Introduction:

Gas Dispersion Analysis is the use of computational models to predict the behavior of an accidental or unplanned release of flammable or toxic gas/vapor into the atmosphere. It is a core component of Quantitative Risk Assessment (QRA) and consequence modeling. The analysis simulates how the released material forms a cloud, how it travels downwind, and how its concentration changes due to atmospheric conditions, gravity, and turbulent mixing.

Purpose:

The main purpose is to understand the potential spatial impact of a loss of containment event. Specific aims are to: 

1) Determine the extent of flammable gas clouds (defining zones where concentration is between the Lower and Upper Flammable Limits), 

2) Determine the extent of toxic gas clouds at harmful concentration levels (e.g., IDLH, ERPG levels), 

3) Provide input for other analyses—defining hazard ranges for fire and explosion models, informing Emergency Response Planning, and assessing risk to personnel and off-site populations.

Methodology:

The process is scenario-driven and more complex than vent dispersion:

1. Define Release Scenarios: Based on HAZOP/PHA, select credible loss of containment scenarios (e.g., full bore rupture of a pipeline, leak from a flange). Define source terms (hole size, pressure, inventory, phase—liquid flash or gas).

2.  Select Meteorological Conditions: Model a range of conditions, including worst-case (e.g., low wind speed, stable atmosphere (F stability) which promotes poor dispersion) and more probable conditions (e.g., Pasquill D stability with average wind speed).

3.  Choose & Run Model: Select an appropriate model. For heavy gas dispersion (e.g., LNG, chlorine), use dense gas models (e.g., SLAB, DEGADIS, or CFD). For neutral/buoyant gases, use Gaussian-based models or CFD. The model calculates cloud concentration contours over time and distance.

4. Interpret Results: Generate "footprint" maps showing distances to specific concentration thresholds (e.g., LFL, ½ LFL, ERPG-2). These footprints are used for QRA, building siting assessments, and gas detector placement.

Importance in the Process Industry:

Gas Dispersion Analysis is fundamental to understanding major accident hazards. It provides the scientific basis for determining "how bad" a release could be. This information is critical for land-use planning (how far away should public receptors be?), designing effective fire and gas detection systems (where to place detectors?), validating the location of safe refuges and muster points, and calculating individual risk contours for a QRA. Without this analysis, emergency planning and safety system design would be based on guesswork, potentially leaving catastrophic gaps in protection.