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Active fire protection systems are critical elements of good process safety. Among them, gaseous extinguishing systems provide quick, clean suppression and prolonged protection due to long retention process of the gas. Standard design methods do not provide sufficient tools for optimisation of the retention process, thus the necessity for development and validation of new tools and methods—such as Computational Fluid Dynamics (CFD) simulations. This paper presents a simplified approach to CFD modelling, by the omission of the discharge phase of the gas system. As the flow field after discharge is stable and driven mainly by the hydrostatic pressure difference, buoyancy and diffusion, this simplified approach appears as an efficient and cost-effective approach. This hypothesis was tested through performing CFD simulations, and their comparison against experimental measurements in a bench scale in a small compartment (0.72 m3), for six mixtures that differ in their density. Modelling the retention of the standard IG55 mixture was very close to the experiment. Modelling of mixtures with a density close to the density of ambient air has proven to be a challenge. However, the obtained results had sufficient accuracy (in most cases relative error <10%). This study shows the viability of the simplified approach in modelling the retention process, and indicates additional benefits of the numerical analyses in the determination of the fire safety of protected premises.

The article discusses the aspect of the fire safety of rooms protected by Fixed Gaseous Extinguishing System (FGE-system). On the basis of a literature study, including the analysis of design standards, it was claimed that analytical models of gas outflow from the compartment ignore some parameters that can affect the process of extinguishing gas concentration changes in time. Correct prediction of the gas flow process may affect the retention time value, which is an important determinant of the fire safety of rooms protected by FGE-system. The density of extinguishing gas was indicated as a parameter with a large potential for extending the retention time. It was noted that the density of gas depends on atmospheric conditions like temperature, pressure and humidity, which are omitted in the standard models. In the research part, the concentration distribution of nitrogen and nitrogen-argon mixtures were analyzed using three methods. Obtained experimental data were compared with analytical calculations using a standard model (model N) and a new proposed model extended by an impact of the atmospheric conditions (model PK). Model PK showed greater accuracy of determining the process of extinguishing gas concentration changes. The new proposed model might be a valuable tool for further analysis of gas flow through the room.

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