Power Plant Engineering

HYDRO-ELECTRIC POWER PLANTS 369

high specific speed reaction runner would reduce the size of the runner to such an extent that the dis-
charge velocity of water into the throat of draft tube would be excessive. This is objectionable because
a vacuum may be created in the extreme case.

The runner of too high specific speed with high available head increases the cost of turbine on
account of high mechanical strength required. The runner of two low specific speed with low available
head increases the cost of generator due to the low turbine speed.

An increase in specific speed of turbine is accompanied by lower maximum efficiency and greater
depth of excavation of the draft tube. In choosing a high specific speed turbine, an increase in cost of
excavation of foundation and draft tube should be considered in addition to the efficiency. The weighted
efficiency over the operating range of the turbine is more important in the selection of a turbine instead
of maximum efficiency.

Experience has determined that there is a range of heads and specific speeds for each type of
turbine. Special conditions may sometimes dictate departure from common practice.

The ranges of heads and specific speeds for different types of turbines are tabulated in
Tables 11.3 and 14.4.

Table 11.3

Type of Turbine Range of Head Specific speed in metric units

Pelton (1 nozzle) 200 metres 10—20

Pelton (2 nozzles) to 20—40

Pelton (4 nozzles) 2000 metres 40—50

Turgo impulse turbine as above 50—100

Francis (low speed) 15 metres 80—120

Francis (medium speed) to 120—220

Fracis (high speed) 300 metres 220—350

Francis (express) 350—420

Propeller 5 metres to 30 metres 310—1000

Table 11.4

Type of Turbine Ns in MKS Ns in SI Ns in FPS

Axial flow Slow 300 – 450 14.8 – 22.2 67.5 – 101.2

(Kaplan) Normal 450 – 700 22.2 – 34.6 101.2 – 157.4

Fast 700 – 1200 34.6 – 59.3 157.4 – 270.0

Radial and Mixed Slow 60 – 150 2.9 – 7.4 5.5 – 33.7

flow (Francis Normal 150 – 250 7.3 – 12.4 33.7 – 56.2

and Deriaz) Fast 250 – 400 12.4 – 19.8 56.2 – 90.0

Impulse (Pelton) Slow 4 – 10 0.2 – 0.5 0.9 – 2.3

Normal 10 – 25 0.5 – 1.2 2.3 – 5.6

Fast 25 – 60 1.2 – 3.0 5.6 – 13.5