Evaluation and Mitigation of Risk Connected to Lighter than Air Gaseous Releases in Confined Environment

Notwithstanding the adoption of more and more accurate preventive techniques, hazardous release still represents a serious concem in the process industry. In particular, flammable release can give rise to the formation of flammable mixtures and, when an ignition source is available, to flash fires or even to explosions. Even if the application of Inherently Safer Technology principles is the best choice to achieve hazard reduction and prevent the effects of domino scenarios in process plants, this option cannot be fully applied in many contexts and the probability of the formation of explosive mixtures cannot be excluded at all. So the safe operation is accomplished by the application of active and passive strategies. The traditional safety approach has focused on adding protective measures and mitigation systems to the process to reduce the risk and improve safety. Mitigation of this kind of risk takes many forms and the most idoneous interventions depend upon different factors, e.g.: • characteristics of the accident and of the equipment/plant section involved in the loss of containment (rupture/brake in a pipeline, hole in a storage vessel/ tank etc.); • release definition (rate of discharge or quantity discharged) and hazardous properties ofthe involved materials (including physico-chemical characteristics and energy content); • characteristics ofthe environment (confined, semi-confined, open air). The main focus of protective measures is to limit the mass of the explosive mixture under formation within a threshold value corresponding to acceptable consequences in case of ignition. Clearly, the optimal strategy to cope with this target must be selected and designed according to the specific case under examination. In this paper, it is investigated the peculiar case of lighter than air flammable releases in partially confined environment, by developing ad-hoc an analytical dispersion model.