The Kaplan turbines are a propeller-type water turbine which has adjustable blades. It was developed in 1913 by Austrian professor Viktor Kaplan. who combined automatically adjusted propeller blades with automatically adjusted wicket gates to achieve efficiency over a wide range of flow and water level.
The head ranges from 10–70 metres and the output ranges from 5 to 200 MW. Runner diameters are between 2 and 11 metres. Turbines rotate at a constant rate, which varies from facility to facility.
Kaplan turbines are now widely used throughout the world in high-flow, low-head power production.
Working Principle of Kaplan Turbines
The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. Moreover, Power is recover from both the hydro static head and from the kinetic energy of the flowing water. The design combines features of radial and axial turbines.
The inlet is a scroll-shaped tube that wraps around the turbine’s wicket gate. Water is directed tangentially through the wicket gate and spirals on to a propeller shaped runner, causing it to spin.
The outlet is a specially shaped draft tube that helps decelerate the water and recover kinetic energy.
The turbine does not need to be at the lowest point of water flow as long as the draft tube remains full of water. A higher turbine location, however, increases the suction that is imparted on the turbine blades by the draft tube. The resulting pressure drop may lead to cavitation.
Variable geometry of the wicket gate and turbine blades allow efficient operation for a range of flow conditions. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications.
Current areas of research include computational fluid dynamics (CFD) driven efficiency improvements and new designs that raise survival rates of fish passing through.
Because the propeller blades are rotated on high-pressure hydraulic oil bearings, a critical element of Kaplan design is to maintain a positive seal to prevent emission of oil into the waterway. Discharge of oil into rivers is not desirable because of the waste of resources and resulting ecological damage.
Main Components of Kaplan Turbine
The main parts of Kaplan Turbine are,
1. Scroll Casing
It is a spiral type of casing that has decreasing cross section area. The water from the pen stocks enters the scroll casing and then moves to the guide vanes where the water turns through 90° and flows axially through the runner. It protects the runner, runner blades guide vanes and other internal parts of the turbine from an external damage.
2. Guide Vane Mechanism
It is the only controlling part of the whole turbine, which opens and closes depending upon the demand of power requirement. In case of more power output requirements, it opens wider to allow more water to hit the blades of the rotor and when low power output requires it closes itself to cease the flow of water. If guide vanes is absent than the turbine can not work efficiently and its efficiency decreases.
3. Draft Tube
The pressure at the exit of the runner of Reaction Turbine is generally less than atmospheric pressure. The water at exit cannot be directly discharge to the tail race. A tube or pipe of gradually increasing area is use for discharging water from the exit of turbine to the tail race. This tube of increasing area is call a Draft Tube. One end of the tube is connect with the outlet of runner while the other end is sub-merge below the level of water in the tail-race.
4. Runner Blades
The heart of the component in kaplan turbine are its runner blades, as it the rotating part which helps in production of electricity.Its shaft is connect to the shaft of the generator. The runner of the this turbine has a large boss on which its blades are attached and the the blades of the runner is adjustable to an optimum angle of attack for maximum power output. The blades of the Kaplan turbine has twist along its length.
Application of Kaplan Turbine
- Kaplan turbines are widely use throughout the world for electrical power production.
- It can work more efficiently at low water head and high flow rates as compared with other types of turbines.
- It is smaller in size and easy to construct.
- The efficiency of Kaplan turbine is very high as compares with other hydraulic turbine.
Disadvantage of Kaplan Turbine
- The only disadvantage of kaplan turbine is cavitation, which occurs due to pressure drop in draft tube. Use of draft tube and proper material generally stainless steel for the runner blades may reduce the cavitation problem to a greater extent.