TYPES OF TURBINES
The oldest form of “water turbine” is the water-wheel. The natural  head difference in water level of a stream is utilised to drive it. In its conventional form the water-wheel is made of wood and is provided with buckets or vanes round the periphery. The water thrusts against these, causing the wheel to rotate. Traditional water wheels have been used for centuries, but these large and slow-moving wheels are not suitable for generating electricity. Water turbines used for electricity generation are made from metals, rotate at higher speeds, and are much easier to build and install. Over the years, many turbine designs have been developed to work best in different situations.

Water turbines may be classified in different ways. One way of classification is according to the method of functioning (impulse or reaction turbine); another way is according to the design (shaft arrangement and feed of water). Water turbines may operate as turbines, as pump turbines or as a combination of both. They may be of the single regulated or double regulated type. Turbines may also be classified according to their specific speed.

Impulse turbines use a nozzle at the end of the pipeline that converts the water under pressure into a fast-moving jet. This jet is then directed at the turbine wheel (also called runner), which is designed to convert as much of the jet’s kinetic energy possible into shaft power. Common impulse turbines are Pelton and cross-flow. In reaction turbines the energy of the water is converted from pressure to velocity within the guide vanes and the turbine wheel itself. Spinning of the turbine is a reaction to the action of the water squirting from the nozzles in the arms of the rotor. The typical example of reaction turbine is a  Francis turbine. The advantage of small hydro power reaction turbine is that it can use the full head available at a site. An impulse turbine must be mounted above tailwater level. The advantage of impulse turbine is that it is very simple and cheap and as the water flow varies , water flow to the turbine can be easily controlled by changing nozzle size. In contrast most small reaction turbines cannot be adjusted to accommodate variable  water flow.

Most hydraulic turbines consist of a shaft-mounted water-wheel or “runner” located within a water-passage which conducts water from a higher location (the reservoir upstream from a dam) to a lower one (the river below a dam). Some runners look very similar to a boat propeller, others have more complex shapes. The turbine runner is installed in a water passage that lets water from the reservoir flow pass the runner blades, which makes the turbine spin.

Almost all hydraulic turbine/generator units turn at a constant speed. The constant speed one type of turbine/generator operates at may be considerably different from the speed of another. The best speed for each type of turbine is set during design, and a generator is then designed that will produce usually alternating current at that speed. A device called a governor keeps each unit operating at its proper speed by operating flow-control gates in the water-passage. There are several types of turbine designs like Pelton, Kaplan, Francis or cross-flow turbine.

PELTON TURBINE
The principle of the old water-wheel is embodied in the modern Pelton turbine. This turbine has a similar look and physical principle like a classic water wheel. A Pelton turbine is used in cases where large heads of water are available (more than 40 m). The Pelton turbine is used for heads up to 2000 m. Below 250 m, mostly the Francis turbines are given preference. Today the maximum output lies at around 200 MW.


Together with crosflow turbines, Pelton turbines belong to the impulse type (or free-jet)  turbines, where the available head is converted to kinetic energy at atmospheric pressure and partial admission of flow into the runner. The free jet turbine was invented around 1880 by the American Pelton, after whom it got its name. The greatest improvement that Pelton made was to introduce symmetrical double cups. This shape is basically still valid today. The splitter ridge separates the jet into two equal halves, which are diverted sideways. The largest Pelton wheels have a diameter of more than 5 m and weigh more than 40.000 kg. The wheel must be placed above the tailrace water level, which means a loss of static head, but avoids watering of the runner. In order to avoid an unacceptable raise of pressure in the penstock, caused by the regulating of the turbine, jet deflectors are sometimes installed. The deflector diverts the jet, or part of it, from the runner.

Since then the turbine has been considerably improved in all respects and the output of power has increased. Power is extracted from the high velocity jet of water when it strikes the cups of the rotor (runner). There is a maximum of 40 cup-like paddles jointed in two half-cups each water is being squirted through nozzles onto the blades where it is deflected by 180° and thus gives almost all of its energy to the turbine. By the reversal almost all the kinetic energy is transferred into force of impulse at the outer diameter of the wheel. Because of the symmetry of the flow almost no axial force is created at the runner. From the design point of view, adaptability exists for different flow and  head. Pelton turbines can be equipped with one, two, or more  nozzles for higher output. In manufacture, casting is commonly used for the rotor, materials being brass or steel. This  necessitates an appropriate industrial infrastructure. Pelton turbines require only very little maintenance.

FRANCIS TURBINE
In the great majority of cases (large and small water flow rates and heads) the type of turbine employed is the Francis or radial flow turbine. The significant difference in relation to the Pelton turbine is that Francis (and Kaplan) turbines are of the reaction type, where the runner is completely submerged in water, and both the pressure and the velocity of water decrease from inlet to outlet. The water first enters the volute, which is an annular  channel surrounding the runner, and then flows between the fixed guide vanes, which give the water the optimum direction of flow. It then enters the runner, which is totally submerged, changes the momentum of the water, which produces a reaction in the turbine. Water flows radially i.e., towards the centre. The runner is provided with curved vanes upon  which the water impinges. The guide vanes are so arranged that the energy of the water is largely converted into rotary motion and is  not consumed by eddies and other undesirable flow phenomena causing energy losses. The guide vanes are usually adjustable so as to  provide a degree of adaptability to variations in the water flow rate and in the load of the turbine.
The guide vanes in the Francis turbine are the elements that direct the flow of the water, just as the nozzle of the Pelton wheel does. The water is discharged through an outlet from the centre of the  turbine. In design and manufacture, Francis turbines are much more complex  than Pelton turbines, requiring a specific design for each head/flow condition to obtain optimum efficiency. Runner and housing are usually cast, on large units welded housings, or cast  in concrete at site, are common.
With a Francis turbine, downstream pressure can be above zero. Precautions must be taken against water hammer with this type of turbine. Under the emergency stop, the turbine overspeeds. One would think that more water is going through the turbine than before the trip occurred since the turbine is spinning faster. However, the turbine has been designed to work efficiently at the design speed, so less water actually flows through the turbine during overspeed. Pressure relief valves are added to prevent water hammer due to the abrupt change of flow. Besides limiting pressure rise, the pressure relief valve prevents the water hammer from stirring up sediment in the pipes.
With a big variety of designs, a large head range from about 30 m up to 700 m of head can be  covered. The most powerful Francis turbines have an output of up to 800 MW and use huge amounts of water.

KAPLAN TURBINE
For very low heads and high flow rates a different type of turbine, the Kaplan or Propeller turbine is  usually employed. In the Kaplan turbine the water flows through the propeller and sets the latter in rotation. In this turbine the area through the water flows is as big as it can be – the entire area swept by the blades. For this reason Kaplan turbines are suitable for very large volume flows and they have become usual where the head is only a few meters.

The water enters the turbine laterally, is deflected by the guide vanes, and  flows axially through the propeller. For this reason, these  machines are referred to as axial-flow turbines. They have the advantage over radial-flow turbines that it is technically simpler to vary the angle of the blades when the power demand changes what improves the efficiency of power production. The flow rate of  the water through the turbine can be controlled by varying the distance between the guide vanes; the pitch of the propeller blades must then also be appropriately adjusted. Each setting of the guide vanes corresponds to one particular setting of the  propeller blades in order to obtain high efficiency. Important feature is that the blade speed is greater than the water speed – as much as twice as fast. This allows a rapid rate of rotation even with relatively low water speeds. Kaplan turbines  come in a variety of designs. Their application is limited to heads from 1 m to about 30 m. Under such conditions, a relatively larger flow as compared to high head turbines is required for a given  output. These turbines therefore are comparatively larger.

CROSS-FLOW (BANKI) TURBINE
The concept of the Cross-Flow turbine -although much less  well-known than the three big names Pelton, Francis and Kaplan -is not new. It was invented by an engineer named Michell who obtained  a patent for it in 1903. Quite independently, a Hungarian professor  with the name Donat Banki, re-invented the turbine again at the  university of Budapest. By 1920 it was quite well known in Europe,  through a series of publications. There is one single company who  produces this turbine since decades, the firm Ossberger in Bavaria, Germany. More than 7000 such turbines are installed world-wide,  most of them made by Ossberger.
The main characteristic of the Cross-Flow turbine is the water jet  of rectangular cross-section which passes twice through the rotor  blades -arranged at the periphery of the cylindrical rotor -  perpendicular to the rotor shaft. The water flows through the  blading first from the periphery towards the centre, and then, after crossing the open space inside the runner,  from the inside outwards. Energy conversion takes place twice;  first when water falls down on the blades upon entry, and then when water strikes the blades during  exit from the runner. The use of  two working stages provides no particular advantage except that it  is a very effective and simple means of discharging the water from the runner.
The machine is normally classified as an impulse turbine. This is  not strictly correct and is probably based on the fact that the original design was a true constant-pressure turbine. A sufficiently large gap was left between the nozzle and the runner,  so that the jet entered the runner without any static pressure. Modern designs are usually built with a nozzle that covers a bigger  arc of the runner periphery. With this measure, unit flow is  increased, permitting to keep turbine size smaller. These designs  work as impulse turbines only with small gate opening, when the  reduced flow does not completely fill the passages between blades  and the pressure inside the runner therefore is atmospheric. With  increased flow completely filling the passages between the blades,  there is a slight positive pressure; the turbine now works as a reaction machine.
Cross-Flow turbines may be applied over a head range from less than 2 m to more than 100 m (Ossberger has supplied turbines for heads  up to 250 m). A large variety of flow rates may be accommodated  with a constant diameter runner, by varying the inlet and runner  width. This makes it possible to reduce the need for  tooling, jigs and fixtures in manufacture considerably. Ratios of  rotor width/diameter, from 0.2 to 4.5 have been made. For wide  rotors, supporting discs welded to the shaft at equal intervals  prevent the blades from bending.
A valuable feature of the Cross-Flow turbine is its relatively flat  efficiency curve, which Ossberger are further improving by using a divided gate. This means that at reduced flow, efficiency is still  quite high, a consideration that may be more important than a higher optimum-point efficiency of other turbines. Due to low price and good control these turbines are, however, very successful in the area of small hydro-electric power plants.