Environmental Engineering Reference
In-Depth Information
For smaller capacities, Pelton turbines are sometimes used from 50 m, and Francis
turbines from 6 m, in old small power stations even for heads as low as 2 m.
Depending on the turbine type and size, efficiencies between approximately 85
and 93 % are achieved at the design discharge and head. The turbine efficiency is
defined as the power at the turbine shaft and the available hydraulic power be-
tween the turbine intake and the draft tube outlet, including turbine outlet losses.
As the turbine is only designed for a specific discharge, the efficiency is depend-
ent on the respective availability of water. Fig. 8.9 shows the sometimes very dif-
ferent efficiency curves for the main turbine types. Pelton turbines, for example,
have a very good efficiency even when working at only 20 % of their maximum
discharge. Propeller turbines, however, should not be operated at less than 70 to
80 % of their design flow, as high losses are generated otherwise.
Layout and function of different turbine types are discussed in more detail be-
low.
100
90
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70
60
50
40
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10
0
0.0
0.1
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1.0
Ratio of flow to design flow
Fig. 8.9
Efficiency curve of different turbine types (see /8-6/)
Kaplan, propeller, bulb, bevel gear, S and Straflo-turbines.
The Kaplan turbine
and the designs derived from it in principle operate like a reverse operating pro-
peller. With the exception of vertical shaft Kaplan turbines with radial approach
flow, the flow runs through all these turbines axially; in the case of vertical Kap-
lan turbines the water is directed via the adjustable guide vanes to the rotating
runner. Vertical, horizontal and slanting axis positions can occur. Additionally,
Kaplan turbines and their derivatives have adjustable runner blades (so-called
double regulation turbines). They allow a better adjustment to different flow rates
and thus an efficiency improvement for various operating conditions.
Propeller turbines have rigid runner blades. With these blades they are not very
adjustable to changing flows but show high efficiency at design discharge. They
are mainly used if several machines within a hydroelectric power station are util-
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