In industrial facilities handling flammable liquids, accidental liquid spillage within bunded storage areas or open ground poses a major catastrophic risk: the formation of a pool fire. Determining safe separation distances for personnel and structural assets requires quantitative risk assessment (QRA) models. While pool diameter and fuel properties determine intrinsic burning rates, atmospheric and meteorological conditions dramatically increase or decrease the off-site thermal hazard footprint.
This article presents an analysis based on atmospheric modelling case studies using ALOHA (Areal Locations of Hazardous Atmosphere). The individual sensitivity of three core atmospheric variables is evaluated—wind speed, ambient air temperature, and relative humidity—across three thermal radiation thresholds:
- Red Zone (> 35 kW/m2): 100% immediate fatality
- Orange Zone (> 12.5 kW/m2): Causes extreme pain within 20 seconds. Can be fatal if evacuation is hindered. Estimated fatality rates: 30% indoors / 50% outdoors.
- Yellow Zone (> 6.31 kW/m2): Continuous exposure may cause skin burns. Personnel wearing appropriate PPE (protective clothing/uniforms) can tolerate this level temporarily
Case Study Data Overview
The modelling dataset evaluates a standard liquid hydrocarbon pool fire subject to parametric atmospheric variations while keeping secondary parameters constant. The other parameters using the parameter from previous post.
The exact threat radii measured downwind from the pool centre are summarized below:
The Dominant Impact of Wind Speed
Wind speed shows the most profound geometric impact on thermal radiation hazards. As wind velocity increases from 2.5 m/s to 10:
- The high-intensity burn threshold (Red Zone) expands downwind from 12 m to 20 m, a 66.7% expansion in lethal reach.
- The hazard footprint distorts from a near-concentric circle into an elongated ellipse oriented along the downwind vector.
Physical Mechanism: Flame Bending & View Factor
Crosswind pushes against the buoyant plume, slanting the flame angle toward the ground downwind. This physical tilt brings the radiant flame column much closer to ground-level targets, causing a steep increase in the geometric view factor. At the same time, flame drag stretches the base of the flame downwind beyond the original pool boundaries.
While the near-field high-intensity zone (Red Zone) grows continuously with wind speed, the far-field boundary (Yellow Zone) peaks at 5 m/s before slightly contracting to 31 m at 10 m. Extreme wind tilts the flame so heavily toward the ground that vertical flame height decreases, reducing long-distance line-of-sight exposure even as ground-level flux nearby reaches peak intensity.
Relative Humidity & Infrared Atmospheric Attenuation
Relative humidity acts as a natural shield against thermal radiation across distances. Lower humidity levels result in wider thermal hazard boundaries:
- Under saturated conditions (99% relative humidity), the outer Yellow Zone boundary extends to 31 m.
- Under dry conditions (5% relative humidity), the Yellow Zone reaches 35 m—an increase of 4 m in critical safety distance.
Physical Mechanism: Water Vapor Absorption
Relative humidity operates as an atmospheric filter against thermal radiation. Triatomic water vapor molecules absorb infrared energy emitted by soot particles and combustion gases. Under low relative humidity, reduced moisture raises path transmissivity, enabling thermal radiation to travel farther and expanding the outer safety distance.
Ambient Air Temperature: Minimal Sensitivity
Varying ambient air temperature between 20oC and 34oC produces negligible changes in thermal hazard radii:
- The Red Zone shifts by only (12 m to 13 m).
- The Orange and Yellow Zones remain almost unchanged across all temperature regimes (23-24 m and 32-33 m).
While warmer ambient air temperatures slightly pre-heat the liquid pool (reducing the enthalpy needed for fuel vaporization), the absolute temperature difference in Kelvin (293 K vs 307 K) is minimal compared to the core flame temperature (Tf ~1200-1400 K). Thus, ambient air temperature plays a secondary role in consequence modeling compared to wind speed and relative humidity.
Key Engineering Takeaways for Process Safety & Plant Layout
- Design for Peak Wind Conditions: Layout and separation distances must be evaluated against localized high wind speed vectors rather than calm ambient averages, as wind-induced flame tilt expands severe high-intensity hazard zones by over 66%
- Account for Dry Environments: Facilities situated in desert or low relative humidity climates require larger safety buffer zones due to reduced atmospheric water vapor intensity.
- Implement Downwind Barriers: Passive fire protection (blast walls, fire walls, deluge systems) should target downwind vectors where ground view factors sskyrocket under wind conditions
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