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ALOHA program overview


ALOHA (Area Locations of Hazards Atmosphere) is a program developed by the National Oceanic and Atmospheric Administration (NOAA) and Environmental Protection Agency (EPA).It aids in modelling chemical releases for planners and emergency responders and can predict the dispersion of toxic clouds after a chemical release. ALOHA allows the modelling of many different scenarios such as toxic gas clouds, Boiling Liquid Expanding Vapor Explosions (BLEVE), vapor cloud explosions, pool fires and jet fires.
The type of hazard evaluated by ALOHA depends on the release scenario.
 

For this project, ALOHA is used to determine the maximum downwind distance of toxic vapors formed in the event where a spillage occurs using the IDLH values of the chemicals (if they are available) or ERPGs. The definitions of terms can be found below.
 

The calculations are based on the ALOHA program and it can be downloaded for free here: ALOHA program

 

 

 

  1. Fill in Location details under SiteData tab
    Note: Singapore is not included in the dropdown list,
    the data for it will have to be manually entered.
    Click Add
    - Location is Singapore
    - Elevation: 15m
    - Approximate Location: (Latitude) 1o 22.0 min N
                                               (Longitude) 103o 48.0 min E
    - Offset from STANDARD time to GMT: -8.00 hours
    - Current model: Standard Time
     

  2. Fill in Date and Time details under SiteData tab
    - Manual User input (Affects atmospheric stability class)





    Table 1: Meteorological conditions that define the Pasquill stability class
     

    Note:   - Stability Class D applies to heavily overcast skies at any windspeed day or night.

                 - Windspeed based on 10m above ground

     

    *Strong Solar radiation corresponds to clear skies with the sun high up in the sky (Solar angle >60o approximately 12 noon local time)

    ** Slight Solar radiation corresponds with clear skies with low sun (solar angle >15o and >30o approximately 5pm local time)
     

  3. Choose Chemical that is being studied under Set-Up
    - Select appropriate chemical to be studied
     

  4. Fill in Atmospheric properties















     

  5. Fill in Source properties

    - Choose appropriate Source (Tank option was chosen for our study)

    For Tank Option 
          

    - Select appropriate Type and fill in the dimensions

    - Choose appropriate State of Chemical and Temperature

    - Enter Mass of Chemical or Liquid Level/Volume

    - Choose appropriate tank failure (ALOHA can only calculate maximum downwind distance with Leaking tank, chemical is not burning as it escapes into atmosphere option)

    - Choose the appropriate Shape of Opening

    - Fill in appropriate Opening Diameter of hole

    - Select appropriate level at which the hole is located

    - Select appropriate Ground Type, Ground Temperature and Maximum puddle diameter/area

    (For a chemical store, it is assumed Concrete ground, Air Temperature, and Unknown maximum puddle diameter/area)
     

  6. Generate ThreatZone value under Display tab

     

     

     

     

     

     

























    Note: The text summary will show the different limits (IDLH, ERPGs) for every chemical if it is available.

 

       - Threat Zone values will be displayed




















      

       - Threat Zone values will also be displayed in the Text Summary

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

        - The values calculated by ALOHA can be used to draw boundaries on a scaled map to mark out the distances to different threat zones.

 

      Definition of Terms

 

             

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

              

 

 

 

        

How to use the program ( to be followed in sequential order)

- Fill in the Wind Speed according the appropriate stability class being investigated (Refer to Table 1 above)

- Fill in appropriate Wind Direction (Assumed SSW or NNE for Singapore)

- Set appropriate Ground Roughness (Assumed to be Urban or Forest for Singapore)

- Set appropriate cloud cover according to appropriate stability class (Refer to Table 1 above)

- Set appropriate Air Temp (Assumed to be 30 degrees celsius for Singapore)

- Stability class will be se according to values set above

- Set appropriate Inversion Height Options (Assumed to be No Inversion for Singapore)

- Set Appropriate Humidity (Assumed to be 80% for Singapore)

- Choose Hazard to be Analyzed (Toxic Area of Vapor Cloud)

- Input appropriate values in the appropriate zones to determine the downwind distances
(Use concentration values provided by ALOHA as highlighted or input own values)

IDLH - Immediate Danger to Life and Health Is a limit set by the National Institute of Occupational Safety and Health (NIOSH) for selecting respirators to use in workplaces.

 

List of Chemicals with their respective IDLH values is available here:  http://www.cdc.gov/niosh/idlh/intridl4.html

ERPG - Emergency Response Planning Guideline values provide an estimation of concentration ranges where a person might feel adverse effects as a consequence of being exposed to a specific substance. There are sub-categories ERPG-1, ERPG-2, ERPG-3 which are defined below. 

 

  • The ERPG-1 is the maximum airborne particle concentration which will cause most people to experience short and mild adverse health effects or sense an obvious objectionable odor when they are exposed to it for 1 hour.

  • The ERPG-2 is the maximum airborne particle concentration which will cause most people to experience serious and irreversible health effects or cause them to be unable to take protective action when they are exposed to it for 1 hour.

  • The ERPG-3 is the maximum airborne particle concentration which will cause most people to experience lethal health effects when they are exposed to it for 1 hour.

 

 

List of chemicals with respective ERPG values is available here:
https://www.aiha.org/get-involved/AIHAGuidelineFoundation/EmergencyResponsePlanningGuidelines/Documents/2013ERPGValues.pdf

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