Pressurized Gas Release — Flame Length & Thermal Radiation
Understand jet fire flame models, radiation calculations, wind tilt effects, and safe distance determination
Jet fire flame length is calculated using the Chamberlain model or API 521 correlations. The Chamberlain (Shell FRED) model relates flame length to the mass release rate and effective source diameter: L = 18.5 * (m_dot)^0.41 for natural gas. API 521 provides similar correlations. Flame length depends primarily on the mass flow rate, gas composition, and release velocity.
Safe distance from a jet fire depends on the release rate and gas composition. For personnel, the thermal radiation threshold is typically 4.7 kW/m2 (pain threshold at 60 seconds) or 6.3 kW/m2 (API RP 521 permissible for personnel with escape). For equipment, 15.8 kW/m2 or 31.6 kW/m2 are used depending on the exposure duration and type of equipment.
Wind causes the jet flame to tilt in the downwind direction, which changes the radiation profile. For a vertical release, wind tilts the flame, increasing radiation on the downwind side and decreasing it on the upwind side. The tilt angle depends on the ratio of wind velocity to jet exit velocity. Crosswind conditions are typically the worst case for a receptor located downwind of a vertical release.