Tuesday, 11 October 2011

Difference Between Impulse and Reaction Turbine

In Hydro-Electric Power Plant mainly Impulse or Reaction Turbines are used to generate electric power. From the engineering point of view both have some differences in working or parts. Here in this article I am trying to share the knowledge with you.

Main Differences of Impulse and Reaction Turbine:

1) In impulse turbine the water flows through the nozzles and impinges on the buckets where as in reaction turbine the water is guided by the guide blades to flow over the moving vanes.

2) In impulse turbine the entire water energy is first converted in kinetic energy but there is no complete energy conversion in nozzle takes place in case of reaction turbine.

3) ) In impulse turbine the water impinges on the buckets with kinetic energy where as in reaction turbine the water glides over the moving vanes with partial kinetic and partial pressure energy.

4) In impulse turbine the work is done only by the change in the kinetic energy of the jet but in reaction turbine the work is done partly by the change in the velocity head, but almost entirely by the change in pressure head.

5) In impulse turbine the pressure of flowing water remains unchanged and is equal to the atmospheric pressure but in reaction turbine the pressure of flowing water is reduced after gliding over the vanes.

6) In impulse turbine it is not essential that the wheel should run full. Moreover, there should be free access of air between the vanes and the wheel where as it is essential that the wheel should always run full and kept full of water in reaction turbine.

7) In impulse turbine the water may be admitted over a part of the circumference or over the whole circumference of the wheel but in reaction turbine the water must be admitted over the whole circumference of the wheel.

8) It is possible to regulate the flow of water without loss in impulse turbine but in reaction turbine it is not possible to regulate the flow without loss.

If you have more education on difference please put it by comments.
Some other useful articles…

Types: Different Types of Impulse Turbines
Turbine: Pelton Wheel the Common form of Impulse Turbine
Impulse Turbine: The Equipment used to Generate Electricity at Power Plant

Why Surge Tank is Required in HydroElectric Power Plant?

In HydroElectric Power Plant Water is used to generate Power by using some Hydraulic Machines.For the smooth operation of Power Plant some necessary arrangements are required,Surge Tank is one of them. The requirement of Surge Tank is due to some problems which accrued in Water Flow.First I will cover the possible problems in Flowing Water,these are,


Water Hammer:-
 
Due to Motion,Water possess some Momentum.This Momentum is destroyed,if the Flowing water is suddenly brought to rest,by closing the Valve. A very High Pressure is developed on Valve.This High Pressure is followed by a series of Pressure Vibrations,resulting noise in pipe.This noise is known as "Knocking".
The sudden rise in Pressure has the effect of Hammering Action on the walls of pipe, known as Water Hammer.This Water Hammer can burst the pipe In HydroElectric Power Plant,the requirement of Water goes on changing,that's why it is essential to increase or decrease the Discharge flowing through the pipe line.
Whenever the requirement of Water suddenly decreased,the Valve must suddenly closed,resulting a very High Pressure developed in entire pipe line between the Reservoir and the Turbine.This is happened due to Water Hammer.
To overcome this problem,a Storage Reservoir called as "Surge Tank" is fitted at some opening made on the pipe line(penstock) in order to store Water when the Valve is suddenly closed, or to discharge Water when increased discharged is required.Such a Storage Reservoir is known as "Surge Tank".
Functions of Surge Tank.
01)To control the Pressure Variations,due to rapid changes in the pipeline flow,thus eliminating Water Hammer possibilities.
02)To regulate the flow of Water to the Turbine by providing necessary retarding Head of Water.

The Surge Tanks are placed near to the Turbine.The Height of Surge Tank is generally kept above the maximum Water Level in the supply Level Reservoir.

There are three important types of Surge Tanks used in HydroElectric Power Plant.

01)Simple Surge Tank
02)Restricted Orifice type Surge Tank
03)Differential Surge Tank.

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Monday, 10 October 2011

Hydro Electric Plants - Classification, Advantages and Disadvantages

Hydro Electric Plants - Classification, Advantages and Disadvantages

Classification of Hydro Electric Plants
The classification of hydro electric plants based upon :
(a) Quantity of water available (b) Available head (c) Nature of load

The classification acording to Quantity of water available is
(i) Run-off river plants with out pondage : These plants does not store water; the plant uses water as it comes.The plant can use water as and when available.Since these plants depend for their generting capacity primarly on the rate of flow of water, during rainy season high flow rate may mean some quantity of water to go as waste while during low run-off periods, due to low flow rates,the generating capacity will be low.
(ii) Run-off river plants with pondage : In these plants pondage permits storage of water during off peak periods and use of this water during peak periods.Depending on the size of pondage provided it may be possible to cope with hour to hour fluctuations.This type of plant can be used on parts of the load curve as required,and is more useful than a plant with out storage or pondage.
When providing pondage tail race conditions should be such that floods do not raise tail-race water level,thus reducing the head on the plant and impairing its effectiveness.This type of plant is comparitively more reliable and its generating capacity is less dependent on avilable rate of flow of water.
(iii) Reservoir Plants :A reservoir plant is that which has a reservoir of such size as to permit carrying over storage from wet season to the next dry season.Water is stored behind the dam and is available to the plant with control as required.Such a plant has better capacity and can be used efficiently through out the year.Its firm capacity can be increased and can be used either as a base load plant or as a peak load plant as required.It can also be used on any portion of the load curve as required.Majority of the hydroelectric plants are of this type.

The classification according to availability of water head is
(i) Low-Head (less than 30 meters) Hydro electric plants :"Low head" hydro-electric plants are power plants which generally utilize heads of only a few meters or less. Power plants of this type may utilize a low dam or weir 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 varies with seasonal flows of water in a 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. Hydro-electric facilities with a capacity of less than about 25 MW (1 MW = 1,000,000 Watts) are generally referred to as "small hydro", although hydro-electric technology is basically the same regardless of generating capacity.

(ii) Medum-head(30 meters - 300 meters) hydro electric plants :These plants consist of a large dam in a mountainous area which creates a huge reservoir. The Grand Coulee Dam on the Columbia River in Washington (108 meters high, 1270 meters wide, 9450 MW) and the Hoover Dam on the Colorado River in Arizona/Nevada (220 meters high, 380 meters wide, 2000 MW) are good examples. These dams are true engineering marvels. In fact, the American Society of Civil Engineers as designated Hoover Dam as one of the seven civil engineering wonders of the modern world, but the massive lakes created by these dams are a graphic example of our ability to manipulate the environment - for better or worse. Dams are also used for flood control, irrigation, recreation, and often are the main source of potable water for many communities. Hydroelectric development is also possible in areas such as Niagra Falls where natural elevation changes can be used.

(iii) High-head hydro electric plants :"High head" power plants are the most common and generally utilize a dam to store water at an increased elevation. The use of a dam to impound water also provides the capability of storing water during rainy periods and releasing it during dry periods. This results in the consistent and reliable production of electricity, able to meet demand. Heads for this type of power plant may be greater than 1000 m. Most large hydro-electric facilities are of the high head variety. High head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet the electrical demand on a distribution system.
The classification according to nature of load is
(i) 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.

Power plants are designated base load based on their low cost generation, efficiency and safety at set outputs. Baseload power plants do not change production to match power consumption demands since it is always cheaper to run them rather than running high cost combined cycle plants or combustion turbines. Typically these plants are large enough to provide a majority of the power used by a grid, making them slow to fire up and cool down. Thus, they are more effective when used continuously to cover the power baseload required by the grid.

Each base load power plant on a grid is allotted a specific amount of the baseload power demand to handle. The base load power is determined by the load duration curve of the system. For a typical power system, rule of thumb states that the base load power is usually 35-40% of the maximum load during the year.Load factor of such plants is high.
Fluctuations, peaks or spikes in customer power demand are handled by smaller and more responsive types of power plants.

(ii) Peak load plants :Power plants for electricity generation which, due to their operational and economic properties, are used to cover the peak load. Gas turbines and storage and pumped storage power plants are used as peak load power plants.The efficiency of such plants is around 60 -70%.
Advantages of hydroelectric plants
  • operation , running and maintenance costs are low.
  • Once the dam is built, the energy is virtually free.
  • No fuel is burnt and the plant is quite neat & clean.
  • No waste or pollution produced.
  • generating plants have a long lifetime.
  • Much more reliable than wind, solar or wave power.
  • Water can be stored above the dam ready to cope with peaks in demand.
  • unscheduled breakdowns are relatively infrequent and short in duration since the equipment is relatively simple.
  • hydroelectric turbine-generators can be started and put "on-line" very rapidly.
  • Electricity can be generated constantly
Disadvantages of hydroelectric plants
  • very land-use oriented and may flood large regions.
  • The dams are very expensive to build.However, many dams are also used for flood control or irrigation, so building costs can be shared.
  • Capital cost of generators, civil engineering works and cost of transmission lines is very high.
  • Water quality and quantity downstream can be affected, which can have an impact on plant life.
  • Finding a suitable site can be difficult - the impact on residents and the environment may be unacceptable.
  • fish migration is restricted.
  • fish health affected by water temperature change and insertion of excess nitrogen into water at spillways
  • available water and its temperature may be affected
  • reservoirs alter silt-flow patterns

Pumped Storage Plants

Pumped Storage Plants
"Pumped Storage" is another form of hydro-electric power. Pumped storage facilities use excess electrical system capacity, generally available at night, to pump water from one reservoir to another reservoir at a higher elevation. During periods of peak electrical demand, water from the higher reservoir is released through turbines to the lower reservoir, and electricity is produced . Although pumped storage sites are not net producers of electricity - it actually takes more electricity to pump the water up than is recovered when it is released - they are a valuable addition to electricity supply systems. Their value is in their ability to store electricity for use at a later time when peak demands are occurring. Storage is even more valuable if intermittent sources of electricity such as solar or wind are hooked into a system.


Pumped storage plant is a unique design of peak load plant in which the plant pumps back all or portion of its water supply during lo load period.The usual construction is a lowand high elevation reservoirs connected through a penstock.The generating pumping plant is at the lower end.The plant utilises some of the surplus energy generated by the base load plant to pump water from low elevation to highelevation reservoir during off peak hours.During peak load period this water is used to generate power by allowing it to flow from high elevation reservoir through reversible hydraulic turbine of this plan to low elevation reservoir.Thus same water is used again and again and extra water is required only to take care of evaporation and seepage.

The main important point in this plant is reversible turbine/generator assemblies act as pump and turbine (usually a Francis turbine design).During low load periods it acts as pump and pumps water from low to high elevation reservoir.During peak load periods it acts as turbine when water flows from high to low elevation reservoir.
To see the flash animation of pumped storage plant working Click here
Advantages
  • Without some means of storing energy for quick release, we'd be in trouble.
  • Little effect on the landscape.
  • No pollution or waste
Disadvantages
  • Expensive to build.
  • Once it's used, you can't use it again until you've pumped the water back up. Good planning can get around this problem.
For more details on this topic click here Link1 Link2

How HydroPower Plant works

How HydroPower Plant works


A hydroelectric power plant harnesses the energy found in moving or still water and converts it into electricity.
Moving water, such as a river or a waterfall, has mechanical energy. ‘Mechanical energy is the energy that is possessed by an object due to its motion or stored energy of position.’ This means that an object has mechanical energy if it’s in motion or has the potential to do work (the movement of matter from one location to another,) based on its position. The energy of motion is called kinetic energy and the stored energy of position is called potential energy. Water has both the ability and the potential to do work. Therefore, water contains mechanical energy (the ability to do work), kinetic energy (in moving water, the energy based on movement), and potential energy (the potential to do work.)
The potential and kinetic/mechanical energy in water is harnessed by creating a system to efficiently process the water and create electricity from it. A hydroelectric power plant has eleven main components. The first component is a dam.

The dam is usually built on a large river that has a drop in elevation, so as to use the forces of gravity to aid in the process of creating electricity. A dam is built to trap water, usually in a valley where there is an existing lake. An artificial storage reservoir is formed by constructing a dam across a river.Notice that the dam is much thicker at the bottom than at the top, because the pressure of the water increases with depth.

The area behind the dam where water is stored is called the reservoir. The water there is called gravitational potential energy. The water is in a stored position above the rest of the dam facility so as to allow gravity to carry the water down to the turbines. Because this higher altitude is different than where the water would naturally be, the water is considered to be at an altered equilibrium. This results in gravitational potential energy, or, “the stored energy of position possessed by an object.” The water has the potential to do work because of the position it is in (above the turbines, in this case.)

Gravity will force the water to fall to a lower position through the intake and the control gate. They are built on the inside of the dam. When the gate is opened, the water from the reservoir goes through the intake and becomes translational kinetic energy as it falls through the next main part of the system: the penstock. Translational kinetic energy is the energy due to motion from one location to another. The water is falling (moving) from the reservoir towards the turbines through the penstock.

The intake shown in figure includes the head works which are the structures at the intake of conduits,tunnels or flumes.These structures include blooms,screens or trash - racks, sluices to divert and prevent entry of debris and ice in to the turbines.Booms prevent the ice and floating logs from going in to the intake by diverting them to a bypass chute.Screens or trash-racks(shown in fig) are fitted irectly at the intake to prevent the debris from going in to the take.Debris cleaning devices should also be fitted on the trash-racks.Intake structures can be classified in to high pressure intakes used in case of large storage reservoirs and low pressure intakes used in case of small ponds.The use of providing these structures at the intake is,water only enters and flows through the penstock which strikes the turbine.
Control gates arrangement is provided with Spillways.Spillway is constructed to act as a safety valve.It dischargs the overflow water to the down stream side when the reservoir is full.These are generally constructed of concrete and provided with water discharge opening,shut off by metal control gates.By changing the degree to which the gates are opened,the discharge of the head water to the tail race can be regulated inorder to maintain water level in reservoir.
The penstock is a long shaft that carries the water towards the turbines where the kinetic energy becomes mechanical energy. The force of the water is used to turn the turbines that turn the generator shaft. The turning of this shaft is known as rotational kinetic energy because the energy of the moving water is used to rotate the generator shaft. The work that is done by the water to turn the turbines is mechanical energy. This energy powers the generators, which are very important parts of the hydroelectric power plant; they convert the energy of water into electricity. Most plants contain several generators to maximize electricity production.
The generators are comprised of four basic components: the shaft, the excitor, the rotor, and the stator. The turning of the turbines powers the excitor to send an electrical current to the rotor. The rotor is a series of large electromagnets that spins inside a tightly wound coil of copper wire, called the stator. “A voltage is induced in the moving conductors by an effect called electromagnetic induction.” The electromagnetic induction caused by the spinning electromagnets inside the wires causes electrons to move, creating electricity. The kinetic/mechanical energy in the spinning turbines turns into electrical energy as the generators function.
The transformer, another component, takes the alternating current and converts it into higher-voltage current. The electrical current generated in the generators is sent to a wire coil in the transformer. This is electrical energy. Another coil is located very close to first one and the fluctuating magnetic field in the first coil will cut through the air to the second coil without the current. The amount of turns in the second coil is proportional to the amount of voltage that is created. If there are twice as many turns on the second coil as there are on the first one, the voltage produced will be twice as much as that on the first coil. This transference of electrical current is electrical energy. It goes from the generators to one coil, and then is transferred through an electromagnetic field onto the second coil. That current is then sent by means of power lines to the public as electricity
Now, the water that turned the turbines flows through the pipelines (translational kinetic energy, because the energy in the water is being moved,) called tailraces and enters the river through the outflow. The water is back to being kinetic/mechanical/potential energy as it is in the river and has to potential to have the energy harnessed for use as it flows along (movement.)
To see the flash animation of hydro power plant working Click here
This Link describes eah part of hydro plant. To know more about hydro electric plant see these links Link1 Link2

Francis Turbine

Francis Turbine
The Francis turbine is a type of water turbine that was developed by James B. Francis. It is an inward flow reaction turbine that combines radial and axial flow concepts.
Francis turbines are the most common water turbine in use today. They operate in a head range of ten meters to several hundred meters and are primarily used for electrical power production.

The Francis turbine is a reaction turbine, which means that the working fluid changes pressure as it moves through the turbine, giving up its energy. A casement is needed to contain the water flow. The turbine is located between the high pressure water source and the low pressure water exit, usually at the base of a dam.

The inlet is spiral shaped. Guide vanes direct the water tangentially to the runner. This radial flow acts on the runner vanes, causing the runner to spin. The guide vanes (or wicket gate) may be adjustable to allow efficient turbine operation for a range of water flow conditions.
As the water moves through the runner its spinning radius decreases, further acting on the runner. Imagine swinging a ball on a string around in a circle. If the string is pulled short, the ball spins faster. This property, in addition to the water's pressure, helps inward flow turbines harness water energy.At the exit, water acts on cup shaped runner features, leaving with no swirl and very little kinetic or potential energy. The turbine's exit tube is shaped to help decelerate the water flow and recover the pressure.
Application
Large Francis turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%.Francis type units cover a wide head range, from 20 meters to 700 meters and their output varies from a few kilowatt to 1000 megawatt. Their size varies from a few hundred millimeters to about 10 meters.
In addition to electrical production, they may also be used for pumped storage; where a reservoir is filled by the turbine (acting as a pump) during low power demand, and then reversed and used to generate power during peak demand.
Francis turbines may be designed for a wide range of heads and flows. This, along with their high efficiency, has made them the most widely used turbine in the world.

For more details on this topic Click here

Kaplan Turbine

Kaplan Turbine
The Kaplan turbine is a propeller-type water turbine that has adjustable blades. It was developed in 1913 by the Austrian professor Viktor Kaplan.

The Kaplan turbine was an evolution of the Francis turbine. Its invention allowed efficient power production in low head applications that was not possible with Francis turbines.
Kaplan turbines are now widely used throughout the world in high-flow, low-head power production.


The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. The design combines radial and axial features.



The above figures shows flow in a Kaplan turbine. In the picture, pressure on runner blades and hub surface is shown using colormapping (red = high, blue = low).
The diameter of the runner of such machines is typically 5 to 8 meters.

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 on to 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.

The turbine does not need to be at the lowest point of water flow, as long as the draft tube remains full of water. A higher turbine location, however, increases the suction that is imparted on the turbine blades by the draft tube. The resulting pressure drop may lead to cavitation.

Variable geometry of the wicket gate and turbine blades allow efficient operation for a range of flow conditions. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications.

Applications
Kaplan turbines are widely used throughout the world for electrical power production. They cover the lowest head hydro sites and are especially suited for high flow conditions.

Inexpensive micro turbines are manufactured for individual power production with as little as two feet of head.

Large Kaplan turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%. They are very expensive to design, manufacture and install, but operate for decades.

Variations
The Kaplan turbine is the most widely used of the propeller-type turbines, but several other variations exist:

Propeller turbines have non-adjustable propeller vanes. They are used in low cost, small installations. Commercial products exist for producing several hundred

watts from only a few feet of head.
Bulb or Tubular turbines are designed into the water delivery tube. A large bulb is centered in the water pipe which holds the generator, wicket gate and runner. Tubular turbines are a fully axial design, whereas Kaplan turbines have a radial wicket gate. Pit turbines are bulb turbines with a gear box. This allows for a smaller generator and bulb.
Straflo turbines are axial turbines with the generator outside of the water channel, connected to the periphery of the runner.
S- turbines eliminate the need for a bulb housing by placing the generator outside of the water channel. This is accomplished with a jog in the water channel and a shaft connecting the runner and generator.
Tyson turbines are a fixed propeller turbine designed to be immersed in a fast flowing river, either permanently anchored in the river bed, or attached to a boat or barge.