Estimating the consequence of a chemical released to atmosphere

Parameters of importance

Various parameters affect the dispersion of a material, and these principally include:

• Release quantity and rate. These are the most important parameters in determining the dispersion distance, which does not increase linearly with the quantity.
• Duration of release. This is a function of the release mode. For liquids that form evaporating pools, the duration is dependent on the evaporation rate. In the case of instantaneous releases (say catastrophic rupture), the release duration is very short and the total quantity of material released contributes to the dispersion hazard. For a continuous release, the discharge lasts a long time and the discharge rate is the main parameter.
• Initial density of the release. This is important as it determines the initial spreading rate.
• Elevation of the source. Increased heights lead to reduced ground level concentrations since a high speed jet ensuing from a vent is likely to travel some distance before dispersing in the downwind direction.
• Prevailing atmospheric condition. The Atmospheric Stability and wind speed affect the dispersion. A low wind speed coupled with a Stability category F is usually the worst for dispersion (greater areas affected). A high wind speed helps dilute the airborne material rapidly, reducing the damage downwind distance.
• Surrounding terrain. This affects the dispersion significantly. Rough terrain with trees, shrubs, hills, buildings and structures usually enhance dispersion.
• Toxicity of the released material. The more toxic the material, the more would be the effect distance, though the relationship is not linear.
• Ambient temperature and humidity. These affect the dispersion in several ways . Also, fire damage effects are diminished by the heat absorption due to the humidity.

Damage Criteria

In consequence analysis, use is made of a number of calculation models to estimate the physical effects of an accident (spill of hazardous material) and to predict the damage (lethality, injury, material destruction) of the effects. The calculations can roughly be divided in three major groups:
a) Determination of the source strength parameters;

b) Determination of the consequential effects;
c) Determination of the damage or damage distances.

The basic physical effect models consist of the following.

Source strength parameters

• Calculation of the outflow of liquid, vapour or gas out of a vessel or a pipe, in case of rupture. Also two-phase outflow can be calculated.
• Calculation, in case of liquid outflow, of the instantaneous flash evaporation and of the dimensions of the remaining liquid pool.
• Calculation of the evaporation rate, as a function of volatility of the material, pool dimensions and wind velocity.
  

Consequential effects

Dispersion of gaseous material in the atmosphere as a function of source strength, relative density of the gas, weather conditions and topographical situation of the surrounding area.

Intensity of heat radiation [in kW/ m2] due to a fire or a BLEVE, as a function of the distance to the source.
Energy of vapour cloud explosions [in N/m2], as a function of the distance to the distance of the exploding cloud.
Concentration of gaseous material in the atmosphere, due to the dispersion of evaporated chemical. The latter can be either explosive or toxic.

The damage criteria give the relation between extent of the physical effects (exposure) and the percentage of the people that will be killed or injured due to those effects. The probability of death depends also on the health of the individual apart from the dose and exposure time. The manner in which unequal population distribution is built in is called the % lethality or Probit function. 50% lethality means that 50% of the exposed population would be killed.The knowledge about these relations depends strongly on the nature of the exposure. For instance, much more is known about the damage caused by heat radiation, than about the damage due to toxic exposure, and for these toxic effects, the knowledge differs strongly between different materials. In-Consequence Analysis studies, in principle three types of exposure to hazardous effects are distinguished:

Heat radiation, from a jet, pool fire, a flash fire or a BLEVE.

Explosion

Toxic effects, from toxic materials or toxic combustion products.

In the next three paragraphs, the chosen damage criteria are given and explained.

Heat Radiation

• The radiation energy onto the human body [kW/m2];
• The exposure duration [sec];
• The protection of the skin tissue (clothed or naked body).
  
  

The consequences caused by exposure to heat radiation is a function of:
The radiation energy onto the human body [kW/m2];
The exposure duration [sec];
The protection of the skin tissue (The limits for 1% of the exposed people to be killed due to heat radiation, and for second-degree burns are given in the table below:

Damages to Human Life Due to Heat Radiation (clothed or naked body).

 Since in practical situations, only the own employees will be exposed to heat radiation in case of a fire, it is reasonable to assume the protection by clothing. It can be assumed that people would be able to find a cover or a shield against thermal radiation in 10-sec. time. Furthermore, 100% lethality may be assumed for all people suffering from direct contact with flames, such as the pool fire, a flash fire or a jet flame. The effects due to relatively lesser incident radiation intensity are given as under:

Effects Due To Incident Radiation Intensity

INCIDENT RADIATION ( kW/m2)	TYPE OF DAMAGE
0.7	Equivalent to Solar Radiation
1.6	No discomfort for long exposure
4.0	Sufficient to cause pain within 20 sec. Blistering of skin (first degree burns are likely)
9.5	Pain threshold reached after 8 sec. second degree burns after 20 sec.
12.5	Minimum energy required for piloted ignition of wood, melting plastic tubings etc.

Explosion

In case of vapour cloud explosion, two physical effects may occur:

• a flash fire over the whole length of the explosive gas cloud;
• a blast wave, with typical peak overpressures circular around ignition source.

As explained above, 100% lethality is assumed for all people who are present within the cloud proper.

For the blast wave, the lethality criterion is based on:
A peak overpressure of 0.1 bar will cause serious damage to 10% of the housing/structures.
Falling fragments will kill one of each eight persons in the destroyed buildings.

The following damage criteria may be distinguished with respect to the peak overpressures resulting from a blast wave: 

Damage Due To Overpressures

Peak Overpressure (bar)	Damage Type
0.83	Total Destruction
0.30	Heavy Damage
0.10	Moderate Damage
0.03	Significant Damage
0.01	Minor Damage

Intoxication

The consequences from inhalation of a toxic vapour/gas is determined by the toxic dose. This dose D is basically determined by:

concentration of the vapour in air;

exposure duration.

Furthermore, of course, the breathing rate of the victim, as well as the specific toxic mechanism unto the metabolism play an important role.
The dose is defined as D = Cn.t, with:

C = concentration of the toxic vapour, in [ppm] or [mg/m3];
t = exposure duration, in [sec] or [min];
n = exponent, mostly > 1.0; this exponent takes into account the fact that a high concentration over a short period results in more serious injury than a low concentration over a relatively longer period of exposure.

The given definition for D only holds if the concentration is more or less constant over the exposure time; this may be the case for a (semi) continuous source. In case of an instantaneous source, the concentration varies with time; the dose D must be calculated with an integral equation :

D = ò Cn.dt

For a number of toxic materials, so-called Vulnerability Models (V.M.) have been developed. The general equation for a V.M. (probit function) is:

Pr = a + b.ln (Cn.t), with
Pr = probit number, being a representation of the percentage of people suffering a certain kind of damage, for instance lethality
Pr = 2.67 means 1% of the population;
Pr = 5.00 means 50% of the population;
a and b material dependent numbers;
Cn.t = dose D, as explained above.

The values for a and b are mostly derived from experiments with animals. Although much research in this field have been done over the past decades, only for a limited number of toxic materials consequence models have been developed. Often only quite scarce information is available to predict the damage from an acute toxic exposition. Data transformation from oral intoxication data to inhalation toxicity criteria, is sometimes necessary. Generally, in safety evaluations pessimistic assumptions are applied in these transformation calculations. The calculated damage (distance) may be regarded as a maximum.