Quantifying Industrial Risk in Process Hazard Analysis

Process Hazard Analysis (PHA) is conducted to identify hazards present in industrial processes and to develop measures to reduce associated risks. A PHA evaluates potential consequences that may arise during operations, including fire, explosion and release of toxic chemicals. Fire scenarios may include flash fires, pool fires and jet fires, while explosion scenarios may include vapour cloud explosions and dust explosions. Toxic consequences may arise from gas dispersion from process vents, flares or accidental leaks.

Traditional PHA methods are described as being heavily dependent on expert judgement, lacking consequence quantification, presenting difficulty in prioritising scenarios and offering limited visualisation of impacts.

The PHAST is developed by Det Norske Veritas (DNV). It performs consequence modelling for fire, explosion and toxic dispersion events. The software is used in the oil and gas and chemical industries and aligns with global safety standards.

Hazard analysis calculations in PHAST are divided into three stages. The first stage consists of discharge calculations, which model the release from a vessel or pipe. The second stage consists of dispersion calculations, which model the behaviour of the released material, including cloud or pool formation. The third stage consists of effects calculations, which predict potential consequences such as fire and explosion.

The PHAST includes modelling for several fire scenarios. Fireball modelling applies to instantaneous releases and short-duration continuous releases. Jet fire modelling applies to continuous releases. Pool fire modelling applies to delayed ignition of a pool of flammable liquid after rainout. Flash fire modelling applies to delayed ignition of a flammable vapour cloud. Version 8.6 includes a jet fire model for hydrogen based on a modification by DNV of the model reported by Miller et al. (2017). The flame shape in this model is represented by two line segments described as momentum dominated and buoyancy dominated.

The PHAST provides four models for vapour cloud explosions. These include the Trinitrotoluene (TNT) model, the Baker–Strehlow–Tang (BST) model and the Netherlands Organisation for Applied Scientific Research (TNO) Multi Energy model in both user-defined configuration and uniform confined configuration, the latter being identified as default. Explosion calculations can provide overpressure results for equipment items using these models.

The PHAST software tool has the ability to place graphical results for dispersion and effects over maps using GIS capability. Results can be displayed for dispersion and effect cases, different wind directions, dynamic animation and multiple releases. Users can view the shape and effect zones associated with a release scenario.

Traditional PHA is described as consisting of hazard identification, consequence analysis, likelihood estimation and recommendation of safeguards. The PHAST can be used in the consequence analysis step for modelling dispersion of toxic and flammable materials, jet fire and pool fire analysis, Boiling Liquid Expanding Vapour Explosion (BLEVE) and vapour cloud explosion modelling, footprint mapping and time-based results. Scenario configuration is included within this modelling framework.

A real-world accident involved catastrophic tank failure resulting in the release of approximately 22 tonnes of ammonia. Simulations were conducted under neutral atmospheric stability conditions with a wind speed of four metres per second. The modelling results indicated an Emergency Response Planning Guideline Level 3 (ERPG-3) distance of approximately 1,500 metres downwind.

The Emergency Response Planning Guideline Level 2 (ERPG-2) footprint extended approximately 4,200 metres with a width of approximately 4,000 metres. The visible plume extended approximately 900 metres downwind.

Traditional PHA or Hazard and Effects Management Analysis (HEMA)-derived dispersion assumptions are described as typically considering distances of 500 to 600 metres for high-toxicity releases. The PHAST is used for quantitative consequence modelling within the PHA process. It applies a structured modelling approach consisting of discharge calculations, dispersion calculations and effects calculations. The tool includes capabilities for modelling fire scenarios, explosion scenarios and toxic dispersion events.

A case illustration involving the release of approximately 22 tonnes of ammonia under neutral atmospheric stability and a wind speed of 4 metres per second demonstrates modelling outputs, including Emergency Response Planning Guideline Level 3 (ERPG-3) and Emergency Response Planning Guideline Level 2 (ERPG-2) distances and visible plume extent.

This article is based on a technical paper presented by Nagmani Kumar Sinha, Sales Specialist, DNV, at Global Refining & Petrochemicals Congress 2025 organised by ENCIS and co-organised by ITEN Media.