Computational Fluid Dynamics (CFD) simulations in combination with droplet population balances lead to a practical standard for calculating the flow curve – and thus the residence time distribution – in horizontal separators of any size on the basis of available process data. By implementing the droplet behavior, the calculation of a droplet-specific separator efficiency is also made possible. The method was developed with identical systems at three different locations. The CFD simulations based on this were successfully validated with experimental data from the industrial partners involved.