Industrial fires and explosions are neither infrequent nor inconsequential. In consequence of the complexity of the phenomena, which are involved in an unconfined vapor cloud explosion (UVCE), a variety of prediction methods and calculation procedures is available to estimate the consequences (overpressure) of the flammable cloud explosion. These methodologies range from simplified methods, which relatively require few calculations, to complicated numerical models, that can involve millions of calculations. In the present paper an ammonia vapor cloud explosion is studied. The cloud is generated by an accidental release of aqueous ammonia (evaporation from pool) in a DeNOX plant, which is installed in a steam power plant. The analysis is carried out by a prediction method, that requires a specification of the chemical reactivity, congestion (obstacles density) and confinement in order to establish the maximum flame speed. The study has been focused on effects of area congestion on overpressure profile. In general for realistic plants geometries a high level of congestion results in higher turbulence and therefore higher flame speeds and overpressures. This parameter often is the most difficult element to assess in consequence of the plants complexities and has important effects on overpressure peak. In fact the definition of the congestion level requires meticulous and painstaking inspections of the plant areas. The results of the study are presented and compared. The goal of the paper is the illustration of a methodology, which can provide useful elements (an estimate of the plant congestion) to enrich the preliminary knowledge of the explosion consequences in order to improve the process safety during the operation of the DeNOX plants.