# Presentation: 775120

## Comparison of EPIcode & ALOHA Calculations for Evaporation and Chemical Atmospheric Transport & Dispersion

**David C. Thoman**
**	****Washington Safety Management Solutions LLC****Phone:  803.502.9721**
**email: dave.thoman@wsms.com**
**Presented to:**
**15****th**** Annual SAWG Workshop**
**Santa Fe, NM**
**April 30 – May 5, 2005**

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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 **

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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*

- Chemical Evaporation and Dispersion

- May 4, 2005

## Code Dispersion Algorithms

**EPIcode Version 7.0**
**Gaussian **

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

- Chemical Evaporation and Dispersion

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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

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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**

- Chemical Evaporation and Dispersion

- May 4, 2005

## Gaussian Plume Formula

- Chemical Evaporation and Dispersion

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## Gaussian Plume Formula (continued)

**Simpler form for:**
- Ground level release (H=0)
- Receptor at ground level (z=0)
- Receptor on plume centerline (y=0)

- Chemical Evaporation and Dispersion

- May 4, 2005

## Dispersion Coefficients

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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

- Chemical Evaporation and Dispersion

- May 4, 2005

## Illustrative Example (continued)

-  y = ~4.0 m
-  z = ~1.6 m
-  u = 1 m/s
-  Q = 1000 mg/s
-  (100,0,0) = (1000 mg/s]) / [(4.0 m) (1.6 m) (1 m/s)]
-                        = ~51 mg/m3

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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

- Chemical Evaporation and Dispersion

- May 4, 2005

## ALOHA Replication

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

- Chemical Evaporation and Dispersion

- May 4, 2005

## EPIcode & ALOHA Replication

**Rural Terrain: **
- 	 ALOHA results = EPIcode results
**Urban Terrain:**
- 	ALOHA results > EPIcode results

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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

- Chemical Evaporation and Dispersion

- May 4, 2005

## Wind Speed Measurement Height

- Chemical Evaporation and Dispersion

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## Wind Speed Profile

- “p” is a function of atmospheric stability class
- EPIcode – function of terrain type

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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

- Chemical Evaporation and Dispersion

- May 4, 2005

## Averaging Time

- Chemical Evaporation and Dispersion

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## Averaging Time – EPIcode Sensitivity

- Chemical Evaporation and Dispersion

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## Deposition – EPIcode Sensitivity

- Chemical Evaporation and Dispersion

- May 4, 2005

## Dense Gas Dispersion – ALOHA Sensitivity

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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**

- Chemical Evaporation and Dispersion

- May 4, 2005

## ALOHA Evaporation Model

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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

- Chemical Evaporation and Dispersion

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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 23:59
- Cloud cover of 50%
- 50% relative humidity
- Ground temperature of 25 oC
- “Default” ground type

- Chemical Evaporation and Dispersion

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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)

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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

- Chemical Evaporation and Dispersion

- May 4, 2005

## 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 > factor of 10
- 100-m concentrations for urban terrain cases:
- No clear trend
- Differences within a factor of 2 – 3

- Chemical Evaporation and Dispersion

- May 4, 2005

## Recommendations for Safety Analysis

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

- Chemical Evaporation and Dispersion

- May 4, 2005

## Concluding Remarks

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

- Chemical Evaporation and Dispersion

- May 4, 2005