The main type of impulse turbine in use today is the Pelton turbine, patented by the American engineer Lester Allen Pelton in 1889. It is found mainly in applications where a high head of water is available. Another type, called the Turgo turbine, has also been developed, again for high-head applications. In both cases, the head of water will normally be greater than 450 m, although the Pelton turbine is applicable for heads of between 200 and 1000 m. (For heads higher than 1000 m there will probably be two turbines, each exploiting half the head.)
A high head of water will generate an enormous pressure at its base. If the water is released through a narrow nozzle, the pressure of water will generate a fierce jet of water. The impulse turbine harnesses this energy of motion.
The Pelton turbine has bucket-shaped blades. The high-pressure jet of water is directed into the buckets at an angle that ensures that the energy in the water is virtually all converted into rotary motion of the turbine wheel. This conversion process can achieve an efficiency of nearly 95%, under ideal conditions, so little energy is wasted.
One of the keys to the operation of an impulse turbine is that it must rotate in the air. If it becomes submerged, its rotation is hampered. This is in direct contrast to the second type of turbine, the reaction turbine, which must be submerged to operate efficiently.
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