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PERFORMANCE CHARACTERISTICS OF HYDRAULIC TURBINES

Turbines are always designed for specific conditions of head, speed and output. However a turbine may actually be required to operate under conditions widely different from those for which it has been designed. It is, therefore, imperative to determine and have a complete info about the performance of the turbine over a wide range of operating conditions. The info is obtained from tests conducted either on the actual turbine at the site or on a small scale turbine model in a research laboratory. The experimental results are represented either in a tabular form or depicted graphically by means of curves which are called the performance characteristics or curves.

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1.MAIN OR CONSTANT HEAD CHARACTERISTICS:

• Head and gate opening of the turbine are maintained at constant and the speed is varied by allowing a variable quantity of water to flow through the inlet opening, the brake power P is measured mechanically by dynamometer. The basic data is then used to calculate the overall efficiency and the unit quantities which are plotted against unit speed as abscissa.

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Following information can be obtained from these plots: There is a separate curve for each gate opening. Unit discharge curves for a Pelton turbine lines

are horizontal lines indicating that unit discharge varies with gate opening but is independent of the unit speed.

For Kaplan turbine the discharge curve rises s the speed increases.

Power and efficiency curves are parabolic in nature; maximum value occurs at a particular speed.

With pelton turbine the max. efficiency for different gate openings occur at the same speed.

With reaction turbines the max. efficiency for different gate settings usually occurs at different speeds.

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2. Operating or constant speed characteristics:

For each gate opening speed is kept constant : discharge Q and head H may vary according to their availability: the brake power P is measured mechanically by a dynamometer. Overall efficiency is then calculated from the measured values of discharge, head and power. Results are graphically represented by plotting ηο against the %age of full load. The term %age of full load is the ratio of measured power to full load.

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3.Constant efficiency curves

These are also called the iso-efficiency curves, indicate the value of constant efficiency.

For a particular efficiency say 70% a horizontal line is drawn which intersects the curves for different gate settings. From the points of intersection the corresponding unit speed values are obtained. This info is then transferred to the main curve for the corresponding gate openings. Points of the same efficiency are smoothly joined to get a constant efficiency curve. A curve fort best performance is obtained when the peak points of various iso-efficiency curves are joined.

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PERFORMANCE CHARACTERISTICS OF CENTRIFUGAL PUMPS:

1. MAIN CHARACTERISTICS: To obtain the test data for main characteristics, the pump is run at a constant speed and the discharge is varied over the desired range. Measurements are taken for head and shaft power for each discharge and calculations are made for the pump efficiency. Curves are then plotted for head , power and efficiency against discharge.

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2.Operating characteristics: Pumps are generally designed for maximum efficiency and that occurs when the pump operates at the design speed. Evidently for optimum performance, the pump needs to be operated at the design speed. A particular set of main characteristics corresponding to the design speed are called the operating characteristics. These help to obtain the design discharge and head; this corresponds to the point of maximum efficiency.

3.Constant efficiency curves: These depict the performance of a pump over its entire range of operation. Data for plotting these curves is obtained from the main characteristics curves. For a particular efficiency a horizontal line is drawn which intersects the curves for different pump speeds. From the point of intersection, the corresponding discharge values are obtained. A curve for best performance is obtained when the peak points to various iso-efficiency curves are joined.

Constant efficiency curves helps to locate the regions where the pump would operate with max. efficiency.