Modern impulse turbines are generally know as Pelton type and are restricted to relatively high-head applications . One or more jets of water impinge on a wheel containing many curved buckets. The jet stream is directed inward, sideways, and outward, thereby producing a force on the bucket, which in turn results in a torque on the shaft. All kinetic energy leaving the runner is "lost".
Pelton Wheel Display
A draft tube is generally not used since the runner operates under approximately atmospheric pressure and the head represented by the elevation of the unit above tailwater cannot be utilized. Since this is a high-head device, this loss in available head is relatively unimportant. As will be shown later, the Pelton wheel is a low specific speed device. Specific speed can be increased by the addition of extra nozzles, the specficc speed increasing by the square root of the number of nozzles. Specific speed can also be increased by a change in the manner of inflow and outflow.
Most Pelton wheels are mounted on a horizontal axis, although newer vertical-axis units have been developed. Because of physical constraints on orderly outflow from the unit, the maximum number of nozzles is generally limited to six or fewer. While the power of a reaction turbine is controlled by the wicket gates the power of the Pelton wheel is controlled by varying the nozzle discharge by means of an automatically adjusted needle, as illustrated below.
Cross section of a single wheel, single jet Pelton turbine. This is the third-highest-head pelton turbine in the world, H = 1447 m, n = 500 rpm, P = 35.2 MW, Ns ~ 0.038. (Courtesy of Vevey Charmilles Engineering Works, Adapted from J. Raabe, Hydro Power: The Design, Use, and Function of Hydromechanical, Hydraulic, and Electrical Equipment , VDI Verlag, Dusseldorf, Germany.)
Jet deflectors, or auxiliary nozzles are provided for emergency unloading of the wheel. Additional power can be obtained by connecting two wheels to a single generator or by using multiple nozzles. Since the needle valve can throttle the flow while maintaining essentially constant jet velocity, the relative velocities at entrance and exit remain unchanged, producing nearly constant effciency over a wide range of power output.
Specific Speed Calculations:
The specific speed nsof a turbine dictates the turbine's shape in a way that is not related to its size. This allows a new turbine design to be scaled from an existing design of known performance. The specific speed is also the main criterion for matching a specific hydro-electric site with the correct turbine type.
The formula suggests that the Pelton turbine is most suitable for applications with relatively high hydraulic head H, due to the 5/4 exponent being greater than unity, and given the characteristically low specific speed of the Pelton.