Hydroturbine

(also hydraulic turbine or water turbine), a vane-type motor that converts the mechanical energy of water (the energy of its level, pressure, and velocity) into the energy of a rotating shaft. Hydroturbines are classified according to their operating principle as impulse and reaction types.

The main operating element of a hydroturbine, which converts the energy, is the rotor. Water is supplied to the rotor in impulse turbines through a nozzle and in reaction turbines through a guiding device. In an impulse hydroturbine (Figure 1) the water is at atmospheric pressure both ahead of and after the rotor. In a reaction hydroturbine (Figure 2) the

Figure 1. Diagram of an impulse hydroturbine: (a) rotor, (b) nozzles

water pressure ahead of the rotor is higher than atmospheric pressure, but it may be higher or lower than atmospheric pressure after the rotor.

The first reaction hydroturbine was invented in 1827 by the French engineer B. Fourneyron; his design had a 6-hp rotor, but such hydroturbines are no longer used because of their poor power characteristics. In 1855 the American engineer J. Francis invented a hydroturbine with a fixed-blade radial-axial rotor, and in 1887 the German engineer Fink proposed a guiding device with rotating blades. In 1889 the American engineer L. A. Pelton patented a hydroturbine that bears his name, and in 1920 the Austrian engineer V. Kaplan obtained a patent on a rotating-blade hydroturbine. The rotating-blade, radial-axial, and Pelton hydroturbines are widely used to produce electric power.

Figure 2. Diagram of a reaction hydroturbine: (a) rotor, (b) guiding device

In order to calculate the profile of a hydroturbine rotor blade that is turning at a constant angular velocity, the following equation is used (Figure 3):

where H is the hydroturbine’s operating head (the energy stored in 1 kg of water—that is, the difference between the water-level marks ahead of the intake to the structure of the power plant and at its outlet minus the loss to resistance in all the structures but not including the loss in the hydroturbine itself), U₁ and U₂ are the peripheral blade velocities at the rotor’s water intake and outlet, V₁ and V₂ are the absolute water velocities at the intake and outlet, α₁ and α₂ are the angles between the directions of the peripheral and absolute velocities at points corresponding to a flow surface averaged with respect to energy (in degrees), and g is the acceleration of gravity (in m/sec²).

The factor ŋ_ŋ, which represents the hydraulic efficiency of the hydroturbine, is introduced into the left-hand part of the equation. Part of the power acquired by the rotor is expended in overcoming mechanical resistances; these losses are accounted for in the mechanical efficiency ŋ_m of hydroturbines. The leakage of water in the rotor bypass is taken into account by the volume efficiency ŋ₀ of the hydroturbine. The overall efficiency of the hydroturbine, ŋ = ŋ_ŋ·ŋ_m·ŋ₀, is the ratio between the useful power imparted to the turbine shaft and the power of the water passed through the hydroturbine. In modern units the overall efficiency is 0.85-0.92; under favorable operating conditions, the best designs reach 0.94-0.95.

Figure 3. Velocity triangles at the intake and outlet of the rotor of a hydroturbine

The geometric dimensions of a hydroturbine are characterized by the nominal diameter D₁ of the rotor. Hydroturbines of different sizes form a turbine series if they have the same type of rotor and geometrically similar elements in the circulating portion. After determining the required parameters of one of the hydroturbines of a given series, it is possible to calculate from similarity formulas the same parameters for any hydraulic turbine of this series. Each turbine series is characterized by a power-speed coefficient, which is numerically equal to the rate of the hydroturbine’s shaft rotation at which a power of 0.7355 kilowatts (kW), or 1 hp, is developed at a head of 1 m. The larger this coefficient, the greater the rate of shaft rotation at a given head and power. The cost of hydroturbines and electrical generators decreases as their rate of rotation increases, so that the tendency is to construct hydroturbines having the greatest possible power-speed coefficients. However, in reaction hydroturbines this is hindered by the phenomenon of cavitation, which causes the machine to vibrate, reduces the efficiency, and damages the hydroturbine.

Curves showing the relationships of the variables that characterize hydroturbines are called turbine characteristics. Figure 4 shows hydroturbine characteristics at a constant head and rate of rotor rotation but with various loads and water flow rates. Under real conditions hydroturbines operate under a variable head; in this case their behavior is depicted by universal characteristics for a model and by operating characteristics for a full-scale hydroturbine. Universal characteristics are plotted using laboratory studies of a model whose flow section is geometrically similar to that of the full-scale version.

Figure 4. Characteristics of a hydroturbine with constant head and speed of rotation of rotor: ŋ is the efficiency, Q is the water discharge, and N is the load

On the universal characteristics (Figure 5), which are based on model testing conditions and have as coordinates the normalized variables of flow rate Q’₁, in liters/sec, and rate of rotation n’_u in rpm (which are typical for hydroturbines of a given series with a rotor diameter of 1 m operating at a head of 1 m), isolines are plotted for efficiencies of ŋ percent, cavitation coefficients ç, and openings of the guiding device a₀.The performance characteristics (Figure 6) are constructed on the basis of the universal curves and show how the efficiency of a full-scale turbine ŋ percent depends on the load N (in MW) and the head H (in m) at a nominal rate of rotation n = const. A curve for the maximum power, which expresses the relationship between the guaranteed power and the head, is also usually plotted on this graph. Lines for equal permissible suction heads H₈ (in m), which

Figure 5. Universal characteristics for a simulated hydroturbine

indicate the submersion of the rotor under the water level in the tail water (the difference between the level of the rotor’s location and the level of the tail water), are also shown on these characteristics.

Figure 6. Performance characteristics for a full-scale hydroturbine

The flow section of reaction hydroturbines consists of the following main elements (Figure 7): the scroll casing of the hydroturbine; the guiding device, which regulates the water flow rate; the rotor; and the suction tube, which discharges the water from the hydroturbine.

Figure 7. Flow section of a reaction hydroturbine

Depending on the direction of the flow in the rotor, reaction hydroturbines are divided into axial and radial-axial types, and according to the method of power regulation, they may have single or double control. The single-control hydroturbines include those with gate apparatus having rotating vanes through which the water is delivered to the rotor (here control is effected by varying the angle of rotation of the vanes in the guiding device), and also those with blade control, in which the rotor blades can be rotated around their axes (here control is effected by varying the angle of rotation of the rotor blades). Double-control hydroturbines have both a guiding device with rotating vanes and a rotor with rotating blades. Rotating-blade hydroturbines are used for heads up to 150 m and can be either axial or diagonal types. A modification of the axial type are the double-bladed hydroturbines, in which two blades are located on each flange instead of one. Single-control radial-axial hydroturbines are used for heads up to 500-600 m. Impulse hydroturbines are built mainly in the bucket type; they are used at heads over 500-600 m and are classified as partial and nonpartial. In the partial type water is delivered to the rotor in the form of a jet through one or more nozzles and therefore operates on one or more rotor blades simultaneously. In the nonpartial type water is delivered to a ring-shaped jet; therefore, all the rotor blades operate simultaneously. There are no suction tubes and scroll casings in impulse hydroturbines, and the flow rate is controlled by nozzle devices using needles that move inside the nozzle, thus altering the area of the output section. Large hydroturbines are equipped with automatic speed regulators.

Hydroturbines are divided according to the rotor shaft placement into vertical, horizontal, and inclined types. The combination of a hydroturbine with a hydrogenerator is called a hydraulic power unit. Horizontal hydraulic power units with rotating-blade or propeller hydroturbines can be constructed within a single housing.

Reversible hydraulic power units are extensively utilized for pumped-storage and tidal electric power plants. They consist of a turbine-pump (a hydraulic machine capable of operating in both the pumping and turbine modes) and a motor-generator (an electrical machine that operates in both the motor and generator modes). Only reaction hydroturbines are used in reversible hydraulic power units. Single-housing hydraulic power units are used in tidal electric power plants.

A method of classification for rotating-blade and radial-axial hydroturbines that provides a system of hydroturbine types and sizes together with their basic hydraulic and structural characteristics (see Table 1) was developed in the USSR in 1962. This system is based on the regular variation of the geometric and hydraulic parameters of the rotor as a function of the head.

The primary directions of hydroturbine developments are an increase in the unit power, the shift of each type of hydro-

turbine in the direction of higher heads, the improvement of existing models and the development of new types, and the improvement of the quality, reliability, and service life of the equipment. In the USSR, radial-axial hydroturbines with a power of 508 MW, a rated head of 93 m, and a rotor diameter of 7.5 m have been developed and are operating successfully at the Krasnoiarsk Hydroelectric Power Plant; hydroturbines of the same type with a unit power of 650 MW, a rated head of 194 m, and a rotor diameter of 6.5 m are being developed for the Saian Hydroelectric Power Plant.

Great advances in the design of hydroturbines have been made by Hitachi, Mitsubishi, and Toshiba in Japan; English Electric in Great Britain; and Voit in the Federal Republic of Germany. For example, the Japanese company Toshiba is designing hydroturbines with a unit power of 600 MW, a head of 87 m, and a rotor diameter of 9.7 m for the Grand Coulee III Hydroelectric Power Plant.