Kaplan is also known as propeller turbine. Kaplan turbine is a propeller type water turbine along with the adjustable blades. Mainly it is designed for low head water applications. The Kaplan turbine consists of propeller type of blades which works reverse. By using shaft power displacing the water axially and creating axial thrust in the turbine. The water flows axially and it creates axial forces on the Kaplan turbine blades to produce generating shaft power.
In earlier stages the Kaplan turbine is suitable for the large heads of water. The sources are receives from the rivers which are at the low heights.Advertisement:
Design of a Kaplan Turbine:
Mainly the Kaplan turbine is used to create the power from the low head turbines. Large flow rate is required from the Kaplan turbine. The main purpose of the Kaplan turbine is used to provide loading at large flow rates. The design of the Kaplan turbine and Francis turbine are very much similar. The water flow in the Kaplan turbine is in the radial direction the flow is entered and exists axially. In the inlet of the turbine guide vanes are fixed. We can see the passage in between the rotor and guide vane which the flow is in the radial direction. Initially the flow must be in radial direction but the radial direction is forced to move in the axial direction. We can observe the similarity in between the rotor and propeller of a ship. To the central shaft of the turbine rotor blades are attached. With the help of moveable joints blades are connected to the shaft. The blades are rotated according to the water flow rate and the water head available. Compare to the other axial flow turbines the blades of the Kaplan turbine are not planer. So they are designed with a twist along the total length so it allows rotation of the water flow at the inlet and leaves at the axial flow.
Parts of Kaplan turbine:
In the turbine the shaft is arranged vertically. The shaft lower end is made up of large so it is known as hub. The hub is also called as boss.
Scroll casting: from the pen stock the water leaves into the scroll casing and then moves to the guide vanes. In the guide vanes the water rotes 90 degrees and to the runner the water flows axially.Advertisement:
Guide Vane Mechanism:
On the hub guide vanes are fixed. By opening the guide vane the power requirement is high and by closing the guide vane the power requirement is low.
Blade control mechanism:
Blades at the joints have movable attachable joints. The main purpose of this mechanism is used to control the angle of attack. But which the water will strike the blades by rotating the blades because it pf their movable joint.
Compared to the atmospheric pressure the pressure at the exit of the runner is less. At the exit the water cannot be discharged directly to the tail race. For discharging of water a tube or pipe with gradually increasing area is used for the water discharge from the turbine exit to the tail race. Increasing area of the tube is known as draft tube. On the tube one end is connected to the outlet of the runner and the other end is submerged below the water level in the tail race.
Blades are like propellers in the Kaplan turbine. In other turbines blades are planer but in Kaplan the blades are not planer.
Working of the Kaplan turbine:
Due to the low water heads it allows the water flow at larger in the Kaplan turbine. With help of the guide vane the water enters. So the guide vanes are aligned to give the flow a suitable degree of swirl. The swirl is determined according to the rotor of the turbine. The water flow from the guide vanes are passes through the curved structure which forces the radial flow to direction of axial. The swirl is imparted by the inlet guide vanes and they are not in the form of free vortex. With a component of the swirl in the form of axial flow are applies forces on the blades of the rotor. Due to the force it loses both angular and linear momentum.
Advantages of Kaplan turbine:
- Runner vanes are adjusted in the Kaplan
- Very low heads are required
- Very small no of blades are used nearly 3 to 8 blades
- Less resistance has to be overcome.
- Position of the shaft is only in vertical direction
- Speed of the turbine is 250 to 850
- High speed generator is required
- Large Flow rate must be required.