Toxic Release Assessment and Dispersion Modelling
PHAST, SAFETI, and ALOHA toxic dispersion with AEGL, ERPG, and SLOT receptor analysis
Toxic Release Assessment
and Dispersion Modelling
Toxic release modelling determines whether a chlorine leak, an ammonia rupture, or a hydrogen fluoride scenario reaches AEGL 2, which is disabling, or AEGL 3, which is life threatening, at your fenceline, your occupied buildings, and the community receptors around your site. It is the foundation for EPA RMP Off Site Consequence Analysis, for the toxic risk demonstration in a COMAH safety case, for Indian MoEFCC hazard analysis, and for emergency response zone definition. The discipline rests on accurate source term derivation covering orifice, pipe, and vessel rupture per CCPS and the Yellow Book, two phase flashing for refrigerated liquefied gases, and aerosol fraction for HF and amines, on atmospheric dispersion modelling that is Gaussian for neutral and passive releases and dense gas using SLAB, DEGADIS, DRIFT, and Britter McQuaid for chlorine, HF, ammonia, and propylene oxide, and on toxic load calculation as concentration to the power n multiplied by time per the HSE SLOT and SLOD methodology. Modern practice also integrates the DNV PHAST 8.x and SAFETI 9.x cloud and dense gas modules, CFD validation for complex topography or congestion using FLACS Dispersion and GEXCON, and the lessons from the methyl mercaptan, methyl isocyanate, and hydrogen fluoride incidents that drove the inclusion of toxic release as an API RP 754 Tier 1 indicator. Our team brings all of this to bear on the credible releases at your facility.
How the study is executed
A structured, facilitated process that runs from scope definition through close out and produces defensible, actionable outputs.
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.
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 to 10 m/s).
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.
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.
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.
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.
What the study covers in full
Outcomes of Toxic Release Assessment and Dispersion Modelling
- We quantify AEGL 2 and AEGL 3 reach for credible releases across your fenceline and community
- We surface inventory reduction opportunities ahead of any regulatory imposition
- We drive toxic gas detector siting and ESD response design
- We anchor a realistic shelter in place and evacuation strategy
- We produce EPA RMP Subpart C Off Site Consequence Analysis that holds up under inspection
- We withstand COMAH and Seveso III competent authority toxic risk examination
- We satisfy India MoEFCC EIA and PESO hazard analysis
- We provide UK HSE SLOT and SLOD compliant land use planning evidence
- We drive toxic gas detector grid design for chlorine, ammonia, HF, and hydrogen sulphide
- We support realistic mutual aid and emergency response zones
- We identify critical release inventories that merit inherently safer design substitution
- We inform operator training scenarios for toxic event response
- We reduce inventory carrying cost through prioritised reduction
- We help you pre empt EPA RMP enforcement, currently 66,633 dollars per violation per day
- We cut community impact mitigation cost through targeted reduction
- We improve underwriter pricing on toxic handling assets
Codes & standards we work to
Triggers that signal the need
Where Toxic Release Assessment and Dispersion Modelling applies
Wellheads, separators, gas compression, FPSO topsides, produced water systems.
Distillation columns, reactors, heat exchangers, storage spheres, LPG handling.
Cryogenic exchangers, liquefaction trains, BOG compressors, storage and sendout.
Reactive systems, batch reactors, solvent handling, runaway reaction scenarios.
Boilers, HRSGs, steam headers, hydrogen systems, ammonia SCR units.
Sterile vessels, CIP/SIP, pressure fermenters, solvent recovery, spray dryers.
Tangible deliverables
- Toxic release assessment report with scenario register
- Source term calculation pack for each release event
- Dispersion modelling output from PHAST, SAFETI, SLAB, or DEGADIS
- Concentration and toxic load contour plots
- AEGL, ERPG, and SLOT receptor analysis
- EPA RMP OCA documentation in RMP*Comp format
- Emergency response zone maps with concentration time profiles
- Inventory reduction opportunity register
- CFD validation where applicable
Ready to start your project?
Speak with our team to scope an engagement tailored to your facility, regulatory context, and lifecycle stage.