dispersion models
DESCRIPTION
chemical safetyTRANSCRIPT
Toxic Release and Dispersion Models
Chapter 5
An Equal Opportunity University
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Toxic releases• In an accident, significant quantities of
toxicants can be released from process equipment– Disperses throughout the plant and
neighboring community– Bhopal MIC incident is an example of the
consequences of unmitigated dispersion of toxicants
• Our jobs as engineers to prevent the existence of release situations
and reduce impact should one occur
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What affects dispersion?• Dispersion models describe airborne
transportation of toxic materials from the source
• Wind carries away material in a plume– Maximum concentration at release point
• Parameters affecting dispersion– wind speed– Atmospheric stability– Ground conditions (buildings, water,
trees)– Height of release above ground level– Momentum and buoyancy
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How these parameters contribute
• Wind speed– Increasing wind speed results in narrower and longer
plumes• Atmospheric stability
– Air/ground temperature affects vertical mixing of the air– Three classes
unstable– Sun heats the ground faster than heat can be removed (early morning)– Enhanced turbulence
neutral– Air above ground warms and wind speed increases to reduce solar
warming effects– Air temperature difference does not affect turbulence
stable– Sun cannot heat ground as fast as it is cooling (temperature near the
ground is lower than upper air temperatures)– Influence of buoyance suppresses turbulence
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How these parameters contribute
• Ground conditions– Affects mechanical mixing at the surface
and the wind profile with height – Vertical obstacles increase mixing
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How these parameters contribute
• Release height significantly affects ground level concentrations
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How these parameters contribute
• Buoyant and momentum of the material released change the effective height of the release
Sufficiently mixed with air to be neutrally
buoyant
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Neutrally Buoyant Dispersion Models
• Estimate the concentration downwind of a release in which the gas is mixed with fresh air to the point the resulting mixture is neutrally buoyant
• plume model– Steady-state concentration of material
released from a continuous source
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• puff model– Temporal concentration of a material from
a single release of a fixed amount
Neutrally Buoyant Dispersion Models
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Other things to consider…• Often want to
consider multiple conditions to determine worst cases– sitting– Design mitigation– Emergency
response plans
http://www.geoassociates.net/
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Worst Case Scenario• Worst case scenarios to consider
– Highest concentration in a plume is at the release point
– Highest concentration in a puff is at the center of the puff
– In design, assume weather conditions resulting in the highest concentration estimates
• Keep in mind the applicability of the model you choose to the release concentration (i.e., only a few ppm or do you need a more robust model)
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Toxic Effect Criteria• You’ve chosen/developed a model of
dispersion, how do we determine how dangerous it is?
• TLV-TWA is too conservative for emergency conditions
• Alternatives?
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Toxic Effect Criteria• Emergency response planning guidelines
(ERPG) for air contaminants – American Industrial Hygiene Association (AIHA)– ERPG-1
Max airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour with only mild, transient effects
– ERPG-2 Max airborne concentration below which it is believed nearly
all individuals could be exposed for up to 1 hour without experiencing/developing irreversible health effects impairing ability to take protective action
– ERPG-3 Max airborne concentration below which it is believed nearly
all individuals could be exposed for up to 1 hour without experiencing/developing life-threatening health effects
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Toxic Effect Criteria• Others…
– NIOSH Immediately Dangerous to Life or Health (IDLH)
– National Academy of Sciences/National Research Council’s emergency exposure guidance levels (EEGLs) and short-term public emergency guidance levels (SPEGLs)
– ACGIH’s TLV-STEL and TLV-C– OSHA’s PEL– New Jersey Dept. of Environmental Protection
Toxicity Dispersion (TXDS)– EPA’s toxic endpoints as part of RMP
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Release Mitigation• Part of the consequence modeling procedure• Inherent safety
– Inventory reduction, chemical substitution, process attenuation
• Engineering design– Spill containment, emergency relief devices
• Management– Policies, procedures, maintenance programs
• Warning systems• countermeasures
– Curtains (air, water, steam), deliberate ignition of explosive cloud
• Emergency response