Assuming a specific speed of, say, 500, the power per machine
(NsH5/4/N)^2 11 377 kW.Therefore the number of units is 30 000/11 3772.64. Therefore, choose
three units, each having an installed capacity of 10 MW. Note that the
number of units depends on other factors such as the variability of power
demand, breakdown–maintenance works, the availability of national grid
power supply in case of emergencies, etc.
The specific speed
Ns 150
1
0
00
0
/165/4468.The discharge per unit is 10 000/0.949.8116 (assuming an effi-
ciency of 94% Table 12.1)67.75 m^3 s^1. Therefore the runner diameter,
D4.57(Q/N)1/33.50 m, and the inlet velocity (i.e. the exit velocity at the
runner) is 67.75/(π/4)3.53.57.04 m s^1 ; hence the inlet velocity head
is 2.53 m. The exit velocity head is 1.516/1000.24 m.
Applying Bernoulli’s equation between the inlet of the draft tube
and the tailwater level (Fig. 12.16)
Yspa/ g pv/ g (V^21 /2g V^22 /2g hfd)10.3 3.0 0.85(2.53 0.24)5.35 m above TWL.From Thoma’s cavitation limiting conditions,
YsB H, 0.58 (from Table 12.4),givingYs1.02 m above TWL. In the absence of further data, Thoma’s cri-
terion may be adopted.
Worked Example 12.7
A run-of-river plant uses a mean head of 10 m and generates approxi-
mately 30 MW. The load factor of the installation is 40%.
- Determine the number, type, and specific speed of the turbines.
- Design the scroll case for one of the units.
Solution
- For low heads and large discharges the use of Kaplan-type units is
suggested. Also, these units can cope with the variable demand very
efficiently.