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8/10/2019 Pwr Plant Eng.. http://slidepdf.com/reader/full/pwr-plant-eng 1/78 Power Plant Engineering Introduction Power plant engineering deals with the study of energy, its sources and utilization of energy for power generation. The power is generated by prime movers (example Hydraulic turbines, steam turbines, diesel engines). Large amount of power is generated using prime movers in a site or layout called power plants, where all the equipments and machineries required for power generation is located. Energy: Energy may be defined as the capacity to do work. Energy exists in various forms, such as Mechanical Energy, thermal energy, electrical energy, solar energy etc. Electricity is the only form of energy, which is easy to produce, easy to transport, easy to use and easy to control. Electricity consumption per capita is the index of the living standard people of a place or country i.e. the utilization of energy is an indication of the growth of the nation.

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    Power Plant EngineeringIntroduction

    Power plant engineering deals with the study of energy, itssources and utilization of energy for power generation. The

    power is generated by prime movers (example Hydraulicturbines, steam turbines, diesel engines). Large amount of

    power is generated using prime movers in a site or layout calledpower plants, where all the equipments and machineries

    required for power generation is located.

    Energy: Energy may be defined as the capacity to do work.Energy exists in various forms, such as Mechanical Energy,

    thermal energy, electrical energy, solar energy etc. Electricity is

    the only form of energy, which is easy to produce, easy totransport, easy to use and easy to control. Electricityconsumption per capita is the index of the living standard peopleof a place or country i.e. the utilization of energy is an indication

    of the growth of the nation.

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    Power and Power Plant:

    Power is primarily associated with mechanical work andelectrical energy. Therefore, power can be defined as the rate

    of flow of energy and can state that a power plant is a unitbuilt for production and delivery of a flow of mechanical or

    electrical energy. In a common usage, a machine orassemblage of equipment that produces and delivers a flow ofmechanical or electrical energy is a power plant. Hence an

    internal combustion engine is a power plant; a water wheel isa power plant, etc. However, what we generally mean by the

    term power plant is that assemblage of equipment,permanently located on some chosen site which receives rawenergy in the form of a substance capable of being operatedon in such a way as to produce electrical energy for deliver

    from the power plant.

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    PRESENT POWER POSITION IN INDIA The present power position in India is alarming as there are major power

    shortages in almost all states of the country leading to crippling of

    industries and hundreds of thousands of people losing jobs and a heavyloss of production.

    The overall power scene in the country shows heavy shortages almost inall states. The situation is going to be aggravated in coming years as thedemand is increasing and the power industry is not keeping pace with the

    increasing demand. Many of the states in India depend to a large extent on hydro generation.

    The increase in demand has far outstripped the installation of new plants.Also there is no central grid to distribute excess energy from one region toanother. The experience in the operation of thermal plants is inadequate.

    All these have led to heavy shortages and severe hardship to people. Very careful analysis of the problem and proper planning and execution is

    necessary to solve the power crisis in our country. Suitable hydrothermal mix, proper phasing of construction of new plants,

    training personnel in maintenance of thermal plants.

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    Energy Resources

    Conventional sources of energy Non conventional sources ofenergy

    (Or) orNon renewable sources of energy Renewable source of


    Examples: - Examples:-Fuels like coal, oil, Sun, wind, waves, tides, energy

    Natural gas, nuclear fuels etc. From earth core, hydro electricPower etc.

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    Gas Turbine Power plant

    A gas turbine power plant may be defined as one in whichthe principal prime mover is of the turbine type and theworking medium is a permanent gas .

    A simple gas turbine plant consists of the following:

    compressor Intercooler

    Regenerator Combustion chamber Gas turbine Reheating unit

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    In gas turbine plant, the axial and centrifugal flowcompressors are used. In most of the gas turbine powerplant, two compressors are used. One is low pressurecompressor and the other is high pressure compressor.In low pressure compressor, the atmospheric air isdrawn into the compressor through the filter. Themajor part of the power developed by the turbine(about 66%) is used to run the compressor. This lowpressure air goes to the high pressure compressorthrough the intercooler. Then the high pressure air goesinto the regenerator.

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    The intercooler is used to reduce the work of thecompressor and it is placed in between the highpressure and low pressure compressor. Intercoolers are

    generally used when the pressure ratio is very high. Theenergy required to compress the air is proportional tothe air temperature at inlet. The cooling of compressedair in intercooler is generally done by water.

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    Regenerators are used to preheat the air which isentering into the combustion chamber to reduce thefuel consumption and to increase the efficiency. This is

    done by the heat of the hot exhaust gases coming outof the turbine.

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    Combustion chambers

    Hot air from regenerator flows to the combustionchambers and the fuel like coal, natural gas or keroseneare injected into the combustion chamber. After the

    fuel injection, the combustion takes place. This highpressure, high temperature products of combustion arepassed through the turbine.

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    Gas turbine

    Two types of gas turbines are used in gas turbineplant.

    High pressure turbine Low pressure turbine

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    High Pressure turbine

    In the beginning, the starting motor runs thecompressor shaft. The burnt gases (product ofcombustion) expand through the high pressure turbine.It is important to note that when the turbine shaftrotates it infact drives the compressor shaft which iscouples to it. Now, the high pressure turbine runs thecompressor and the starting motor is stopped. About

    66% of the power developed by the turbine is used torun the compressor and only 34% of the powerdeveloped is used to generate electric power.

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    Low pressure turbine The purpose of the low pressure turbine is to produce electric

    power. The shaft of the LPT is coupled with the generator. Theburnt fuel (gases) after leaving the HPT is again sent to acombustion chamber where it further undergoes combustion. Even if there is any left out unburnt fuel from the previousturbine it gets fully burnt in the combustion chamber. The burntgases run the low pressure turbine (LPT). The shaft of theturbine is directly coupled with the generator for producingelectricity. The exhaust hot gases after leaving the LPT passes through theregenerator before exhausted through the chimney into theatmosphere. The heat from the hot gases is used to preheat theair leaving the HPC before it enters the combustion chamber.This preheating of the air improves the efficiency of thecombustion chamber.

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    Advantages For a gas turbine plant, Natural gas is a very suitable fuel. It would be ideal to

    install gas turbine plants near the site where natural gas is readily available. Gas turbine plants can work economically for short running hours. Storage of fuel requires less area and handling is easy. Gas turbine plant is small and compact in size as compared to steam powerplants. It can be started quickly and can be put on load in a very short time. The cost of maintenance is less. It is simple in construction. There is no need for boiler, condenser and otheraccessories as in the case of steam power plants. The gas turbine can operate at high speed since there are no reciprocatingparts Cheaper fuel such as kerosene, paraffin, benzene and powdered coal(cheaper than petrol and diesel) can be used. Gas turbine plants can be used in water scarcity areas Less pollution and less water is required.

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    Disadvantages of gas turbine power

    plant 66% of the power developed is used to drive the compressor;the gas turbine unit has a low thermal efficiency The running speed of the gas turbine is in the range of (40,000 to 1, 00,000 rpm) and the operating temperature is as

    high as 20000

    C, for this reason special metals and alloys haveto be used for the various parts of the turbine. Special cooling methods are required for cooling the turbineblades. It is difficult to start a gas turbine as compared to a diesel

    engine in a diesel power plant The life of a gas turbine plant is upto 10 years, after which itsefficiency decreases to less than 10 percent.

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    Applications of gas turbine powerplant

    To drive generators and supply loads in steam,diesel or hydro plants To work as combination plants with conventionalsteam boilers Thermal process industries Petro chemical industries Power generation (used for peak load and as stand

    by unit) Aircraft and ships for their propulsion. They are notsuitable for automobiles because of their very highspeeds.

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    Classification of Gas Turbine

    Gas turbines may be broadly classified as: Open cycle gas turbine

    Closed cycle gas turbine

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    Open cycle gas turbine In the open cycle gas turbine, air is drawn into the compressor from the

    atmosphere. The compressed air is heated by directly burning the fuel in the air atconstant pressure inside the combustion chamber. The high pressure hot gasesfrom the combustion chamber drive the turbine and the power is developed whenthe turbine shaft rotates.

    Gas turbines are not self starting. A starting motor drives the compressor till fuel isinjected inside the combustion chamber, once the turbine starts gaining speed the

    starting motor is disengaged.

    Part of the power developed by the gas turbine (about 60%) is used to drive thecompressor and the remaining is used to drive a generator or other machinery.

    In the open cycle, system, the working fluid i.e. air and the fuel must be replacedcontinuously as they are exhausted into the atmosphere. Thus the entire flowcomes from the atmosphere and is returned to the atmosphere, hence it is calledopen cycle.

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    Closed cycle gas turbine

    In this the compressed air from the atmosphere is heated in air heater(heat exchanger). Heat is added to the air heater from some externalsource (oil or coal) at constant pressure. High pressure working fluidexpands through the turbine and power is developed. The exhaustworking fluid is cooled in a pre cooler before the same fluid is sent into thecompressor again.

    In a closed cycle gas turbine the same working fluid is continuouslycirculated. The fuel required for adding heat from an external source canbe any fuel ranging from kerosene, to heavy oil and even peat and coalslurry without reducing the efficiency.

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    Diesel power plant


    This is a fossil fuel plant since diesel is a fossil fuel. Diesel enginepower plants are installed where supply of coal and water is notavailable in sufficient quantity. These plants produce the power in the range of 2 to 50 MW. They are used as standby sets for continuity of supply such ashospitals, telephone exchanges, radio stations, cinema theatres andindustries. They are suitable for mobile power generation and widely used in

    railways and ships. They are reliable compared to other plants. Diesel power plants are becoming more popular because ofdifficulties experienced in construction of new hydel plants andthermal plants

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    Diesel engine

    This is the main component of a diesel power plant.The engines are classified as two stroke engine andfour stroke engines. Engines are generally directly

    coupled to the generator for developing power. Indiesel engines, air admitted into the cylinder iscompressed. At the end of compression stroke, fuelis injected. The fuel is burned and the burning gasesexpand and do work on the piston. The shaft of theengine is directly coupled to the generator. Afterthe combustion, the burned gases are exhausted tothe atmosphere.

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    Air filter and supercharger

    The air filter is used to remove the dust fromthe air which is taken by the engine. Air filtersmay be of dry type, which is made up of felt,wool or cloth. In oil bath type of filters, the airis swept over a bath of oil so that dustparticles get coated. The function of the

    supercharger is to increase the pressure of theair supplied to the engine and thereby thepower of the engine is increased.

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    Engine starting system

    Diesel engine used in diesel power plants isnot self starting. Engine starting systemincludes air compressor and starting air tank.This is used to start the engine in coldconditions by supplying the air.

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    Fuel system It includes the storage tank, fuel pump, fuel transfer pump, strainers and

    heaters. Pump draws diesel from the storage tank and supplies it to the small daytank through the filter. Day tank supplies the daily fuel need for the engine. The daytank is usually placed high so that diesel flows to engine under gravity.

    Diesel is again filtered before being injected into the engine by the fuel injection pump.

    The fuel injection system performs the following functions.

    Filter the fuel Meter the correct quantity of the fuel to be injected Time the injection process Regulate the fuel supply Secure fine atomization of fuel oil Distribute the atomized fuel properly in the combustion; chamber. The fuel is supplied to the engine according to the load on the plant.

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    Lubrication system It includes oil pumps, oil tanks, coolers and pipes. It is used to reduce the

    friction of moving parts and reduce wear and tear of the engine parts suchas cylinder walls and piston. Lubrication oil which gets heated due to thefriction of the moving parts is cooled before recirculation.

    In the lubrication system the oil is pumped from the lubricating oil tankthrough the oil cooler where the oil is cooled by the cold water enteringthe engine. The hot oil after cooling the moving parts return to thelubricating oil tank.

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    Cooling system

    The temperature of the burning fuel inside the enginecylinder is in the order of 1500 0 C to 2000 0 C. In order to lowerthis temperature, water is circulated around the engine. Thewater envelopes (water jacket) the engine, the heat from the

    cylinder, piston, combustion chamber etc, is carried by thecirculating water. The hot water leaving the jacket is passedthrough the heat exchanger. The heat from the heatexchanger is carried away by the raw water circulated through

    the heat exchanger and is cooled in the cooling tower.

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    Governing system

    It is used to regulate the speed of the engine.This is done by varying the fuel supplyaccording to the engine load.

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    The air and fuel mixture act as a working medium indiesel engine power plant. The atmosphere air entersinside the combustion chamber during the suctionstroke and the fuel is injected through the injectionpump. The air and fuel is mixed inside the engine andthe charge is ignited due to high compression inside theengine cylinder. The basic principle in diesel engine isthat, the thermal energy is converted into mechanicalenergy and this mechanical energy is converted intoelectrical energy to produce the power by usinggenerator or alternator.

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    Applications of Diesel Engines inPower Field

    (a) Peak load plant: Diesel plants can be used in combination with thermal orhydro-plants as peak load units. They can be easily started or stopped at ashort notice to meet the peak demand.

    (b) Mobile plant: Diesel plants mounted on trailers can be used for temporaryor emergency purposes such as for supplying power to large civil engineeringworks.

    (c) Standby unit: If the main unit fails or cannot cope up with the demand, a

    diesel plant can supply the necessary power. For example, if water available ina hydro-plant is not adequately available due to less rainfall, the diesel stationcan operate in parallel to generate the short fall in power.

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    (d) Emergency plant: During power interruption in a vital unit like a keyindustrial plant or a hospital, a diesel electric plant can be used togenerate the needed power.

    (e) Nursery station: In the absence of main grid, a diesel plant can be

    installed to supply power in a small town. In course of time, whenelectricity from the main grid becomes available in the town, the dieselunit can be shifted to some other area which needs power on a small

    scale. Such a diesel plant is called a "nursery station". (f) Starting stations: Diesel units can be used to run the auxiliaries (like FD

    and ID fans, BFP, etc.) for starting a large steam power plant. (g) Central stations :Diesel electric plants can be used as central station

    where the capacity required is small

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    Advantages of diesel power plant It is easy to design and install these electric stations. They are easily available in standard capacities. They can respond to load changes without much difficulty. There are less standby losses. They occupy less space. They can be started and stopped quickly. They require less cooling water. Capital cost is less. Less operating and supervising staff required. High efficiency of energy conversion from fuel to electricity. Efficiency at part loads is also higher. Less of civil engineering work is required. They can be located near the load centre. There is no ash handling problem. Easier lubrication system.

    d ll d l

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    Disadvantages in installing dieselunits for power generation.

    High operating cost. High maintenance and lubrication cost. Capacity is restricted. Cannot be of very big size. Noise problem. Cannot supply overload. Unhygienic emissions. The life of the diesel power plant is less (7 to 10 years) as

    compared to that of a steam power plant which has a life spanof 25 to 45 years. The efficiency of the diesel plant decreasesto less than 10% after its life period.

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    Hydroelectric power plant

    Working principleHydroelectric power plant (Hydel plant) utilizes the potentialenergy of water stored in a dam built across the river. Thepotential energy of the stored water is converted into kinetic

    energy by first passing it through the penstock pipe. Thekinetic energy of water is then converted into mechanicalenergy in a water turbine. The turbine is coupled to theelectric generator. The mechanical energy available at theshaft of the turbine is converted into electrical energy bymeans of the generator.

    Because gravity provides the force which makes the water fall,the energy stored in the water is called gravitational potentialenergy.

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    C f H d l i

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    Components of Hydro electricplant

    Water reservoir Dam Spillway Gate Pressure tunnel Surge tank Penstock Water turbine Draft tube Tail race level Power house

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    Water reservoir: In a reservoir the water collected from the catchmentarea during rainy season is stored behind a dam. Catchment area gets itswater from rains and streams. Continuous availability of water is a basicnecessity for a hydroelectric power plant. The level of water surface in the

    reservoir is called Head water level. The water head available for powergeneration depends on the reservoir height.

    Dam: the purpose of the dam is to store the water and to regulate the outgoing flow of water. The dam helps to store all the incoming water. It alsohelps to increase the head of the water. In order to generate a requiredquantity of power, it is necessary that a sufficient head is available.

    Spillway: Excess accumulation of water endangers the stability of damconstruction. Also in order to avoid the overflow of water out of the damespecially during rainy seasons spillways are provided. This prevents therise of water level in the dam. Spillways are passages which allow theexcess water to flow to a different storage area away from the dam.

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    Gate: A gate is used to regulate or control the flow of water from the dam. Pressure tunnel: It is a passage that carries water from the reservoir to

    the surge tank.

    Surge tank: A surge tank is a small reservoir or tank in which the waterlevel rises or falls due to sudden changes in pressure. There may suddenincrease of pressure in the penstock pipe due to sudden backflow ofwater, as load on the turbine is reduced. This sudden rise of pressure inthe penstock pipe is known as water hammer.

    A surge tank is introduced between the dam and the turbine andserves the following purposes: To reduce the distance between the free water surface in the dam and the

    turbine, thereby reducing the water hammer effect. Otherwise, penstockwill be damages by the water effect.

    To serve as a supply tank to the turbine when the water in the pipe isaccelerated during increased load conditions and as a storage tank whenthe water is decelerating during reduced load conditions.

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    Layout of Hydro electric Power plant

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    Penstock: Penstock pipe is used to bring water from the dam to the hydraulicturbine. Penstock pipes are made up of steel or reinforced concrete. The turbine isinstalled at a lower level from the dam. Penstock is provided with a gate valve atthe inlet to completely close the water supply. It has a control valve to control the

    water flow rate into the turbine. Water turbine or hydraulic turbine (Prime mover): The hydraulic turbine converts

    the energy of water into mechanical energy. The mechanical energy (rotation)available on the turbine shaft is coupled to the shaft of an electric generator andelectricity is produced. The water after performing the work on turbine blade isdischarged through the draft tube. The prime movers which are in common useare Pelton wheel, Kaplan turbine, Francis turbine.

    Draft tube: Draft tube is connected to the outlet of the turbine. It converts the

    kinetic energy available in the water into pressure energy in the diverging portion.

    Thus, it maintains a pressure of just above the above the atmospheric at the endof the draft tube to move the water into a tail race. Water from the tail race isreleased for irrigation purposes.

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    Tail race level: Tail race is a water path to lead the waterdischarged from the turbine to the river or canal. The waterheld in the tail race is called Tail race water level.

    Power House: The power house accommodates the water

    turbine, generator, transformer and control room. As thewater rushes through the turbine, it spins the turbine shaft,

    which is coupled to the electric generator. The generator has arotating electromagnet called a rotor and a stationary partcalled a stator. The rotor creates a magnetic field thatproduces an electric charge in the stator. The charge is

    transmitted as electricity. The step up transformer increasesthe voltage of the current coming from the stator. Theelectricity is distributed through power lines.

    Cl ifi ti f H d l t i

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    Classification of Hydro electricpower plant

    Hydro electric power plants are usually classified according to the available ofhead of water

    High head power plants Medium head power plants Low head power plants High head power plants: When the operating head of water exceeds 70 meters,

    the plant is known as High head power plant. Pelton wheel turbine is the primemover used.

    Medium head power plants: When the water ranges from 15 to 70 meters, then

    the power plant is known as Medium head power plant. It uses Francis Turbine. Low head power plants: When the head is less than 15 meters, the plant is named

    as Low head power plant. It uses Francis or Kaplan turbine as prime mover.

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    Advantages Water source is perennially available. No fuel is required to be burnt to generateelectricity. It is aptly termed as 'the white coal'. Water passes through turbines toproduce work and downstream its utility remains undiminished for irrigation of farmsand quenching the thirst of people in the vicinity. The running costs of hydropower installations are very low as compared to thermal ornuclear power stations. 1n thermal stations, besides the cost of fuel, one has to takeinto account the transportation cost of the fuel also. There is no problem with regards to the disposal of ash as in a thermal station. Theproblem of emission of polluting gases and particulates to the atmosphere also doesnot exist. Hydropower does not produce any greenhouse effect, cause the perniciousacid rain and emit obnoxious NO. The hydraulic turbine can be switched on and off in a very short time. In a thermal ornuclear power plant the steam turbine is put on turning gear for about two days

    during start-up and shut-down. . The hydraulic power plant is relatively simple in concept and self-contained inoperation. Its system reliability is much greater than that of other power plants. Modern hydropower equipment has a greater life expectancy and can easily last 50years or more. This can be compared with the effective life of about 30 years of athermal or nuclear station.

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    Disadvantages Hydro-power projects are capital-intensive with a low rate of return. Theannual interest of this capital cost is a large part of the annual cost of hydro-power installations. The gestation period of hydro projects is quite large. The gap between thefoundation and completion of a project may extend from ten to fifteen years. Power generation is dependent on the quantity of water available, whichmay vary from season to season and year to year. If the rainfall is in time andadequate, then only the satisfactory operation of the plant can be expected. Such plants are often far way from the load centre and require longtransmission lines to deliver power. Thus the cost of transmission lines andlosses in them are more. Large hydro-plants disturb the ecology of the area, by way of deforestation,destroying vegetation and uprooting people. Strong public opinion against.Erection of such plants is a deterrent factor. The emphasis is now more onsmall, mini and micro hydel stations.

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    Hydro electric power plant in India

    Srisailam Hydel power plant AP 770 MW Upper sileru Hydor electric project AP - 120 Kodayar hydro electric power plant TN 100 MW Iddiki hydel project Kerala 800 MW

    l l

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    Nuclear power plant


    As large amounts of coal and petroleum are being used to produceenergy, time may come when their reserves may not be able to meetthe energy requirements. Thus there is tendency to seek alternativesources of energy. The discovery that energy can be liberated by thenuclear fission of materials like uranium (U), Plutonium (Pu), hasopened up a new source of power of great importance. The heatproduced due to fission of U and Pu is used to heat water to generatesteam which is used for running turbogenerator. It has been found that one kilogram of U can produce as much energy

    as can be produced by burning 4500 tonnes of high grade coal. Thisshows that nuclear energy can be successfully employed for producinglow cost energy in abundance as required by the expanding andindustrializing population of future.

    Factors which go in favor of nuclear

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    Factors which go in favor of nuclearenergy

    Hydro electric power is of storage type and is largely dependent ofmonsoons. The systems getting power from such plants have to shed loadduring the period of low rainfall.

    Oil is mainly needed for transport, fertilizers and petrochemicals and thuscannot be used in large quantities for power generation.

    Coal is available only in some parts of the country and transportation ofcoals requires big investments.

    Nuclear power is partially independent of geographical factors, the onlyrequirement being there should be reasonably good supply of water. Fueltransportation networks and larger storage facilities are not needed andnuclear power plant is a clean source of power which does not pollute theair if radioactive hazards are effectively prevented.

    Large quantity of energy is released with consumption of only a smallamount of fuel.

    N l fi i

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    Nuclear fission

    The fuel inside the reactor is a metal called uranium.Uranium exists as an isotope in the form of U 235 , U234 andU238 . Out of these isotopes U 235 is more unstable. When aneutron is captured by a nucleus of an atom of U 235 , itsplits up roughly into two equal fragments and about 2.5neutrons are released and a large amount of energy(nearly 200 million electron volts MeV) is produced. Thisis called fission process. The neutrons so produced are

    very moving neutrons and can be made to fission othernuclei of U 235 thus enabling a chain reaction to take place.When a large number of fission occurs, enormousamount of heat is produced.

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    From the fig, 2.5 neutrons are released in fissionof each nucleus of U 235 , out of these oneneutron is used to sustain the chain reaction,0.9 neutrons is absorbed by U 238 and becomesPu 239 . The remaining 0.6 neutrons escapefrom the reactor. Moderators are provided toslow down the neutrons from the highvelocities but not to absorb them. The

    moderators which are commonly used areordinary water and Heavy water.

    M i f l

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    Main components of a nuclearpower plant

    The main components of a nuclear power plant are

    Nuclear fuel Nuclear reactor Steam generator Moderator Control rods Reflector Turbine Condenser Shielding

    N l F l F l f l h ld b fi i bl i l

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    Nuclear Fuel: Fuel of a nuclear reactor should be fissionable materialwhich can be defined as an element or isotope whose nuclei can becaused to undergo nuclear fission by nuclear bombardment and toproduce a fission chain reaction. It can be one or all of the following U 235 ,U233 and Pu 239

    Nuclear reactor: A nuclear reactor may be regarded as a substitute for theboiler furnace of a steam power plant. Heat is produced in the reactor dueto nuclear fission of the fuel. During the fission process, the large amountof heat is liberated. This large amount of heat is absorbed by the coolantand it is circulated through the core.

    The various types of reactors used in nuclear power plant is

    Boiling water reactor Pressurised water reactor Fast breeder reactor

    St t Th h t lib t d i th t i t k b th

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    Steam generator: The heat liberated in the reactor is taken up by thecoolant circulating through the core. The purpose of the coolant is totransfer the heat generated in the reactor core and use it for steamgeneration. Ordinary water or heavy water is a common coolant.

    Moderator: It is used to reduce the kinetic energy of fast neutrons intoslow neutrons and to increase the probability of chain reaction. Graphite,heavy water and beryllium are generally used as moderator. A moderatorshould possess the following properties:

    It should have high thermal conductivity It should be available in large quantities in pure form It should have high melting point in case of solid moderators and low

    melting point in case of liquid moderators. Solid moderators should also

    possess good strength and machinability. It should provide good resistance to corrosion It should be stable under heat and radiation It should be able to slow down neutrons

    C t l d Th g l t th t f h i ti Th d f

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    Control rods: They regulate the rate of a chain reaction. They are made ofboron, cadmium or other elements which absorb neutrons. Control rodsshould posses the following properties:

    They should have adequate heat transfer properties They should be stable under heat and radiation They should be corrosion resistant They should be sufficient strong and should be able to shut down the

    reactor almost instantly under all conditions. They should have sufficient cross sectional area for the absorption.

    Reflector: The neutrons produced during the fission process will be partlyabsorbed by the fuel rods, moderator, coolant or structural material etc.Neutrons left unabsorbed will try to leave the reactor core and will be lost.Such loss is minimized by surrounding the reactor core by a material calledreflector which will send the neutrons back into the core. The returnedneutrons can then cause more fission and improve the neutrons economyof the reactor. Generally the reflector is made up of graphite and


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    Turbine: The steam produced in the steam generator ispassed to the turbine. Work is done by the expansion of stemin the turbine.

    Condenser: The exhaust steam from the turbine flows to the

    condenser where cooling water is circulated. The exhauststeam is condensed to water in the condenser by cooling. Thecondensate is pumped again into the steam generator by thefeed pump.

    Shielding: The reactor is a source of intense radioactivity.

    These radiations are very harmful and shielding is provided toabsorb the radioactive rays. A thick concrete shielding and apressure vessel are provided to prevent the radiationsescaped to atmosphere.

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    Working of a Nuclear Power plant

    The reactor of a nuclear power plant is similar to the furnaceof steam power plant. The heat liberated in the reactor due tothe nuclear fission of the fuel is taken up by the coolantcirculating through the reactor core. Hot coolant leaves the

    reactor at top and then flows through the tubes of steamgenerator (boiler) and passes on its heat to the feed water.The steam produced is passed through the turbine and afterwork has been done by expansion of steam in the turbine,steam leaves the turbine and flows to condenser. Pumps areprovided to maintain the flow of coolant, condensate andfeed water.

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    Boiling water reactor (BWR)

    In this reactor, enriched uranium (enriched uranium containsmore fissionable isotope U 235 than the naturally occurringpercentage 0.7% as nuclear fuel and water is used as coolant.Water enters the reactor at the bottom. It takes up the heat

    generated due to the fission of fuel and gas converted intosteam. Steam leaves the reactor at the top and flows into theturbine. Water also serves as moderator. Indias first nuclearpower plant at Tarapur has two reactors (each of 200 MWcapacity) of boiling water reactor type.

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    Pressurised Water reactor (PWR)

    A pressurised water nuclear plant is shown in fig. It usesenriched uranium as fuel. Water is used as coolant andmoderator. Water passes through the reactor core and takesup the heat liberated due to nuclear fission of the fuel. In

    order that water may not boil (due to its low boiling point212 0 F at atmospheric conditions) and remain in liquid state, itis kept under a pressure of about 1200 p.s.i.g in thepressuriser. This enables water to take up more heat from thereactor. From the pressuriser, water flows to the steamgenerator where it passes on its heat to the feed water whichin turn gets converted into steam.

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    Fast breeder reactor (FBR)

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    Fast breeder reactor (FBR)

    In this reactor the core containing U 235 is surrounded by a blanket (a layer offertile material placed outside the core) or fertile material U 238 . In this reactorno moderator is used. The fast moving neutrons liberated due to fission ofU235 are absorbed by U 238 which gets converted into fissionable materialPu 239 , Pu 239 is capable of sustaining chain reaction. Thus the reactor isimportant because it breeds fissionable material from fertile material U 238 available in large quantities. This reactor uses two liquid metal coolantcircuits. Liquid sodium is used as primary coolant when circulated through thetubes of intermediate heat exchanger transfer its heat to secondary coolantsodium potassium alloy. The secondary coolant while flowing through thetubes of steam generator, transfer its heat to feed water.

    Fast breeder reactors are better than conventional reactor both from thepoint of view of safety and thermal efficiency. For India which already is fastadvancing towards self reliance in the field of nuclear power technology, thefast breeder reactor becomes inescapable in view of the massive reserves ofthorium and the finite limits of its uranium resources.

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    Advantages of nuclear power plant

    The fuel used in nuclear power plant is uranium; it does not releasechemical or solid pollutants into the air during use. Space required is less when compared with other power plants. Fuel consumption is very less. Fuel transportation cost is low and no large storage area for fuel isrequired. The plant is not affected by weather conditions. The plant canfunction throughout the year (Hydel power plants depends onmonsoon) By using nuclear fuel we can conserve the fossil fuels like coal, oil, gas

    etc for other purposes. For example coal can be used to power steamengines, oil can be used for running vehicles, and gas be used forcooking.

    Disadvantages of nuclear power

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    Disadvantages of nuclear powerplant

    1.Nuclear plants cost more to build than thermal orhydro electric power plants of the same capacity.

    2.Radioactive wastes must be disposed carefully,

    otherwise it will adversely affect the health ofworkers and the environment as a whole.

    3.Maintenance cost of the plant is high.

    4.Not suitable for varying load conditions5.Well trained persons are required to operate theplant.

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    Nuclear power stations in India

    Tarapur Nuclear power station (Bombay) Boilingwater reactor 200 MW Rana Pratap Sagar Nuclear power station (Kota in

    Rajasthan) Two 200 MW Kalpakkam Nuclear power station Two 235 MW Pressurised water reactor Narora Nuclear power station (Uttar Pradesh) Two 235 MW CANDU reactor Kakarpur Nuclear power plant (Gujarat) 4 235MW CANDU type reactor