Definition of a shortcut methodology for assessing flood-related Na-Tech risk

Abstract. In this paper a qualitative methodology for the initial assessment of flood-related Na-Tech risk was developed as a screening tool to identify which situations require a much more expensive quantitative risk analysis (QRA). Through the definition of some suitable key hazard indicators (KHIs), the proposed methodology allows the identification of the Na-Tech risk level associated with a given situation; the analytical hierarchy process (AHP) was used as a multi-criteria decision tool for the evaluation of such qualitative KHIs. The developed methodology was validated through two case studies by comparing the predicted risk levels with the results of much more detailed QRAs previously presented in literature and then applied to the real flood happened at Spolana a.s., Neratovice, Czech Republic in August 2002.</p

Influence of Ground on Jet Fire Extension

A common accident in the industrial process industry is the puncturing of storage tanks or rupture of process pipelines containing gases. In these scenarios, the gas will escape the piece of equipment producing a single-phase gas jet. If the fluid is flammable, an ignition source is most probably encountered during the accidental scenario and a jet-fire can follow the leak. Free jets of hazardous gases and free jet-fires have been extensively analyzed in the past literature to assess their shape and extension for safety purposes. Similar analyses have been conducted to observe the effect on shape/extension of neutral jets if obstacles were present. Also, the effect of the ground proximity to the jet source has been studied. In general, the presence of obstacles and the proximity to the ground lead to enlarged hazardous areas, mainly because of the Coandă effect. In this work, flammable jets igniting and forming a jet-fire were considered. The effect of the ground proximity was analyzed, to observe the extension of the flame. Two opposed phenomena were supposed to act on the fire, differently from non-ignited jets: the Coandă effect having an attractive nature towards the ground and the buoyancy effect on the opposite direction. The relevant methane jet-fires case study was considered carrying out computational fluid dynamics (CFD) simulations using the Fire Dynamics Simulator software. The study considered both the jet source height from the ground and the gas relief flowrate effects. CFD results were summarized basing on simple dimensionless parameters to determine the eventual variation of jet-fire extension for preliminary safety analyses

Study of the Interaction Between a High-Pressure Jet and Horizontal Tanks using CFD

Accidental high-pressure flammable gas releases are among the most relevant hazards in the process safety, and consequences could be severe. In the recent decades, there have been numerous efforts to study high-pressure jets in open field (i.e., free jets). Easy-to-use mathematical models have been developed, to rapidly assess the main physical variables involved in safety evaluations. However, in a realistic scenario, the accidental leak may involve either the ground or a piece of equipment. As demonstrated by recent works, when a jet interacts with an obstacle, its behavior can significantly change. Therefore, the mathematical models extrapolated for the free jet scenario could be a source of incorrect predictions. Focusing on the scenario of an accidental high-pressure unignited flammable jet, this work shows how the presence of one or two obstacles, placed at a different distance from the source of the leak, can influence the lower flammability limit cloud extent of methane. Varying the height of the source term, the effect of the interaction among the jet, both the obstacles, and the ground was systematically studied through a Computational Fluid Dynamics analysis

Modelling of indoor air pollutants dispersion: New tools

Ventilation systems are used for create a thermally comfortable environment and good indoor air quality. It is therefore essential to have adequate tools for predicting the performance of these systems. Among the various approachs, the computational fluid dynamics could be a useful tool for the design of the ventilation system. When dealing with pollutants dispersion problems, a steady state averaged simulation can be misleading because it is not able to properly predict and model peak concentrations, which can be relevant even if temporary. An interesting approach is the use of LES (Large Eddy Simulations) simulations to obtain a better description of concentrations oscillations. In this framework, the aim of this work is the validation of simulation carried out using the FDS (Fire Dynamic Simulator) software with an actual case study, already studied with a mock-up. Secondly, two new configurations of the ventilation system are proposed, in order to stress the capacity of the software to describe complex and different features, classical of HVAC (Heating, Ventilation and Air Conditioning) systems. Interesting conclusions about efficiency are drawn from the comparison, highlighting the potentiality of the software

High-pressure methane jet: Analysis of the jet-obstacle interaction

The study of unplanned high-pressure gas releases is of paramount importance in the industrial safety framework because of the possible large consequences, both in case of flammable and toxic substances leakage. In addition, if an obstacle is involved in the release, it is known that the main effect on the jet behavior is the enhancement of the risk area. Pointing out the importance to consider the obstacle presence, among the various available numerical approaches, the sole reliable tool able to correctly model the scenario of a jet interacting with an obstacle seems to be the Computational Fluid Dynamics (CFD). This work lies in the context outlined through the examination of a realistic unignited high-pressure methane jet interacting with a realistic obstacle placed along its axis via CFD simulations: a stationary 65-bara unignited methane jet outflowing from a one-inch diameter hole and a medium size horizontal cylindrical tank are the building blocks of the realistic scenario. The aim is to deeply investigate how the distance between obstacle and jet orifice modifies the jet behavior. In particular, the final purposes are: i) to establish when the obstacle most influences the jet cloud extent and, ii) to assess when the obstacle influence expires. Moreover, a sensitivity analysis on the obstacle shape and size is conducted for comparison purposes

Influence of Buoyancy Effects on the Mixing Process and RTD in a Side-Injection Reactor Equipped with Static Mixers

The mixing process and the residence time distribution (RTD) of molecules inside reactors are well-known topics in chemical engineering; good radial mixing and poor axial mixing of chemical species are the essential conditions to achieve a plug flow behavior in a tubular reactor, which is often highly desirable. While the influence of viscosity and spatial velocities on mixing and RTD has been investigated in the literature, the influence of density differences between the streams to be mixed has been much less investigated, especially considering laminar regimes. Thus, the mixing and RTD of two miscible liquids with different densities and viscosities in a side-injection tubular reactor equipped with Sulzer Static Mixers were studied by RANS-based computational fluid dynamics (CFD) simulations. The results obtained show that if adequate configurations are used, it is possible to well-approximate radial mixing and a plug flow behavior, even when large differences in densities are involved. Moreover, graphics for a fast estimation of the maximum mixing length involved as well as for the corresponding Pe-1 value were obtained as a function of the Re number

Ground interaction on high-pressure jets: Effect on different substances

Due to the severity of their consequences, accidental high-pressure flammable gas releases are relevant hazards in the process safety. In the recent decades, several are the efforts spent on the study of highpressure jets in open field (i.e., free jets). In particular, easy-to-use mathematical models have been developed. These, by hand calculations, allow to quickly assess various physical variables that are of paramount importance in safety evaluations. However, it is easily as possible that, in a realistic accidental scenario, the unwanted leak may involve either the ground or an equipment placed in its vicinity. As demonstrated by recent works, when a jet interacts with an obstacle, its behavior can significantly change. Hence, in the safety assessment of this situation, the mathematical models derived for the free jet scenario can lead to incorrect predictions. Focusing on the scenario of an accidental high-pressure unignited flammable jet, this work shows how the proximity to the ground can influence the lower flammability limit cloud extent of different substances. Varying the height above the ground of the source term, the effect of the ground was systematically studied through a Computational Fluid Dynamics analysis considering high-pressure unignited methane, propane and hydrogen jets. The main achievement is the demonstration that releases of compounds with similar or larger molecular weight than that of air are similarly affected by the ground while, releases of compounds lighter than air interact with the ground in a sensibly different way