• Hydroelectric Power Plants


    Hydroelectric power plant is an electric power generating station that captures the potential energy of water at a high level to generate electricity. A turbine converts the kinetic energy of falling water into mechanical energy. Then a generator converts the mechanical energy from the turbine into electrical energy.

    Water flowing in the river is comprised of kinetic energy and potential energy. In hydroelectric power plants the potential energy of water is utilized to produce electricity. The different parts of a hydroelectric power plant are:


    Figure: Hydroelectric Dam

    1. Dam:

    A dam is a barrier which stores water and creates water head.  Dams are built of concrete or stone masonry, earth or rock fill. The dam is built on a large river that has abundant quantity of water throughout the year. It should be built at a location where the height of the river is sufficient to get the maximum possible potential energy from water. The height of water in the dam is called head race. Dam can be classified as follows:

    A. Masonry dams:  Masonry dams are dams made out of masonry — mainly stone and brick, sometimes joined with mortar. They can be further classified as follows:

    a. Gravity Dam: This type of dam is considered massive in size and made from concrete or stone. They are designed to hold back enormous amounts of water. The use of concrete allows the dam to resist the horizontal thrust of the water as it pushes against it. It is suitable for most of the site location. The height of dam depends on the strength of subsoil strata.

    b. Arch Dam: In an arch dam, the entire structure is curved with the convexity towards the upstream side. These types of dams are best suited for narrow canyons. Arch dam transmits a major portion of its water pressure horizontally to abutments by the arch action.

    c. Buttress dam: This dam has an upstream face and buttress that transmit the water pressure and weight of dam to the foundation. They are also sometimes called hollow dams because the buttress does not form a solid wall that stretches across the river valley. Such dams are more suitable for week foundation and earthquake prone sites.

    Figure: Hydroelctric Dam

    B. Earth Dam: Just as the name implies, an earth dam is constructed of soil and is built up by compacting layers of earth. A facing of crushed stone helps to prevent erosion by the water or the wind. It has very large base as compared to its height. A spillway is also constructed; usually this is made of concrete. Am earth dam is able to resist the forces that are pushing against it because of the strength of the soil. Such dam is cheaper than masonry dams and fit best in nature surrounding.


    2. Water Reservoir:

    The water reservoir is the place behind the dam where water is stored. The height of water in the reservoir decides how much potential energy the water possesses. The higher the height of water results in more potential energy. The high position of water in the reservoir also enables it to move downwards effortlessly. The height of water in the reservoir is higher than the natural height of water flowing in the river. This also helps to increase the overall potential energy of water, which helps ultimately produce more electricity in the power generation unit.


    3. Intake or Control Gates:

    These are the gates built inside the dam. The water from reservoir is released and controlled through these gates. These are called inlet gates because water enters the power generation unit through these gates. When the control gates are opened the water flows due to gravity through the penstock and towards the turbines. The water flowing through the gates possesses potential as well as kinetic energy.


    4. Penstock:

    The penstock is a long pipe that carries the water flowing from the reservoir towards the power generation unit, comprised of the turbines and generator. The water in the penstock possesses kinetic energy due to its motion and potential energy due to its height. The total amount of power generated in the hydroelectric power plant depends on the height of the water reservoir and the amount of water flowing through the penstock, which is controlled by the control gates. A penstock can be made of concrete for small size power plant or made of steel for large size power plant.


    5. Surge Tank:

    The main function of surge tank is to reduce the water hammering effect in pipes which can cause damage to pipes.

    When there is a sudden increase of pressure in the penstock due sudden decrease in the load demand on the generator, the turbine gates admitting water to the turbine closes suddenly owing to the action of the governor. This sudden rise in the pressure in the penstock will cause the positive water hammering effect. This may lead to burst of the penstock because of high pressures.

    When there is sudden increase in the load, governor valves opens and accepts more water to the turbine. This results in creation of vacuum in the penstock resulting into the negative water hammering effect. Therefore, the penstock should have to withstand both positive and negative water hammering effect created due to sudden change in water requirement. In order to protect the penstock from these water hammering effects, surge tank is used in hydroelectric power station.


    Figure: Forebay, Surge Tank, and Penstock

    A surge tank is introduced in the system between dam and the power house nearest. Surge tank is a tank provided to absorb any water surges caused in the penstock due to sudden loading and unloading of the generator. When the velocity of the water in the penstock decreases due to closing of turbine valves, the water level in the surge tank increases and fluctuating up and down till its motion is damped out by the friction. Similarly when the water accelerates in the penstock, water is provided by the surge tank for acceleration. Surge tank water level falls down and fluctuates up and down absorbing the surges.


    6. Forebay:

    It is an enlarge body of water to store water temporarily to meet the hour-to-hour load fluctuations on the station. The other purpose of forebay is to allow the last particles to settle down before the water enters the penstock.


    7. Spillway:

    The function of spillway is to provide safety of the dam. Spillway should have the capacity to discharge major floods without damage to the dam and at the same time keeps the reservoir levels below some predetermined maximum level.



    8. Trash Rack:

    The water intake from the dam or from the forebay is provided with trash rack to prevent the entry of any debris (small pieces of rock) which may damage the turbine runners or choke-up the nozzles of impulse turbine. But during winter season when water forms ice, to prevent the ice from clinging to the trash racks, they are often heated electrically.


    9. Water Turbines:

    Water flowing from the penstock is allowed to enter the power generation unit, which houses the turbine and the generator. When water falls on the blades of the turbine the kinetic and potential energy of water is converted into the rotational motion of the blades of the turbine. The rotating blade causes the shaft of the turbine to also rotate. The turbine shaft is enclosed inside the generator. In most hydroelectric power plants there is more than one power generation unit.

    There is large difference in height between the level of turbine and level of water in the reservoir. This difference in height, also known as the head of water, decides the total amount of power that can be generated in the hydroelectric power plant. There are various types of water turbines such as Kaplan turbine, Francis turbine, Pelton wheels etc. The type of turbine used in the hydroelectric power plant depends on the height of the reservoir, quantity of water and the total power generation capacity.



    10. Draft Tube:

    The draft-tube is a pipe of gradually increasing area which one end is connected to the outlet of the runner of the reaction turbine while the other end is submerged below the level of water in the tail race. Thus draft tube reduces the velocity of the discharged water to minimize the loss of kinetic energy at the outlet or we can say that it converts a large portion of the kinetic energy rejected at the outlet of the turbine into useful pressure energy because draft tube increase the net head on the turbine.  This permits the turbine to be set above the tail water without any appreciable drop of available head.


    11. Generators:

    It is in the generator where the electricity is produced. The shaft of the water turbine rotates in the generator, which produces alternating current in the coils of the generator. It is the rotation of the shaft inside the generator that produces magnetic field which is converted into electricity by electromagnetic field induction. Hence the rotation of the shaft of the turbine is crucial for the production of electricity and this is achieved by the kinetic and potential energy of water. Thus in hydroelectricity power plants potential energy of water is converted into electricity.


    Some facts about Hydroelectric power plant

    Long life and low operating costs: Hydro power plants are expensive to build. But once the plant is in operation, hydro power is extremely inexpensive. The plants are almost entirely automated, no fuel needs to be purchased and maintenance costs are low. In addition, the useful life of a hydro power plant is long. Many of the plants in operation today were built over 50 years ago and their useful life will continue for many years to come. Investment costs are quickly recouped once the plant is in operation.

    Hydro – a balancing power: Hydro power plants can be used both to generate baseload power (the amount of electricity that is always needed) and as balancing power (electricity output that can quickly be turned on to meet variations in demand).

    A problem with electricity is that it cannot be stored to any great extent. Water, on the other hand, can be. Water reservoirs next to hydro power plants function as large "batteries". Energy can be stored during the times of the year when water inflow is high and electricity demand is low, and the energy can then be used when demand is greatest.


    Quick start-up: An important characteristic of hydro power is that it generates a great deal of electricity as soon as the water is released, and is not dependent on weather, wind or long complicated start-up processes. Hydro power generation can be increased, for instance, to cover shortfalls from wind power and other types of energy that cannot be directly controlled, or from nuclear and coal power plants which take longer to get started.


    Hydroelectric Power Generation Efficiency

    Hydroelectric power generation is by far the most efficient method of large scale electric power generation. Energy flows are concentrated and can be controlled. The conversion process captures kinetic energy and converts it directly into electric energy. There are no inefficient intermediate thermodynamic or chemical processes and no heat losses. The overall efficiency can never be 100% however since extracting 100% of the flowing water's kinetic energy means the flow would have to stop. The conversion efficiency of a hydroelectric power plant depends mainly on the type of water turbine employed and can be as high as 95% for large installations. Smaller plants with output powers less than 5 MW may have efficiencies between 80 and 85 %. It is however difficult to extract power from low flow rates.


    Water Turbines

    A water turbine is a rotary machine that converts kinetic energy and potential energy of water into mechanical work. Water turbines were developed in the 19th century and were widely used for industrial power prior to electrical grids. Now they are mostly used for electric power generation. Water turbines are mostly found in dams to generate electric power from water kinetic energy.


    Turbine Types

    The type of turbine used depends on the size of the power plant, the rate of water flow, the head or pressure of water and other conditions. The Francis and Kaplan turbines are the most common types, used chiefly in hydro power plants with medium and low head respectively. Hydro power plants with higher head normally use a Pelton turbine. Like steam turbines, water turbines may depend on the impulse of the working fluid on the turbine blades or the reaction between the working fluid and the blades to turn the turbine shaft which in turn drives the generator.


    1. Impulse Turbines:

    The impulse turbine generally uses the velocity of the water to move the runner and discharges to atmospheric pressure. The tangential water flow on one side of the turbine runner hits each bucket on the runner. There is no suction on the down side of the turbine, and the water flows out the bottom of the turbine housing after hitting the runner. An impulse turbine is generally suitable for high head, low flow rate applications. Example: Pelton turbine (for high head: HH), Turgo turbine (for MH & LH), Cross-flow turbine (for LH), Jonval turbine, Reverse overshot water-wheel, Screw turbine (for LH).

    a. Pelton Turbine:

    The Pelton turbine has one or more jets (nozzles) that direct forceful, high-speed streams of water against a rotary series of spoon-shaped buckets, also known as impulse blades, which are mounted around the circumferential rim of a drive wheel—also called a runner. As the water jet impinges upon the contoured bucket-blades, the direction of water velocity is changed to follow the contours of the bucket. Water impulse energy exerts torque on the bucket-and-wheel system, spinning the wheel; the water stream itself does a "u-turn" and exits at the outer sides of the bucket, decelerated to a low velocity. In the process, the water jet's momentum is transferred to the wheel and hence to a turbine. Thus, "impulse" energy does work on the turbine. For maximum power and efficiency, the wheel and turbine system is designed such that the water jet velocity is twice the velocity of the rotating buckets. A very small percentage of the water jet's original kinetic energy will remain in the water, which causes the bucket to be emptied at the same rate it is filled, and thereby allows the high-pressure input flow to continue uninterrupted and without waste of energy. Typically two buckets are mounted side-by-side on the wheel, which permits splitting the water jet into two equal streams. This balances the side-load forces on the wheel and helps to ensure smooth, efficient transfer of momentum of the fluid jet of water to the turbine wheel. Depending on water flow and design, Pelton wheels operate best with heads from 15–1,800 metres (50–5,910 ft), although there is no theoretical limit.


    Figure: Pelton Turbine


    Spear: It’s used to increase and decrease the speed of water entering into the turbine, which is control by governor. If more water is needed spear move back and allow more to enter into the turbine and if less water is needed spear moves forward to spot excess water from entering in to the turbine.

    Brake nozzle: It’s used to stop the runner of this turbine by putting water pressure at the back of the buckets.

    Deflector: It’s used to stop turbine’s runner in case of emergency without water hammering.


    2. Reaction Turbines:

    A reaction turbine develops power from the combined action of pressure and moving water. The runner is placed directly in the water stream flowing over the blades rather than striking each individually. Reaction turbines are generally used for sites with lower head and higher flows than compared with the impulse turbines. Example: Francis turbine (for LH & MH), Kaplan turbine (for LH), Tyson turbine, Gorlov helical turbine.

    a. Francis Turbine:

    In the modern Francis turbine, water flow enters in a radial direction towards the axis and exits in the direction of the axis i.e., it is a mixed-flow reaction turbine that combines radial and axial flow concepts. Francis Turbines are generally installed with their axis vertical and water enters the turbine through the spiral casing with a small and fairly constant speed and exerts a pressure, varying from maximum at the top to a small value at the bottom. As the water passes through the spiral casing, it loses a part of its pressure in the volute (spiral casing) to maintain its speed. Then water passes through guide vanes where it is directed to strike the blades on the runner at optimum angles. As the water flows through the runner its pressure and angular momentum (or speed) reduces.


            Figure: Francis Turbine        

    This reduction imparts reaction on the runner and power is transferred to the turbine shaft. After doing work, water is discharged to the tailrace through a closed tube of increasing cross section, called draft tube. Large scale turbines used in dams are capable of delivering over 500 MW of power from a head of water of around 100 meters with efficiencies of up to 95%.


    b. Kaplan Turbines:

    The Kaplan turbine has adjustable blades and was developed on the basic principles of the Francis turbine. The main advantage of Kaplan turbines is its ability to work in low head (6 to 15 m) sites which was not possible with Francis turbines. Thus, these turbines are widely used in high-flow, low-head power production. The Kaplan turbine is an axial flow reaction turbine, which means that the working fluid flows in the direction parallel to the axis of the shaft. In Kaplan turbine, the working fluid changes pressure as it moves through the turbine and gives up its energy.

    The inlet is a scroll-shaped tube that wraps around the turbine’s wicket gate. Water is directed tangentially through the wicket gate and spirals onto a propeller shaped runner, causing it to spin. The outlet is a specially shaped draft tube that helps decelerate the water and recover kinetic energy. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications.

    Figure: Kaplan Turbine


    c. Propeller Turbine:

    The propeller turbine is another example of an axial flow reaction turbine and has no provision for changing the runner blade angle while turbine is in motion. It is similar in form to a ship's propeller and is the most suitable design for low head water sources with a high flow rate such as those in slow running rivers. Though its construction or design is simple and optimised for a particular flow rate but its efficiency drops rapidly if the flow rate falls below the design rating. Therefore, such a turbine is kept fully loaded for efficient operation. Its efficiency is about 92% at full load and drops to 65% at half load.


    Types of Hydropower Plants

    1. Impoundment Power Plant (Potential Energy):

    A hydroelectric dam installation uses the potential energy of the water retained in the dam to drive a water turbine which in turn drives an electric generator. The available energy therefore depends on the head of the water above the turbine and the volume of water flowing through it. Turbines are usually reaction types whose blades are fully submerged in the water flow.


    Available Power

    Potential energy per unit volume = ρgh

    Where ρ is he density of the water (103 Kg/m3 ), h is the head of water and g is the gravitational constant (10 m/sec2)

    The power P from a dam is given by

    P = ηρghQ

    Where Q is the volume of water flowing per second (the flow rate in m3/second) and η is the efficiency of the turbine.

    For water flowing at one cubic meter per second from a head of one meter, the power generated is equivalent to 10 kW assuming an energy conversion efficiency of 100% or just over 9 kW with a turbine efficiency of between 90% and 95%.


    2. Run-off River (Diversion) Power plant:

    Run-of-the-river power plants are typically used for smaller schemes generating less than 10 MW by using Kaplan or bulb type turbine. Run-of-the-river hydroelectricity generation uses the natural flow and elevation drop of a river to generate electricity.

    A possible alternative to conventional hydroelectric dams is the run-of-the-river turbine, which seeks to mitigate most, if not all, of the political, economic, and environmental issues associated with the hydroelectric dam and reservoir model. In a typical run-of-river structure, a portion of the river is diverted through a turbine system that turns with the river flow and powers a generator that then converts that mechanical energy into electricity. Such installations also have a minimal effect on the rivers themselves – as the project scale is smaller and there are fewer infrastructures involved, run-of-river turbines inherently alter less of the natural environment than hydroelectric reservoirs. Water flow is also allowed to continue downstream as normal upon exiting the penstock pipes instead of being collected in reservoirs, further reducing the effects of run-of-river turbines on downstream aquatic life and ecosystems.

    The available energy therefore depends on the quantity of water flowing through the turbine and the square of its velocity. Kaplan and bulb turbines are common types of turbine for run-of-the-river power plants with small drop heights of 6 to 15 m and high volume flows. They are suitable for fluctuating water volumes.

    Run-off river projects are much less costly than dams because of the simpler civil works requirements. They are however susceptible to variations in the rainfall or water flow which reduce or even cut off potential power output during periods of drought. During flood conditions the installation may not be able to accommodate the higher flow rates and water must be diverted around the turbine losing the potential generating capacity of the increased water flow. Because of these limitations, if the construction of a dam is not possible, run of river installations may need to incorporate some form of supply back-up such as battery storage, emergency generators or even a grid connection.

    Available Power

    The maximum power output from a turbine used in a run of river application is equal to the kinetic energy (½mv2) of the water impinging on the blades. Taking the efficiency η of the turbine and its installation into account, the maximum output power Pmax is given by

    Pmax =½ηρQv2

    Where, v is the velocity of the water flow and Q is the volume of water flowing through the turbine per second.

    Q is given by,  Q = A v

    Where, A is the swept area of the turbine blades.


    Pmax =½ηρAv3

    This relationship also applies to shrouded turbines used to capture the energy of tidal flows and is directly analogous to the equation for the theoretical power generated by wind turbines. Note that the power output is proportional to the cube of the velocity of the water.

    Thus the power generated by one cubic meter of water flowing at one meter per second through a turbine with 100% efficiency will be 0.5 kW or slightly less taking into account the inefficacies in the system. This is only one twentieth of the power generated by the same volume flow from the dam above. To generate the same power with the same volume of water from a run of river installation the speed of the water flow should be √20 meters per second (4.5 m/sec).


    3. Pumped Storage Power Plant:

    A pumped hydroelectric storage plant is a variation on a traditional hydropower plant that operates with two reservoirs: a lower and an upper one. Such a plant utilizes gravity to "store" electricity in the form of potential energy. In generating mode, water flows in traditional fashion from the upper reservoir to the lower, driving turbines and generating electricity. When there is a surplus of electricity in the grid, for example, when demand is low or wind/solar are producing more than needed, electricity is drawn from the grid and used to pump water from the lower reservoir to the upper reservoir. Thus, the system acts like a giant battery to store electricity until needed.

    Figure: Pumped Storage Hydroelectric Power Plant


    A pumped hydroelectric storage plant typically uses reversible pump/turbines that can either generate electricity or pump water. And, although such equipment can be very efficient, the plant is still a net user of electricity. I.e., it takes more electricity to pump water from the lower to upper reservoir than is generated by the same amount of water flowing from the upper to lower reservoir. Some estimates put the energy loss at 15-30%; however, this is good for a storage system and comparable to battery storage. And, with the ability to store power for use when needed and take advantage of electricity price differentials between peak and off-peak hours, these plants can be very cost effective. Essentially, electricity is generated when demand is high for peak price, and electricity is stored when demand is low for reduced price. Managed properly, such price differentials will more than offset losses in efficiency.


    Classification According to Availability of Water Head

    1. Low-head (less than 30 m) hydro-electric plants:

    ‘‘Low head’’ hydro-electric plants are power plants which generally utilize heads of less than 30 meters. Power plants of this type may utilize a low dam or weir (a low dam built across a river to raise the level of water upstream or regulate its flow) to channel water, or no dam and simply use the ‘‘run of the river’’. Run of the river generating stations cannot store water, thus their electric output vary with seasonal flows of water in the river. A large volume of water must pass through a low head hydro plant’s turbines in order to produce a useful amount of power. Since the head of water is very small in these hydroelectric power plants, they have lesser power producing capacity of less than about 25 MW, which are generally referred to as ‘‘small hydro’’.


    2. Medium-head (30–300 m) hydro-electric plants:

    The hydroelectric power plants in which the working head of water is more than 30 meters but less than 300 meters are called medium head hydroelectric power plants. These hydroelectric power plant are usually located in the mountainous regions where the rivers flows at high heights, thus obtaining the high head of the water in dam becomes possible. In medium head hydroelectric plants dams are constructed behind which there can be large reservoir of water. Water from the reservoir can be taken to the power generation system where electricity is generated.


    3. High-head hydro-electric plants:

    The hydroelectric power plants in which the working head of water is more than 300 meters and can extend even up to 1000 meters are called high head hydroelectric power plants. These are the most commonly constructed hydroelectric power plants. In the high head hydroelectric power plants huge dams are constructed across the rivers. There is large reservoir of water in the dams that can store water at very high heads. Water is mainly stored during the rainy seasons and it can be used throughout the year. Thus the high head hydroelectric power plants can generate electricity throughout the year. The high head hydroelectric power plants are very important in the national grid because they can be adjusted easily to produce the power as per the required loads.


    Classification According to Nature of Load

    1. Peak Load Plants:

    The peak load plants are used to supply power during the peak demand period. Since such plants are kept running on only peak load, load factor of such plants are therefore low. Run-off river plants with pondage and pumped storage plants can be employed as peak load plants. They store water during off-peak periods and operated during peak load periods. Their efficiency varies between 60–70%.


    2. Base load plants:

    A base load power plant is one that provides a steady flow of power regardless of total power demand by the grid. These plants run at all times through the year except in the case of repairs or scheduled maintenance. Such plants are usually of large capacity. Since such plants are kept running on constant load, load factor of such plants are therefore high. Plants having large storage can best be used as base power plants. For example, Run-off river plants without pondage and reservoir plants.


    Advantages of Hydro Power plant

    1. It does not require fuel as water is used for the generation of electrical energy.

    2. It is quite neat and clean as no smoke or ash is produced.

    3. It requires very small running charges (Rs ≈2/kWh) because water is the source of energy which is available free of cost.

    4. It is comparatively simple in construction and requires less maintenance.

    5. It does not require a long starting time like a steam power station. In fact, such plants can be put into service instantly.

    6. It is robust and has a longer life.

    7. In addition to power generation, it can be used for irrigation purposes.

    8. Although such plants require the attention of highly skilled persons at the time of construction, yet for operation, a few experienced persons may do the job well.


    Disadvantages of Hydro Power plant

    1. It involves high capital cost because dams are extremely expensive to build and must be built to a very high standard.

    2. There is uncertainty about the availability of sufficient amount of water due to dependence on weather conditions.

    3. Construction of hydroelectric power plant can lead to imbalances in ecosystems and landscape change, and over time can be reduced river flow.

    4. Skilled and experienced hands are required to build the plant.

    5. Sediment is retained behind the dam so they are subject to erosion.

    6. Large hydro plant may involve great risk of flood due to rupture (bursting) of the dam.

    7. It requires high cost of transmission lines as the plant is located in hilly areas which are quite away from the consumers.


    Site Selection for Hydroelectric Power Plant:

    The following points should be taken into account while selecting the site for a hydro-electric power station:

    1. Availability of water: Since the primary requirement of a hydro-electric power station is the availability of huge quantity of water, such plants should be built at a place (e.g., river, canal) where adequate water is available at a good head.

    2. Quality of water: Polluted water may cause excessive corrosion and damage to metallic structure of the plant. This may make unreliable and uneconomical operation of the plant. So it is necessary to have the water is of good quality.

    3. Storage of water: There are wide variations in water supply from a river or canal during the year. This makes it necessary to store water by constructing a dam in order to ensure the generation of power throughout the year. The storage helps in equalising the flow of water so that any excess quantity of water at a certain period of the year can be made available during times of very low flow in the river. This leads to the conclusion that site selected for a hydro-electric plant should provide adequate facilities for erecting a dam and storage of water.

    4. Cost and type of land: The land for the construction of the plant should be available at a reasonable price. Further, the bearing capacity of the ground should be adequate to with- stand the weight of heavy equipment to be installed.

    5. Transportation facilities: The site selected for a hydro-electric plant should be accessible by rail and road so that necessary equipment and machinery could be easily transported.

    It is clear from the above mentioned factors that ideal choice of site for such a plant is near a river in hilly areas where dam can be conveniently built and large reservoirs can be obtained.


    Differences between Impulse and Reaction Turbines


    Impulse Turbine

    Reaction Turbine


    In impulse turbine all hydraulic energy is converted into kinetic energy by a nozzle and it is the jet so produced which strikes the moving blades.

    In reaction turbine only some amount of the available energy is converted into kinetic energy before the fluid enters the moving blades.


    The velocity of fluid decreases, while the pressure remains constant as fluid passes through moving blades.

    Both pressure and velocity decreases as fluid passes through moving blades. Pressure at inlet is much higher than at outlet.


    Water-tight casing is not necessary. Casing has no hydraulic function to perform. It only serves to prevent splashing and guide water to the tail race.

    The runner must be enclosed within a watertight casing.


    Water is admitted only in the form of jets. There may be one or more jets striking equal number of buckets simultaneously.

    Water is admitted over the entire circumference of the runner.


    The turbine doesn’t run full and air has a free access to the bucket.

    Water completely fills at the passages between the blades and while flowing between inlet and outlet sections does work on the blades.


    The turbine is always installed above the tail race and there is no draft tube used.

    Reaction turbine is generally connected to the tail race through a draft tube which is a gradually expanding passage. It may be installed below or above the tail race.


    Flow regulation is done by means of a needle valve fitted into the nozzle.

    The flow regulation in reaction turbine is carried out by means of a guide-vane assembly. Other component parts are scroll casing, stay ring runner and the draft tub.


    Example of impulse turbine is Pelton wheel.

    Examples of reaction turbine are Francis turbine, Kaplan and Propeller Turbine, Deriaz Turbine, Tubular Turbine, etc.


    Impulse turbine has more hydraulic efficiency.

    Reaction turbine has relatively less efficiency.


    Impulse turbine operates at high water heads.

    Reaction turbine operates at low and medium heads.


    Water flow is tangential direction to the turbine wheel.

    Water flows in radial and axial direction to turbine wheel.


    Needs low discharge of water.

    Needs medium and high discharge of water.


    Degree of reaction is zero.

    Degree of reaction is more than zero and less than or equal to one.


    Impulse turbine involves less maintenance work.

    Reaction turbine involves more maintenance work.