HYDRAULIC TURBINEINTRODUCTION

• Hydraulic turbines are the machines which convert hydraulicenergy into mechanical energy. The mechanical energydeveloped by a turbine is used in running an electricgenerator which is directly coupled to the shaft of theturbine. The generator develops electric power which is alsosometimes known as hydroelectric power. In general, a waterturbine consists of a wheel called runner which is providedwith specially designed blades or buckets. The waterprossessing large hydraulic energy when strikes the runnerand causes it to rotate.

TYPES OF HYDRAULIC TURBINES

(i) IMPULSE OR VELOCITY TURBINES(ii) REACTION OR PRESSURE TURBINES

IMPULSE OR VELOCITY TURBINES

• In the case of impulse turbine, all the available potential energy or head isconverted into kinetic energy or velocity head by passing it through acontracting nozzle or by guide vanes before it strikes the buckets of theturbine.

• The wheel revolves free in air and water is in contact with only a part of thewheel at a time.

• The pressure of water all long is atmospheric.

• In order to prevent splashing and to guide the water discharged from the bucket

to the tail race, a casing is provided.

Characteristic an impulse turbine

(i) The wheel passages are not completely filled.

(ii) The water acting on the wheel vanes is under atmospheric

pressure.

(iii) The water is supplied at a few points at the periphery of the wheel

(usually one point, occasionally two or more points).

(iv) Energy applied to the wheel is wholly Kinetic.

• An impulse turbine is essentially a low speed wheel and is used for

relative high heads, ordinarly beginning in the neighbourhood of 150

m and so far being limited to about 1000m, through a head of over

1770m has been utilised by impulse wheels in Switzerland.

Examples of the impulse turbines

(i) Pelton Wheel turbine.

(ii) Turgo-impulse wheel turbine.

(iii) Girad turbine.

(iv) Banki turbine.

(v) Jonval turbine.

Out of these, the Pelton Wheel turbine is predominantly used at present.

Pelton wheel turbine

Turgo-impulse turbine

Girard turbine

Banki turbine

Jonval turbine

REACTION OR PRESSURE TURBINES

• In case of reaction turbine, only a part of the available potentialenergy is converted into velocity head, at the entrance to the runnerand the balance that forms a substantial portion remains a pressurehead.

• The pressure at the inlet to the turbine is much higher than the pressure atthe outlet and it varies throught the passage of water through the turbine.

• Major part of the power is derived from the difference in pressure actingon front and back of runner blades and only a minor part from the dynamicaction of velocity.

• The water being under pressure, the entire flow from head race to tail race takes place in a closed conduit system and the atmospheric air has no access to any point.

Characteristic of Reaction Turbine

(i) The wheel passages are completely filled with water.

(ii) The water action on the wheel wheel is under pressure greater than atmospheric.

(iii) The water enters all around the periphery of the wheel.

(iv) The energy in the form of both pressure and kinetic is utilised by the wheel.

Examples of the Reaction Turbine

(i) Fourneyron turbine

(ii) Thompson turbine

(iii) Propeller turbine

(iv) Francis turbine

(v) Kaplan turbine etc.

Out of these, Francis and Kaplan turbines are predominantly used at present.

Fourneyron turbine

Thompson turbine

Propeller turbine

Francis turbine

Kaplan turbine

Classification of turbines on the basis of direction of flow of water in the runner

(i) Tangential flow turbine.

(ii) Radial flow turbine.

(iii) Axial flow turbine

(iv) Mixed flow turbine

• In the tangential flow turbine, the water strikes on the run along the

tangent to the part of rotation of the runner, such as in the Pelton

wheel turbine.

• In radial flow turbine, the water flows along the radial direction and

remains in a plane normal to the axis of rotation, as it passes through

the runner.

• The radial flow may be inward (i.e. is water entering the outer

circumference and flowing radially inward) such as in the Francis

turbine, Thomson turbine or Girad turbine.

• The floor may be out word (i.e. water entering at the centre and flowradially outward), such as in Fourneyron turbine or Boyden turbine.

• In an axial flow turbine 0r parallel flow turbine, water enters and leaves therunner along the direction parallel to the axis of the turbine shaft.

• Example of the type of flow arein Jonval turbine, Girad turbine, Propellerturbine and Kaplan turbie.

• In the mixed flow turbine, water enters the runner at the outer peripheryin the radial direction and leaves it at the centre in the direction parallel tothe axis of the shaft. Example is modern Francis turbine.

CLASSIFICATION OF TURBINES ON THE BASIS OF SPECIFIC SPEED

Table shows the suitability of types of turbine, head and specific speed

S. no. Head (meters)

Types of turbine Specific Speed

1 300 or more Pelton wheel (single jet) 10 - 35

2(a) 150 -300 Pelton or Franis 35 – 100

2(b) 200 – 300 Pelton wheel (multiple jets) 35 – 56

2(c) 150 -200 Francis turbine 60 – 100

3 60 – 150 Francis turbine 100 – 220

4(a) < 60 Francis, Kaplan or Propeller 220 -1000

4(b) 30 – 60 Francis turbine 220 – 300

4(c) 16 - 30 Kaplan turbine 300 – 1000

4(d) < 16 Kaplan or Propeller (for no load variation) 300 -1000

SELECTION OF SUITABLE TYPE OF TURBINE

• selection of suitable type of turbine depends on the available headand the quantity of water required.

• On this basis turbines may be classified as follows:-

(i) Low head turbine.

(ii) Medium head turbine.

(iii) High head turbine.

LOW HEAD TURBINE

• Low head turbines are those which are capable of working underhead less and their heads less than 30 m (Example :- PropellerTurbine and Kaplan turbine).

• Propeller turbine and Kaplan turbine used large quantity of water.

• Medium had turbines are capable of working under the headsranging from 30 to 150m.

• Modern Francis turbine is coming under this categories.

• High head turbines are those turbines which are capable of workingunder very high head ranging from 150 to over 1000 metres.

• Impulse turbines are high head turbines and requirerelatively less quantity of water.

• As far as possible, a turbine with highest permissible specificspeed should be chosen, which will not only with thecheapest in itself but it selectively small size and highrotational speed will reduce the size of the generation aswell as the power house.

• The specific speed cannot be increased indefinitely becausehighest specific speed of turbine is generally more liable tocavitation.

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