Francis Turbine: Components and Working

Francis Turbine Components and Working

A Francis turbine is a type of hydroelectric turbine used in hydropower plants. It is a combination of impulse and reaction turbines.

In the Francis turbine, the blades rotate by both reaction and impulse force of water. This type of turbine is mostly used in medium or large-scale hydropower plants.

The Francis turbines are suitable for the head as low as 2m to as high as 300m.

These turbines are favorable as they work equally when positioned horizontally as they are oriented vertically.

The water going through a Francis turbine losses pressure and stays at more or less the same speed. Therefore, it would be considered a reaction turbine.

A diagram of the Francis turbine is shown in the figure below.

Francis turbine
Francis turbine

Main Components of Francis Turbine

The main components of Francis turbines are listed below;

  • Scroll Casing
  • Guide Mechanism
  • Runner and Shaft
  • Draft Tube

Scroll Casing

The water from the penstock flows into the outer scroll casing of the turbine. And the casing is made in a spiral shape.

The casing is used to provide surrounding the runner, guide vanes, and shaft.

To distribute the water evenly surrounding of runner, the cross-section of the casing is made of the decreasing area and helps the circumference of the runner almost at constant velocity and pressure.

The material used for the casing depends on the head at which it works. Genially, it is made up of concrete for less than 30m head, welded rolled steel plates up to 100m head, and for more than 100m head, it is made up of cast steel.

Stay vanes are provided inside the casing to support it. It also helps in guiding the water from casing to guiding vanes.

Therefore, the water is equally distributed around the periphery without the formation of any eddies.

Guide Mechanism

The Francis turbine consists of a stationary ring (in a form of a wheel) known as a guide wheel. It is fixed to an inner surface of the casing and it surrounds the outer periphery of a runner.

In between the outer and inner ring of the guide wheel, it carries a series of guide vanes or wicket gates of an airfoil section.

These vanes are used to have a number of passages between the casing and runner blades. These vanes are fixed but they can be rotated about their respective pivots.

The function of guide vanes is;

  • It is used to direct the water from casing to runner at the inlet without shock. The relative velocity of water must be kept tangent to the inlet tip of a runner to achieve smooth and shockless entry in a runner.
  • The load on the turbine is used to regulate the discharge.

The above functions are achieved by swinging the guide vanes about their pivots by opening or closing the guide vane passages.

Swing action of guide vanes can be obtained by the regulating rod connected to regulating shaft. And it is operated by a servo-motor governing mechanism.

Runner and Shaft

The runner is keyed to the main shaft of the turbine. The shaft is made up of steel and it is supported by thrust bearings and it can be horizontal or vertical accordingly the turbine is called a horizontal turbine or vertical turbine.

The runner consists of airfoil-designed blades. Hence, the water enters and leaves without shock. Generally, the number of blades varies from 16 to 24.

For the low head, the runner is made up of cast iron and for a high head turbine, it is made up of stainless steel.

Draft Tube

The graft tube is used to discharge water passing over the runner blades. It discharges the water to tailrace through a gradually increasing area.

The small end of the draft tube is fixed to the outer opening of the casing. And its bigger end is deeply submerged into a tail race by at least 1m depth from tail race level.

The entire passage of a reaction turbine from head race to tail race is enclosed and it does not communicate with the atmosphere.

The function of the draft tube is as below;

  • It is used to convert the kinetic energy of water at the exit of the runner into useful pressure energy. And it helps the water discharge into a tail race at very low velocity.
  • It improves the efficiency of the turbine.
  • The draft tube increases the head on the turbine by an amount equal to the height of the runner outlet above the tail race.

Working of Francis Turbine

The water from the penstock enters to spiral casing and then it passes through the stay vanes and guide vanes.

To maintain the flow pressure, the diameter of the spiral casing decreases.

The stay vanes are stationary and it tries to make the flow of water more linear to be deflected by adjustable guide vanes.

The water flows at an optimum angle by guide vanes and strikes the runner blades. The performance of the turbine depends on the runner blade and the angle of water created by the guide vanes.

In the Francis turbine, the lower half of the runner blade is in the shape of a small backet (hence, it uses the impulse action of water) and the upper half of the blade uses the reaction force of water.

Hence, the Francis turbine rotates with the principle of impulse and reaction principle. And the runner blades of the Francis turbine use the pressure and kinetic energy of water. It makes the turbine more efficient.

To increase the pressure of water, the draft tube is used to coming out water to the tail race.

 213 total views,  7 views today

Leave a Reply

Your email address will not be published. Required fields are marked *