Introduction
This article discusses hydroelectric turbine types, their designs, categorizations, and typical applications. Impulse and reaction turbines are discussed, and the properties of each. Francis, Kaplan, and Pelton hydro turbines are discussed in detail, along with their working principles.
Short History Lesson
Waterwheels are the ancestor of modern hydroelectric turbines. The modern equivalent of a waterwheel today is referred to as a ‘turbine runner’. Turbine runner designs may have changed considerably over the past thousand years, but their purpose has not. The purpose of a hydroelectric turbine runner is to convert potential energy to mechanical energy. This mechanical energy can be used to perform useful work, such as rotate pump impellers in pumps, or rotors in electrical generators. In this video, we are going to focus on the common hydroelectric turbine runner designs that are used in the power engineering industry.
Waterwheel
Common Turbine Runners
The three most common turbine runners are the Francis, Kaplan, and Pelton turbine runners. The Francis and Pelton turbines were invented in the 1800s by James Francis and Lester Pelton respectively. The variable pitch propeller type runner was invented by Victor Kaplan in the early 1900s. Fixed propeller type runners exist, but only the variable pitch propeller type is referred to as a Kaplan turbine runner. There are other designs of runner such as the Deriaz and cross-flow designs, but these are less common. Each runner has different operating characteristics, and each runner is suitable only across defined operating heads and flow rates; it is however possible that several runner designs may be suitable for a shared application.
Hydroelectric Turbine Runner Classification
Hydroelectric turbine runners can be categorized in various ways:
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The three common hydroelectric turbine designs are the Kaplan, Pelton, and Francis designs.
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There are reaction (Kaplan and Francis) and impulse (Pelton) turbines, which are pressure and pressure-less respectively.
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By the type of flow through or onto the runner blades or buckets (axial, mixed, tangential, and radial flow turbines).
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By the volumetric flow rate fed to the runner.
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By the specific speed of the runner.
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By the operating pressure head.
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Classification by Energy
Reaction turbines are pressure type turbines and rely upon a continuous body of water from the headwater to the tailwater. Reaction turbines use the shape of the runner blades to create a pressure differential across the suction and discharge sides of the blades. The change in pressure across the blades represents the amount of energy that is converted to mechanical energy, although kinetic energy is also converted to mechanical energy by this type of turbine. Francis and Kaplan runners are reaction type runners.
Reaction Turbine Operating Principle
Impulse turbines are pressure-less type turbines. Impulse turbines rely upon water jets that are directed tangentially at buckets. As the water impacts upon each bucket, the buckets move away from each jet and torque is applied to the runner shaft which causes it to rotate. Pelton turbine runners may be driven by one or multiple jets of water. The Pelton runner is the most common type of impulse runner.
Impulse Turbine Operating Principle
Classification by Flow
Runners can be classified as axial, mixed, radial, cross, or tangential flow. Where water flows parallel across the runner, it is an axial flow runner. Where water flows radially across the runner, either inwards or outwards, it is a radial flow runner. Runners that utilize both axial and radial flow, are referred to as mixed flow runners. In practice, reaction type runners are usually either axial or mixed flow. Kaplan runners are classified as axial flow runners, whilst Francis runners are classified as mixed flow runners. Pelton runners are tangential flow runners, because the water strikes the buckets tangentially. Cross-flow turbine runners are categorized as using transverse flow, because the water flows on one side of the runner and out of the other.
Classification by Speed
The specific speed is given in revolutions per minute (rpm), it relates to the speed of the runner should it be reduced to a size where it generates one unit of power. Reducing the runner down to a size where it generates only one unit of power, allows us to compare the specific speed of turbine runners of different designs.
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Low specific speed turbines are those with a specific speed of less than 50.
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Medium specific speed turbines have a specific speed of between 50 to 250.
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High specific speed turbines operate at specific speeds in excess of 250.
Classification by Pressure Head
The pressure head is measured by the difference in elevation between the headwater and tailwater. A low head turbine has an operating head of 30m or less, a medium head turbine has an operating head of between 30 to 300m, and a high head plant has an operating head in excess of 300m.
Factors Effecting Efficiency
Irrespective of the turbine runner design, the amount of potential energy that can be converted to mechanical energy depends upon three main variables:
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Available head
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Flow rate
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Turbine runner efficiency
The head is the difference in elevation between the headwater and tailwater. The flow rate is determined by the cross-section area of the water conductors and the flow velocity. The turbine efficiency is dependent upon the turbine runner design and application, if the correct runner is used for the correct application, efficiencies in excess of 90% can often be achieved for all common runner designs.
Hydro Turbine Runners
Impulse turbines, reaction turbines, and the three most common hydro turbine runner designs have been briefly discussed. This article will now look at each runner design in more detail.
Francis Turbines
Francis runners consist of a series of fixed blades connected to a runner crown at the top, and runner band at the bottom. This type of runner converts both pressure energy, and kinetic energy, into mechanical energy.
Francis Turbine Working Principle
The appearance of Francis runners can vary considerably, this is due to the conditions and speed in which the runner is expected to operate. Francis runners have a wide operating range and can be used for many head and flow applications, which makes them a very common type of runner.
Unlike other turbine runner designs, the Francis turbine does not reduce markedly in efficiency until the load reduces to around 40%, and it has the unique feature of being able to act as both a pump, and a turbine.
Francis Turbine Runner
Francis runners can be used across a wide range of pressures and flow rates; they have been used for applications exceeding 800 MW.
Kaplan Turbines
Kaplan runners consist of a series of blades mounted onto a central hub. Each blade can rotate within its mounting, which means the pitch is adjustable; this is the differentiating factor between Kaplan turbines and fixed propeller type turbines. Typically, Kaplan turbines utilise between three to six blades, although up to ten blades are possible. The adjustable pitch blades can be used to regulate the rotational speed of the runner, and thus also regulate the amount of potential energy that can be extracted from the flowing water.
Kaplan Turbine Working Principle
Kaplan turbines are used for low head applications where a medium to high flow rate is present; this makes them an ideal choice for run-of-the-river, and tidal hydroelectric power plants. Kaplan runners have been used for applications exceeding 200 MW.
Kaplan Turbine Runner
Pelton Turbines
Pelton runners have the most distinctive appearance of all hydroelectric turbines. The outer periphery of the runner contains a series of buckets connected to a circular disc. A jet of water is sprayed at these buckets from a spray nozzle, and the resultant impulsive force applied to the runner causes it to rotate. The number of jets depends upon the size of the Pelton runner, although typically between one to six jets are used.
Pelton Turbine Working Principle
A ‘needle’ is used to start, stop, and regulate the water flow through the nozzle. The needle is a cone shaped item that can be inserted or retracted into the nozzle in order to regulate flow; it is usually manually or electrically actuated. It is the nozzle that converts the pressure energy of the water to kinetic energy, which is then converted to mechanical energy by the runner. Note that the Pelton runner does not convert pressure energy to mechanical energy, because it is operating under atmospheric conditions. Instead, water is discharged from the Pelton runner buckets directly to a discharge pit, then the tailrace.
Pelton turbines can be installed with a horizontal or vertical orientation. Smaller units tend to be installed with a horizontal orientation whilst larger units are installed with a vertical orientation.
Pelton turbines are used for high head, low flow applications only. Pelton runners have been used for applications exceeding 400 MW.
Summary
In this article, we have learnt how impulse and reaction turbines work. We have learnt how Francis, Kaplan, and Pelton turbines work, and for which application each turbine is suitable.
The content of this article is taken from our Introduction to Hydro Power Plant Engineering Course.
Additional Resources
https://en.wikipedia.org/wiki/Water_turbine