MECHANICAL THEORY

Kaplan Turbine & Francis Turbine-Components, Working, Adv & Limitations, Applications

Kaplan Turbine & Francis Turbine-Components, Working, Adv & Limitations, Applications:

In the last session, we had discussed the Introduction to Hydraulic Turbines, Pelton Wheel Turbine, Centrifugal pump & Reciprocating pump in a detailed way. Therefore, in this article, we can discuss both the Kaplan Turbine and Reaction Turbine so that we can complete the session of Hydraulic Turbines.

Kaplan turbine is an axial flow reaction turbine which is suitable for low heads and hence requires a large quantity of water to develop a large amount of power. It is more compact then Francis turbine which can run faster and maintains high efficiency. The Kaplan Turbine is located between the high-pressure water source and the low-pressure water exit.

This Kaplan Turbine was developed in1913 by Viktor Kaplan, an Austrian Professor. In his design, he combined automatically adjusted propeller blades and guided vanes to obtain efficiency over a wide range of water flow.

Principle:

Since it is a reaction turbine, the reaction force of leaving the water is used to turn the runner of the Kaplan turbine. As the water flows through the twisted blades, a lift force is generated in the opposite direction of the leaving water and that lift force causes the blades to rotate.

Kaplan turbine
Kaplan turbine-Image Source

Components of the Kaplan Turbine:

The main components of a Kaplan turbine are

1.Scroll Casing
2.Guide Vanes and Guide Mechanism
3.Runner, Runner Blades, and Guide Vanes
4.Draft Tube

The explanation of the above parts is as follows.

1.Scroll Casing:

The inlet is through the scroll casing which is in the form of a spiral. This ensures constant velocity of water flows along the path after entering into the casing.

2.Guide Vanes and Guide Mechanism:

Water gets distributed by the guide vanes and can flow on to the runner blades in an axial direction. The blades are so shaped that water flows axially through the runner.

3.Runner, Runner Blades, and Guide Vanes:

The runner blades, as well as guide vanes, are adjustable while the turbine is in motion. Guide vanes are turned through a certain angle to regulate the flow. The axial flow of water acting on the runner vanes causes the runner to rotate.

Blades of Kaplan Turbine
Blades of the Kaplan Turbine

4.Draft Tube:

Finally, water is discharged to the tailrace through a gradually expanding tube called the Draught tube. A Kaplan Turbine runner has 4 to 6 blades.

When both the Guide vanes angle and the Runner blade angle may thus be varied, higher efficiency can be maintained over a wide range of operating conditions.

Working of Kaplan Turbine:

Kaplan turbine 2D Sketch
Kaplan turbine 2D Sketch

The water from the penstock is to be allowed into the scroll casing of the Kaplan Turbine. The scroll casing is designed in such a way that the pressure flow is maintained. The two types of blades which are used in this turbine are Guide Vanes and Runner blades.

The Guide vanes are fixed along the spiral casing which makes the water to flow on to the runner whereas the runner blades are fixed to the Hub or Boss(runner) which rotates due to the reaction force of water hitting the blades of the runner. 

The guide vanes are adjustable w.r.t the flow rate. The water takes a 90-degrees turn so that the direction of the water is axial to that of runner blades.

From the runner blades, the water enters into the draft tube where its pressure energy and kinetic energy decreases. Kinetic energy is gets converted into pressure energy results in increased pressure of the water.

Due to the rotation of the runner, the shaft of the turbine rotates which thereby generates electricity.

Applications of Kaplan Turbine:

  1. Kaplan turbines are widely used for electrical power production.
  2. It can work more efficiently at low heads and high flow rates.
  3. Its construction is very easy because of the smaller size.
  4. The efficiency of the Kaplan turbine is very high when compared to other hydraulic turbines.

Disadvantages of Kaplan Turbine:

The only disadvantage of the Kaplan turbine is “cavitation”, which occurs due to the pressure drop in the draft tube.



FRANCIS TURBINE-Components, Working, Adv & Dis Advantages, Applications:


“Francis Turbine is a mixed flow reaction Turbine in which water enters radially at its outer periphery and leaves axially at its center”.

Components of Francis Turbine:

The components of Francis Turbine are as follows.

1.Scroll Casing (spiral casing)

2.Speed ring (stay ring)

3.Guide vanes

4.Runner

5.Draft tube

6.Tailrace

Working of Francis Turbine:

francis turbine
Francis turbine

Water passes through various components of the Francis turbine to rotate the runner and generates electricity. The components are arranged in sequential order of how water enters into the spiral casing and makes the runner rotate and how it exits from the draft tube to the tailrace generating electricity was explained below.

Scroll casing–>speed ring–>Guide vanes–>Runner–>Draft tube–>Tailrace.

Let’s discuss in detail…

1.Scroll Casing (spiral casing):

In Francis Turbine, Water from the Penstock enter a scroll casing (spiral casing) which completely surrounds the runner.

The purpose of the casing is to provide an even distribution of water around the circumference of the runner, maintaining constant velocity for the water by gradually reducing the cross-sectional area of the casing.

2.Speed ring (stay ring):

From the scroll casing, water passes through a speed ring or stay ring which consists of a series of fixed vanes called stay vanes.

The no. of stay vanes is usually taken as half the no. of guide vanes.

The function of the speed ring is to direct the water from casing to the guide vanes or wicket gates and further resist the load imposed upon it.

3.Guide vanes:

From the speed ring, water passes through a series of guide vanes provided around the periphery (aerofoil shaped) of the runner.

Their function is to regulate the quantity supplied to the runner and direct the water on to the runner at an angle appropriate to the design.

The vanes are so shaped that water enters the runner radially at the outer periphery and leaves it axially at the inner periphery.

4.Runner:

The most important part of Francis Turbine is the runner. It is fitted with a collection of complex-shaped blades. In runner, water enters radially and leaves axially.

Francis Turbine-Axial and Radial Flow of water into the runner
Francis Turbine-Axial and Radial Flow of water into the runner

Francis Turbine blades are specially shaped that it has thin aerofoil cross-section, so when water flows over it, low pressure will be produced on one side and high pressure will be on another side. This will result in a lift force.

Lift Force-Blade experiences both High pressure and low pressure in Francis Turbine
Lift Force-Blade experiences both High pressure and low pressure in Francis Turbine

One more peculiar thing about the blade is it is having a bucket type of shape towards the outlet so water will hit and produce impulse force before leaving the runner.

Impulse force and Reaction Force in Francis Turbine
Impulse force and Reaction Force in Francis Turbine

Animated Images-Source

Both Impulse force and lift force will make the runner rotate. So Francis Turbine is not a pure reaction turbine but a portion of force comes from the Impulse action also.

Therefore, it is a combination of Impulse and Reaction forces and thereby Francis turbine is also called a Mixed Flow turbine.

5.Draft tube:

A draft tube is a large pipe with increasing cross-section area which connects the runner exit to the tailrace.

It converts a large portion of velocity energy rejected from the runner into useful pressure energy.

6.Tailrace:

The water after passing through the runner flows to the tailrace through a draft tube.

This is the detailed explanation of the working of Francis Turbine. If you have any doubts, you can ask in the comments section.

Advantages of the Francis turbine:

  • Its control is easy even with variable heads
  • It has a small runner size
  • Very little change in efficiency with time

Disadvantages of Francis Turbine

  • Inspection is relatively difficult
  • Problem of cavitation

Applications of Francis Turbine:

  • It works over a wide range of head and flow rate
  • One of the most efficient hydro-turbines
  • Most widely used turbines to generate electricity

If you have any doubts, you can ask from the comments section. If the article is helpful to you, appreciate us from the comments section.

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