A combined approach to improve safety performance on existing process plants. Practical application according to Seveso II

The process of Hazop is well known as an effective highly respected method of hazard identification; on the other side, it is considered a long and expensive process. It is usually carried out along or near the end of the design process, allowing to achieve early identification of hazards, process changes in order to eliminate or reduce them, specifically designed control systems and comparison of alternatives. The problems encountered in carrying out a Hazard Review on existing plants deal with some difficulties related to different factors [1]. Often older plants have no hazard studies at the design stage; moreover many plants suffered changes as time went on and the original design notes may no longer apply properly to the new conditions. On the other hand, Seveso II Directive (96/82/EC), dealing with the control of major accidents and acknowledged by Italian law D.L. 334/99, implies the need of developing adequate tools for the evaluation and mitigation of industrial risk. Retrospective Hazop is considered suitable for safety assurance, but it is rejected since it suffers limitations and involves considerable time [1,2]. Further difficulties arise when dealing with batch processes, in that the usual approach, consisting in dividing line by line the plant, must be performed considering as well three operative stages: charge, reaction and discharge. This paper illustrates a practical example of a combined technique, which can overcome the above mentioned limitations and difficulties. This technique has been developed ad hoc, starting from Hazop and Process Hazard Review and applied to a process of acrylic polymerization along the whole production cycle. A preliminary study, based on the Index Method, is carried out on the plant, in order to identify the most hazardous units and to provide a quantification of the risk. The Hazop will be applied only to those sections of a plant where it has been determined that a deviation from the design intent could lead to serious accidents and it will suggest the action required to reduce the risk of accidents. The evaluation of risk by Index Method allows to obtain a univocal interpretation of the hazards of an existing plant, related to both the intrinsic characteristics and the environmental conditions. Moreover, this technique tries to overcome Hazop limitations taking into account aspects usually under-evaluated in the traditional studies: plant comparison with current best practice, adoption of safe system of work, safety critical procedures, human factor precautions etc