Pelton Turbines
This chapter deals with the history and the latest developments of Pelton turbines. Together with Turgo and crosflow turbines, Pelton turbines belong to the impulse type. The common characteristic is the pressureless and partial admission of flow into the runner.
On the Pelton turbine the torque is generated by deflection of the free jet in the double buckets of the runner. That is why the Pelton turbine is also called free jet turbine.
The free jet turbine was invented around 1880 by the American Pelton, after whom it got its name. The greatest improvement that Pelton made was to introduce symmetrical double buckets. This shape is basically still valid today. The splitter ridge separates the jet into two equal halves, which are diverted sideways. Since then the turbine has been considerably improved in all respects and the output of power has increased.
The free jet turbine is used for heads up to 2000 m. Below 250 m, mostly the Francis turbines are given preference. Today the maximum output lies at around 200 MW.
Depending on the discharge, head and quality of the water, Pelton turbines are installed with a horizontal shaft, with 1 or 2 jets per runner, as single or twin turbine, or with a vertical shaft with up to 6 jets. Generally the electrical generator is coupled directly to the shaft. The speed of small turbines may be adjusted by belt drives or gear boxes.
Basically the turbine consists of the following parts: nozzle, runner and housing. The runner (photograph 1 ) is mostly a one piece steel casting, containing 13 % chrome. The largest Pelton wheels have a diameter of more than 5 m and weigh more than 40'000 kg.
In the nozzle (illustration 8) the pressure of the water is converted into velocity. The nozzle consists of a nose-piece which is fixed to a pipe bend, and a needle which can be moved inside the pipe bend. The nose cone and nose piece, which are subject to wear, are made out of high quality material and are easy to exchange.
The buckets are shaped in such a way that the ridge in the middle divides the free jet into two equal parts which are reversed by almost 180°(illustration 2).
By the reversal almost all the kinetic energy is transferred into force of impulse at the outer diameter of the wheel. Because of the symmetry of the flow almost no axial force is created at the runner.
The wheel must be placed above the tailrace water level, which means a loss of static head, but avoids watering of the runner. In order to avoid an unacceptable raise of pressure in the penstock, caused by the regulating of the turbine, jet deflectors are sometimes installed (illustration 1 2). The deflector diverts the jet, or part of it, from the runner.