If the turbines are to drive electricity generators their speeds must
correspond to the nearest synchronous speed for a.c. machines and the
correct speed physically possible for d.c. machines. For synchronous
running, the speed Nis given by
N 120 f/p (12.4)wherefis the frequency of the a.c. supply in Hz (50–60 Hz) and pis the
number of poles of the generator (divisible by 4 for heads up to 200 m or
by 2 for heads above 200 m). The recommended (Mosonyi, 1987) normal
speeds for 50 Hz a.c. supply machines are 3000, 1500, 1000, 750, 600, 500,
375, 300, 250, 214, 188, 167, 150, 125, 107, 94, 83, 75 and 60 rev min^1.
If the external load on the machine suddenly drops to zero (sudden
rejection) and the governing mechanism fails at the same time, the turbine
will tend to race up to the maximum possible speed, known as runaway
speed. This limiting speed under no-load conditions with maximum flow
rate must be considered for the safe design of the various rotating com-
ponents of the turbogenerator unit. The suggested runaway speeds of the
various runners for their appropriate design considerations are given in
Table 12.3.
HYDRAULIC TURBINES AND THEIR SELECTION 509
Table 12.2 Q–H–Nsdata (after Raabe, 1985): Q(m^3 s^1 );H(m);P(kW);
D(m);N(rev min^1 )
Type of runner Specific speed, Maximum Unit Unit speed
NsNP1/2/H5/4 head, discharge, NuND/H1/2
H (m) QuQ/D^2 H1/2
Impulse (Pelton) 007–11 1650–1800 0.011–0.007 39.4–39.8
011–17 700–1650 0.024–0.011 38.9–39.4
017–26 350–700 0.055–0.024 37.6–38.9
Francis:
slow 051–107 410–700 0.35–0.1 63.6–60.8
medium 107–150 240–410 0.59–0.35 67.5–63.6
150–190 150–240 0.83–0.59 72.6–67.5
fast 190–250 090–150 1.13–0.83 81–72.6
250–300 064–90 1.28–1.13 92.2–81
Kaplan 240–450 050 1.22–0.93 145–85
330–560 035 1.61–1.29 155–100
390–690 020 2.0–1.6 170–110
490–760 015 2.35–2.0 180–120
570–920 006 2.45–2.35 200–135