The average output of the plant is 300.412 MW. The units will
be designed to produce this power most efficiently and any variation
in demand will be provided by additional units. There are several
alternatives: one 12 MW unit and three units of 6 MW each, five units
of 6 MW each, three units of 10 MW each, or two units of 15 MW
each, as it is better to have fewer larger units than many smaller
units, provided that the units are capable of working efficiently under
varying load conditions. More details on the nature of demand vari-
ability would provide better answers.
- As an example, let us design the scroll casing for a 6 MW unit.
Assuming a specific speed of 800, the operational speed of the unit
NNsH5/4/ 6
0
0
0
183 rev min^1.
Adopt a synchronous speed of 150 rev min^1 , as lower specific speeds
are desirable in order to minimize the cavitational problems. This
gives the specific speed, Ns653. The discharge Q per unit is
6000/0.99.81 10 68 m^3 s^1. Note that an efficiency of 90% is
used, which corresponds to high specific speeds. Therefore the
runner diameter, D4.57(68/150)1/33.50 m.
Adopting a maximum entrance velocity of 1.7 m s^1 (Zipparo and
Hasen, 1993), the inlet area of the flume entrance to the scroll is
68/1.740 m^2. Assuming a height of 5 m, the width of the entrance
flume is 40/58m.
SCROLL CASE DESIGN
A concrete scroll with a contracting section is adopted. Equation (12.19)
gives R values as a function of the position angle of the spiral
(Fig. 12.13). The vortex strength
K 30 gH/πN5.61 m^2 s^1.
The outer diameter of the guide vane assembly ( 2 r 0 )1.5D5.25 m.
Thereforer 0 5.25/22.625 m. The height of the inflow section at guide
vane assembly, h 0 0.4D1.40 m. 7.47 with 1 2 15°. For the nose
angle, a partial spiral is suitable because of large discharges and low heads.
Adopting a 240° spiral, the heights H 0 (Fig. 12.13(b)) can be calculated for
different values of .
Assuming the maximum spiral height to be 5 m (to conform to the
entrance height adopted) and assuming a linear variation between h 0 and
H0max, i.e. between 1.4 m and 5 m, the following table gives the values of
H 0 ():
544 HYDROELECTRIC POWER DEVELOPMENT
