CHAPTER 2. Basic Concepts Of Water Francis Turbine

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CHAPTER 2. Basic Concepts of Water Francis TurbineCHAPTER - 22.1 IntroductionFrancis turbines inward flow reaction turbine. In old Francis turbine, thedischarge was radial. The modern Francis turbine is an inward mixed flow(radial and axial) reaction turbine. In this turbine, water under pressure entersthe runner through the guide blade radially in inward direction and leaves therunner axially.The turbine runner consists of a series of curved vanes uniformly fittedaround the circumference. The vanes are smooth and their profile ensuresefficient performance and freedom from “cavitations”.Water from the reservoir is led to the turbine through the penstock. Fromthe penstock, water enters the “scroll casing” which surrounds the guide vanes(called “wicket gates”) and the runner. Flowing through the guide vanes, waterradially strikes the runner blades and causes the runner to rotate. After doingwork in the runner vanes, water leaves the turbine axially through a straightdivergent cone, called “draft tube” and finally flows through the tail race.Note : Francis turbine is so called, because it was invented by Francis, anAmerican engineer [1].2.2 Theory of TurbineDivided theory of turbine into:a- reaction turbineb- impulse turbineReaction Turbine:In any reaction turbine the water at inlet possesses both pressure energyand kinetic energy. As the water flows through the turbine its pressure energy istransformed into kinetic energy and ultimately energy this water leaves theturbine at atmospheric pressure or at a pressure greater than atmosphericpressure.The runner of the turbine is enclosed in an air-tight casing and the runner isfull of water. Reaction turbines are also called pressure turbines.2.3 Classification of Reaction TurbineReaction turbines are classification as follows:1-Radial flow turbines.2-Axial flow turbine.12

CHAPTER 2. Basic Concepts of Water Francis Turbine3-Mixed flow turbines.Radial flow reaction turbines are those turbines in which water flowsradially.Radial flow reaction turbines are sub-divided into (a) inward flow reactionturbine and (b) Outward flow reaction turbine.Inward flow reaction turbine, water flows radially from outward towards theaxis of rotation of the turbine shaft. In outward flow reaction turbine, water flowsradially from inwards towards outwards.2.4 Main Components of a Radial Flow Reaction Turbine.A radial flow reaction turbine consists of the following main components:1- Casing2- Guide mechanism3- Runner4- Draft tube13

CHAPTER 2. Basic Concepts of Water Francis TurbineFig (2.1) main of components of radial flow reaction turbine.The material with which the spiral casing is made depends upon the headwater under which the turbine works.HEAD OF WATERMATERIAL USED IN MAKINGTHE CASINGUp to 30 mConcreteBetween 30 m to 100 mWelded rolled steel plateAbove 100 mCast steelIMPULSE WATER TURBINEREACTION WATER TUBINE1- All the available pressure head is 1- Only part of the available pressureconverted into kinetic head before head is converted into kinetic headstriking on the buckets.before striking on the runner vanes.2-As the water moves over the 2-The velocity and pressure bothbuckets the pressure throughout from change during flow of water over theinlet to exit remains atmospheric.vanes.3-The wheel must not run full.3-The runner must run full.4-The turbine must be placed above 4-The turbine must be submerged inthe tail race or at the foot of the fall. the tail race or it can be placed abovethe foot of the fall by connecting thedischarge end with draft tube.5-Water may be admitted over part 5-Water is admitted over the wholeor whole of the wheel circumference. circumference of the runner.6-Flow of water can be regulated 6-Flow of water cannot be regulatedwithout loss of efficiency.without loss of efficiency.7-Work is done due to change in 7-Work is done partly by the changekinetic energy of water entering into in kinetic energy and party by thethe buckets.change in pressure energy of waterentering into the runner.8-Components of the turbine are 8-Components of the turbine are noteasily accessible. Hence repairs are easily accessible since the runner iseasy.completely enclosed in an air-tightcasing.9-It dose not require any a draft tube. 9-It requires a draft tube10-Working speed is less. Hence for 10-Working speed is more. Hencethe same power developed size of the for the same power developed size ofturbine is large.the turbine is less.14

CHAPTER 2. Basic Concepts of Water Francis TurbineTable (2.1) Relation between impulse & reaction turbine.FfFig (2.2) Inlet and outlet velocity diagrams for Francis turbine.15

CHAPTER 2. Basic Concepts of Water Francis TurbineIn the velocity diagrams for Francis turbine,u1 peripheral speed of the runner at inletu2 peripheral speed of the runner at outletv1 absolute velocity of water at inletv2 absolute velocity of water at outletvr1 relative velocity of water at inletvr2 relative velocity of water at outletvf1 velocity of flow at inletvf2 velocity of flow at outletvw1 velocity of whirl at inletα1 inlet angle of waterα2 outlet angle of water 90 θ1 inlet vane angleθ2 outlet vane angle16

CHAPTER 2. Basic Concepts of Water Francis Turbine2.5 Advantages and Disadvantages of Hydroelectric power plants2.5.1 Advantagesi. No fuel requirement: No fuel is required to generate electricity. Watersource is perennially available.ii. Low runner cost: the electricity per kWh is very cheap as compared tothermal or nuclear.iii. No problem of disposal of ash: since no fossil fuel is used, so there is noproblem of disposal of ash.iv. Pollution free electricity generation: the electricity generated byhydropower dose not produces any type of pollution.v. Easily switched on and off in a short period: the hydraulic turbine isswitched on and off in a very short period unlike thermal or nuclear plants.The steam turbine of thermal or nuclear plant is put on turning gears forabout 48 hours during start-up and shut-down.vi. Simple in concept, self-contained and reliable in operation. The designconcept of hydraulic power plant is simple in concept and the operation isself-contained and reliable as compared to thermal or nuclear power plants.vii. Greater life expectancy. Modern hydraulic power plants have greater lifeexpectancy (about 50 years) as compared to thermal or nuclear plants(about 30 years).viii. Act as ideal spinning reserve :Owing to greater ease of taking up andthrowing off the load, the hydro-power can be used as an ideal spinningreserve in a system of mix of thermal, hydro and nuclear power stations.ix. Higher plant efficiency: the plant efficiency is high (about 85 to 94%) overa considerable range of load.x. Ancillary benefits; hydroelectric power plants provide ancillary benefitslike irrigation, flood control, a forestation, navigation and aqua-culture.xi. Less skilled workers: Owing to simple design and operation, hydroelectricpower plants do not require highly skilled workers. Even the manpowerrequirements are much less as compared to thermal or nuclear powerplants.xii. Quick response to the change of load: the outstanding features of hydroelectric plant are quick response of the change of load compared withthermal or nuclear. The rapid fluctuating loads are served mosteconomically by hydro-plants.17

CHAPTER 2. Basic Concepts of Water Francis Turbine2.5.2 Disadvantages.The following are the disadvantages of hydroelectric power plants.i.High capital cost: hydroelectric power plants are capital intensive with alow rate of return. The interest rate of this capital cost is a large of annualcost of hydraulic power station.ii.Power dependent on quantity of water available: power generated by thehydro-plants is only dependent on the quantity of water available whichin turn depends on the natural phenomenon of rain. The dry year is moreserious for the hydro-electric project.iii.Site selection dependent on water availability: the sit is selected on thecriterion of water availability at economical load. Such sites are usuallyaway from the load centers. Long transmission lines are needed totransmit power from station to consumers.iv.Long erection time: the completion of hydroelectric power plants takes amuch longer period (about 10 years) as compared to thermal power plants(about 3 years).v.Disturbed ecology of the area: the large hydro-plants disturb the ecologyof the area by way of deforestation, destroying vegetation and uprootingpeople. There is a strong public opinion against large hydro-plants. Now,emphasis is on small, mini and micro level hydro plants.Presently, Government of India is planning to interconnect all the rivers ofIndia. Since national grid is now a reality so if rivers are interconnected it willnot only help in power production but in irrigation and flood control also.2.6 Theory of Francis water turbinesFrancis turbine is widely used world over. It operates under the headvarying from 30 to 500 m. the single unit may develop power as high as 750MW. The specific speed ranges from 60 to 400. It is reaction turbine. Formerlyits specific speed was limited to about 60 and it was radial inward flow type butat present they are the mixed flow type with radial entry and axial exit.18

CHAPTER 2. Basic Concepts of Water Francis TurbineFig. (2.3) A schematic of a Francis water turbine:19

CHAPTER 2. Basic Concepts of Water Francis TurbineFig (2.4) Velocity diagram for a Francis turbine:2.7 Cavitation Problem in Water TurbineCavitations are one of the undesirable problems in the operation of waterturbines. The formation of water vapour and air bubbles on the water surfacedue to reduction of pressure and sudden collapse are known as cavitations. Inany part of the turbine if the pressure drops below the vapour pressure at thetemperature some of the liquid flashes into vapour .the bubbles formed duringvaporization are carried by the water steam to higher pressure zones, where thebubbles condense into liquid forming a cavity or vacuum. The surroundingliquid rushes towards the cavity giving rise to a very high level pressure as highas 700 atm.the rapid formation and collapsing of the bubbles cause the pitting ofthe metallic surface accompanied with variation and noise . This phenomenonis known as cavitations. It also reduces the hydraulic efficiency of turbine.Since the chance of reduction of pressure below atmospheric occurs only atthe exit of runner blade of turbine or inlet of draft tube so the cavitations occurat the exit of runner. The energy balance at the exit of runner(er) and exit ofdraft tube (ed) yields//Per / δ Pa δ – [h (C²er C² ed 2g)20– hf]

CHAPTER 2. Basic Concepts of Water Francis TurbineWhere h is height between turbine exit and tailrace. Hf is the head lossbetween turbine and draft tube exit pa is the atmospheric pressure exerting on thetailrace, per is the pressure of water at the exit of runner, Cer and Ced are the watervelocities at the exit of runner and draft tube respectively.2.7.1 Cavitations Factor.Prof. D. thaws of Germany suggested a cavitations factor to determine thezone where the turbines can work without any danger of cavitations. The criticalvalue of cavitations factor is given byσc (Ha Hv) /HWhere Ha atmospheric pressure head (m), Hv vapour pressure (m) atwater temperature, H working head of turbine (m) h high of turbine outletabove tailrace level (m) the values of σ c depend upon specific speed andturbine which are given in table (2.1) below explained values of σc for variousof Ns and Different turbine [6].Francis turbineKaplan 500.14700-8001.053000.23500.27Table (2.2) The value of σc:AdvantageDisadvantageNO1Pollution free, therefore no Comparatively high investments.pollutants2No consumption of natural Frequently large distance betweenresourcesfavorable water power locations andconsumer centers3Small delivery of warmthEnergy production is inconsistentwith wave power stations in open sea4High efficiency (about 90%)Over congestion of otherwise usablesurfaces and ecologically valuation21

CHAPTER 2. Basic Concepts of Water Francis Turbinehabitat.5Long life spam of a plantSociological effects due to resetting6Simple and proven technologyInterruption and restriction of thehabitat for moving fish7Low operating cost due to Disturbance of the water regime insmallrequirementat the environmentmaintenance and operation8Speed of employingturning a plant off9Advantage during general- Disturbance the habitat of manypurposeuse(irrigation animals species living in water andnavigation flood protection plant typeswater supply)10Energy storage possibility11Elevation of the ground-water No flooding- no fertile mudleveldistribution and disturbance of thebiological equilibriumand Reduction of the velocity of flowwhich leads demolition of the areaabove the water thus retainingstructure to deposits and below to theerosionWater changes occur how: oxygenpoverty temperature change, changeof the flow conditions and increasein nutrients too inadvertent growthTable (2.3) Advantage and disadvantage of the hydraulic power plant[6].22

CHAPTER 2. Basic Concepts of Water Francis TurbineFig (2.5) The pump-fed power station:Facts:Head:up to 800 metersAchievement:up to 750 MWEfficiency:up to 90% [7].Power (kW) Head (meters) x Flow (m3/second) x Gravity (9, 81) x Efficiency(0, 6)Head Nethead GrossHere the overall efficiency was set at 60%.head-losses(m)2.8 Hydroelectric Power Plant TechnologyIn hydro power plants the kinetic energy of falling water is captured togenerate electricity. A turbine and a generator convert the energy from the waterto mechanical and then electrical energy. The turbines and generators areinstalled either in or adjacent to dams, or use pipelines (penstocks) to carry thepressured water below the dam or diversion structure to the powerhouse. Thepower capacity of a hydropower plant is primarily the function of two variables:(1) flow rate expressed in cubic meters per second (m3/s), and (2) the hydraulic23

CHAPTER 2. Basic Concepts of Water Francis Turbinehead, which is the elevation difference the water falls in passing through theplant. Plant design may concentrate on either of these variables or both.Fig (2.6) connection turbine and generator by control:From the energy conversion point of view, hydro power is a technologywith very high efficiencies, in most cases more than doubles that ofconventional thermal power plants. This is due to the fact that a volumeof water that can be made to fall a vertical distance represents kinetic energywhich can more easily be converted into the mechanical rotary power needed togenerate electricity, than caloric energies. Equipment associated withhydropower is well developed, relatively simple, and very reliable. Becauseno heat (as e.g. in combustion) is involved, equipment has a long life andmalfunctioning is rare. The service life of a hydroelectric plant is well In excessof 50 years. Many plants built in the twenties - the first heyday of hydroelectricpower - are still in operation.Since all essential operating conditions can be remotely monitored andadjusted by a central control facility, few operating personnel are required onsite. Experience is considerable with the operation of hydropower plants inoutput ranges from less than one kW up to hundreds of MW for a single unit.Fig (2.7) Explained in this chart power production24

CHAPTER 2. Basic Concepts of Water Francis TurbineFig (2.8) The relation between Flow and headIn the fig (2.8) load factor and reducing the cycling of its base load units.In most cases, pumped storage plants run a full cycle every 24 hours.2.9 stages of velocity trianglesThe flow geometry at the entry and exit of a turbomachine stage isdescribed by the velocity triangles at these stations. A minimum number of dataon velocity vectors and their directions are required to draw a complete set ofvelocity triangles.All type of turbo machines has a finite cross-section at the entry and exit.Therefore, the magnitudes of velocity vectors and their directions vary overthese sections. Because of this, an infinite number of velocity triangles would berequired to fully describe the flow. This is obviously not possible. On the otherhand, a single pair of velocity triangles will only represent a one-dimensionalflow through the stage. In view of this, mean values of velocity vectors and theirdirections are defined for blade roes of given geometries and flow conditions.These values make it possible to draw the mean velocity triangles for aturbomachine contain, besides the peripheral velocity (u) of the rotor bladesboth the absolute (c) and relative (w) fluid velocity vectors. These velocities arerelated by the following well known vector equation:25

CHAPTER 2. Basic Concepts of Water Francis TurbineAbsolute velocity vector peripheral velocity vector relative velocityvectorC u wThis simple relation is frequently used and is very useful in drawing thevelocity triangles for turbomachines. For instance, velocity triangles have beendrawing using this relation. The angles are from the axial direction (thereference direction). For axial machines u1 u2 u3 constant.2.10 Design conditionsLike other machine, turbo machines are also designed for some prescribedrunning conditions at which they should have high or maximum efficiency.The design conditions among other things fix the geometry of the flow; atthe design point operation of a turbomachine there is some correspondencebetween the blade angles and the mean fluid angles. The ratio of the peripheralvelocity of the rotor to some fluid velocity is also fixed by design conditions.Aerodynamic or aerothermodynamics design conditions should fully take intoaccount the strength considerations ease in manufacture material and economicaspects [9].2.11 General Layout of a Hydro-Electric Power PlanFig (2.10) shows a general lay-out of a hydro-electric power plant whichconsists of:Fig (2.9) General layout of hydro-electric power plant26

CHAPTER 2. Basic Concepts of Water Francis Turbine(i) A dam constructed across a river to store water.(ii) Pipes of large diameters called penstocks, which carry water under pressurefrom the storage reservoir to the turbines. These pipes are made of steel orreinforced concrete.(iii) Turbine having different types of vanes fitted to the wheels.(iv) Tail race, which is a channel which carries water away from the turbinesafter the water has worked on the turbines. The surface of water in the tail raceis also known as tail race.2.12 Radial Flow Reaction Turbines:Radial flow turbines are those turbines in which the water flows in theradial direction. The water may flow radially from outwards to inward (i.e.,towards the axis of rotation) or from inwards to outwards. If the water flowsfrom outwards through the runner, the turbine is known as inward radial flowturbine. And if the water flows from inward to outwards the turbine is known asoutward radial flow turbine.Reaction turbine means that the water at the inlet of the turbine possesseskinetic energy as well as pressure energy. As the water flows through the runnerapart of pressure energy goes on changing into kinetic energy. Thus the watertrough the runner is under pressure. The runner is completely enclosed in an airtight casing and casing and the runner is always full of water.2.12.1 Main parts of a radial flow reaction turbine.The main parts of a radial flow reaction turbine are:1-casing3- runner and2- Guide4- draft tube.1-Casing:As mentioned above that in case of reaction turbine casing and runner arealways full of water. The water from the penstocks enters the casing whichis of spiral shape in which area of cross-section of the casing goes ondecreasing gradually. The casing completely surrounds the runner of theturbine. The casing as shown in fig below is made of spiral shape so thatthe water may enter the

Reaction turbines are also called pressure turbines. . 2.5 Advantages and Disadvantages of Hydroelectric power plants . 2.5.1 Advantages . concept of hydraulic power plant is simple in concept .

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