A Near Field-Far Field model for assessing Oxygen Deficiency Hazard

Oxygen Deficiency Hazard (ODH) due to inert gas releases can be assessed by the use of predictive models. Several models are available in the literature: the majority of them can be classified as “well-mixed” models because they assume the existence of completely and instantaneously well mixed air. In order to provide a more precise estimation of the indoor oxygen level in the breathable air close to a release point, we propose a Near Field-Far Field (NF-FF) model in which the Near Field volume is an output and can vary over time. The trend of the Near Field size can be a useful data for the risk assessor in order to determine the safety distance from point source releases, and improve the emergency response plan. Starting from balances of mass of air and moles of oxygen both in the Near Field and in the Far Field, the objective of our model is to predict the volume of the Near Field that contains a limit value for the oxygen concentration at every time instant. The approach includes several analytical formulas that model the different flows occurring in each field and between the two fields. In particular, we assume the existence of inert gas releases, forced and natural ventilation airflows, interzonal airflows, and air that has to move from the Far Field to the Near Field, or vice versa, for assuring a limit value for the oxygen concentration in the Near Field. Finally, examples of the application of this model in some case studies available in the literature are presented and discussed.