What is Kaplan Turbine?
A Kaplan turbine is a flow turbine with an adjustable blade axial. The Kaplan turbine focuses on the principle of axial flow reaction. According to this, the liquid flows in the direction parallel to the axis of rotation through the runner. The water inside the inlet of the Kaplan turbine has both pressure and kinetic energy, which results in the effective rotation of blades.
Austrian professor Victor Kaplan created this turbine in 1913. The Kalpan turbine was created by combining automatically adjusted propeller blades and wicket gates to achieve efficiency over a wide range of flow or water levels. This full reaction turbine generates a lift on the impeller blades due to its aero file form. The Kaplan turbine is referred to as a key product in the industrial or electricity industry.
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In this kind of turbine, the lower end of the shaft is thick and therefore forms a hub or boss. The Kaplan turbine is used for hydroelectric plants, and what makes these blades special is their ability to preserve maximum efficiency regardless of the water flow rate.
Kaplan turbine: Parts
The Kaplan turbine is made after ensuring that large quantities of water can flow through these turbines without the possibility of damage. Hence, these turbines are constructed a bit differently as compared to other turbines.
First, the Kaplan turbine is much smaller than the other ones. In a radial direction, the water is injected through a flow tube. On the other hand, functional guide vanes and permanent blades direct this water into the turbine with an axial orientation. Here are the parts of the turbine that make this work efficiently:
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1. Scroll casing
The scroll casing refers to a spiral-shaped casing with a decreasing cross-section area. The spiral casing and guide vanes in a Kaplan turbine are similar to that of a Francis turbine. Initially, the water in the penstocks is shifted into the scroll casing, which then travels to the guide vanes. After this, the water is turned 90 degrees and flows through the runner efficiently. This protects the runner and runner blades and the guide vanes from being exposed to any external damage. The primary goal of this casing is to maintain that the constant water velocity is maintained at each point of entrance.
2. Guide vane mechanism
The guide vane mechanism is the only controlling part of the turbine and is responsible for opening and closing depending on the power demand. It opens wider to let in more water to hit the blades of the motor in case of more power output requirements. Similarly, it closes itself to stop the water flow when low power output is required. With the presence of the guide vane mechanism, the turbine can move efficiently, reducing its speed.
3. Draft tube
The area that is expanded out of the tube or pipe is referred to as the draft tube. One end of the draft tube is attached to the runner outlet while the opposite is submerged underwater. It is important to note that the draft tube is only present in the reaction turbine. The draft tube is made to prevent gushing out of the water from the outlet of reaction turbine runners due to the pressure generally being smaller than the atmospheric pressure. Therefore the water requires a wider area to move out.
4. Runner blades
Referred to as the heart of the Kaplan turbine components, the runner blades are the rotating parts of the machine that help produce electricity. This blade’s shaft is connected to the generator’s shaft. The blades of this component are adjustable for an optimum angle of attack to ensure maximum power output. They rotate automatically around pivots, following a governor servo mechanism.
How does the Kaplan turbine work?
The working of the turbine starts with the water in the penstock. It then enters the scroll casing, which is shaped into a medium that allows the flow pressure to be even. Following this, the water is then guided by the guide vanes towards the runner blades. These vanes are adjustable and can automatically adjust based on pressure and flow rate requirements. After this, the water is made to take a 90-degree turn, turning it axial to the direction of the runner blades.
The runner blades start rotating as soon as the water hits, owing to the reaction force of the water. These blades also have a twist along their length, which is constructed to ensure an optimum angle of attack. This ensures greater efficiency on all cross-sections of the blades. After this, the water enters the draft tube. This is where the pressure energy of the water decreases along with the kinetic energy.
To produce electricity, the generator’s shaft is rotated with the help of the rotation of the turbine. This is to ensure that the water moved via the Kaplan turbine remains a little different compared to other turbines.
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Advantages of the Kaplan turbine
- The Kaplan turbine works much more efficiently with a low water head and high flow rates than other turbines.
- Kaplan turbine has been of greater choice across the world when there is a question of electrical power production.
- The Kaplan turbines are smaller in size compared to other turbines and are also much easier to construct.
- When compared to various other hydraulic turbines, the efficiency of the Kaplan turbine remains very high.
- For individual power production, economical microturbines can be manufactured worldwide with only two feet of head.
- The Kaplan turbine can be individually designed for any site to operate with the highest possible efficiency, usually above 90%.
- These turbines last for decades with low maintenance.
Disadvantages of the Kaplan turbine
- The shaft inside the turbine can only be positioned vertically.
- This kind of turbine is designed for a large flow rate and cannot function with a low flow rate.
- One of the most significant disadvantages posed by this kind of turbine is cavitation. To prevent cavitation, the use of a draft tube and stainless steel can be effective. However, this will just slow down the cavitation process.
Kaplan turbine: Types of draft tubes used
There are four types of draft tubes associated with Kaplan turbines, namely:
- Conical draft tube
In conical draft tube form used in Kaplan turbine, the flow direction remains straight and divergent. These kinds of tubes are constructed out of steel plates. The shape of these tubes is tapered, and the diameter of the outlet remains greater than the diameter of the inlet of the draft tube.
- Simple elbow draft tube
The shape of this draft tube used in Kaplan turbine is like an elbow and has curricular inlets and outlets. This draft tube is to be mounted and placed next to the tail race and is used at low-head positions. The advantage of this draft tube is that it minimises the cost of drilling. The exit diameter should be maintained to be wide to recover the kinetic energy present at the runner outlet. The moderate efficiency of the simple elbow draft tube is approximately 60%.
- Moody draft tube
The outlet is separated into two sections in the moody draft tube. This kind of draft tube used in Kaplan turbine is similar to a conical draft tube. It has a central core component that is responsible for dividing the outlet into two parts. In Moody draft tubes, there are two exits and one inlet with one aim to reduce the swirling motion of water in the machine. The efficiency of moody draft tubes ranges to approximately 88%.
- Elbow draft tube with varying cross-section
The elbow draft tube used in Kaplan turbine with varying cross-sections is an improvement of a simple elbow draft tube. The outlet of this draft tube is rectangular, while the inlet of this draft tube is circular. The horizontal section dedicated to this draft tube is inclined upwards to prevent air from approaching the exit area. While the outlet in this kind of draft tube remains beneath the tail race, and the performance efficiency when used with the Kaplan turbine is approximately around 70%.
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What is the difference between Kaplan turbine and Francis turbine?
- Kaplan turbine is comparatively compact in cross-section and low rotational speed as compared to Francis turbine.
- While water flows axially in and axially out in Kaplan turbine, in Francis turbine water flows radially in and axially out.
- Kaplan turbine has higher efficiency than Francis turbine.
- Specific speed in Kaplan turbine is high ranging from 300- 1000 rpm, whereas specific speed in Francis turbine is medium ranging between 60 -300 rpm.
FAQs
Who created Kaplan turbines?
Kaplan Turbines were created in the year 1913 by Victor Kaplan.
How many types of draft tubes can be used for the construction of Kaplan turbines?
There are four types of draft tubes that can be used in the construction of Kaplan turbines: a conical draft tube, a simple elbow draft tube, a moody spreading draft tube, and an elbow draft tube with a varying cross-section.
Where are Kaplan turbines used?
Kaplan turbines are often used in the industrial and electricity sector.
