Atmospheric dispersion

The gas or vapour released will be dispersed in the surrounding area under the influence of the atmospheric turbulence. The concentrations of the gas or vapour released in the surrounding area can be calculated by means of the atmospheric dispersion models. These concentrations are important for determining whether, for example, an explosive gas cloud can form or whether injuries will occur in the case of toxic gases. Within EFFECTS a first differentiation is made between the following three types of dispersion models:

In the dispersion models account is taken of the atmospheric stability, the so-called Pasquill classes (A to F) and a certain wind velocity. The source dimensions are taken into account by means of an imaginary (virtual) point source wind upwards, for which the dispersed dimensions at the point of the actual source are equal to the actual source dimensions. The dispersion models apply only to open terrain. However allowance is made for the roughness of the terrain. The influence of trees, houses, etc. on the dispersion can be determined by means of a class of the roughness length.

Neutral gas dispersion

The neutral gas dispersion model is based on the Gaussian plume model and no account is taken of the difference in density between the ambient air and the gas. Because of this, the model must only be used for gases with a density approximately the same as air, or if the gas concentration at the point of release is low.

The direction of the release is always taken as horizontal to the wind direction.

Both Neutral and Dense gas dispersion models are available for the following type of releases: Concentration, Explosive mass and Toxic dose.

For the neutral gas and heavy gas dispersion models the following type of calculations can be carried out:

Concentration contour The model calculates:

– the dimensions of the contour (length and max. width) at given height

– the maximum concentration and corresponding distance at time t: only for neutral gas dispersion, instantaneous release and semi-continuous release when cloud has drifted away from its release point.

– graphical presentation of the contour in X-Y directions – graphical presentation of concentration with distance.

For semi-continuous and instantaneous releases the concentration contour is calculated for one specified time t.

Explosive mass The model calculates:

– the explosive mass, for concentrations higher than LEL or between LEL and UEL – the dimensions of the LEL contour

– the dimensions of the UEL contour (not for heavy gas dispersion)

– whether the source is at ground level or the plume touches the ground level or it is a free plume.

For semi-continuous and instantaneous releases the explosive mass is calculated for one specified time t.

For neutral gas dispersion the explosive mass and the dimensions of the LEL and UEL contour are calculated for a height equal to the source height.

For heavy gas dispersion these parameters are calculated for a height equal to zero (ground level).

Toxic load

The model calculates:

· the toxic load, Cⁿ.t, with C = concentration in mg/m³ and t = duration in min at position (x, y, z) for neutral gas dispersion

· at position (x, 0, 0) for heavy gas dispersion for a certain exponent n and for a given maximum exposure time after arrival cloud

· the maximum concentration at position (x, y, z), and for semi-continuous and instantaneous releases the corresponding time at position (x, y, z)

· the arrival time and departure time of the cloud at position (x, y, z) (not for continuous releases)

For these parameters a concentration equal to 1% of the maximum concentration is assumed.

Continuous, Semi-continuous or Instantaneous

Within the model itself, the user has to choose for continuous, Semi-continuous or Instantaneous.

For rather long releases the continuous release dispersion model has to be used and for very short releases the instantaneous release dispersion model. In general the following is used to judge whether the source can be considered as continuous or instantaneous [Yellow Book]:

Continuous: at distances < 1.8 * wind velocity * duration of release Instantaneous: at distances > 18 * wind velocity * duration of release Semi-continuous: intermediate cases.

The dispersion calculations for the semi-continuous releases could be rather time consuming.

Dense gas dispersion

If the gas has a higher density than air (because of a high molecular weight or of a low temperature) it will tend to spread in a radial direction because of gravity, resulting in a ‘gas pool’. As a result of this, in contrast to a neutral gas, the gas released may spread against the direction of the wind.

In combined models, the selection criteria for using dense gas is: Dense gas is situation where density of mixture (possibly including liquid droplets with very high density is 10%

heavier than air.

Heavy gas dispersion models are available for the following type of releases:

Instantaneous gas release: instantaneous release of gas, vapour or flashing liquid.

Pool evaporation: vapour source is formed by evaporation from a pool

Jet, horizontal or vertical: (semi-)continuous release of gas, vapour or spray release in vertical or horizontal direction

Turbulent free jet

When a gas or vapour releases and the Reynolds number under the release conditions is greater than about 2.5.10⁴ (e.g. high release velocity) a jet occurs. Another condition is the absence of obstacles in the jet. Turbulent free jet dispersion occurs when the gas velocity at the release equals or is close to the velocity of sound.