What is gas dispersion modelling?
Gas dispersion modelling predicts the concentration of a released gas as a function of distance, time and atmospheric conditions. It answers the questions that drive layout and emergency planning: how far does the flammable cloud extend, and where does the toxic concentration drop below a harm threshold?
Neutral, buoyant and dense gas
A gas that has the same effective density as air disperses passively and is well described by a Gaussian plume or puff model. A buoyant release lifts away from the ground. A dense release — heavy hydrocarbons, chlorine, cold flashing vapours, or any gas cold enough that its density exceeds air — slumps, spreads laterally and travels much further at ground level before diluting.
Getting this classification wrong is one of the most common modelling errors, because a dense cloud can produce hazard distances several times longer than a passive model predicts.
The Gaussian plume model
For a continuous, passive release the concentration field is described by a Gaussian distribution in the crosswind and vertical directions, scaled by the release rate and inversely by wind speed and the dispersion coefficients. The dispersion coefficients grow with downwind distance and depend on atmospheric stability.
- Higher wind speed dilutes faster but carries the cloud further downwind before it disperses.
- Stable conditions (Pasquill F) give the longest hazard distances; unstable daytime conditions (A/B) give the shortest.
- Surface roughness increases turbulence and shortens hazard distances over congested plant compared with open water.
Concentration thresholds that matter
- LFL and half-LFL for flammable extent — half-LFL is commonly used to account for concentration fluctuations.
- Toxic endpoints such as ERPG-2, AEGL-2 or IDLH for emergency planning.
- Detector set points, typically expressed as a fraction of LFL for flammable gas detection.
- Occupied building infiltration criteria for toxic ingress assessment.
What dispersion modelling cannot do
Integral models assume a broadly uniform wind field and simple terrain. They do not resolve individual obstacles, recirculation behind large structures, or confined congested regions — that requires CFD. They also do not capture the momentum-dominated near field of a high-pressure jet in detail; a jet release should be modelled with the jet phase before handing over to the passive or dense phase.
Frequently asked questions
What is the difference between toxic and flammable dispersion?
The physics is the same; only the endpoint differs. Flammable assessment looks for the LFL or half-LFL contour and cares about the cloud at the moment of ignition. Toxic assessment tracks dose — concentration integrated over exposure time — against health-based thresholds, so cloud duration matters as much as peak concentration.
Which weather case should be used?
Studies typically run a stable low-wind case (for example 2 m/s, Pasquill F) and a neutral moderate-wind case (for example 5 m/s, Pasquill D), then take the worst credible result or weight cases by their annual frequency in a QRA.
Why is half-LFL used instead of LFL?
Dispersion models predict time-averaged concentration, but real clouds fluctuate. Using half of the lower flammable limit is an accepted allowance so that instantaneous pockets above LFL beyond the mean contour are captured.
Calculators for this topic
- Gas Dispersion — Atmospheric gas dispersion calculator
- Gas Discharge — Gas discharge rate calculator for pressurised releases
- Gas Detection Mapping — Gas detection coverage mapping