API 521 Flare Stack Height & Thermal Radiation Calculator
Calculate minimum required flare stack height, thermal radiation contours at grade, flame length, and wind tilt angles per API Standard 521.
Relief Gas Flow & Heating Value
API 521 Radiation Limits & Fraction
Stack Height & Radiation Sizing
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Frequently Asked Questions
What are the API 521 thermal radiation limits for personnel exposure?
API 521 recommends K = 500 BTU/hr·ft² for continuous unrestricted personnel exposure without special clothing, 1,000 BTU/hr·ft² for operating personnel performing routine tasks for several minutes, 1,500 BTU/hr·ft² for emergency escape paths (maximum 1 to 2 minutes), and 3,000 to 5,000 BTU/hr·ft² for unmanned structural steel equipment.
How does wind speed influence the required flare stack height?
Crosswinds tilt the flame downwind toward grade. As wind speed increases, the flame center shifts horizontally and downward, significantly reducing the distance between the radiant fireball and personnel on the ground. Stack height must be increased to maintain safe radiation limits during design wind conditions.
What is the radiation fraction (F) in flare calculations?
The radiation fraction F is the portion of total combustion energy emitted as thermal radiation. Clean, low-molecular-weight gases like hydrogen or methane have F ~ 0.15 to 0.20, whereas heavy hydrocarbon gases that produce sooty, luminous flames can reach F = 0.28 to 0.35 unless smokeless steam or air assist is injected.