Today, all modern industrial units acknowledge the necessity of e fficient and eff ective safety, health, and en-vironment -HSE- systems. To become practical, these systems must be localized and customized to serve the exact needs of the industry. Nevertheless, most HSE plans are developed upon a set of common presumptions. In the water industry, gas chlorination units require strong HSE plans to mitigate the possibility of chlorine ex-plosion and leak. This study aimed to provide an efficient HSE system for gas chlorination process within water treatment plants. This goal was achieved through a case study performed on a water treatment plant in Razavi-Khorasan province, Iran. In the fi rst stage of this study, the researchers made combined use brainstorming sessions and modi fied Delphi technique to identify the risk factors of gas chlorination units and classify them into six groups in terms of association with chlorination unit building, gas cylinder storage, technical details of gas cylinders, gas cylinder transport, chlorinator connections, and chlorination unit management. In the second stage, the extracted factors were analyzed by Failure Mode E ff ects Analysis -FMEA- and Shannon Entropy ap-proaches using two different panels of experts, and the results were compared for validation. Finally, the analysis results were structured by Petri Net modeling. The results showed that, according to FMEA, the risk factors with risk priority number -RPN- of over 46 are of highest importance for the studied unit. Once observed, these factors necessitate shutting down the operation until a risk mitigation solution is reached. Among the analyzed factors, -i - the presence of compounds such as NH 3 ,O2 , gas and liquid hydrocarbons and oil in gas chlorine cylinders and - ii - non-vertical and non-mechanized handling of full and empty cylinders during loading and unloading, with RPNs of respectively 160 and 120, were found to be signifi cantly more important than others. In the SE analysis, in addition to the above factors, poor implementation of airflow control mechanism inside the chlorination chamber -W = 0.359-, storage of chlorine cylinders near electrical and mechanical installations such as elevators or power panels -W = 0.327-, poor pipe placement for connecting the injector to the water inlet and the possibility of air suction -W = 0.433-, and failure to provide scienti fic and practical training to the chlorination sta ff -W = 0.342- were found to be of highest importance.

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