The prime movers or turbines in hydroelectric power plants converts the kinetic energy of the water into mechanical energy which in turn converts into electrical energy. As per the action of water on the turbines, they are classified as impulse turbine and reaction turbine. In case of impulse turbine, the pressure energy of water is converted to kinetic energy when passed through the nozzle and forms the high velocity get of water. The formed water jet is used for driving the wheel. In case of reaction turbine, the water pressure combined with the velocity works on the runner. The power is in the turbine is the combined action of pressure and velocity of the water that completely fills the runner and water passage.
The casing of the impulse turbine operates at the atmospheric pressure whereas the casing of the reaction turbine operates under high pressure. The pressure acts on rotor and vacuum underneath it. This is why the casing of the reaction turbine is made completely leak proof.
Pelton Wheel Turbine in Hydroelectric power plant:
Pelton wheel is a special type of axial flow impulse turbine generally mounted on horizontal shaft. A number of buckets are mounted round the pheripery of the wheel. The water is directed towards the wheel through nozzles. The flow of nozzle is generally controlled by the special regulating system. The water jet after imprinting on the buckets is deflected through an angle of 160o and flows axially in both the directions thus avoiding the axial thrust on the wheel. The hydraulic efficiency of pelton wheel lies between 85 to 95%. Pelton wheels are generally used where the head is above 2000 meters.
In most of the pelton wheel plants, single jet with horizontal shaft is used. The number of jets adopted depends on the specific speed required.
Any impulse turbine achieves the maximum efficiency when the velocity of the bucket at the center the line of the jet is slightly under half the jet velocity. Hence, for maximum speed of rotation, the mean diameter of the runner should be as small as possible. There is a limit to the size of the jet which can be applied to any impulse turbine runner without reducing the efficiency (D/d is about 10:1). The jet of the pelton turbine strikes the splitter edge of the bucket, bifurcates and is discharged at either side.
Francis Turbine in Hydro power plant:
In Francis turbine, the water enters into the casing with relatively lower velocity, passes through the guide vanes located around the circumference and flows through the runner and finally discharges into the draft tube sealed below the tailrace water level. The water passage from the headrace to tailrace is completely filled with water which acts upon the whole circumference of the runner. A large part of the power is obtained from the difference in the pressure acting upon the front and back of the runner buckets, only a total part of the power is delivered from the dynamic action of the water.
There are two types of Francis turbines known as open flume type and closed flume type.
In open flume type, the turbine is immersed under the water of the headrace in a concrete chamber and discharges into the tailrace through the draft tube. The main disadvantage of this type is that, the runner and the guide-vane mechanism is under the water and they are not open for either for inspection or repair without draining the chamber.
In closed type, the water is led to the turbine through the penstock whose end is connected to the spiral casing of the turbine.
The guide vanes are provided around the runner to regulate the water flowing through the turbine. The guide vanes provide gradually decreasing area of flow for all gate openings, so that no eddies are formed and efficiency does not suffer much even at part load conditions. The majority of the Fracis turbines are inward radially flow type and most preferred for medium heads. The inward flow turbine has many advantages over the outward flow turbines:
The chances of eddy formations and pressure loss are reduced as the area of flow becomes gradually convergent.
The runaway speed of the turbine is automatically checked as the centrifugal force acts upwards while the flow is inwards.
The guide vanes can be located on the outer periphery of the runner, therefore better regulation is possible.
The frictional losses are less as the water velocity over the vanes is reduced.
The inward flow turbine can be used for fairly high heads without increasing the speed of the turbine as centrifugal head supports the considerable part of the supply head.
Propeller Turbine in Hydro power plant:
The propeller turbine runner may be considered as a development of Francis type in which the number of blades is greatly reduced and the lower band omitted. It is an axial flow turbine having a small number of blades from three to six. The propeller turbine may be fixed blade type or movable blades type known as Kaplan turbine.
The fixed blade propeller type turbine has high efficiency (88%) of the full load but its efficiency rapidly drops with decrease in the load. The efficiency of the unit is hardly 50% at 40% of full load operation. The use of propeller turbine is limited to the installation where the units run at the full load conditions all the times. The use of propeller turbine is further limited to the low head installations of 5 to 10 meters.
Kaplan Turbine in Hydro power Plant:
In Kaplan Turbine, the blades are rotated to the most efficient angle by the hydraulic servo motor. A cam on the governor is used to change the blade angle with the gate position so that high efficiency is always obtained at almost any percent of the load
These Kaplan turbines are constructed to run at a speeds varying from 60 to 220 rpm and to work under varying heads from 2 to 60 meters.These are particular suitable for varying heads and for varying flows and where the ample quantity of the water is not required to maintain constant flow.
The velocity o the water flowing through the Kaplan turbine is high as the flow in large and therefore the cavitation is more serious problem in Kaplan turbine than in Francis Turbine
The propeller type turbines have an outstanding advantage of higher speed which results in lower cost of the runner, generator and smaller power house substructure and superstructure.
The capital of maintenance of the Kaplan turbine is much higher than fixed blade propeller type units operating at a point of maximum efficiency.
For a low head development with fairly constant head and requiring a number of units, it is always advisable to install fixed blade propeller type runners for most of them and Kaplan type for only one or two units. With this combination, the fixed blade units can be operated at maximum efficiency and Kaplan units could take the required variations in the loads. Such an operation is particularly suitable to a large power system consisting of multiplicity of units
Francis Turbine vs Pelton Wheel:
The Francis turbines are used for all available heads on the other hand, pelton wheels are used for very high heads only (200 mts to 2000 mts).
The Francis Turbines are preferred over Pelton wheel for following reasons:
Advantages of Francis Turbine:
- The variation in the operating head can be more easily controlled in Francis turbine than in Pelton wheel turbine
- The ratio of maximum and minimum operating head can be even be two in the case of Francis Turbine
- The operating head can be utilized even when the variation in the tail water level is relatively large when compared to the total load
- The size of the runner, generator and power house required is small and economical if the Francis turbine is used instead of Pelton wheel for the same power generation.
- The mechanical efficiency of the pelton wheel decreases faster with wear than Francis turbine
Drawbacks of Francis Turbine:
- Water which is not clean can cause very rapid wear in high head Francis turbine. In passing through the guide vanes and cover facings, it can quickly reduce overall efficiency of the turbine by several percentage. The effect is much more serious in turbines of smaller diameter than in larger ones
- Particles of solid matter in the water will wear the tip of the spear, the nozzle and after several years the runners also. The first two components are easily removable, renewable and repairable. The runner repairing by welding can often be done without removing the runner from the shaft or casing
- The inspection and overhaul of a Francis Turbine is much more difficult job than that of the equivalent pelton turbine.
- Cavitation is an ever present danger in Francis Turbine as well as in all the reaction turbines. The raising of the power house floor level to reduce the danger of flooding may be followed by the endless cavitation troubles
- Usually below 60% load, the pelton wheel have much better efficiency than the Francis turbine of lower specific speeds. If there is a possibility of running the prime mover below 50% load for long time, the Francis will not loose efficiency but cavitation danger will become more serious
- The water hammer effect with the Francis turbine is more troublesome than the pelton turbine
Kaplan Turbine vs Francis Turbine:
The advantages of Kaplan over Francis Turbine are listed below:
Advantages of Kaplan Turbine:
- It is more compact in construction and smaller in size for the same power developed
- Its part load operating efficiency is considerably high. The efficiency curve of Kaplan turbine remains flat for over the whole load range
- The frictional losses passing through the blades considerably lower due to small number of blades used in Kaplan Turbine