Francis Reaction Turbines

1. Water from penstock flows into the outer scroll casing of turbine which is made in spiral shape.
2. The casing surrounds the guide vanes, the runner, the shaft etc.
3. The casing and runner always run full of water.
4. The cross section of the casing is made of decreasing area so that the water is evenly distributed around the circumference of runner almost at constant velocity and pressure.
5. The material used for casing of a reaction turbine depends on the head it works.
6. The casing is usually made of concrete for less than 30 m head, of welded rolled steel plates upto 100 m head and of cast steel for more than 100 m head.
7. In large units, stay vanes are provided inside the casing to support it.
8. Stay vanes also help in guiding the water from casing to guiding vanes so that water is equally distributed around the periphery without formation of any eddies.

(2) Guide Mechanism :

1. It consists of a stationary ring in the form of a wheel called guide wheel.
2. It surrounds the outer periphery of runner and is fixed to inner surface of casing.
3. In between the outer and inner ring of guide wheel, it carries a series of guide vanes or wicket gates of aerofoil section.
4. These vanes form the number of passages between the casing and runner blades.
5. Though these vanes are fixed in position but they can be rotated about their respective pivots.
6. The guide vanes have the following functions to perform :

• To direct the water from casing to moving vanes of runner at inlet without shock. In order to achieve smooth and shockless entry to runner, the relative velocity of water must be kept tangent to inlet tip of runner.
• To regulate the discharge according to load on the turbine. It is achieved by swinging the guide vanes about their own pivots by opening or closing the guide vane passages, Swinging action of guide vanes is obtained by the regulating rod connected to regulating shaft operated by Servo-motor governing mechanism.

(3) Runner and shaft :

1. The runner is keyed to the main shaft of the turbine.
2. Shaft may be horizontal or vertical, accordingly the turbines are called horizontal turbines and vertical turbines.
3. The shaft is made of steel and it is supported in thrust bearings.
4. The runner consists of suitable designed blades of aerofoil section so that the water enters and leaves the blades without shock.
5. The numbers of blades usually vary between 16 to 24. The surfaces of moving vanes are made very smooth to reduce friction losses.
6. The runners are made of cast iron for low head turbines and it is made of stainless steel for high head turbines.
7. A runner for a mixed flow turbine is shown in Figure B.

1. Water passing over the runner blades is discharged to tail race through a gradually increasing area, called draft tube.
2. The small end of draft tube is fixed to outer opening of the casing and its bigger end is deeply submerged into tail race by atleast 1 m depth from tail race level.
3. It is evident from discussion that the entire passages of a reaction turbine from head race to tail race are totally enclosed and it does not communicate with the atmosphere. It is so necessary since reaction turbines run under pressure.
4. The functions of a draft tube are :

• It large converts the kinetic energy of water at exit of runner into useful pressure energy and the water discharges into tail race at very low velocity. Therefore, increases the pressure head.
• It increases the head on the turbine by an amount equal to the height of runner outlet above the tail race.
• It improves the efficiency the efficiency of the turbine .