In industrial installations, most accidents occur due to the loss of containment of hazardous chemical substances in pipes and units used to transport or store gas or liquid materials. The accidental release of these hazardous chemical substances might pose a threat to health and the environment.
If the substance released into the atmosphere can lead to the generation of a gas or vapour cloud, the prediction of its dispersion behaviour is of paramount importance. By predicting the behaviour of a dispersing cloud, we can get very relevant information, such as the variation in concentration of the substance involved in the accident, at relevant points as a function of time. This information allows the estimation of the accident’s effects on people, the environment, and equipment, which is crucial when designing safety measures and emergency plans.
In the context of the generation of a gas or vapour cloud, hazardous chemical substances can be classified as flammable or toxic.
Flammable substances are defined as substances that will ignite and continue to burn when brought in contact with an ignition source. The ignitability of a chemical depends on its flash point, auto-ignition temperature, and flammability limits. Flammable vapour clouds can accumulate in poorly ventilated rooms or highly congested areas, which can lead to an explosion event.
Toxic substances are defined as substances that can be poisonous or cause health effects. These substances are often toxic at very low concentrations when one is exposed to them for a certain period. Products that we use daily such as household cleaners, prescription and over-the-counter drugs, pesticides, and cosmetics, can also be toxic. Any chemical can be toxic or harmful under certain conditions.
But what makes a chemical toxic?
Chemical toxicity
Dose
In general, the larger the amount of a toxic substance that enters your body, the bigger its effect on you.
For example, organic solvents such as toluene, acetone, and trichloroethylene all affect the brain in the same way but to different degrees at different doses. The effects of these solvents are similar to those that result from drinking alcoholic beverages. You may feel nothing or a mild feeling of relaxation or drowsiness at a low dose. A larger dose may cause dizziness or headache. With an even larger dose, you may become drunk, pass out, or even stop breathing.
When you inhale a toxic chemical, the dose you receive depends on two main factors:
- The level (concentration) of chemicals in the air
- How long the exposure lasts
It is safest to keep exposure to any toxic substance as low as possible. However, since some chemicals are more toxic than others, it is necessary to distinguish acceptable dose levels per type of chemical.
Exposure duration
The longer you are exposed to a chemical, the more likely you will be affected by it. However, the concentration of the chemical in the air is still important. You may not experience any effects at very low concentration levels no matter how long you are exposed. You may not be affected at higher concentrations following a short-term exposure, but repeated exposure over time may cause harm.
Chemical exposure, which continues over a long period, is particularly hazardous because some chemicals can accumulate in the body. The body has several systems, such as the liver, kidney, and lungs, which are organs that can transform these chemicals into a less toxic form to eliminate them from your body. If your rate of exposure to a chemical exceeds the rate at which the body can eliminate it, some of the chemicals will accumulate in your body. For that purpose, typical occupational exposure limits, like MAC (Maximum Allowable Concentration) values, can be used. Specific emergency response and direct lethality relations should be used for accidental releases, which may give a short duration but high concentration levels.
The toxic dispersion model in EFFECTS
Gexcon’s consequence modelling software EFFECTS allows you to calculate a cloud’s dispersion behaviour based on concentration thresholds, flammability levels or toxic exposure.
The toxic dispersion model calculates (among others):
- The toxic dose, Cn•t, where C is the concentration in mg/m3 and t the duration of exposure, at different downwind distances from where the accidental release occurs.
- The maximum concentration along the centreline of the toxic cloud and at different study heights.
- The arrival time and departure time of the cloud at different positions (x, y, z).
This allows you to have a complete three-dimensional understanding of the cloud’s trajectory, concentration over distance, etc.
Concentration vs dose in EFFECTS
As you can see from the screenshot below, EFFECTS reports both the hazard distance to a threshold concentration and to a dose. But what is the difference between these two results?
The hazard distance to a threshold concentration (red contour) will always be greater than when the hazard distance is calculated for a dose-based value. That is because exposure to a low concentration for a very short instance of time might not lead to fatalities. As previously mentioned, a toxic concentration poses a threat to people’s health when the person is exposed to it for a certain period (which is determined by the exposure duration). That is the reason why this hazard distance will reach many other areas without leading to fatalities.
The hazard distance to a dose value (blue contour) presents a smaller contour. That is because the exposure to the concentration of toxic material for a certain exposure duration will lead to more fatalities in a smaller radius from where the accidental release occurs.
For more information about calculating dose, please refer to the blog post “Predicting consequences of toxic dispersion”.
Mortality/Probit calculator in EFFECTS
The Mortality/Probit calculator is a tool available within EFFECTS that can be used to quickly estimate the fraction of mortality when exposed to a toxic concentration. This calculator also presents the dose value, at the given concentration and exposure duration.
As you can see from the screenshot below, a concentration of 205.6 mg/m3 of chlorine would lead to a mortality of 1% when the exposure duration is 30 minutes. The corresponding dose at which one would be subject to those conditions is 1.3E-07 s•(kg/m3)n.
Ammonia, however, is a much less toxic chemical than chlorine. A concentration of 1696.4 mg/m3 would produce a 1% mortality for the same exposure duration. Therefore, the dose required would also be much larger (i.e., 0.00518 s•(kg/m3)n).
Therefore, this tool gives you a very quick overview of how toxic a chemical can be and can also be used to understand the results given by the toxic dispersion model.
For example, going back to the top view dispersion contour shown in Figure 1:
- The red contour depicts the 1% lethality concentration contour for a chlorine dispersion cloud, which is the distance for a concentration that would cause 1% mortality when a person is continuously exposed to chlorine for 30 min (which is 205.6 mg/m3 as shown in Figure 2). In reality, this concentration level does not occur that long at those distances, which is why its corresponding dose contour is small.
- The blue contour depicts the 1% lethality dose contour for that same chlorine dispersion cloud, which is the distance for a dose that would cause 1% mortality for the given exposure duration (which is 1.31E-07 s•(kg/m3)n as shown in Figure 2).
Exposure duration in EFFECTS
The exposure duration is an input parameter of the EFFECTS toxic dispersion model and can be based on three different concepts:
- Time limit of release: the toxic dispersion calculation is performed based on a non-limited exposure. The person is exposed to the toxic cloud for the entire release duration.
- Time limit for cloud exposure: the toxic dispersion calculation is performed so that the person starts being exposed to a toxic concentration when the cloud arrives at the person’s location.
- Time limit until sheltering: the toxic dispersion calculation is performed based on the fact that exposure lasts as long as the person is looking for shelter. Once they have protected themselves indoors, exposure to the toxic cloud stops.
The toxic dispersion calculation will lead to different dose values depending on which type of exposure is selected.
Toxic indoor calculation
But what would be the toxic exposure indoors if the person has found shelter? Would they be exposed to the toxic cloud? And if so, what would be the equivalent fatalities occurring indoors?
A person is typically more protected indoors, but there will still be exposure to the toxic cloud if the building is ventilated, as this will allow toxic concentrations to enter the building.
A simplified approach assumes that the lethality indoors is 10% the lethality outdoors. However, this 10% approach is based on a very simple rule of thumb and can highly underestimate the resulting effects of the toxic cloud on people indoors.
That is why the EFFECTS toxic dispersion model is adapted to calculate the indoor dose based upon concentration build-up due to ventilation rate. This ventilation rate will highly affect the toxic dose occurring indoors. The more ventilated the building is, the higher the toxic concentration indoors will be as well (thus, more fatal). The figure below shows how underestimating this “10% of outdoor lethality” rule of thumb can be and how important it is to calculate indoor lethality based on ventilation rate instead.
The screenshot below shows the same chlorine toxic dispersion cloud as depicted in Figure 1, where the lethality dose indoors has been added. A ventilation rate of 1 time per hour has been used in this case.
As you can see from the figure above, the lethality dose contour indoors reaches a smaller area, as people indoors are partially protected from exposure to the toxic cloud occurring outdoors. The partial protection is due to a slower build-up of the toxic concentration as well as a slower decay.
Dynamic concentration grid
The dispersion models in EFFECTS also allow the calculation of a dynamic concentration grid, as seen from the video below, which gives a very clear overview of the trajectory of the cloud with time.
Other grid results
The EFFECTS toxic dispersion model also allows the user to show a maximum concentration grid, an indoors/outdoors lethality grid and an indoors/outdoors dose grid projected on top of a background map (which can be an internet server). You can then use the crosshair tool to read in grid values directly from the map, a handy functionality in EFFECTS.
The screenshot below shows the top view lethality outdoors grid for the same chlorine plume. The lethality outdoors is 0.82 at the selected coordinate in the map.
In short, EFFECTS contains many different features that will allow you to simulate toxic dispersion scenarios easily and to comply with rules and regulations worldwide.
Would you like to see an example of a toxic dispersion simulation in EFFECTS?
Download the project file below, which contains an example of the simulation of a toxic cloud.
You can open the project file with the EFFECTS free viewing demo, available for download via the button below.
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