Introduction:

Flare Radiation and Dispersion Analysis is a specialized engineering study that assesses the safety and environmental impact of operating a flare system. Flares are critical safety devices used to combust waste or emergency-relieved gases in a controlled manner. This analysis has two key components: 1) Radiation Analysis to calculate thermal radiation levels at ground level from the flare flame, and 2) Dispersion Analysis to model the release and atmospheric dispersion of un-combusted gases (e.g., during a flare ignition failure or during smokeless operation with steam/air assist).

Purpose:

The dual-purpose is to ensure personnel safety from thermal radiation and to prevent hazards from unburned gas plumes. Objectives are to: 

1) Verify that calculated radiation levels at grade and on nearby equipment do not exceed allowable limits for personnel exposure (typically 1.58 kW/m² for short-term escape, 4.73 kW/m² for emergency actions) or equipment integrity, 

2) Ensure that dispersion of unburned gases, particularly during low-flow, low-heating value, or flame-out conditions, does not create a flammable or toxic cloud at ground level, and 

3) Optimize flare tip design and assist medium (steam/air) rates.

Methodology:

The analysis uses integrated modeling:

1.  Define Flare Scenarios: Consider various operating cases—steady-state purge/pilot, normal smokeless operation, emergency relief (single largest contingency, multiple contingencies), and flame-out/puff scenarios.

2.  Flame & Radiation Modeling: Use empirical methods (e.g., API 521, Brzustowski & Sommer) or computational fluid dynamics (CFD) to predict flame length, shape, tilt, and the resulting thermal radiation intensity isopleths on the ground. This requires data on gas composition, flow rate, exit velocity, and wind speed.

3.  Dispersion Modeling for Uncombusted Releases: For scenarios where gases are not fully combusted, conduct a gas dispersion analysis (see Gas Dispersion Analysis) to plot ground-level concentration contours of flammable/toxic components.

4.  Evaluation & Mitigation: Compare results against safety criteria. If radiation levels are too high, increase flare stack height or establish a restricted area (flare radiation fence). If dispersion results show hazardous ground-level concentrations, review flare operability (reliability of pilots, ignition systems) and consider design changes.

Importance in the Process Industry:

The flare is the ultimate safety device for managing overpressure events. However, the flare itself can become a hazard if not properly designed. This analysis is crucial for ensuring that the final safeguard does not create new, unacceptable risks. It directly informs safe plot plan layout, defines exclusion zones for personnel, and ensures compliance with environmental regulations regarding combustion efficiency and emissions. It is a key part of the flare system design package, required to demonstrate that the plant's last line of pressure relief is safe under all operating scenarios.