Studies in support of a quantitative approach to hazardous area classification

A study was made of the feasibility of putting hazardous area classification (HAC) on a more quantitative basis. A review of current HAC practice showed that the widespread policy of setting fixed zone distances around sources of hazard was subjective and sometimes led to inconsistencies between different codes of practice when applied to the same situation. Fatality and injury statistics were used to show that there is a significant risk to workers from the ignition of flammable atmospheres. which should be reduced. Data were researched and compiled to fit into a proposed framework for the quantification of HAC. These included information concerning leak source inventory: source leak frequency: and source leak size distribution. Mathematical models were collected which could be used to describe the emission and dispersion of flammable releases. Example calculations were performed for typical leak scenarios to illustrate the variation in hazard distances. Estimates were made of the ignition and explosion probabilities of flammable leaks. which depended princi pally on emission size. To compensate for uncertainties in the researched data. a fire and explosion model was devised to estimate the ignition frequency on a typical process plant. The model was applied to a "standard" plant which was formulated from researched data. By iteratively checking the estimated ignition frequencies against historical data it was concluded that reasonable agreement was achieved with some adjustment of the input data. The special problems of HAC of indoor plants were also addressed. It was concluded that the results of this study provided a basic framework for the quantification of HAC. although the quality of currently available data necessary for quantification is generally poor. The acquisition of better quality leak and ignition data should provide a platform from which the current work may progress. Further work should include the further refinement of the basic fire and explosion model to account for ignitions which HAC cannot protect against such as autoignitions. It was also noted that the behaviour of indoor releases requires clarification. together with the concept of a minimum flammable inventory below which there is negligible risk of ignition.