The diagonal turbine (also Dériaz turbine) is a double regulated reaction turbine for medium head range. (Range of operation up to about 80 m head) This innovative product has been built by Geppert since 1999 for small hydro power plants which has been successfully installed in dozens.
Adjustable runner- and guide vanes maintain a high efficiency over a wide flow range for this medium pressure turbine with resulting high partial-load efficiencies, comparable with Kaplan turbines. With seasonal strongly variable discharge a higher annual production can be reached with this turbine type than with comparable Francis turbines.
This turbine type is often installed with seasonal strongly variable discharge in order to avoid regarding electrical equipment and civil works costly solutions with more Francis units. Also high positioned Kaplan turbines have been replaced by Diagonal turbines as these have turned out to be less cavitation sensitive.