Things you need to know about Kaplan turbine

With the vast majority of turbine out there, Kaplan fall to be one. It works on the principle of axial flow reaction, though some other types work on this principle. In axial flow turbines, water flows through the runner along the direction parallel to the axis of rotation of the runner. Stick with me, this will be further explained!

Today you’ll get to know the definition, applications, function, components, diagram, and working principle of a Kaplan turbine. You’ll also get to know the advantages and disadvantages of this Kaplan turbine.

Contents

What is a Kaplan turbine?

A Kaplan turbine is a propeller-type water turbine that works on the principle of axial flow reaction. Unlike others, the Kaplan turbine has adjustable blades. Its invention allowed efficient power production in low-head applications. This head ranges from 10 to 70 meters (33 to 230 ft) and the output ranges from 5 to 200 MW. The use of Kaplan turbines is in high-flow, low-head power production.

Kaplan turbines are the evolution of the Francis turbine, allowing efficient power production in low head applications. This is not possible with Francis turbines, and that is where the modification begins.

An Austrian professor Viktor Kaplan developed this turbine in 1913. He combined automatically adjusted propeller blades with automatically adjusted wicket gates so that efficiency can be achieved over a wide range of flow and water levels.

Applications of Kaplan turbine

Below are the applications of a Kaplan turbine:

• Kaplan turbines are widely used for the production of electrical power.
• Its small version or model is produced for individual power production.
• Large Kaplan turbines are designed for sites to operate at the highest possible efficiency.
• Recently, Kaplan turbines are used offshore for wave energy generation.

Note: just as with other turbines, the primary function of a Kaplan turbine is for electrical power generation.

Read more: Types of wind turbine

Components of Kaplan turbine

Below are the components of a Kaplan turbine and their functions.

Scroll casing:

This is a spiral type of casing, having decreasing cross-section area. Water from the penstocks enters this component and then moves to the guide vanes where it turns through 900. It also flows axially through the runner. The runner, runner blades guide vanes, and other internal parts of the turbine are protected from external damage.

Guide vane mechanism:

Kaplan turbines use the guide vane mechanism for controlling the whole turbine. It opens and closes depending upon the power requirement. In cases where more power output is required, it opens wider to allow more water to hit the blades of the rotor. At lower power output, it closes to cease the flow of water. In the absence of these components in Kaplan turbines, efficiency extremely decreases.

Draft tube:

The function of a draft tube is for discharging water from the exit of a turbine to the tailrace. This is because the pressure at the exit of the runner of reaction turbines is generally less than atmospheric pressure. The water at the exit cannot be directly discharged to the tailrace. Therefore, a tube or pipe of gradually increasing areas is used for the water discharging.

In Kaplan turbines, runner blades are one of the essential components. In fact, it is considered the heart of Kaplan turbines since it helps with electrical power generation. The blade is designed with a shaft that is connected to the shaft of the generator. There is a large boss mounted on the runner which allows the blades to tightly sit. These blades are adjustable to an optimum angle of attack for maximum power output. Kaplan turbines have twists along their length.

Working principles of Kaplan turbine

The working of a Kaplan turbine is less complex and can be easily understood. In its working, the water coming from the pen-stock enters into the scroll casing. This scroll casing is designed in a shape that the flow pressure is not lost. A guide vane which is adjustable directs the water to the runner blades. These vanes can adjust themselves depending on the flow rate requirement.  The water takes a 90-degree turn so that the water direction is axial to the runner blades.

The water strikes due to the reaction force of the water, as the runner blades start to rotate. For greater efficiency of the turbine, the runner blades have twists along their length. Therefore, an optimum angle of attack for all cross-sections of blades is obtained.

Water enters into the draft tube from the runner blades, where its pressure energy and kinetic energy decreases. The increased pressure of the water is experienced when the kinetic energy gets converted into pressure energy.

This turbine rotation is what rotates the shaft of the generator for electrical power production.

Below are the benefits of Kaplan turbine in their various applications:

• Design is less complex and can be easily constructed.
• Less space is required for installation.
• Have very high efficiency compared to others
• More efficient at lower head
• The numbers of blades are less
• Stainless steel is the blade material that can reduce cavitation.

Despite the good advantages of the Kaplan turbine, some limitations still occur. Below are the disadvantages of a Kaplan turbine in its various applications.

• Cavitation is a major drawback of a Kaplan turbine. It occurs due to the pressure drop in the draft tube.
• The shaft can only be positioned in the vertical direction.
• Its operation required a large flow rate.