Francis Turbine

The Francis turbine is the most common water turbine in use. It is mostly preferred at medium or large-scale hydroelectric plants with a head difference of 2 – 300 meters. Its high-rated speed capacity makes it a preferable choice. Francis turbine contains four main parts: spiral casing, runner blades, guide and stay vanes, and draft tube.

The runner is put inside the circular space of the spiral casing. Guide and stay vanes are located between the runner and spiral casing to adjust the velocity.

The water goes through the spiral casing and discharges from the draft tube. During the time water travels inside the spiral casings, a specific amount is transferred to the runner by the guide and stay vanes to rotate it. However, the speed of the water does not decrease but remains the same because of the spiral casing’s design. The spiral casing gets smaller and smaller to keep the velocity of the water the same.

Q̇ = V A, in other words, flow rate = velocity x area

The flow rate in spiral casing drops over time, which leads velocity to decrease if the area is the same. However, the area is not the same but also decreases as water goes deeper, which ensures that the speed of water is the same in every spot.

Runners are the heart of the turbines. They receive the strike of fluid, causing it to rotate. This rotation is necessary to transmit water’s kinetic energy to the shaft connected to a generator.

Finally, water reaches the draft tube to discharge smoothly.

The material of each part made can differ. Runner blades are made of stainless steel, sometimes nickel-based alloys. Guide and stay vanes must maintain their structural integrity to regulate the water flow despite varying flow conditions. The spiral casing is built with cast iron or steel. Stainless steel is used in conditions where the risk of corrosion is high. Lastly, steel and concrete are used for draft tubes. Concrete is preferred for large-scale turbine applications, whereas steel is for smaller turbines.

Runner

Spiral casing

Eren Erden