Consequence modelling requires carefully defined scenario and understanding of chemical properties of substance
This article is written by Asta Karpiola, HSE Specialist at AFRY Process Industries
Do you know why careful specification of a scenario and understanding the chemical characteristics of a substance in consequence modelling are so important?
Consequence analyses are executed for multiple reasons.
The most common reason for modelling certain scenarios is to fulfil the requirements of identifying and mitigating possible major accident hazards in facilities that handle or store hazardous chemicals and substances. Identification and prevention of major accident hazards are instructed by regulations and guidelines in many countries. For example, in Finland, identification of major accident hazards and defining safety distances are required for facilities which have large-scale operations. The basic requirements for identifying possible chemical hazards, preventing accidents caused by chemicals and limiting their consequences for people, property and the environment are set out in the "Act on the Safe Handling and Storage of Dangerous Chemicals and Explosives 390/2005" and in "Government Decree on Safety Requirements for Industrial Handling and Storage of Dangerous Chemicals 856/2012". The guide “Location of industrial plant” (2015) by the Finnish Safety and Chemicals Agency (Tukes) gives guidance and requirements for consequence modelling.
Why is it necessary to pay attention to the careful specification of the scenario in well instructed consequence modellings?
The instructions for consequence modelling given in the guide “Location of industrial plant” (2015) create a basic structure for modelling. More detailed information helps to execute a more versatile and comprehensive estimation of the safety distances for potential hazardous scenarios, whether it is the dispersion of hazardous or toxic gases, heat radiation intensities of a fire or overpressure wave of an explosion. Especially when modelling the dispersion of toxic gases, small details in specifying the scenario can affect the safety distances considerably.
For example, let’s consider modelling a pipe bridge leakage of liquefied gas with lower density than air. The basic parameters creating the foundation for the dispersion modelling scenario are
- diameter of the pipe
- height of the pipe bridge,
- characteristics of chemical substances in the pipeline (toxicity, flammability limits, density, dispersion rate, boiling point, vapour pressure etc.)
- flow rate,
- process pressure and temperature.
The parameters that may cause significant variation in dispersion modelling results are, for example, diameter and the location of leakage. Often in risk assessments, the reasons for pipe bridge leakages are either external mechanical impact to the pipeline or corrosion damage. In the modelling, the diameter of the leakage orifice is considered to be millimetres. The location of the leakage in the pipeline was examined more carefully, and the diameter of the orifice was kept constant. This scenario examined two different release directions from the pipe bridge: leakage directed horizontally from the pipe bridge and vertically downwards, impinging to the ground (Figure 1).
Figure 2 below describes two different releases of substance that is lighter than air. The parameters for release were the following:
- orifice diameter 2 mm
- process pressure 7 bar and
- release location in 6 meters (pipe bridge).
In figure 2 below, blue colour illustrates the side view effect zone of a horizontal release from the pipe bridge, and purple colour shows the side view effect zone of a down impinging release from the pipe bridge. The height of interest for presented results of a modelled scenario is normally 1 – 2 meters which is chosen to demonstrate the effect of consequences on people.
From Figure 2, it can be seen that the hazardous concentration does not reach the height of interest 1,5 m from the horizontal release. Hence, there would not be health hazards in this scenario to people. On the other hand, the down impinging release forms a pool to the ground first and the evaporated chemical forms a concentrated cloud with hazardous concentration at the height of interest 1,5 m. When the weather conditions stay constant, the chemical evaporated from the pool disperses further because the mixing of chemical and air takes longer at lower height and the cloud concentration dilutes slowly. With horizontal release from 6 meters, the mixing of chemical with air is faster, and the concentration dilutes earlier, thus decreasing the distance of hazardous concentration.
In this example scenario, there would not be any hazardous consequence if we would only consider the horizontal release from the pipe bridge. With down impinging release, the safety distance would be in 60 meters. This could possibly have an impact on e.g. emergency planning at the plant area. This shown scenario is just one of many possible examples to demonstrate the effect of small variables to final results and the importance of identifying the correct scenarios in risk assessments. The consequence analysis was implemented using Phast modelling software from DNV.
As a summary, important factors in consequence analyses of major accident hazards are related to
- identification of likelihoods and most potential scenarios,
- knowing the characteristics of used chemical to interpret the results correctly,
- defining properly the conditions of modelled scenario and
- further identification of the necessary safety measures to prevent or mitigate the effects.