508 HYDROELECTRIC POWER DEVELOPMENT
Fig. 12.10 Turbine performance curves
load condition) it is better to use a greater number of smaller machines for
widely variable operating conditions. The load variation is then met by
putting machines into and out of service, so that individual sets do not
have too wide a variation of load and therefore always operate as effi-
ciently as possible.
12.8.4 Operating or normal speed, Nand runaway speed
All runners having the same geometrical shape, regardless of size, have the
same specific speed; specific speed is a useful parameter for the selection of a
runner for particular duties. Charts or tables (Table 12.2) can be used for this
selection. If the head and flow rate are known, the speed of the unit Ncan be
calculated from equations (12.1) and (12.2) (see worked example 12.3).
