This work is aimed at investigating the fluid dynamic behaviour of a vapor-liquid separator treating a two phase flow mixture of liquid water and steam for a petroleum industry application. The investigation is carried out on the realm of a Computational Fluid Dynamic (CFD) approach, consisting in the numerical solution of the Reynolds-Averaged Navier-Stokes equations (RANS) in the Eulerian framework for each phase, coupled with a two-phase extension of the very well know k-ε turbulence model. The calculations are referred to a computational domain describing very closely the three dimensional geometrical features of a knock-out drum adopted in an oil refinery process. The velocity fields of both phases, the pressure and the hold-up distributions of water and steam as predicted by the numerical model are discussed. The results allow to highlight the effect of a water slug inlet, whose entrance represents the most critical working condition for the drum, causing vibrations. The dynamic behaviour of the drum equipped with two different inlet devices is critically analyzed and the possible geometrical optimization for the reduction of the intermittent pressure on the vessel wall due to the entrance of the slug is discussed.