Hazard Studies & Risk Assessment

Toxic Release Assessment & Dispersion Modelling

PHAST / SAFETI / ALOHA toxic dispersion with AEGL / ERPG / SLOT receptor analysis

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What this study delivers

Toxic Release Assessment
& Dispersion Modelling

Toxic release modelling determines whether a chlorine leak, an ammonia rupture, or a hydrogen-fluoride scenario reaches AEGL-2 (disabling) or AEGL-3 (life-threatening) at fenceline, occupied buildings, and community receptors — the foundation for EPA RMP Off-Site Consequence Analysis, COMAH safety case toxic-risk demonstration, Indian MoEFCC hazard analysis, and emergency response zone definition. The discipline rests on accurate source-term derivation (orifice / pipe / vessel rupture per CCPS / Yellow Book, two-phase flashing for refrigerated liquefied gases, aerosol fraction for HF and amines), atmospheric dispersion modelling (Gaussian for neutral / passive, dense-gas SLAB / DEGADIS / DRIFT / Britter-McQuaid for chlorine, HF, ammonia, propylene oxide), and toxic-load (Cn × t) calculation per HSE SLOT / SLOD methodology. Modern practice now also integrates DNV PHAST 8.x and SAFETI 9.x cloud / dense-gas modules, CFD validation for complex topography or congestion (FLACS-Dispersion, GEXCON), and the post-Methyl-Mercaptan / Methyl-Isocyanate / Hydrogen-Fluoride incident learnings that drove API RP 754 toxic-release Tier-1 indicator inclusion.

Toxic Release Assessment & Dispersion Modelling — Overview
Study execution

How the study is executed

A structured, facilitated process — from scope definition through close-out — producing defensible, actionable outputs.

Toxic Inventory & Source Term Build

Compile toxic inventory per unit / vessel / pipeline with concentration, quantity, temperature, pressure; characterise source terms per release scenario — hole size (10mm, 25mm, 100mm, FBR per CCPS / API 581), release height, momentum, buoyancy.

Release Scenario Catalogue

Catalogue release scenarios per CCPS / API 581 / COMAH — instantaneous (catastrophic vessel rupture), continuous (leak, flange failure), pulsed (relief actuation); specify weather (Pasquill stability A-F), wind speed (typical 2-10 m/s).

Dispersion Modelling (Gaussian / CFD)

Model toxic dispersion per Pasquill-Gifford Gaussian (PHAST, ALOHA, SAFETI) for far-field, dense gas (DEGADIS, SLAB) for heavy gases, CFD (FLACS, Kameleon) for congested geometry; align with EPA RMP / CCPS / COMAH guidance.

Toxic Endpoint Application

Apply toxic endpoints — AEGL-1/2/3 (Acute Exposure Guideline Levels), ERPG-1/2/3 (Emergency Response Planning Guidelines), IDLH (NIOSH), TEEL-1/2/3 (Protective Action Criteria); align with EPA / OSHA / DOE jurisdictional endpoints.

Risk Integration & F-N Curve

Integrate toxic risk into QRA — calculate IRPA, F-N curve, PLL, location-specific individual risk; compare against tolerability criteria per UK HSE R2P2 / Dutch / Singaporean / Indian; align with corporate risk tolerability matrix.

Risk Reduction & ERP Integration

Recommend risk-reduction measures — inventory reduction (ISD), detection (toxic gas), scrubbing / abatement, building isolation, evacuation distance; integrate with ERP scenario set and toxic refuge / shelter-in-place design.

Toxic Release Assessment & Dispersion Modelling — Scope
Study scope

What the study covers in full

Toxic inventory mapping with Appendix A regulated chemicals (RMP) and site-specific additions
Source-term modelling — orifice / pipe / catastrophic rupture, two-phase flashing, aerosol fraction
Pasquill-Gifford stability classification with sensitivity (D5, F2 worst-case per RMP)
Dense-gas dispersion modelling (DEGADIS, SLAB, Britter-McQuaid) for Cl₂, NH₃, HF, propylene oxide
Passive / Gaussian dispersion for neutral releases above ambient density
Toxic load (Cn × t) calculation per HSE SLOT (Dangerous Toxic Load) and SLOD methodology
AEGL-1/-2/-3 receptor analysis with 10-min, 30-min, 1-hr, 4-hr, 8-hr exposure binning
ERPG-1/-2/-3 and IDLH benchmarking for emergency response design
EPA RMP worst-case (catastrophic, F2, 1.5 m/s) and alternative-case scenario sets
CFD validation (FLACS-Dispersion, GEXCON) for complex topography, building wake, congestion
Why it matters

Outcomes of Toxic Release Assessment & Dispersion Modelling

Toxic Release Consequence Accuracy
  • Quantifies AEGL-2 / AEGL-3 reach for credible releases — fenceline and community impact
  • Surfaces inventory reduction opportunities ahead of regulatory imposition
  • Drives toxic-gas detector siting and ESD response design
  • Anchors realistic shelter-in-place and evacuation strategy
EPA RMP / COMAH Toxic Zone Defence
  • EPA RMP Subpart C Off-Site Consequence Analysis defensible under Inspection
  • Withstands COMAH / Seveso III competent-authority toxic-risk examination
  • Satisfies India MoEFCC EIA and PESO hazard analysis
  • Provides UK HSE SLOT / SLOD compliant land-use planning evidence
Operational Zone & Control Clarity
  • Drives toxic-gas detector grid design (chlorine, ammonia, HF, hydrogen sulphide)
  • Supports realistic mutual-aid and emergency response zones
  • Identifies critical-release inventories meriting ISD substitution
  • Informs operator training scenarios for toxic-event response
Toxic Incident Liability & Cost Reduction
  • Reduces inventory carrying cost through prioritised reduction
  • Pre-empts EPA RMP enforcement (currently $66,633 per violation per day)
  • Cuts community-impact mitigation cost through targeted reduction
  • Improves underwriter pricing on toxic-handling assets
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