CHEMICAL ENGINEERING TRANSACTIONS VOL. 26, 2012 A publication of The Italian Association of Chemical Engineering Online at: www.aidic.it/cet Guest Editors: Valerio Cozzani, Eddy De Rademaeker Copyright © 2012, AIDIC Servizi S.r.l., ISBN 978-88-95608-17-4; ISSN 1974-9791 Consequences Assessment of an Accidental Toxic Gas Release Through a CFD Tool: Effect of the Terrain and Major Obstacles Marco Pontiggia *1 , Valentina Busini 2 , Marco Gattuso 3 , Giovanni Uguccioni 1 , Renato Rota 2 1 D'Appolonia S.p.A. - via Martiri di Cefalonia 2, 20097 – San Donato Milanese (MI) - Italy 2 Politecnico di Milano - Dip. di Chimica Materiali e Ingegneria Chimica "G. Natta" - via Mancinelli 7 - 20131 Milano, Italy 3 D'Appolonia S.p.A. - via Farabola Est 32, 55049 Viareggio (LU) - Italy marco.pontiggia@dappolonia.it The main aim of this study is the establishment of a new methodology for the assessment of terrain and structures geometry effects on gas dispersion. Significant obstacles (such as the plant structures, buildings, or the terrain elevation) play a major role in gas dispersion, due to the eddies, wakes, stagnation and recirculation points they can introduce. A comparison between CFD simulations and integral model predictions have been worked out for a realistic case-study in order to point geometry role in gas dispersion. Moreover, obstacles and terrain geometries are often not available in a suitable format. The proposed methodology uses easy-accessible data (such as SRTM data and geo-referenced aerial photography) to work out the required inputs, to reduce the time and cost associated to CFD modelling and make it practically applicable in industrial cases (design of new installations or assessment of existing ones). 1. Introduction In safety studies concerning consequences analysis of gas releases, integral methods are widely used in order to obtain previsions of the dimensions of the area involved by the dispersion (Bernatik and Libisova, 2004). They are easy and low time-consuming tools, but they are liable to some deficiencies (one-dimensional modelling) and obey to certain assumptions. On the other hand, powerful computational tools based on fluid dynamics methods have recently been developed (Computational Fluid Dynamics, CFD) allowing for an integrated approach on complex scenarios and/or physicochemical phenomena. CFD codes perform three-dimensional computations of fluid properties variation, turbulence modelling, chemical reactions, in addition to accurately represent the geometry of the flow field. However, this level of details could be time-consuming. The prediction of an accidental gas release, in term of both people and area involved by a toxic or flammable cloud dispersion, is of paramount importance for the definition of safety plans and actions to be undertaken to avoid and/or mitigate the consequences of such an accident; recent works (Pontiggia et al., 2011; Vianello et al., 2011; Cozzani et al., 2011) have pointed out that CFD approach cannot be given up when geometrically complex scenarios are considered. Nevertheless, geometry data are generally not available in a suitable format: incompatible 3D geometry encoding, too small level of 537