In the developed countries the installation of new hydro electric power stations is nowadays very difficult and in many cases impossible. Here the upgrading of old power stations through replacement of critical components is the right measure to considerably increase the production of electricity by use of renewable resources. The development of such improved and more powerful components, most of all the turbine runner, is today possible based on numerical flow simulation and other modern CAE-tools. More than 50 % of old power plant installations have operational problems such as cavitation erosion, vibration of the structure due to vortices at off-design operation or severe noise. As an example the numerical engineering is described, that was carried out in order to upgrade a low head small hydro electric power station in Germany to increase power production by 30 % and to avoid cavitation erosion in the turbine runner. The new components have been installed and successfully put to operation. Now the turbine is performing well and is running surprisingly smooth.