Toxic Release and Dispersion

Toxic release models

These are used to estimate the effects of a release on the plant and community environments.

Dispersion models

These describe the airborne transport of toxic materials away from the accident site and into the plant and community.

Plume, puff

After a release, the airborne toxic material is carried away by the wind in a characteristic ___ or a ___.

At the release point

Where does the maximum concentration of toxic material occur?

True

TRUE OR FALSE. Concentrations downwind are less, because of turbulent mixing and dispersion of the toxic substance with air.

To provide a tool for performing release mitigation

What is the purpose of a toxic release model?

1. Wind speed

2. Atmospheric stability

3. Ground conditions (buildings, water, trees)

4. Height of the release above ground level

5. Momentum and buoyancy of the initial material released

Enumerate the five (5) parameters affecting atmospheric dispersion of toxic materials.

Neutrally buoyant dispersion models

These are used to estimate the concentrations downwind of a release in which the gas is mixed with fresh air to the point that the resulting mixture is neutrally buoyant.

Neutrally buoyant dispersion models

These models apply to gases at low concentrations, typically in the parts per million range.

1. Plume models

2. Puff models

Enumerate the two (2) types of neutrally buoyant vapor cloud dispersion models.

Plume model

TYPES OF NEUTRALLY BUOYANT VAPOR CLOUD DISPERSION MODELS. Determine what is being asked below.

It describes the steady-state concentration of material released from a continuous source.

Puff model

TYPES OF NEUTRALLY BUOYANT VAPOR CLOUD DISPERSION MODELS. Determine what is being asked below.

It describes the temporal concentration of material from a single release of a fixed amount of material.

Puff model

TYPES OF NEUTRALLY BUOYANT VAPOR CLOUD DISPERSION MODELS. Determine what is being asked below.

This model is used when you have essentially an instantaneous release and the cloud is swept downwind.

Plume

TYPES OF NEUTRALLY BUOYANT VAPOR CLOUD DISPERSION MODELS. Determine what is being asked below.

It is the release of continuous puffs.

True

TYPES OF NEUTRALLY BUOYANT VAPOR CLOUD DISPERSION MODELS. Determine what is being asked below.

TRUE OR FALSE. Puff model can be used to describe a plume.

Plume model

(ADDITIONAL: A smokestack looks like this.)

TYPES OF NEUTRALLY BUOYANT VAPOR CLOUD DISPERSION MODELS. Determine what is being asked below.

The model was originally developed for dispersion from a smokestack.

True

TRUE OR FALSE. A plume can develop if there is a leak in a large tank.

FAMILIARIZE (Plume model).

FAMILIARIZE (Plume model).

FAMILIARIZE (Puff model).

FAMILIARIZE (Puff model).

Pasquill-Gifford or Gaussian Dispersion models

This model assumes that the materials spread out in a normal Gaussian-type distribution.

Pasquill-Gifford or Gaussian Dispersion models

This dispersion model applies only to neutrally buoyant dispersion of gases in which the turbulent mixing is the dominant feature of the dispersion.

0.1 to 1 km

Pasquill-Gifford or Gaussian Dispersion models are typically valid only for a distance of ___ to ___ km from the release point.

Dense gas dispersion (Britter and McQuaid Model)

This model is best suited for instantaneous or continuous ground-level releases of dense gases where the release is assumed to occur at ambient temperature and without aerosol or liquid droplet formation.

Dense gas

It is defined as any gas whose density is greater than the density of the ambient air through which it is being dispersed.

1. Initial cloud volume

2. Initial plume volume flux

3. Duration of release

4. Initial gas density

5. Wind speed at the height of 10 m

6. Distance downwind

7. Ambient gas density

Britter and McQuaid model requires (7):

Dense gas

This result can be due to a gas with a molecular weight greater than that of air or a gas with low temperature resulting from auto refrigeration during release or other processes.

Release mitigation

It is defined as “lessening the risk of a release incident by acting on the source (at the point of release) either (1) in a preventive way by reducing the likelihood of an event that could generate a hazardous vapor cloud or (2) in a protective way by reducing the magnitude of the release and/or the exposure of local persons or property".”

1. Inherent safety

2. Engineering design

3. Management

4. Early vapor detection

5. Countermeasures

6. Emergency response

Release mitigation can be divided into six (6) aspects:

READ.

Under inherent safety,

1. Inventory reduction

2. Chemical substitution

3. Process attenuation

READ.

Under inherent safety,

1. Inventory reduction

2. Chemical substitution

3. Process attenuation

READ.

Under engineering design,

1. Physical integrity of seals and construction

2. Process integrity

3. Emergency control

4. Spill containment

READ.

Under engineering design,

1. Physical integrity of seals and construction

2. Process integrity

3. Emergency control

4. Spill containment

READ.

Under management,

1. Policies and procedures

2. Training for vapor release

3. Audits and inspections

4. Equipment testing

5. Routine maintenance

6. Management of change

7. Security

READ.

Under management,

1. Policies and procedures

2. Training for vapor release

3. Audits and inspections

4. Equipment testing

5. Routine maintenance

6. Management of change

7. Security

READ.

Under early vapor detection, 

1. Sensors

2. Personnel

READ.

Under early vapor detection, 

1. Sensors

2. Personnel

READ.

Under countermeasures,

1. Water sprays and curtains

2. Steam or air curtains

3. Deliberate ignition

4. Foams

READ.

Under countermeasures,

1. Water sprays and curtains

2. Steam or air curtains

3. Deliberate ignition

4. Foams

READ.

Under emergency response, 

1. On-site communications

2. Emergency shutdown

3. Site evacuation

4. Safe havens

5. PPE

6. Medical treatment

7. On-site emergency plans, procedures, training, and drills

READ.

Under emergency response, 

1. On-site communications

2. Emergency shutdown

3. Site evacuation

4. Safe havens

5. PPE

6. Medical treatment

7. On-site emergency plans, procedures, training, and drills