An asymptotic analysis of a mechanistic Equal Velocity Unequal Temperature (EVUT) model has been developed to predict critical flashing flows. The obtained results have been compared with those coming from the Homogeneous Equilibrium Model (HEM) and some experimental data extracted from Marviken test series. In spite of its simplicity, the developed non-equilibrium model not only gives a good approach to experimental measures but also explains the complex behavior shown by critical blowdowns of pressurized vessels.