Based on data collected on hazardous substances, hazardous activities and possible points of accident in the production process and facilities, it is necessary to simulate the possible developments, which includes consideration of the potential scope and consequences of the accident to the life and health of people and the environment, as well as the size of the damage. According to the defined accident, a scenario is calculated and modelled for the effects of the accident and determines the width of the vulnerable zone.
Creating the model requires the following data and parameters:
- Parameters derived from the nature of chemical compounds and their physical-chemical, toxicological, eco-toxicological and other properties;
- The quantities of chemical substances and physical states in which the substances are;
- The mode of action of hazardous substances (explosion, fire, the release into the atmosphere, soil or water);
- Information about the place in which accidents occur: Open space; Time of Day; Topographic; Characteristics of terrain; Hydro geological characteristics of the terrain; and other…
Meteorological conditions in general affect the shape and size contaminated cloud formation, direction, speed and depth of propagation (horizontal and vertical), as well as the retention time of concentration dangerous to life and health of people and the environment in vulnerable locations.
Also, the current weather conditions have a significant impact if there are discharges of large quantities of hazardous substances in the soil, because they depend largely on stability and volatility of hazardous substances, and therefore the presence of harmful concentrations in surface air.
The most important meteorological parameters, which influence the formation and behaviour of clouds contaminated and thus harmful consequences for humans and the environment are:
- The temperature of air and soil,
- Wind direction and velocity,
- Vertical stability of air,
- Cloudiness, humidity and rainfall.
The stability of the atmosphere is a parameter that is extremely important for the spatial distribution of air pollution, despondent because of the intensity of the process of turbulent mixing in the lower layer. In a stable atmosphere, this process is weak and reduces the diffusion of foreign matter, i.e. air pollution. Due to slow spread of air pollution, there are high concentrations of polluted air around the source. In an unstable atmosphere, turbulent vortex can spread air pollution significantly faster, which leads to a rapid decrease in the concentration.
Calculation of the spatial distribution of air pollution is carried out by using the equation of atmospheric diffusion. These equations are used as the basis of mathematical models for different types of sources. Overview stability class is shown in the following table.
|Wind (m/s) on 10 m||Day||Night|
|Solar radiation – Daily insulation||The negative balance of radiation|
|Strong||Medium||Poor||Low cloud cover > 4/8||High cloud cover < 3/8|
|2 – 3||AB||B||C||E||F|
|3 – 5||B||BC||C||D||E|
|5 – 6||C||CD||D||D||D|
A – Very unstable
B – Moderately unstable
C – Slightly unstable
D – Neutral
E – Slightly stable
F – Moderately stable
G – Very stable
The topography of the soil is an important factor in assessing the possible consequences of an accident. Depending on the type of surface-soil terrain topography can be:
- urban topography – terrain full of obstacles
- rural topography – flat terrain
Modelling the effects of the explosion and fire
In the process of modelling the effects of explosion and/or fire, it is necessary to calculate and determine the zone in which they will exhibit all the harmful effects of accidents (fragmentation effect of the explosion, demolition, expressed the shock wave overpressure transfer of fire-emitted heat-burns) as well as a safe zone for people and objects.
A Thermal Radiation Level of Concern (LOC) is a threshold level of thermal radiation, usually the level above which a hazard may exist. In use are three threshold values (kW/m2) to create the default threat zones:
- Red: 10 kW/m2 (potentially lethal within 60 sec);
- Orange: 5 kW/m2 (second-degree burns within 60 sec); and
- Yellow: 2 kW/m2 (pain within 60 sec).
The thermal radiation effects, that people experience, depend upon the length of time they are exposed to specific thermal radiation level. Longer exposure durations, even at a lower thermal radiation level, can produce serious physiological effects. The threat zones displayed represent thermal radiation levels; the accompanying text indicates the effects on people who are exposed to those thermal radiation levels but are able to seek shelter within one minute.
Overpressure. A major hazard associated with any explosion is overpressure. Overpressure, also called
a blast wave, refers to the sudden onset of a pressure wave after an explosion. This pressure wave is caused by the energy released in the initial explosion-the bigger the initial explosion, the more damaging the pressure wave is. Pressure waves are nearly instantaneous, traveling at the speed of sound. An Overpressure Level of Concern (LOC) is a threshold level of pressure from a blast wave, usually the pressure above which a hazard may exist.
- Red: 8.0 psi (destruction of buildings);
- Orange: 3.5 psi (serious injury likely);
- Yellow: 1.0 psi (shatters glass).
VCE – Vapor Cloud Explosion. When a flammable chemical is released into the atmosphere, it forms a vapor cloud that will disperse as it travels downwind. If the cloud encounters an ignition source, the parts of the cloud where the concentration is within the flammable range (between the LEL and UEL) will burn. In some situations, the cloud will burn so fast that it creates an explosive force (blast wave). The primary hazards are overpressure and hazardous fragments.
Boiling Liquid Expanding Vapour Explosion. BLEVEs typically occur in closed storage tanks that contain a liquefied gas, usually a gas that has been liquefied under pressure. A gas can be liquefied by either cooling (refrigerating) it to a temperature below its boiling point or by storing it at a high pressure. Although both flammable and nonflammable liquefied gases may be involved in a BLEVE.
Fireball. During the modelling of a BLEVE, it can be assumed that a fireball will form. The fireball is made up of both the chemical that flash-boil when the tank fails and the chemical that spray out as an aerosol during the explosion.The primary hazard associated with a fireball is thermal radiation.
Pool Fire. A pool fire occurs when a flammable liquid forms a puddle on the ground and catches on fire. Thermal radiation is the primary hazard associated with a pool fire. Other potential pool fire hazards include smoke, toxic byproducts from the fire, and secondary fires and explosions in the surrounding area. In some cases, heat from the pool fire may weaken a leaking tank and cause it to fail completely—in which case, a BLEVE may occur.
Jet fires. A jet fire occurs when a flammable chemical is rapidly released from an opening in a container and immediately catches on fire – much like the flame from a blowtorch. A two-phase jet fire occurs when a gas that has been liquefied under pressure is released. Thermal radiation is the primary hazard associated with a jet fire. Other potential jet fire hazards include smoke, toxic byproducts from the fire, and secondary fires and explosions in the surrounding area. In some cases, heat from the jet fire may weaken the tank and cause it to fail completely—in which case, a BLEVE may occur.
Flash fires (flammable area) When a flammable vapor cloud encounters an ignition source, the cloud can catch fire and burn rapidly in what is called a flash fire. Potential hazards associated with a flash fire include thermal radiation, smoke, and toxic byproducts from the fire. The flammable area is bounded by the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL).
Modelling the effects of the explosion of pressure vessels Two main causes need to be considered concerning the vessel rupture. The first one is a fast rise in pressure inside the vessel due to an overfilling or an overheating of the vessel. The second cause of rupture is a reduction of the vessel strength caused by corrosion, overheating, material defect or external impact.
Determination of accidents
- I level – consequences of the accident are not expected outside the installation (plant level). Effects of the accident are limited to one building or facility and can be controlled by the staff responsible for managing the process. Consequences for the local community, are not expected.
- II level – consequences of the accident are not expected outside the installation (plant level). Harmful effects have engulfed several buildings and facilities or transferred to the industrial complex. Consequences for the local community, can be expected.
- III level – consequences of the accident may expand outside the complex (local level).
- IV level – consequences of the accident can be extended to the territory of the region (regional level). Refers to the larger and more serious accidents that have regional significance.
- V level – consequences of the accident may extend beyond the borders of the State (international level). Applies to very large-scale accidents where its negative consequences can be transferred outside the borders of the State.
Risk is a certain level of probability that an activity, directly or indirectly, causes a danger to the environment, human health and life.
Risk assessment of hazardous activities is the process of determining the risk based on:
- Assess the probability of accidents, and
- The possible consequences for the life and health of people and the environment.
Risk (R) is a function of the probability of an accident (p) and the potential losses (L) and can be summarized as follows:
R = ƒ [p, L]
Probability of accidents (p) – Criteria