Kaplan Turbine | How does a Kaplan Turbine Work?

The reaction turbines are used all over the world for electricity production. The reaction turbines have multiple types, but the most major types are the Kaplan turbine and 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 the high flow rate of water and low head with maximum efficiency that is not possible in 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.

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.

Also read: How does a Francis Turbine work?

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:

Kaplan Turbine working

  • First of all, the water introduces into the volute/scroll casing from the pen-stock.
  • 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 converts 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. 

For better understanding, watch the following video:

See also: Different types of the Reaction Turbines

Components of the Kaplan Turbine

The components of the Kaplan turbine are given below in detail.

Components of kaplan turbine

  1. Runner or Impeller 
  2. Hub
  3. Draft Tube
  4. Runner Blades
  5. Shaft
  6. 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 have not 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 includes in the essential components of the Kaplan turbine. The blades mountain on the hub of the turbine. It controls the rotation of blades. And blades follow it for their movement. It connects with the central turbine shaft.

3) Shaft

The one end of the turbine shaft is linked with the turbine runner, while the other end is linked with 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 has a very vital role in the Kaplan turbine working. The runner or impeller 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. It includes in essential components of the Kaplan turbine.

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 upto 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

  1. These turbines use for the production of electricity.
  2. Microturbines are inexpensive that design for single power production that can work for at least 24 inches head
  3. Large Kaplan turbines are designed individually for each location to achieve maximum efficiencies of up to 90%.
  4. Very expensive to install, manufacture, and design, but they work for many years.
  5. These turbines work more effectively at a high flow rate and low head.

Nowadays, the demand for the Kaplan turbine is increasing all over the world. These turbines are widely used for electricity production in different hydropower plants. So, the main reason for the popularity of these turbines is their easy construction and small size. These turbines require a low head that is a big of these turbines over other reaction turbines. So, the Kaplan turbine has high efficiency as compared to the Francis turbine. That is a significant advantage of these turbines over Francis turbines.

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 use in hydroelectric powerplants to produce electricity. These turbines use for high flow rate and low head.

What is the efficiency of a Kaplan turbine?

The Kaplan turbines have efficiencies between 90% to 93%.

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.

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

Do you Know:

  1. Different types of Turbines 
  2. What is a Reaction turbine and its different types?
  3. How does an Impulse turbine work?

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