This paper presents an alternative method for the quantification of the irreversible phenomena that characterize the flashing process in a stage of a MSF desalination plant, and that lead to an increase in the brine temperature of the flashing tank with respect to the ideal equilibrium conditions. The current calculation of the irreversibilities is based on the determination of a factor, the non-equilibrium allowance (NEA), representing the difference between the real temperature of the brine exiting the stage and the temperature corresponding to thermodynamic equilibrium with the prevailing pressure in the flashing vacuum. We propose instead to use a stage efficiency, in some aspects similar to the polytropic efficiency, commonly adopted in gas in a turbine and compressor. The results show that the use of this efficiency leads to a more physically complete problem formulation. The numerical values for design specifications can be chosen from a range of assigned values, in a similar fashion to the use of the polytropic efficiency. The results of a numerical application show that the interval of allowable values is rather limited and that the set of operational parameters influencing the choice is small.