THMG277 – It Depends…Part 4

In this series of shows we will be tackling something that is typically an Operations concern. Behavior modeling and exploring how it questions the age-old answer, “It Depends?!?”

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A.          Dispersion and Engulfment

1.    The dispersion of material is sometimes referred to as engulfment

2.     Dispersion of the hazardous material, energy, and container components depends on the type of release, which include

a.     A solid, liquid, or gas/vapor

b.    Mechanical, thermal, or chemical energy and ionizing radiation

c.      Product characteristics and environmental conditions (such as weather and terrain)

i.       Physical/chemical properties
ii.      Prevailing weather conditions
iii.    Local topography
iv.    Duration of the release
v.     Control efforts of responders

3.     The shape and size of the dispersing hazardous material also depends on how the material emerges from its container — Whether as an instantaneous “puff,” a continuous plume, or a sporadic fluctuation

4.    The outline of the dispersing hazardous material, sometimes called its dispersion pattern, can be described in a number of ways

5.     Common dispersion patterns include

a.     Hemispheric — Semicircular or dome-shaped pattern of airborne hazardous material that is still partially in contact with the ground or water

i.      A hemispheric release generally results from a rapid release of energy (such as detonation, deflagration, and violent rupture)
ii.      The following elements are common to hemispheric releases
(a)      Energy — Generally travels outward in all directions from the point of release
(b)      Dispersion of energy — Affected by terrain and cloud cover (Solid cloud cover can reflect the detonation shock wave, increasing the explosion impact)
(c)       Energy release — May propel the hazardous material and container parts; however, this dispersion may not be hemispherical; Large container parts generally (but not always) travel in line with the long axis of the container

b.    Cloud — Ball-shaped pattern of the airborne hazardous material that collectively rises above the ground or water

i.       Gases, vapors, and finely divided solids that release quickly (puff release) can disperse in cloud form under minimal wind conditions
ii.      Terrain and/or wind effects can transform a cloud into a plume

c.    Plume — Irregularly shaped pattern of an airborne hazardous material where wind and/or topography influence the downrange course from the point of release

i.       Dispersion of a plume (generally composed of gases and vapors) is affected by vapor density and terrain (particularly if vapor density is greater than 1) as well as wind speed and direction.
ii.     Other plume dispersion elements include the following
(a)      Puff release — When all of the material is released at one time, the concentration of gas or vapor in the cloud or plume decreases over time
(b)      Ongoing release — Concentration increases over time until the leak stops or all of the product releases; then it decreases

d.  Cone — Triangular-shaped pattern of a hazardous material with a point source at the breach and a wide base downrange

i.       An energy release may be directed (based on the nature of the breach) and may project solid, liquid, or gaseous material in a three-dimensional cone-shaped dispersion
ii.      Examples of cone-shaped dispersions include container failures in a BLEVE or a pressurized liquid or gas release

e.    Stream — Surface-following pattern of liquid hazardous material that is affected by gravity and topographical contours

i.       Liquid releases flow downslope whenever there is a gradient away from the point of release

f.     Pool — Three-dimensional (including depth), slow-flowing liquid dispersion; Liquids assume the shape of their container and pool in low areas

i.       As the liquid level rises above the confinement provided by the terrain, the substance flows outward from the point of release
ii.      If there is a significant gradient or confinement due to terrain, this flow forms a stream

g.    Irregular — Irregular or indiscriminate deposit of a hazardous material (such as that carried by contaminated responders)

6.     In the event of a release, facility preincident surveys may contain plume dispersion models to help estimate the size of an endangered area

a.    Computer software such as CAMEO (Computer-Aided Management of Emergency Operations), ALOHA (Area Locations of Hazardous Atmospheres), and HPAC (Hazard Prediction and Assessment Capability) can also assist in the prediction of plume dispersion patterns

b.First responders may consult the ERG for isolation and evacuation distances

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