Comparison of EPIcode

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Comparison of EPIcode ALOHA Calculations for
Evaporation and Chemical Atmospheric Transport
Dispersion
David C. Thoman Washington Safety Management
Solutions LLCPhone 803.502.9721 email
dave.thoman_at_wsms.com Presented to 15th Annual
SAWG Workshop Santa Fe, NM April 30 May 5, 2005
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Overview

• DOE Toolbox Codes for Chemical Consequence
  Analysis
• EPIcode Version 7.0
• ALOHA Version 5.2.3
• Code Guidance Reports located at
  www.eh.doe.gov/sqa/central_registry.htm

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Code Source Term Algorithms

• EPIcode Version 7.0
• Direct (Term or Continuous)
• Liquid Spill
• Fire
• Explosion

• ALOHA Version 5.2.3
• Direct (Instantaneous or Continuous)
• Puddle
• Tank
• Pipe

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Code Dispersion Algorithms

• EPIcode Version 7.0
• Gaussian

• ALOHA Version 5.2.3
• Gaussian
• Heavy Gas (Dense Gas)

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Issues

• EPIcode Evaporation Model
• EPA-550-B-99-009 (Risk Management Program) Model
• 2.68 factor increase mass transfer coefficient of
  water
• ALOHA Model
• More complex evaporation algorithm
• Dense gas dispersion algorithm

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Outline

• Atmospheric Transport Dispersion
• Gaussian plume hand calculation
• EPIcode ALOHA replication
• EPIcode ALOHA sensitivity results
• Pool Evaporation
• Algorithm overview
• EPIcode ALOHA results
• Evaporation rate
• Concentration
• Recommendations Conclusions

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Gaussian Plume Formula
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Gaussian Plume Formula (continued)

• Simpler form for
• Ground level release (H0)
• Receptor at ground level (z0)
• Receptor on plume centerline (y0)

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Dispersion Coefficients
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Illustrative Example Direct Release

• Problem statement
• 1 g/s release of a chemical at ground level
• Rural terrain
• F atmospheric stability class 1-m/s wind speed
• Desired result
• Ground level concentration at 100 m

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Illustrative Example (continued)

• sy 4.0 m
• sz 1.6 m
• u 1 m/s
• Q 1000 mg/s
• c(100,0,0) (1000 mg/s) / p (4.0 m) (1.6 m)
  (1 m/s)
• 51 mg/m3

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EPIcode Replication

• EPIcode Version 7.0
• Measurement height for wind speed - 2 m
• Averaging Time - 10 minutes
• Deposition velocity - 0 cm/s
• Receptor Height - 0 m

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ALOHA Replication

• ALOHA Version 5.2.3
• Measurement height for wind speed - 3 m
• Dispersion model - Gaussian

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EPIcode ALOHA Replication

• Rural Terrain
• ALOHA results EPIcode results
• Urban Terrain
• ALOHA results gt EPIcode results

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Urban Terrain Sensitivity

• 100-m Base Case Concentration 51 mg/m3 for rural
  terrain
• Sensitivity Results urban terrain
• 4.0 mg/m3 for EPIcode Version 7.0
• 11 mg/m3 for ALOHA Version 5.2.3

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Wind Speed Measurement Height
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Wind Speed Profile

• p is a function of atmospheric stability class
• EPIcode function of terrain type

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Measurement Height Sensitivity

• 100-m Base Case Concentration 51 mg/m3 for 1-m/s
  wind speed at reference height
• Sensitivity Results 1-m/s at 10 meters
• 120 mg/m3 for EPIcode Version 7.0
• 84 mg/m3 for ALOHA Version 5.2.3

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Averaging Time
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Averaging Time EPIcode Sensitivity
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Deposition EPIcode Sensitivity
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Dense Gas Dispersion ALOHA Sensitivity
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Outline - Refresher

• Atmospheric Transport Dispersion
• Gaussian plume hand calculation
• EPIcode ALOHA replication
• EPIcode ALOHA sensitivity results
• Pool Evaporation
• Algorithm overview
• EPIcode ALOHA results
• Evaporation rate
• Concentration
• Recommendations Conclusions

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ALOHA Evaporation Model

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EPIcode Evaporation Model

• Constant temperature pool
• EPA evaporation model
• Evaporation rate proportional to reference
  (water) mass transfer coefficient (Kref)
• Old Kref cm/s 0.25 x u0.78
• New Kref cm/s 0.67 x u0.78

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Example Pool Evaporation

• Problem statement
• 21-gallon spill
• Uniform depth of 1 cm (7.95 m2 surface area)
• Pool temperature of 25 oC
• Additional ALOHA inputs
• Date/Time of 6/29/04 and 2359
• Cloud cover of 50
• 50 relative humidity
• Ground temperature of 25 oC
• Default ground type

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Example Pool Evaporation (continued)

• 16 cases
• 4 chemicals (nitric acid, benzene, chlorine,
  ammonia)
• 2 atmospheric conditions (stability class F
  1-m/s wind speed, stability class D 2-m/s wind
  speed )
• 2 terrain types (rural, urban)

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Example Pool Evaporation (continued)

• Desired Results
• Evaporation rates
• Ground level concentration at 100 m
• Gaussian plume results EPIcode ALOHA
• Dense gas results ALOHA
• Appropriate dispersion model
• Source Richardson Number - ALOHA

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Summary - Pool Evaporation Results

• General Trends (Caveat limited data set!)
• Higher sustained evaporation rates with EPIcode
  (differences up to factor of 2.7)
• 100-m concentrations for rural terrain cases
• EPIcode results higher
• Typically factor of 2 4 higher
• One case gt factor of 10
• 100-m concentrations for urban terrain cases
• No clear trend
• Differences within a factor of 2 3

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Recommendations for Safety Analysis

• Code selection
• Input data modeling assumptions
• Site specificity
• Consistency between source term dispersion
  calculations

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Concluding Remarks

• Results sensitivity
• Modeled phenomena (code capabilities)
• Variability often case dependent
• Variability magnitude
• Assessing conservatism