Interazioni tra fumi e sistemi sprinkler: analisi fluidodinamica durante un incendio

Gli incendi in galleria costituiscono uno degli scenari incidentali di maggior rischio e sono stati oggetto di ampi studi in letteratura, volti ad indagarne lo sviluppo o l’influenza di vari parametri, come ad esempio l’effetto delle ostruzioni sulla velocità critica di ventilazione. Uno sviluppo minore è invece riscontrabile per quanto riguarda l’utilizzo e gli effetti di impianti di tipo sprinkler per migliorare la sicurezza dei tunnel stradali. Un esempio recente è costituito dallo studio di Zheng e Ingason, in cui l’impiego di sprinkler è stato testato su una riproduzione in scala di una galleria stradale. Questo studio si propone di indagare l’interazione tra i fumi e i sistemi antincendio fissi ad acqua in galleria, problema che risulta essere tuttora non completamente risolto

Preliminary CFD analysis of a ventilated chamber for candles testing

As candles have grown in popularity with consumers over the last few years, so has the potential safety concern with their use in indoor environments. Carbon monoxide, particulate matter and different volatile and semi-volatile species can be found in candles emissions. Currently it is not possible to predict theoretically which emissions will be produced by a specific candle and in order to quantify real emissions is still necessary to proceed with experimental tests. A common way to quantify released pollutants is to burn candles in a well-controlled environment, such as a laboratory-scale test chambers. Obviously, it is required that these chambers are able to reproduce the environmental combustion regime of the candles, so as to guarantee that an equal level of emissions is produced and measured. Another crucial point is related to the measurements themselves: generally, air quality is measured in a single point inside the chamber with the assumption that the air and the exhausts in that point are representative of the whole ambient. This work aims to reproduce one of these chambers by means of a CFD (Computational Fluid Dynamics) model, with the purpose of obtaining an adequate tool to analyze and design more efficient test chambers. A comparison with an ad hoc experiment is performed to validate the CFD model

Advanced turbulence models and boundary conditions for flows around different configurations of ground-mounted buildings

When dealing with Atmospheric Boundary Layer (ABL) simulations, commercial computational fluid dynamics (CFD) acquires a strategic resonance. Thanks to its good compromise between accuracy of results and calculation time, RANS still represents a valid alternative to more resource-demanding methods. However, focusing on the models’ performances in urban studies, LES generally outmatches RANS results, even if the former is at least one order of magnitude more expensive. Consequently, the present work aims to propose a variety of approaches meant to solve some of the major problems linked to RANS simulations and to further improve its accuracy in typical urban contexts. All of these models are capable of switching from an undisturbed flux formulation to a disturbed one through a local deviation or a marker function. For undisturbed flows, a comprehensive approach is adopted, solving the issue of the erroneous stream-wise gradients affecting the turbulent profiles. Around obstacles, Non-Linear Eddy-Viscosity closures are adopted, due to their prominent capability in capturing the anisotropy of turbulence. The purpose of this work is then to propose a new Building Influence Area concept and to offer more affordable alternatives to LES simulations without sacrificing a good grade of accuracy

Experimental and numerical study of an air lock purging system

High concentrations of H2S in offshore wells represent a major concern for personnel safety: if a significant external H2S contamination occurs, depending on wells and process conditions, it may prove impossible an effective evacuation, and thus Temporary Refuge (TR) provisions must be set-up to provide prompt availability of safe and reliable protection to personnel. Air Locks (ALs) to enter the TR may be necessary to ensure isolation of the safe internal environment when entering into the TR. ALs modelling is essential to verify that sufficient time for entering the TR is available to all personnel in case of accident. Nevertheless, due to the extreme conditions (high toxicity, short characteristic times, high purging air velocity etc.), experimental modeling of the AL can prove difficult and very expensive. Given the importance of ALs efficiency in a real emergency situation, simulation of its performances in realistic condition and optimization of the design of air purges to ensure the required efficiency is a factor of extreme importance for the overall safety of the installation. In this work, the purging efficiency of a typical AL has been analyzed through a combined approach of experimental tests and CFD simulations, to prove the capability of CFD modeling to analyse real AL conditions. A scaled model have been realized and analyzed using CO2 as tracing gas to determine the concentration field; even if the realization constraints above mentioned do not allow for a full scaling of all the involved variables, fluid-dynamic conditions have been set to reproduce real AL purging capabilities as close as possible. Experimental results have been used for a fine tuning and validation of the CFD tool in an operative range close to a real configuration, through the comparison of the obtained flow and concentration fields with those predicted by the CFD simulations of the experimental set-up. Subsequently, the tuned CFD approach has been used to simulate a typical AL and to check its performances

Thermal behavior of a semibatch reactor during upset conditions as a function of dosing and temperature controller type

In fine chemical industries, potentially runaway reactions are often carried out in semibatch reactors to better control the heat evolution. For such processes, an uncontrolled temperature increase can trigger secondary undesired reactions or, worse, decompositions of the reacting mixture with consequent reactor pressurization and, eventually, physical explosion. For this reason, during years, it has been tried to simulate how a runaway phenomenon evolves as a consequence of a number of upset operating conditions: e.g. dosing errors, cooling system failure or external fire. In this work, a dedicated software has been developed and used to simulate a dosing error occurring during an industrial synthesis. Particularly, it has been analyzed the effect of the different industrial temperature control modes (isoperibolic and isothermal) and their related controller parameters onto the time evolution of the main process variables. Theoretical simulations have shown that dif-ferent scenarios can arise as a function of these control features

Integrating Recursive Operability Analysis with Different Risk Assessment Methods: Analysis of the Historical Bp American Refinery Explosion

The British Petroleum (BP) American Refinery accident, back in 2005, was one of the most severe explosions recorded in any industrial accident database. According to both the reconstruction and the interviews with the company, it was found that the causes of the accident where both technical, with the failure of a level controller, which was also badly designed for the isomerization unit, and human, with a very stressed and undersized personnel. In this work, a Quantitative Risk Assessment (QRA) based on the Recursive Operability Analysis (ROA), as hazards and accidental scenarios identification tool, was performed on the unit (BP isomerization unit) involved in the accident. The analysis was carried out exploiting many different techniques, to provide a proper assessment. The quantification of all node-deviation-variables (necessary to establish the real behaviour of the system) was performed by implementing the BP plant in CoCo simulator. Basic events were identified using a simplified Failure Mode and Effects Analysis (FMEA). Then, the magnitude of fire and explosion was estimated basing on the simulation results provided by the ALOHA software. Finally, a Fault Tree Analysis for the BP isomerization unit was performed, quantifying the probability of occurrence of all the most credible scenarios. Probabilities, magnitudes, and risk indexes (function of the distance with respect to the source point) were also estimated. From the analysis, the importance of redundant measurements of the most crucial variables, such as liquid level, and the impact of human errors was highlighted

Characterization of Removable Coatings for Graphite-moderated Nuclear Reactors Decommissioning

In this study the behavior and characteristics of two removable coatings to be deposited on a nuclear grade graphite substrate were analyzed, with the aim to evaluate their possible application on graphite bricks during dismantling operations of graphite-moderated nuclear reactors.Indeed, nowadays many shut-down reactors are still in decommissioning phase, and effective measures should be taken to guarantee safe dismantling operations. One option could be the use of coating application techniques, which mitigate the risk of graphite dusts spreading and loose contamination, protecting clean surfaces.Tested coatings, both polymeric mixtures, were selected according to their previous application history, availability in commerce and easiness in handling, whereas substrates used were non-irradiated nuclear Virgin Atcheson Graphite Ordinary Temperature (AGOT) graphite samples from L-54M Politecnico di Milano research reactor, which is in decommissioning phase. Thermal characterization of the coatings was carried out before deposition using a Thermogravimetric and Differential Thermal Analysis (TGA-DTA) equipment, tests were performed to obtain a preliminary estimation of the drying time and degradation conditions. After deposition, mechanical properties, such as hardness, of the coating were assessed. The preliminary experimental campaign showed that coating painting could be a feasible option to prevent the spread of highly contaminated graphite dusts during decommissioning of graphite components of nuclear reactors, thus ensuring clean and safe working conditions

Assessment of the indoor odour impact in a naturally ventilated room

Indoor air quality influences peopleâ\u80\u99s lives, potentially affecting their health and comfort. Nowadays, ventilation is the only technique commonly used for regulating indoor air quality. CO2 is the reference species considered in order to calculate the air exchange rates of indoor environments. Indeed, regarding air quality, the presence of pleasant or unpleasant odours can strongly influence the environmental comfort. In this paper, a case study of indoor air quality monitoring is reported. The indoor field tests were conducted measuring both CO2 concentration, using a photoacoustic multi-gas analyzer, and odour trends, using an electronic nose, in order to analyze and compare the information acquired. The indoor air monitoring campaign was run for a period of 20 working days into a university room. The work was focused on the determination of both CO2 and odour emission factors (OEF) emitted by the human activity and on the evaluation of the odour impact in a naturally ventilated room. The results highlighted that an air monitoring and recycling system based only on CO2 concentration and temperature measurements might be insufficient to ensure a good indoor air quality, whereas its performances could be improved by integrating the existing systems with an electronic nose for odour detection

Evaluation of workers accidents through risk analysis

Among the available methods for the analysis of accidents/injuries, the AEB approach (Accident Evolution and Barrier Analysis) and the 3CA-Form B method (Control Change Cause Analysis) have been used to analyze a typical accident, which often occurs, in which have been involved several workers that entered in a confined space containing toxic and/or inert gases, causing the workers death. A critical analysis of the investigated methods has been performed in order to identify the best available methodology; in particular, it was evidenced a problem of events representation with the AEB method because the large part of the causes was due to human factors, so the entire sequence of the events that led up to the accident is represented in a linear way, on a same column, by this approach making difficult to relate events that took place simultaneously. The 3CA-Form B method allows, on the other hand, a good description of the events sequence taking into account simultaneously all aspects and issues, both human and technical, that characterize it