Flare Radiation Analysis

Flare Type Selection

• Single Point: Standard elevated, subsonic exit.
• Sonic: High-pressure, choked flow at tip.
• Steam/Air Assisted: Reduced smoke & radiation (lower F).
• Multipoint: Ground flares, distributed burners.

F-Factor Selection

Gas Type	F Range
Hydrogen (H₂)	0.10 – 0.17
Methane (CH₄)	0.15 – 0.20
Natural Gas	0.20 – 0.25
Propane/Butane	0.28 – 0.33

Note: Steam/air assist reduces F-factor by 40–50%.

Gas Properties Guidance

• Molecular Weight: Lower MW = higher flame temp, generally lower F.
• Carbon Content: More carbon = more soot = higher radiation.
• Compressibility Factor (Z): Use 1.0 for low-pressure flares (<50 psia). Calculate Z for high-pressure sonic tips.
• Mixtures: Calculate molar-weighted average MW and mass-weighted LHV.

Environmental Notes

API RP 521 Default Conditions:
• Wind: 20 mph with 45° tilt angle
• Humidity: 50% (conservative for transmissivity)

API RP 521 Radiation Limits

Exposure Type	Limit (BTU/hr-ft²)
Continuous (8 hr)	500 (1.58 kW/m²)
Emergency (2-3 min)	1,500 (4.73 kW/m²)
Short-term (~30 sec)	2,000 (6.31 kW/m²)
Equipment (no insul.)	4,000 (12.6 kW/m²)

Typical Gas Properties

Gas	MW	LHV (BTU/lb)	Carbon (wt%)
Hydrogen (H₂)	2	51,600	0
Methane	16	21,500	75
Natural Gas	18–20	20,000–22,000	73–75
Ethane	30	20,400	80
Propane	44	19,900	82
Butane	58	19,600	83

Assessment Point Guidance

• Common Assessment Points:
  • Property line / fence: Continuous limit (500 BTU/hr-ft²)
  • Equipment / cable trays: Equipment limit (3000 BTU/hr-ft²)
  • Escape routes: Short-term limit (2000 BTU/hr-ft²)