An impulse turbine is a type of turbine often found in steam power plants. The impulse turbine was invented in the late 19th century by Swedish inventor Gustaf de Laval, and it has been improved significantly and employed in a variety of ways ever since. Turbines, however, are not exclusive to steam; an impulse turbine is any turbine whose rotation is created by fixed nozzles, where jets of the working fluid — whether it is steam, water or other gases — push the buckets on a rotor. This is very different from a reaction turbine, whose nozzles rotate with the rotor itself. A good example of a reaction turbine would be an automatic garden sprinkler, and an impulse turbine is more like a toy pinwheel.
The most common type of impulse turbine is the impulse steam turbine, where fixed nozzles help create high-velocity jets of steam. The kinetic energy from the steam velocity is then used to rotate the shaft of the steam turbine at very high speeds. The fixed nozzles in front of the spinning blades help to increase the velocity of steam at the expense of a decrease in pressure. This is a very important part of the process, because velocity and pressure both play equally important roles in turbine efficiency. This is much unlike a reciprocating steam engine, where engine revolutions per minute is mainly dependent on steam pressure alone.
Another type of impulse turbine is the impulse water turbine, which is used in hydroelectric power plants around the world. These types of turbines are known as Pelton wheels, and the rotor consists of many buckets to pick up the flowing water. Pelton wheels, however, are very different from a typical water wheel; water wheels rely on the weight of water for rotation, but the Pelton wheel relies on the flow. Pelton wheels are incredibly efficient, because they use nearly all of the energy from the velocity of water.
Impulse turbines come in a wide variety of types and sizes. Sizes range anywhere from small devices of less than one horsepower to large industrial turbines capable of delivering more than 2 million horsepower. Smaller turbines are a rarity, having been used only for smaller jobs, such as generating electricity on steam locomotives. Larger turbines, however, are responsible for about 80 percent of the world's electricity. These turbines often are a combination of both impulse and reaction turbines.