Design and Development of an Integrated Chemical Accident System for Monitoring Risk Potential of Hazard Installation

Mathematical models are extremely useful tools to predict the impacts of chemical process accidents. The applications of these models require competency in mathematics and computer programming. Therefore the procedural and methodology for the calculation of consequences and risks from installations handling toxic or flammable materials have been computerized by using a software called the Simulation of Chemical Industrial Accidents (SCIA). The SCIA software was successfully developed using Visual Basic (VB) programming language with integration of GIS tools. The explosion, fire and toxic release models are implemented in an interactive VB environment. The first step to evaluate the hazard consequence by using the SCIA software is to select whether the material is a toxic or flammable substance which is accidentally released into the environment, the specification of the position of the installations, and of all the damage states under analysis. For each and every plant damage state the following tasks are performed. An outflow model calculates the rate of release of the hazardous substance. For a toxic material the software will precede the dispersion calculations, in order to estimate the concentration of the substance in the environment. In the case of a flammable substance a distinction is made on whether an immediate or a delayed ignition will occur. If the flammable substance along with the plant damage state results in an immediate ignition, an appropriate model is called to calculate the resulting thermal radiation. If delayed ignition is appropriate, the software will proceed to dispersion calculations to determine the explosive mass and its position in the resulting cloud. In the second step, the explosion, fire and toxic release models are used to calculate the overpressure, radiation and concentration effects around the site. Assessment of the dose follows the calculation of concentrations, thermal fluxes and overpressures as appropriate. The third step is to estimate the probability of fatality or injury that an individual will die or suffer as a result of its exposure to extreme phenomenon which is estimated in terms of dose-response models. All dose-response models employed in SCIA are based on a “probit” function for the substance and phenomenon. The results obtained from all three scenario models (explosion, fire and toxic release) were compared with results from previous software or data from real accidents, spreadsheets and manual calculations. However slight differences can be noticed with those results obtained from earlier studies. The final step is to present the hazard zones located in the vicinity of the accident area. This can be done by using GIS functionality to present the results as a circle around the point of release from the source. The software is capable of handling multiple and alternative accident scenarios, complex terrain dispersion, uncertain quantification (including parameter and model uncertainty) and is characterized by a user-friendly Graphic User Interface (GUI). Furthermore the software is linked to GIS for screening hazard displays. The development is done by customizing ArcGIS Engine using Visual Basic (VB). With ArcGIS Engine user can efficiently build and deploy custom desktop ArcGIS applications and use embedded GIS logic in non-GIS centric applications. Using the ArcGIS applications, users can create and manage maps used in custom applications which will save in development and effort. The results from SCIA software were extensively validated and compared with other commercial softwares such as: FRED (developed by Shell Global company, 2004), BIS (developed by ThermDyne Technologies Ltd, 2003), EFFECT (developed by TNO, 1987) and MAXCRED (developed by Khan and Abbasi, 1998) and with established data. It was observed that the difference between results from the SCIA software in comparison to the others can be considered insignificant. Furthermore the SCIA software has been developed successfully to be utilized as a stand alone application, which allows users to run the software independently on a PC. The SCIA software is useful due to various reasons; the cost of developing the software is cheap, the program size is small which enables users to run the application instantly, compatible to download for all windows operating systems, work as a stand alone application and finally contains a flexible database to add or delete records in the future. Furthermore the application includes an internal help option to provide and guide users with instructions to use the software. Therefore SCIA is considered as a user-friendly application.

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