What is Kaplan Turbine : Working & Its Applications

The first Kaplan Turbine was invented by “Viktor Kaplan” in the year 1913 based on the turboprop engine design. Thus, it works on the reverse principle of the propeller. He merged propeller blades with wicket gates which are automatically adjusted to attain efficiency over an extensive range of water flow & level. This was the first hydroelectric turbine that could work at low head & high water flow. The Kaplan turbine is also known as a propeller turbine because its blades look like propellers & function in the opposite direction with a similar phenomenon. So this turbine is applicable in the river & low head regions. This article discusses an overview of a Kaplan turbine and its working with applications.


What is Kaplan Turbine?

A Kaplan turbine is specifically a reaction turbine or a type of propeller hydro turbine used in hydroelectric plants. In this turbine, the water flows both in & exist from the turbine in an axial direction. This turbine works at a high water flow rate & low head with the highest efficiency.

The main feature of the Kaplan turbine is that the blades in the turbine will change their location when required to maintain maximum efficiency in different water supply conditions. When water flows through this turbine then it loses its pressure, so this is known as a reaction turbine. The Kaplan turbine diagram is shown below.

Kaplan Turbine
Kaplan Turbine

The working principle of a Kaplan Turbine mainly depends on the axial flow reaction principle because, in axial flow types turbines, the water supplies throughout the runner along the direction which is parallel to the rotation axis of the runner. In the turbine, the water at the inlet possesses both pressure & kinetic energy for effective blades rotation within a hydro-power station.

Kaplan Turbine Construction

Kaplan turbine design can be done to allow large amounts of water supply through them without breaking them. As compared to other types of turbines, these turbines are designed slightly differently.
These turbines are designed with several rotor blades which are connected to the central shaft of the turbine directly.

Kaplan Turbine Construction
Kaplan Turbine Construction

These rotor blades are connected through variable joints so that the position can be changed. It is very important to note that, the rotor blades in this turbine are not horizontal but they slightly twisted because the external part of the rotor blade rotates very quickly as compared to the internal part.

Kaplan turbine components mainly include runner or Impeller, hub, daft tube, runner blades, shaft & guide blades.

Runner Blades

The runner blades are essential components in this turbine which looks similar to a propeller. As compared to other axial flow turbines, these turbines don’t have plane blades but they are twisted so that the water flows at the inlet to exit. Once the water hits these blades, then they start rotating which further turns the shaft.

Hub

The shaft of this turbine is vertical and the shaft’s lower end is made larger which is known as a hub. The blades of the turbine are located on the hub to control the revolution of blades.

Shaft

In the turbine, one end of the shaft is simply connected to the runner of the turbine, whereas the other end is connected to the generator coil. When the runner turns because of the rotation of the blades, then the shaft also rotates, further, this rotation can be transmitted to the generator coil.

Once the generator coil turns then it generates electricity. The turbine shaft should include heat-resistant properties because it rotates at a high speed which ranges from 1800rpm to 3600 rpm. The material used in the turbine shaft is structural steel.

Guide Vane

The guide vane in the turbine is a regulating component that turns ON & OFF based on the power requirement. Guide vanes turn at a precise angle to control the flow of water. If the requirement of power is more, then it opens more so that it allows a large amount of water to hit the rotor blades. When the requirements of power decrease, then it opens less so that it allows less amount of water to hit the blades. The turbine efficiency can be increased through guide vanes otherwise it cannot function efficiently.

Runner

The runner or impeller in the turbine plays an essential role. It is a rotating component that helps in generating electricity. The axial water flow on the turbine blades can cause the revolution of the impeller to rotate the shaft.

Mechanism of Blade Control

The blade of the turbine 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 essential components of the Kaplan turbine.

Scroll Casing or Volute Casing

The whole turbine can be surrounded by a scroll casing so that it decreases the cross-sectional area. Initially, the water supplies from the penstock to the volute casing; then, it supplies into the area of the guide vane.

The water turns up to 90° from the guide blade & goes axially through the impeller. Here, the casing of the turbine avoids the essential components like impeller blades, guide vanes, a runner from damage because of external load.

Draft Tube

The accessible force at the exit of the turbine’s runner is usually smaller as compared to the atmospheric force. Thus, the exit water cannot be released directly to the tailrace. A tube can slowly enhance the area & this is used in discharging water from the Kaplan turbine to the tailrace.

So, the increasing area of the tube is called a Draft tube. This draft tube’s one end is connected to the exit of the runner whereas the other end is submerged under the level of water within the tailrace. The Draft tube is only used in the Reaction turbine.

Kaplan Turbine Working

The water flowing from the pen-stock will enter into the scroll casing of the turbine where scroll casing is made in such a way without losing the flow pressure. The guide vanes in the turbine will push the water into the runner blades. The vanes are changeable according to the necessity of the flow rate of water. The water supply takes a 90-degree twist so that the water direction is axial toward runner blades.

When the water hits runner blades then it starts rotating because of the reaction force of the water supply. These blades have twisted through their length to include optimum angle of attack always for all cross-sections of blades to attain greater efficiency. 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]

Specifications

The specifications of the Kaplan turbine include the following.

  • The nominal flow is 10 m³/s
  • The net water jump is 4 m
  • The specific speed is 255 rpm
  • Rotary speed is 255 rpm
  • The diameter of the propeller is 1.54 m
  • The diameter of Hub is 0.63 m
  • No. of propeller blades are 4
  • No. of distributor vanes are 17
  • The highest axial drive is 8 Tn
  • Volumetric o/p is 0,93
  • Mechanical o/p is 0,93
  • Total o/p is 0,865

Kaplan Turbine Solved Problems

Example1: The flow ratio of the Kaplan turbine is 0.5 & the available head is 25meter 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/s^2

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

Example2: The flow velocity of a Kaplan turbine is 15 m/s and the external runner diameter is 6 m whereas the hub diameter is 3 m. So, find the flow rate volume for the Kaplan turbine within m3/s?

We know that the outer or external runner diameter Do = 6m

The hub diameter Dh = 3m

The flow velocity V = 15m/s

The volume rate of flow can be measured as Q = π/4 (Do – Dh)*V

= π/4(6-3)*15 => 3.14/4 (3)*15

= 0.785 (45) = 35.3 m^3/sec

Example3: The Kaplan turbine’s velocity flow is 20 m/s and the available turbine’s head is 50 m. So, calculate the turbine’s flow ratio.

g = 10 m/s^2.

H = 50 m

Vf1 = 20 m/s

ψ = Vf1/√2*g*H)

Substitute the values in the above equation, we can get

ψ = Vf1/√2*g*H)

ψ = 20/√2*10 *50 = 20/√1000

ψ = 20/ 31.62 = 0.632

Characteristics

The characteristics of the Kaplan turbine include the following.

  • Size is small and compact.
  • Simple construction.
  • Runner vanes are fixed, so cannot be changed.
  • Vane ranges from 4 to 8
  • Specific speed ranges from 600 to 1000.
  • The type of flow is axial.
  • The direction of the shaft is vertical.
  • Appropriate for low heads.
  • Friction loss is less.
  • Maximum flow rate.
  • Efficiency is high.

Advantages

The advantages of the Kaplan turbine include the following.

  • Kaplan turbine works at low head.
  • It includes fewer blades.
  • It occupies less space.
  • It includes flexible runner vanes.
  • Simple construction.
  • Its efficiency is very high as compared to other turbines.
  • Its size is not large.
  • Applicable for high discharge-based applications.

Disadvantages

The disadvantages of the Kaplan turbine include the following.

  • Cavitation can be occurred because of the pressure drop within the draft tube.
  • Requires high flow rate.
  • Manufacturing and installation cost is high.
  • The material used for runner blades is stainless steel which may decrease the cavitation problem to some level.

Applications

The applications of the Kaplan turbine include the following.

  • These turbines are used for the production of electrical power.
  • These turbines work very efficiently at high flow rates & low heads.
  • These turbines are used where the head is low & discharge is high.

Thus, this is all about an overview of Kaplan turbines and their working with applications. These turbines work on the axial flow reaction principle and their main function is to generate electrical power. Here is a question for you, what is an impulse turbine?

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