The reaction turbines are used all over the world for electricity production. The reaction turbines have multiple types, but the major types are the Kaplan turbine and the Francis turbine. In the previous article, we discussed the Francis turbine.
Therefore, the main objective of this article is to explain the different aspects of the Kaplan turbine.
What is a Kaplan Turbine?
The Kaplan turbine is an axial flow turbine. In the Kaplan turbine, the water enters and exits the turbine through the runner’s axis of rotation (axial flow). In simple words, the water enters and exits the turbine in an axial direction but this water flows in a direction parallel to the runner’s axis of rotation.
The Kaplan turbines work at a high flow rate of water and low head with maximum efficiency which is not possible in the Francis turbine. A unique feature of Kaplan turbines is that their blades change their position as needed to maintain the highest efficiency under different water flow conditions. The water flowing through these turbines loses its pressure.
The area that water requires to enter into the Kaplan turbine is large, which corresponds to the total area of the blades. Because of the large area of the turbine, these turbines are also most useful in areas having a dam with high water flow rates. This was especially important before the development of the Kaplan turbine.
Before the development of Kaplan turbines, the maximum number of turbines was only appropriate for a large head. Viktor Kaplan invented 1st Kaplan turbine in 1913. Viktor Kaplan designed the Kaplan turbine based on the turboprop design and therefore it works on the opposite principle of the propeller.
This was the 1st hydroelectric turbine that could operate at high water flow as well as a low head. The Kaplan turbine is also called a propeller turbine because its blades resemble propellers and work in the reverse direction with the same phenomenon. This makes it suitable for use in the river and low head areas.
Kaplan Turbine Working Principle
The Kaplan turbine working principle is very simple. These turbines design for a low water head so that a high flow rate of water is permitted. The Kaplan turbine scheme for hydropower is the same as for the Francis turbine.
Kaplan turbine works on the principle of the axial flow reaction. In an axial flow turbine, the fluid moves by the impeller in a direction parallel to the impeller’s axis of rotation. A Kaplan turbine works in the following way:
- First of all, the water introduces into the volute/scroll casing from the penstock.
- As water flows inside the volute casing, guide blades direct the water from the casing toward the impeller blades. These blades are flexible and may change their position based on flow requirements.
- As the water enters into the impeller area, it takes a turn of 90o so that it can strike the impeller blades in an axial direction.
- When the water strikes the impeller blades, these blades start revolving because of the water reaction force.
- These blades convert the K.E of the water into speed and increase the speed of the water.
- After passing through the impeller blades, the water reaches the draft tube, where the kinetic and pressure energies of the water reduce.
- This draft tube converts the kinetic energy or speed into pressure energy and increases the pressure of water.
- When the water pressure increases according to the requirements, the water delivers into the tailrace.
- The increased pressure of the water rotates the turbine. A generator is coupled with the turbine shaft. The rotation of the turbine further rotates the generator coil. According to Faraday’s 1st law, “when a conductor rotates in a magnetic field then electricity produces,” and in hydroelectric powerplants, electricity produces by using the same phenomena.
See also: Different types of the Reaction Turbines
Components of the Kaplan Turbine
The components of the Kaplan turbine are given below in detail.
- Runner or Impeller
- Hub
- Draft Tube
- Runner Blades
- Shaft
- Guide Blades
1) Runner Blades
The blades are the key components of the turbine. The Kaplan turbine blade looks like a propeller. Other axial flow turbines have plane blades, while Kaplan blades don’t have plane blades but are of twist shape lengthways so that the water swirls at the inlet-outlet.
When the water strikes these blades, they start rotatory motion, which further rotates the shaft.
2) Hub
Hub is one of the essential components of the Kaplan turbine. The blades are installed on the turbine hub. It controls the rotation of blades. And blades follow it for their movement. It connects with the central turbine shaft.
3) Shaft
One end of the turbine shaft is linked to the turbine runner, while the other end is linked to the generator coil.
As the runner rotates due to the rotation of the blades, the shaft also rotates, which further transmits its rotation to the generator coil. As the generator coil rotates, it produces electricity.
4) Guide Vane
The guide vane is a regulating component of the entire turbine. It switches on and off according to the requirements of power. Guide vanes rotate at a specific angle to regulate the water flow.
If the power requirement is more, it opens more so that a large volume of water can strike the rotor blades. As the power requirements reduce, it opens lower so that a low amount of water can strike the blades. The guide vanes increase the efficiency of the turbine. Without the guide vanes, the turbine can’t operate efficiently, and the turbine efficiency will reduce.
5) Runner
The runner plays a very vital role to ensure the efficient performance of the Kaplan turbine. It is a rotating component of the turbine. It provides help for electricity production. The axial water flow acting on the blades causes the rotation of the impeller, which further rotates the shaft.
6) Mechanism of Blade Control
The blade has a movable axis at the connection point. The blade control mechanism controls the attack angle as the water hits the blade, caused by the movable blade connection.
7) Scroll or Volute Casing
The entire turbine mechanism is surrounded by a housing called a scroll casing. The scroll casing reduces the cross-sectional area. First of all, the water flows from the penstock into the volute casing; after that, it flows into the guide vane area.
The water rotates up to 90° from the guide blade and moves axially by the impeller. The turbine casing prevents the guide vanes, impeller blades, runner, and other inner essential components from damage due to any external load.
8) Draft Tube
In the case of a Kaplan turbine, the atmospheric pressure is higher than the pressure at the runner outlet area.
Therefore, the fluid from the turbine outlet can’t discharge directly into the tailrace. Due to this reason, a tube having a progressively rising area uses to discharge the fluid from the outlet into the tailrace. This increasing area tube is known as a Draft Tube.
The draft pipe attaches the outlet of the runner to the tailrace and drains the water from the turbine. The primary function of the draft tube is to decrease the flow rate and minimize the loss of K.E. at the exit. They reduce the water speed by raising the water surface.
Advantages and Disadvantages of Kaplan Turbine
| Advantages | Disadvantages |
| The impeller blades are flexible | These turbines have a cavitation problem. |
| This turbine requires a shallow head |
It is expensive to manufacture. |
| These turbines require only three to eight blades | The Kaplan turbine is expensive to install |
| This kind of reaction turbine is extremely efficient compared to other kinds of hydroelectric turbines | These turbines need a very high-cost of installment |
| It has a small size | For high efficiency, it requires a high flow rate that is not possible everywhere |
| Takes up less space | In these turbines, a heavy-duty generator is needed. |
| It has a simple construction |
Applications of Kaplan Turbines
- Industrial applications: The industry that requires a large volume of water for cooling or other processes may also use a Kaplan turbine to regain energy from the water flow and decrease energy costs.
- Pumped-storage hydroelectric power plants: In these plants, water is pumped to an upper reservoir during periods of low electricity demand, and then released through Kaplan turbines to generate electricity during periods of high demand.
- Hydroelectric power plant: Kaplan turbines are commonly used in hydroelectric power plants to generate electrical power.
- Tidal power generation: They are also employed used in the tidal power plant to produce electrical power from the motion of tides.
- Irrigation system: Kaplan turbines may be used in irrigation systems to produce electrical power from the water flow utilized for irrigation.
- Navigation locks or canals: They can be installed into canals or navigation locks to produce electrical power from the water flow utilized to empty or fill the locks.
FAQ Section
Who Invented Kaplan Turbine?
Viktor Kaplan invented 1st Kaplan turbine in 1913.
Which turbine has the highest speed?
A Kaplan turbine has 2 to 3 times more speed than a Francis turbine.
Where is used Kaplan Turbine?
Kaplan turbines are employed in hydroelectric power plants to produce electricity. These turbines are usually used for high-flow rate and low-head applications.
What is the efficiency of a Kaplan turbine?
The Kaplan turbines have efficiencies between 90% to 93%.
Why is Kaplan Turbine known as a reaction turbine?
A Kaplan turbine is known as a reaction turbine because, in this turbine, the turbine rotates due to the reaction force of the water
What is the difference between Kaplan and Francis Turbine?
A Francis turbine is a mixed-flow turbine, while a Kaplan turbine is an axial flow turbine.