A hydraulic turbine is a machine that converts the energy from moving water into the rotational force of a shaft. The rotating shaft can be connected to a generator to create electricity. The main distinction of one hydraulic turbine from another is whether it is a reaction or impulse type. These categories are further classified into specific designs including Francis, Kaplan, Pelton, Turgo, and cross-flow turbines.
A reaction turbine creates rotation by enclosing the shaft with its attached wheel in a casing to contain the water pressure and by operating immersed within the water flow. The angled blades around the shaft on a reaction hydraulic turbine cause lift to take up pressure and force from the water and convert it to rotation. Reaction turbines generally operate with low to moderate head, or water pressure, levels.
One type of reaction hydraulic turbine is the Francis design. A spiral-shaped inlet directs water into the wheel, or runner. Adjustable vanes guide the water against the runner at the desired angle. The pressure and flow of the water are changed by the reaction with the runner, thus creating torque.
The Kaplan, or propeller, turbine is another variation on the reaction type. The design incorporates vanes to guide the water flow and blades on the rotor shaft similar to a ship's propeller. The blades are adjustable to maximize efficiency through a range of pressure levels if there is enough water flow, thereby making this design very flexible.
An impulse turbine creates rotation through the force of a stream of water directed toward a series of buckets, or cups, surrounding the shaft. The buckets are not immersed in water, but are moved by the water stream. The velocity of the water is increased by forcing it through a nozzle before it strikes the buckets. Impulse turbines can operate at high head levels and do not require the wheel to be encased.
The Pelton version of the impulse hydraulic turbine, i.e., the Pelton wheel, has a series of buckets in a ring around the shaft. The buckets are divided in the middle so the force of water from the nozzle is not blocked by one bucket from reaching the next. The water curves around the shape of each rounded bucket through almost 180° and its velocity decreases substantially. The slowed water drops into a discharge channel below the turbine.
Another impulse turbine is the Turgo design. This unit is similar to the Pelton wheel, but the buckets are flat on one edge surrounded by an encircling band. The nozzle directs the water against the buckets so that it enters on one side and discharges on the other. The Turgo handles a higher speed than the Pelton configuration using a smaller wheel because the inlet water doesn't mix with the discharge.
Cross-flow turbines operate with a drum-shaped rotor capped at either end and joined with rounded slats that are sometimes in a helix or double-helix design. The nozzle of this hydraulic turbine directs the water across the rotor such that the water contacts the blades twice before expending its energy and falling into the discharge. Cross-flow turbines can operate with very little head, even with a large flow.