WATER POWER ENGINEERING

WATER POWER ENGINEERING

Types of water power developmentA water power development is essentially to utilize the available power in the fall of a river or a canal, through a portion of its course, by means of hydraulic turbines.A water power development scheme is known as hydroelectric scheme or Hydel-scheme.

Classification of hydel plants according to their function as follows:-Run-off-river plantsStorage plantsPumped storage plantsRun-off-river plant-These plants utilize the river flow having no pondage at its upstream.A weir or a barrage is constructed across the river to raise the water level slightly.Such a scheme is a low head scheme & is adopted in the case of a perennial river.Run-off-river plants are of two types-Those that utilize the varying flowThose that operate on minimum available discharge & work throughout the year Storage plants-Such a plant has a storage reservoir at upstream end. A dam is constructed across the river and water is stored during the periods of excess supply.In India, most of the major plants are of this category.

Pumped storage plants-Such plants stores power in the form of potential energy of water. The scheme consists of a storage reservoir at higher level, and a turbine-cum-pump installation.This scheme is used to generate the power only during peak hours of demand.During peak demand , water flows from the reservoir to the turbine & power is generated. During off-peak hours, excess power is available from the thermal power plant or from other hydro-plant.This power is utilized in pumping the water from low reservoir back to the storage reservoir at the higher elevation and water is thus stored. This water is utilized again during hours of peak demand , so no water is wasted.

Classification of hydro electric scheme Depending upon the available headThe high head needed for high head schemes can be obtained by one of the following methods-Construction of high damUtilization of natural fallConveying water to a lower valley

Principal components of hydroelectric scheme-A hydroelectric work comprises of the following components-ForebayIntake structureSurge tankPenstocksPower-houseTurbines and governorsGeneratorsTransformersTransmission linesFOREBAY Forebay is an enlarged body of water just in front of the intake.The main function of forebay is to store, temporarily, the water rejected by the plant when the load is reduced and to meet the instantaneous increased demand when the load is instantaneously increased.When a canal leads water to turbine, the canal itself serves as a forebay and whwn the penstocks directly take the water from the reservoir, the reservoir acts as the forebay

INTAKE STRUCTURE-The water is conveyed from the forebay to the penstocks through the intake structure.Following are the accessories of an intake structure-Trash rack:- There rack prevents the debris getting into penstocks.Rakes to clear the trash rack.Ice removal equipment.Penstock closing gates with hoists.PENSTOCKS-Penstocks are the pipes of large diameter which carry water under pressure from the storage reservoir to the turbine.Penstock may be subjected to water hammer pressure so short length of penstocks are designed , and in the case of long penstocks surge tank is provided to reduce water hammer.The intake of the penstock at the forebay must be at a level low enough to provide an adequate water seal under all conditions to prevent the entry of air SURGE TANK When the load on the turbine is reduced, the governor automatically close the inlet gates partially, to reduce the inflow to the turbines. This retards the water flowing in the penstocks, leading to water hammer in the penstocks.Surge tank is provided to reduce the water hammer.

The functions of surge tank are as follows:-It furnished a reservoir surface very near to the discharge end of the penstock to dampen the water hammer pressure.It temporarily stores water when load is reduced until such time that the velocity has fallen to the steady value.It temporarily supplies more water when the load is increased until such time that the velocity has increased to a fresh steady value.It calms down effectively and rapidly the surging in the water level.

TURBINES Hydraulic turbines are the machines which convert hydraulic energy into mechanical energy. The mechanical energy is used to in running an electric generator which develop electric power (hydro-electric power) Hydraulic turbines may be classified under two heads:-Impulse or velocity turbinesReaction or pressure turbines

A. Impulse or velocity turbines-All the available potential energy or head is converted into kinetic energy or velocity head by passing it through a contracting nozzle or by guide vanes before it strikes the buckets of the turbine.An impulse turbine has the following characteristic feature-The wheel passages are not completely filledThe water acting on the wheel vanes is under atmospheric pressureThe water is supplied at a few points at the periphery of the wheelEnergy applied to the wheel is wholly kineticReaction or pressure turbines In this only a part of available potentially energy is converted into velocity head.The pressure at the inlet to the turbine is much higher then the pressure at outlet. A reaction turbine has the following characteristic feature-The wheel passages are completely filled with waterThe water action on the wheel vanes is under pressure greater than atmospheric.The water enters all around the periphery of the wheel.Energy in the form of both pressure and kinetic is utilized by the wheel.

The turbines may be classified according to the direction of flow of water in the runnerTangential flow turbineRadial flow turbineAxial flow turbineMixed flow turbine

SPECIFIC SPEED Turbines may also classified according to their specific speed-

Selection of suitable type of turbine-On the basis of available head-Low head turbine - head less than 30m & use large quantity of waterMedium head turbine - head between 30m to 150mHigh head turbine head from 150m to over 1000m

THE POWER HOUSE A power house of a hydel scheme serves as a protective covering for the hydraulic and electrical equipment.The following items of equipment should be provided for the layout of the power-house-Hydraulic equipment-Turbines GatesRelief valves for penstock settingGovernorsFlow measurement equipmentElectrical equipment-Generators: air ductsExcitersTransformersSwitching equipmentsHigh tension systemauxiliaries

Miscellaneous equipment-CraneShopOffice roomsLavatory

SCROLL CASING Scroll casing surrounds the turbine runner and the guide mechanism. The water from the penstock is delivered to the scroll casing.The main purpose of the scroll casing is to maintain nearly uniform velocity at the entry to the guide vanes.

DRAFT TUBE A draft tube is a pipe or passage of gradually increasing cross-sectional area, which connects the runner exit to the tail race.

Draft tube serve following purposes-It permit a negative or suction head to be established at the runner exit, thus making it possible the placing of wheel and connecting machinery at a level above that of water in the tail race under high-water flow conditions of river, without loss of head.It reduces the high velocity of the water discharged by the turbine by gradually enlarging the cross section and thus converts the kinetic energy of the water leaving the runner , into mechanical energy of the wheel.

TAIL RACE Tail race is a channel into which the draft tube is discharges.Where the power house is close to the river , the tail race may be the river itself .The design velocity is usually 0.8 to 1.5 m/s .The tail race channel should be maintained properly so that excessive aggradation and degradation is avoided.

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