Reaction turbines

Reaction turbines Points : Reaction turbines, Main components of a reaction turbine, Spiral casing, Guide mechanism, Turbine runner, Draft tube In a reaction turbine, the water enters the wheel under pressure and flows over the vanes. As the water, flowing over the vanes, is under pressure, therefore wheel of the turbine runs full and may be submerged below the tail race or may discharge into the atmosphere. The pressure head of ‘water, while flowing over the vanes, is converted into velocity head, and is finally reduced to the atmospheric pressure, before leaving the wheel. Main components of a reaction turbine: A reaction turbine has the following main components:
1. Spiral casing,
2. Guide mechanism,
3. Turbine runner,
4. Draft tube 1. Spiral casing: The water, from a pipeline, is distributed around the guide ring in a casing. This casing is designed in such a way that its cross-sectional area goes on reducing uniformly around the circumference. The cross-sectional area is maximum at the entrance, and minimum at the zip. As a result of this, the casing will be of spiral shape; that is why it is called a spiral casing or scroll casing. The spiral casing is provided with inspection holes and pressure gauges. The material of a casing depends upon the head of water, under which the turbine is working, as discussed below:
Concrete - up to 30 m
Welded rolled steel plate - up to 100 m
Cast steel - more than 100 m 2. Guide mechanism: The guide vanes are fixed between two rings in the form of a wheel. This wheel is fixed in the spiral casing. The guide vanes are properly designed in order to.
1. Allow the water to enter the runner without shock (This is done by keeping the relative velocity, at inlet of the runner, tangential to the vane angle).
2. Allow the water to flow over them, without forming eddies.
3. Allow the required quantity of water to enter the turbine. (This is done by adjusting the opening of the vanes).

All the guide vanes can rotate about their respective pivots, which are connected to the regulating ring by some mechanical means. The regulating ring is connected to the regulating shaft by means of two regulating rods. The guide vanes may be closed or opened by rotating the regulating shaft, thus allowing the required quantity of water to flow according to the need. The regulating shaft is operated by means of a governor, whose function is to govern the turbine (i.e. to keep the speed constant at varying loads). The guide vanes are generally made of cast steel. 3. Turbine runner: The runner of a reaction turbine consists of runner - blades fixed either to a shaft or rings, depending upon the type of turbine. The blades are properly designed, in order to allow the water to enter and leave the runner without shock. The runner is keyed to a shaft, which may be vertical or horizontal. If the shaft is vertical, it ‘is called a vertical turbine. Similarly, if the shaft is horizontal, it is called a horizontal turbine. The surface of the runner is made very smooth. The runner may be cast in one piece or may be made of separate steel plates welded together. For low heads, the runner may be made of cast iron. But for high heads, the runner is made of steel or alloys. When the water is chemically impure, the runner is made of special alloy. Draft tube: The water, after passing through the runner, flows down through a tube called draft tube. it is, generally, drowned approximately I m below the tail race level. A draft tube has the following functions:
1. It increases the head of water by an amount equal to height of the runner outlet above the tail race.
2. It increase efficiency of the turbine.