In this paper a smart design procedure for a spiral casing for hydraulic turbines has been developed and described. Two applications are presented for the design of the spiral casing for two existing Francis turbines with specific speeds equal to 81rnd/min and 48rnd/min. Using a commercially available CFD tool the existing geometry of a Francis turbine was analysed; the object being to determine the nominal operating condition of the existing turbine which is used to redesign the same spiral casing. The two new geometries of the spiral casing have been designed for each application and by using the CFD tool the flow through the two spiral casings of each turbine is analysed, in order to select a better geometry. The design procedure of the spiral casing has been numerically tested, and the authors have obtained two new designs which have a smaller area (size).