Kaplan turbine: Construction, Working, Efficiency, Advantages, Applications
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Based on the design of the turboprop engine, “Viktor Kaplan” created the first Kaplan Turbine in 1913. It therefore operates using the propeller’s opposite principle. He combined wicket gates, which are automatically adjusted to achieve efficiency over a wide range of water flow and level, with propeller blades. This was the first hydroelectric turbine to function at both high water flow and low head. Because its blades resemble propellers and work in the opposite direction with a similar phenomenon, the Kaplan turbine is also known as a propeller turbine. This turbine can therefore be used in river and low head regions. An overview of a Kaplan turbine and its use in applications is covered in this article.
What is Kaplan Turbine
A reaction turbine or a particular variety of propeller hydro turbine used in hydroelectric plants is referred to as a Kaplan turbine. The water in this turbine flows axially from the turbine as well as in. This turbine operates most effectively at low head and high water flow rates. So The Kaplan turbine’s key characteristic is its ability to move its blades as needed to maintain maximum efficiency under various water supply conditions. This device is referred to as a reaction turbine because when water passes through it, its pressure is lost. Below is a diagram of a Kaplan turbine.
The axial flow reaction principle underlies the operation of a Kaplan Turbine because in axial flow turbines, water is supplied to the runner in a direction parallel to the runner’s axis of rotation. The water entering the turbine has the pressure and kinetic energy needed for the blades to rotate efficiently inside a hydroelectric plant.
Kaplan Turbine Construction
It is possible to design Kaplan turbines so that heavy water loads can pass through them without breaking them. These turbines are made slightly differently from other types of turbines.
These turbines feature multiple rotor blades that are directly connected to the turbine’s central shaft.
The connection between these rotor blades and the variable joints allows for positional change. Because the external part of the rotor blade rotates much faster than the internal part, it is crucial to note that the rotor blades in this turbine are not horizontal but rather slightly twisted. The main parts of a Kaplan turbine are its runner or impeller, hub, fool tube, runner blades, shaft, and guide blades.
In this turbine that resembles a propeller, the runner blades are crucial parts. These turbines differ from other axial flow turbines in that they don’t have plane blades; instead, they have twisted blades that allow water to flow from the inlet to the exit. Once the water contacts these blades, they begin to rotate, which causes the shaft to turn even more.
This turbine’s vertical shaft has a hub that is larger than the lower portion of the shaft. To control the rotation of the blades, the turbine’s blades are situated on the hub.
One end of the shaft is simply connected to the turbine’s runner, and the other end is connected to the generator coil in a turbine. The shaft rotates when the runner turns as a result of the blades’ rotation, and this rotation can also be transmitted to the generator coil. Electricity is produced when the generator coil turns. Because the turbine shaft rotates at a high speed between 1800 and 3600 rpm, it should have heat-resistant features. The turbine shaft is made out of structural steel.
When more power is needed, the guide vane in the turbine regulates itself by turning ON and OFF. Water flow is regulated by guide vanes, which rotate at a precise angle. When there is a greater need for power, it opens wider, allowing more water to hit the rotor blades. It opens less as a result of a reduction in power requirements, which means that less water strikes the blades. Guide vanes can improve the turbine’s efficiency if they are not functioning properly.
In the turbine, the runner or impeller is crucial. It is a rotating part that aids in the production of electricity. The impeller’s revolution may turn the shaft due to the axial water flow on the turbine blades.
Mechanism of Blade Control
At the connection point, the turbine blade has a movable axis. The movable blade connection, which is what causes the attack angle when the water hits the blade, is controlled by the blade control mechanism. It contains all of the necessary parts for the Kaplan turbine.
Scroll Casing or Volute Casing
A scroll casing can be used to completely enclose the turbine, reducing the cross-sectional area. The water initially flows from the penstock to the volute casing before entering the guide vane area. Water flows axially through the impeller and turns up to 90 degrees away from the guide blade. So Here, the turbine’s casing protects the vital parts from damage caused by an external load, including the impeller blades, guide vanes, and a runner.
The accessible force at the turbine’s runner’s exit is typically less than the atmospheric force. As a result, the exit water cannot be released into the tailrace directly. Water is discharged from the Kaplan turbine to the tailrace using a tube, which can gradually improve the area. The term “Draft tube” thus refers to the tube’s expanding surface area. So One end of this draft tube is attached to the runner’s exit, and the other end is buried beneath the water’s surface in the tailrace. Only the Reaction turbine uses the Draft tube.
Kaplan Turbine Working
The scroll casing of the turbine is constructed in such a way that the water flowing from the pen-stock will enter without losing flow pressure. So The water will be forced into the runner blades of the turbine by the guide vanes. Depending on the requirement of the water flow rate, the vanes can be changed. The water supply is turned 90 degrees so that it is directed axially toward the runner blades. Due to the water supply’s reaction force, when water strikes runner blades, they begin to rotate.
In order to increase efficiency, these blades have been twisted throughout their length to always include the best angle of attack for all blade cross-sections. So The water enters from the runner blades to the draft tube wherever its kinetic energy & force energy is reduced. Once kinetic energy is changed into pressure energy then water pressure can be increased. The turbine rotation can be used to turn the generator’s shaft for the generation of electricity.
Kaplan Turbine Efficiency
The Kaplan turbine efficiency is high approximately 90%.
The formula for efficiency is (ξ) = WD(Work Done)/ρgQH
Work done per every second is = ρAV1[VW1 x u1]
The specifications of the Kaplan turbine include the following.
- The nominal flow is 10 m³/s
- The specific speed is 255 rpm
- The net water jump is 4 m
- The diameter of the propeller is 1.54 m
- Rotary speed is 255 rpm
- No. of propeller blades are 4
- The diameter of Hub is 0.63 m
- The highest axial drive is 8 Tn
- No. of distributor vanes are 17
- Volumetric o/p is 0,93
- Mechanical o/p is 0,93
- Total o/p is 0,865
Kaplan Turbine Examples
The flow ratio of the Kaplan turbine is 0.5 & the available head is 25 meter then what is the approximate velocity for water flow at the inlet of the runner?
Kaplan turbine’s flow rate can be given as (ψ) = Vf/√2*g*H
Here, ‘Vf’ is the flow velocity at the inlet of the runner & ‘H’ is the available head.
We know the values of
Ψ = 0.5
H = 25
g = 9.8m/s2
Substitute these values in the above equation to get Vf value.
(ψ) = Vf/√2*g*H
0.5 = Vf/√2x10x25
1/Vf = 0.5 (√2×9.8×25) = 1/√490×0.5 =1/22.13×0.5 = 1/11.06
Vf = 11.06 m/sec
The characteristics of the Kaplan turbine include the following.
- Simple construction.
- Size is small and compact.
- Vane ranges from 4 to 8
- Runner vanes are fixed, so cannot be changed.
- The type of flow is axial.
- Specific speed ranges from 600 to 1000.
- Appropriate for low heads.
- The direction of the shaft is vertical.
- Maximum flow rate.
- Friction loss is less.
- Efficiency is high.
The advantages of the Kaplan turbine include the following.
- It includes fewer blades.
- Kaplan turbine works at low head.
- It includes flexible runner vanes.
- It occupies less space.
- Its efficiency is very high as compared to other turbines.
- Its size is not large.
- Simple construction.
- Applicable for high discharge-based applications.
The disadvantages of the Kaplan turbine include the following.
- Requires high flow rate.
- The pressure drop inside the draft tube can lead to cavitation.
- So Runner blades are made of stainless steel, which may help to some extent with the cavitation issue.
- Manufacturing and installation cost is high.
The applications of the Kaplan turbine include the following.
- These turbines work very efficiently at high flow rates & low heads.
- These turbines are used for the production of electrical power.
- These turbines are used where the head is low & discharge is high.