WATER (HYDRAULIC) TURBINES
Turbine is a machine wherein rotary motion is obtained by centrifugal forces, which result from a change in the direction of high velocity fluid jet that issues from a nozzle.
Water turbine is a prime mover, which uses water as the working substance to generate power.
A water turbine uses the potential and kinetic energy of water and converts it into usable me-chanical energy. The fluid energy is available in the natural or artificial high level water reservoirs, which are created by constructing dams at appropriate places in the flow path of rivers. When water from the reservoir is taken to the turbine, transfer of energy takes place in the blade passages of the unit. Hydraulic turbines in the form of water wheels have been used since ages; presently their application lies in the field of electric power generation. The mechanical energy made available at the turbine shaft is used to run an electric generator, which is directly coupled, to the turbine shaft. The power generated by utilizing the potential and kinetic energy of water has the advantages of high efficiency, operational flexibility, low wear tear, and ease of maintenance.
Despite the heavy capital cost involved in constructing dams and reservoirs, in running pipelines and in turbine installation (when compared to an equivalent thermal power plant) different countries have tried to tap all their waterpower resources. Appropriate types of water turbines have been installed for most efficient utilization. A number of hydro-electric power plants have and are being installed in India too to harness the available waterpower in the present crisis of fast idling energy resources. Hydro-electric power is a significant contributor to the world?s energy sources.
Water (hydraulic) turbines have been broadly classified as,
MPULSE AND REACTION TURBINES
Hydraulic turbines are required to transform fluid energy into usable mechanical energy as effi-ciently as possible. Further depending on the site, the available fluid energy may vary in its quantum of potential and kinetic energy. Accordingly a suitable type of turbine needs to be selected to perform the required job.
Based upon the basic operating principle, water turbines are categorized into impulse and reaction turbines depending on whether the pressure head available is fully or partially converted into kinetic energy in the nozzle.
Impulse Turbine wherein the available hydraulic energy is first converted into kinetic energy by means of an efficient nozzle. The high velocity jet issuing from the nozzle then strikes a series of suitably shaped buckets fixed around the rim of a wheel (Fig. 1.16). The buckets change the direction of jet without changing its pressure. The resulting change in momentum sets buckets and wheel into rotary motion and thus mechanical energy is made available at the turbine shaft. The fluid jet leaves the runner with a reduced energy. An impulse turbine operates under atmospheric pressure, there is no change of static pressure across the turbine runner and the unit is often referred to as a free jet turbine. Important impulse turbines are: Pelton wheel, Turgo-impulse wheel, Girad turbine, Banki turbine and Jonval tur-bine etc., Pelton wheel is predominantly used at present.
Reaction Turbine wherein a part of the total available hydraulic energy is transformed into kinetic energy before the water is taken to the turbine runner. A substantial part remains in the form of pressure energy. Subsequently both the velocity and pressure change simultaneously as water glides along the turbine runner. The flow from inlet to outlet of the turbine is under pressure and, therefore, blades of a reaction turbine are closed passages sealed from atmospheric conditions.
Fig. 1.17 illustrates the working principle of a reaction turbine in which water from the reservoir is taken to the hollow disc through a hollow shaft. The disc has four radial openings, through tubes, which are shaped as nozzles. When the water escapes through these tubes its pressure energy decreases and there is increase in kinetic energy relative to the rotating disc. The resulting reaction force sets the disc in rotation. The disc and shaft rotate in a direction opposite to the direction of water jet. Important reaction turbines are, Fourneyron, Thomson, Francis, Kaplan and Propellor turbines Francis and Kaplan turbines are widely used at present.
The following table lists salient points of difference between the impulse and reaction turbines with regard to their operation and application.
All the available energy of the fluid is converted into kinetic energy by an efficient nozzle that forms a free jet.
The jet is unconfined and at atmospheric pressure throughout the action of water on the runner, and during its subsequent flow to the tail race.
Blades are only in action when they are in front of the nozzle.
Water may be allowed to enter a part or whole of the wheel circumference.
The wheel does not run full and air has free access to the buckets.
Casing has no hydraulic function to perform; it only serves to prevent splashing and to guide the water to the tail race.
Unit is installed above the tail race.
Flow regulation is possible without loss.
When water glides over the moving blades, its relative velocity either remains constant or reduces slightly due to friction.
Only a portion of the fluid energy is transformed into kinetic energy before the fluid enters the turbine runner.
Water enters the runner with an excess pressure, and then both the velocity and pressure change as water passes through the runner.
Blades are in action all the time.
Water is admitted over the circumference of the wheel.
Water completely fills the vane passages throughout the operation of the turbine.
Pressure at inlet to the turbine is much higher than the pressure at outlet ; unit has to be sealed from atmospheric conditions and, therefore, casing is absolutely essential.
Unit is kept entirely submerged in water below the tail race.
Flow regulation is always accompanied by loss.
Since there is continuous drop in pressure during flow through the blade passages, the relative velocity does increase.