissivity. Use 0.7-0.85 for humid climates.
Ignoring wind tilt: 20 mph wind tilts flame ~45°. Check worst-case wind direction.
Flame center at grade: Use actual flame center height (H_stack + L/2), not stack base.

References

API STD 521, 7th Edition – Table 12 Radiation Limits
Brzustowski & Sommer (1973) – Radiant Heating from Flares
Shell DEP 80.45.10.10 – Flare System Design

Flare Thermal Radiation: API 521 Engineering Guide

500 Btu/hr-ft²

1,500 Btu/hr-ft²

0.15–0.35

Contents

Use this guide to: