Vapour cloud modelling in the risk assessment of major toxic hazards: Effect of relative humidity

During the last ten years there has been a considerable increase in the use of quantified risk assessment to achieve a detailed understanding of complex plants, and for assisting decision making on safety matters stemming from the hazards associated with process. In risk assessments, a variety of predictive models is employed. The final results therefore contain uncertainties which arise from modelling imperfections or the adoptions of assumptions and judgements that are necessary to perform risk analysis. The paper illustrates the procedure required for a quantitative top-down analysis of risks from major toxic hazards to the individuals. The paper also indicates that many items of data or assumptions need to be incorporated. Particular attention is paid to the importance of relative humidity effects on consequence analysis. A computerised procedure developed within the Technical Research Centre of Finland is presented. The sensitivity of the results to assumptions and judgements, and the validity and usefulness of the approach are discussed.