shaft thrust. This also allows for the use of a smaller turbine runner, which can decrease costs and mechanical losses. Typical range of heads • Water wheel • Screw turbine • VLH turbine • Kaplan turbine • Francis turbine • Pelton wheel • Turgo turbine 0.2 < H < 4 (H = head in m) 1 < H < 10 1.5 < H < 4.5 2 < H < 70 10 < H < 300 80 < H < 1600 50 < H < 250 Specific speed The specific speed of a turbine characterizes the turbine's shape in a way that is not related to its size. This allows a new turbine design to be scaled from an existing design of known performance. The specific speed is also the main criteria for matching a specific hydro site with the correct turbine type. The specific speed is the speed with which the turbine turns for a particular discharge Q, with unit head and thereby is able to produce unit power. Affinity laws Affinity laws allow the output of a turbine to be predicted based on model tests. A miniature replica of a proposed design, about one foot (0.3 m) in diameter, can be tested and the laboratory measurements applied to the final application with high confidence. Affinity laws are derived by requiring similitude between the test model and the application. Flow through the turbine is controlled either by a large valve or by wicket gates arranged around the outside of the turbine runner. Differential head and flow can be plotted for a number of different values of gate opening, producing a hill diagram used to show the efficiency of the turbine at varying conditions. Runaway speed The runaway speed of a water turbine is its speed at full flow, and no shaft load. The turbine will be designed to survive the mechanical forces of this speed. The manufacturer will supply the runaway speed rating. Control systems Different designs of governors have been used since the mid-18th century to control the speeds of the water turbines. A variety of flyball systems, or first-generation governors, were used during the first 100 years of water turbine speed controls. In early flyball systems, the flyball component countered by a spring acted directly to the valve of the turbine or the wicket gate to control the amount of water that enters the turbines. Newer systems with