Components of Francis Turbine

THEORY OF OPERATION OF A FRANCIS TURBINE

The Francis turbine is a reaction turbine, which means that the working fluid changes pressure as it moves through the turbine, giving up its energy. A casement is needed to contain the water flow. The turbine is located between the high pressure water source and the low pressure water exit, usually at the base of a dam.

The inlet is spiral shaped. Guide vanes direct the water tangentially to the turbine wheel, known as a runner. This radial flow acts on the runner's vanes, causing the runner to spin. The guide vanes (or wicket gate) may be adjustable to allow efficient turbine operation for a range of water flow conditions.

As the water moves through the runner its spinning radius decreases, further acting on the runner. For an analogy, imagine swinging a ball on a string around in a circle; if the string is pulled short, the ball spins faster due to the conservation of angular momentum. This property, in addition to the water's pressure, helps Francis and other inward-flow turbines harness water energy efficiently.

At the exit, water acts on cup shaped runner features, leaving with no swirl and very little kinetic or potential energy. The turbine's exit tube is shaped to help decelerate the water flow and recover the pressure.

Guide Vane

The guide vane and stem shall be integrally cast in one piece of stainless steel in accordance with ASTM A 743/A743M-06 CA-6 NM or JIS G 5121 SCS 6 and shall be made of the same steel as runner. The guide vane shall have a smooth contour in shape in order to facilitate least hydraulic loss at the runner vane inlet. Each guide vane shall be accurately formed and finished as well as interchangeable.

Turbine Main Shaft

The main shaft shall be made of forged carbon steel properly heat-treated according to
ASTM A 668/A668M-02 class D, E or JIS G3201. It shall be equipped with an integrally
forged out skirt corresponding to the turbine guide bearing and forged out flange couplings
corresponding to the runner and the generator shaft.
The alignment of the turbine shaft is checked with rotation. The tolerances for shaft run out
are half of the tolerances values given in IEEE Standard 810-1989(R2001).

Spiral Case

Spiral case is made of welded steel plates and rolled steel plates for welded structure. After
welding 100% ultrasonic testing is performed on all welded units. Radiographic inspection
is performed for all cross points of welded joints and all doubtful welding defects are
detected by ultrasonic detection.

Main parts of a Francis Turbine

Francis Turbine consists more complex parts than Pelton Turbine.It's operation is also complicated.

Francis type Turbine units cover a wide head range, from 20 meters to 700 meters, and their output power varies from just a few kilowatts up to 1 GW. Francis turbines may be designed for a wide range of heads and flows.

Spiral Case: provides encased water path to contain the water pressure

Stay Ring: Supports total weights of superposed parts, such as generator turbine rotating parts and superposed concrete.

Runner: absorbs the energy from the water and converts it to rotational motion of the main shaft.

Head cover: Supports the weights of guide vanes and confine the vibrations and unnecessary movements of the wicket gates

Bottom Ring: provide drainage space

Shaft Seal: Prevent water leakage between main shaft and head cover.