In process safety engineering and chemical facility design, evaluating the impact of accidental toxic gas releases—such as Hydrogen Sulfide (H2S)—is a fundamental requirement. Carrying out a Quantitative Risk Assessment (QRA) translate hazardous dispersion models into concrete numerical metrics: Location Specific Individual Risk (LSIR), Potential Loss of Life (PLL), and Fatal Accident Rate (FAR).
In this guide, we will break down the mathematical workflow used to determine toxic risk metrics using real-world QRA data and a toxicity-based Probit equation.
Essential Terminology
Probit (Pr): A statistical measure converting toxic dose (concentration and duration) into a probability of fatality.
Pf (Probability of Fatality): The percentage likelihood that an exposed individual will suffer a fatal outcome.
LSIR (Location Specific Individual Risk): The annual fatality risk or serious injury to a person in the vicinity of a hazard, assuming the person is there 365 days a year, 24 hours a day
PLL (Potential Loss of Life): The total expected human fatalities per year across an exposed population (societal risk measure).
FAR (Fatal Accident Rate): The estimated number of fatalities per 108 person-hours of exposure.
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:
Study variables for thermal radiation due to pool fire
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.
Thermal radiation from pool fire (wind speed 2.5 ms-1)Thermal radiation from pool fire (wind speed 5 ms-1)Thermal radiation from pool fire (wind speed 10 ms-1)
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 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.
Thermal radiation from pool fire (RH 5%)Thermal radiation from pool fire (RH 50%)Thermal radiation from pool fire (RH 99%)
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).
Thermal radiation from pool fire (air temp 20 C)Thermal radiation from pool fire (air temp 24 C)Thermal radiation from pool fire (air temp 34 C)
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
In this post, I want to share reverse engineering using ALOHA (Areal Locations of Hazardous Atmosphere) to model thermal radiation and safe fire distances. The original publication can be downloaded here.
The purpose of the study is to assess safe distance of thermal radiation exposure resulting from a tank fire (pool fire) scenario using ALOHA simulation software.
ALOHA (Areal Locations of Hazardous Atmospheres) is an industry-standard atmospheric dispersion modelling tool developed by the EPA and NOAA to predict how chemical releases spread during industrial accidents. By factoring in chemical properties, storage conditions, and ambient weather, it calculates precise threat zones for toxic plumes, explosions, and pool fires. Read More
In this post I want to share with you process parameters for biogas production from industrial liquid waste. By understanding required these parameters, optimal environment conditions for biogas production can be maintained. The parameters include temperature, pH and buffering systems, gas solubility, nutrients, and toxicity.
Process Parameters for Biogas Production from Industrial Liquid Waste
In this post, I’ll explain how to estimate the energy potential of biogas generated from Palm Oil Mill Effluent (POME). To convert POME into energy, it undergoes anaerobic digestion to produce methane-rich biogas. If you’re new to these concepts, feel free to check out my previous post on biogas and biorefineries.
In this post I want to share with you how to size indirect fired water bath heater.
Indirect Fired Water Bath Heaters (IFWBH) safely heat various mediums by transferring energy indirectly. In this system, the process coils are submerged in a heated bath solution—typically a water-glycol mixture—which absorbs heat from the firetube and transfers it to the process media.
How Indirect Fired Water Bath Heater Works
Fuel gas burns inside a firebox submerged in the lower section of the water bath, transferring heat through the firebox wall to maintain the bath at the desired temperature. The process fluid (such as well fluids, natural gas, or oil) flows through a coil immersed in the upper section of the bath, where it is heated indirectly through the tube walls. A temperature controller regulates the fuel gas supply to the firebox to keep the water bath at an optimum operating temperature of 190°F. Operating above this threshold is inefficient, as it leads to excessive water loss through evaporation.
In this post I want to share with you how to estimate greenhouse gas emissions in oil and gas industry.
Greenhouse gas (GHG) emissions from oil and gas production activities must be accurately estimated to enhance accountability and formulate efficient mitigation strategies. Particularly, the oil and gas industry is the largest source of methane (CH4) emissions within the energy sector. As the primary component of natural gas, methane is a significantly more potent GHG than carbon dioxide (CO2), trapping up to 25 times more heat in the atmosphere over a standard timeline. Read More
Mercury (Hg) can be present in feed gas in wide range of concentrations. It is important to remove mercury from the feed gas to avoid mercury attack on the brazed aluminum heat exchangers. Aluminum material is highly reactive with mercury and can be corroded quickly resulting heat exchangers failure and potentially can pose environmental and safety hazards. In addition, mercury will also cause poisoning precious metal catalyst if it presents in petrochemical process. This post outlines the method on mercury removal in oil and gas processing facilities.
Mercury Removal in Oil and Gas Processing Facilities
Mercury presents in most natural gas field in concentration ranging from less than 10 ppb to more than 1 ppm as elemental (metallic), organic, and inorganic compound. Due to its strong affinity for higher molecular weight fractions, mercury predominantly concentrates within the liquid phase rather than the gas stream. Read More
In anaerobic digestion and biogas production, we often hear or read about BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand). Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) are critical parameters used to quantify the organic load of a substrate. This post will explain simply about those two terms.
Biogas biorefineries are an innovative concept in the field of process engineering. As a process engineer, this is my first experience working with biogas biorefineries, and I am eager to explore and understand this subject in depth. This section will help us get acquainted with the basics and significance of biogas biorefineries within the industry.
Definition of Biogas Biorefineries
Biogas biorefineries use sophisticated, combined processes to convert organic waste, energy crops, and agricultural leftovers into biogas, biomethane, and valuable bio-based products through anaerobic digestion.
Biorefineries of biogas may include the following type:
Biogas biorefineries focusing on the production of biogas and fertilizers using energy crops or waste substrates like food waste or food production waste
Sugar crop biorefineries based on sugar beet, sugar cane, etc.
Starch crop biorefineries based on cereals, maize, potato, etc.
Green biorefineries based on wet biomass like grass, clover, etc.
Biorefineries that process all kinds of organic waste, including municipal, food industry, restaurant, and grocery store waste.
Typical Process of Biogas Biorefineries
Typical process of biogas biorefineries is shown below. Read More
