A water turbine is a rotary engine whose job it is to convert kinetic and potential energy of water into mechanical work. Since their inception in the 19th century, they have been used widely to produce industrial power. In today’s world, however, their main function is the generation of electrical power, though they are also used for pumped storage.
Of all kinds of water turbines in use today, Francis turbines are the most commonly used and the most preferred in the manufacture of electric power, as well as the most reliable for use under a wide range of operating conditions. Today, almost 60% of the world’s hydroelectric power generation is carried out through francis turbines.
Francis turbines operate in a water head (measurement of liquid pressure above a geodetic datum) from 40 m to 600 m (130 to 2,000 ft). Francis turbines are most commonly used in those electric generators that have a power output ranging from just a few kilowatts up to 800 MW, even though mini-hydro
Penstocks (a sluice to control water flow) are between 3 and 33 feet (0.91 and 10.06 metres) in diameter. The speed of the turbine varies from 75 to 1000 rpm. Wicket gates are fitted around the outside of the francis turbine’s rotating runner. The purpose of these gates is to control the rate of water flow through the francis turbine for varied power production rates. Francis turbines are usually mounted with the shaft vertical to isolate water from the generator. This system also makes installation and maintenance easier to carry out.
Components of a francis turbine:
Runner: This is the most important part of a francis turbine, and is connected to the generator for manufacture of electricity. The runner is fitted with several complex blades. While the water enters radially and leaves axially, it runs over these blades, bringing down the kinetic and pressure energy of water.
Spiral casing: The runner is nestled inside the spiral casing. The spiral casing is also known as the scroll case. At intervals along the entire length of the casing, there are openings, through which water flows into the runner, losing part of its speed on the way. The design of the casing is such that water reaches the runner at a uniform velocity.
Guide/stay vanes: Their job is to convert pressure energy of the fluid into momentum energy. Thy also monitor the flow of water to the runner.
Draft tube: Water discharged through the exit of the runner passes through the draft tube to the tail race, through which it leaves the turbine. The draft tube, acting as a diffuser, minimizes water loss, and consequently minimizes loss of kinetic energy.