12.8 Hydraulic turbines and their selection
12.8.1 Types of hydraulic turbinesHydraulic turbines may be considered as hydraulic motors or prime
movers of a water-power development, which convert water energy
(hydropower) into mechanical energy (shaft power). The shaft power
developed is used in running electricity generators directly coupled to the
shaft of the turbine, thus producing electrical power.
Turbines may be classified as impulse- and reaction-type machines
(Nechleba, 1957). In the former category, all of the available potential energy
(head) of the water is converted into kinetic energy with the help of a con-
tracting nozzle (flow rate controlled by spear-type valve – Fig. 12.8(a)) pro-
vided at the delivery end of the pipeline (penstock). After impinging on the
curved buckets the water discharges freely (at atmospheric pressure) into the
downstream channel (called the tail race). The most commonly used impulse
turbine is the Pelton wheel (Fig. 12.8(b)). Large units may have two or more
jets impinging at different locations around the wheel.
In reaction turbines only a part of the available energy of the water is
converted into kinetic energy at the entrance to the runner, and a substantial
part remains in the form of pressure energy. The runner casing (called the
scroll case) has to be completely airtight and filled with water throughout the
operation of the turbine. The water enters the scroll case and moves into the
runner through a series of guide vanes, called wicket gates. The flow rate and
HYDRAULIC TURBINES AND THEIR SELECTION 505
Fig. 12.8 Impulse turbine (Pelton wheel)