Flare Thermal Radiation Safe Distance (API 521)

Only a fraction of the heat release leaves as radiation -- the rest goes up with the plume -- and that fraction depends on the gas, from around 0.1 for light hydrocarbons to 0.3 for heavier and sootier ones. Beyond that it is inverse square, which means distances move much more slowly than heat releases: halving the allowable radiation multiplies the required distance by 1.41, not by 2. SOLAR IS ADDED, NOT IGNORED, and it is a large part of the budget. On a clear day the sun contributes on the order of 250 to 300 BTU/h per square foot, which against a 500 BTU/h-sq ft continuous-personnel criterion is more than half the allowance spent before the flare is lit. The distance with the solar term left out is computed here alongside the correct one, because the gap between them is the whole argument and quoting it from memory understates it. THE CRITERION HAS TO MATCH THE TARGET, and this is the error that matters. A fence line where the public may stand, a control room, a walkway an operator uses during the emergency, and a vessel that only has to survive the event are four different numbers, and using the equipment figure where people stand is not a conservative simplification. AND THE STILL-AIR RESULT IS NOT THE GOVERNING CASE: wind tilts the flame, moving the effective radiating centre downwind and lower, and increasing radiation on the downwind side substantially. This is a point-source screen on an entered heat release and radiant fraction. It does not model flame length, flame tilt or the wind case, locate the radiating centre, select the radiant fraction or the criterion for a target, address flare tip design, exit velocity, or smokeless operation, evaluate dispersion of an unignited release, or address the noise and the ground-level concentration a flare also produces. API 521 as adopted, the operator's facility siting standards, and the design engineer govern.

the API 521 point-source relation D = sqrt(F x Q / (4 pi K)), with the solar contribution SUBTRACTED from the allowable level before the distance is taken, because solar adds to the flare's radiation at the target.

A still-air point-source screen. The radiant fraction and the criterion are ENTERED; flame length and wind tilt are not modelled.

One square root and one subtraction.

Estimate. AHJ and licensed professional govern.

Field names used by the API: heat_release_btuh, radiant_fraction, allowable_btuh_ft2, solar_btuh_ft2, available_distance_ft, radiated_btuh, budget_btuh_ft2, required_distance_ft, no_solar_distance_ft, solar_penalty_ft, radiation_at_distance_btuh_ft2, supportable_release_btuh

• Point source, still air wind tilts the flame and increases downwind radiation; that is the design caseAPI 521 as adopted
• The radiant fraction is entered 0.1 to 0.15 for light hydrocarbons, up to 0.3 for sootier gasesthe gas composition
• The criterion is entered fence line, control room, escape route and equipment are four different numbersthe operator's facility siting standards