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Consequence Analysis

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Source Term Modeling

Consequence modeling is conducted in order to quantify the effects and consequences of identified Loss Of Containment (LOC) scenarios, and it entails the characterization of the sources of release of material or energy associated with the hazard being analyzed (i.e., source term), and the quantification of the impacts on a target of interest.

To model the consequences of LOCs, the source strength, duration, and phase must be accurately determined. These quantities are functions of storage or process conditions and the thermo-physical properties of the chemical(s) released, and can be determined from fluid flow equations.

The results from the source term modeling provide the required information (e.g., release phase, rate, temperature, pressure, velocity), not only for consequence modeling, but also for final outcomes identification.

Based on the source term estimated properties, releases can be classified as follows:

  • Ambient liquid release.

  • Refrigerated liquid release.

  • Instantaneous pressurized liquid release.

  • Continuous pressurized liquid release.

  • Instantaneous gas/vapor release.

  • Continuous gas/vapor.

  • Combustible dust.

This classification allows to conduct the identification of all final outcomes that the LOC may cause, procedure that accounts for enabling conditions/events:

  • Immediate/delayed ignition sources.

  • Unconfined congested regions.

  • Confined regions.

  • Active/passive protection layers.

The Event Tree Analysis (ETA) is an inductive methodology typically used for this purpose.

Ventilation / Dispersion Modeling

BC uses powerful and modern tools for addressing:

  • External ventilation and dispersion modeling in outside environments around buildings and structures.

  • Internal Ventilation and dispersion modeling within buildings, enclosures, or compartments.

The capability of ventilation and dispersion modeling accounts for both steady-state and transient simulations, and simulations near source behavior with or without momentum, and far-field behavior without momentum. While in most cases, the steady-state approach is justified, the transient simulations are of interest when analyzing Emergency Shut Down (ESD) systems, BlowDown (BD) and limited inventories.

One of the advanced features of the proposed ventilation modeling and analysis is the identification of areas and wind directions where the air flow may get trapped, identifying which regions are more prone to recirculation and would cause difficulties if a gas cloud disperses near these regions. Examples of application of this analysis are the following:

  • Identification of the most poorly ventilated regions; e.g., design purposes.

  • Identification of worst-case scenarios; i.e., dispersion of gas clouds into these poor ventilation regions.

Additional considered features by BC for ventilation and dispersion modeling are the importance of taking into account forced inlet/outlet flow rates (fans that force air into or out the enclosure), and Heating – Ventilation – Air Conditioning (HVAC)  taking into account the through-flow fan. These capabilities are necessary when performing internal ventilation studies; e.g., adequacy of the ventilation system.

Other examples of application of the technology used by BC are the following:

  • Extend of and LFL and UFL (Lower / Upper Flammability Limits), flammable mass/volume.

  • Toxic profiles and final concentrations at target locations.

  • Hazardous Area Classification (HAC) - ATEX.

  • Strategic decision on gas detector placement/layouts.

  • Guidelines for prevention and mitigation measures.

Fire Modeling

BC uses powerful and modern tools for addressing:

  • Open fires occurring in outside environments around buildings and structures.

  • Internal fires occurring within buildings, enclosures, or compartments.

Fire properties are influenced by leakage rates and their time dependence, type of flammable substance burning, storage and discharge conditions, surrounding topside structures and equipment and ambient wind conditions. Despite the large number of possible fire events, few categories of industrial fires are relevant for consequence/risk assessment, and for escalation leading to domino effect. The most relevant industrial fire types to be identified in a process facility are the following:

  • Pool fires: uncontrolled combustion of vapors generated from a pool of a flammable liquid.

  • Jet fires: characterized by a momentum dominated release and high levels of thermal radiation resulting from well mixed combustion.

  • Cloud fires: relatively slow flame front propagation through a flammable vapor cloud without producing significant overpressure.

  • Fireballs: rapid involvement of a large amount of flammable material, and characterized by high levels of thermal radiation.

 

Modeling open fires is mainly intended to characterize hazards of high heat fluxes and potential accident propagation (escalation) to target locations; i.e., process equipment, buildings, structural elements. An example of application of the escalation analysis due to fires is the estimation of the Time To Failure (TTF), which can be compared with the Time to Effective Mitigation (TEM) for ensuring mitigation plan effectiveness.

Additional features considered by BC is the capacity for internal fire modeling within buildings, enclosures, or compartments. In addition to the well-known hazards of high heat fluxes, there are additional potential hazards to personnel, including the extend of external flaming, impaired visibility along escape routes through smoke, increased Carbon Monoxide (CO) hazard, and explosion hazard from unburned fuel if the fire terminates due to lack of oxygen.

Explosion Modeling

BC uses powerful tools for addressing:

  • Explosions occurring in outside environments around buildings and structures.

  • Explosions occurring within buildings, enclosures, or compartments.

An explosion is a sudden release of energy, and the rate at which this energy is released defines the violence of the explosion. There are several kinds of energy which may be released in an explosion: (1) physical energy, which may take forms such as pressure energy in gases, strain energy in metals or electrical energy; (2) chemical energy; which derives from a chemical reaction; and (3) nuclear energy, which is out of BC scope.

 

Based on these energy categorization, BC addresses the following types of explosions:

  • Physical explosions; e.g., Boiling Liquid Expanding Vapor Explosions (BLEVEs), Rapid Phase Transition Explosions (RPTs)

  • Vapor Cloud Explosions; e.g., vapor, gas, or mist explosion

  • Condensed phase explosions; e.g., high explosives, ammonium nitrate, organic peroxides, sodium chloride

  • Confined explosions with reaction (runaway reactions); e.g., explosion involving vapor combustion, reactor explosions, other explosions involving liquid phase reactions

  • Dust explosions

Based on the study purpose, BC models explosions using different technologies:

  • When the purpose of the explosion assessment allows simplifying the definition of the presence of complex geometries, BC uses simple-correlated methods or one dimensional phenomenological models with the aim to perform screening runs that can be done easily or before detailed design has been completed. The computer time is extremely short, so it is easy to perform many “what if...” runs, testing the effect of design modifications; i.e., sensitivity runs.

  • When detailed explosion modeling results are pursued with the aim to investigate the explosion interaction with complex geometries, BC uses a Computational Fluid Dynamics - CFD-based tool capable to provide fast and detailed results.

Hazards characterization of high overpressure/impulses, and potential accident propagation; sensitivity analysis of installing passive barriers (e.g., fire/blast walls), vent sizing in enclosures, and probabilistic assessments are some examples of explosion modeling applications.

 

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