Power Plant Engineering

HYDRO-ELECTRIC POWER PLANTS 367

2. The inspection and overhaul of a Francis is much more difficult job than that of the equivalent
   Pelton turbine. The badly worn-out parts will have to be replaced by new ones and it will take a consid-
   erable time.


3. Cavitations is an ever-present danger in Francis as well as in all reaction turbines. The raising
   of power house floor level to reduce the danger of flooding may be followed by endless cavitations
   troubles.


4. Usually below 60% load, the Pelton is much better as it gives more efficiency than Francis of
   low specific speed. If there is possibility of running the prime-mover below 50% load for a long period,
   the Francis will not only lose its efficiency but the cavitation danger will become more serious.


5. The water hammer effect with the Francis is more troublesome than the Pelton turbine. Kaplan
   versus Francis Turbine.

The advantages of Kaplan over Francis are listed below :

1. It is more compact in construction and smaller in size for the same power developed.


2. Its part-load efficiency is considerably high. The efficiency curve remains more or less flat
   over the whole load range.


3. The frictional losses passing through the blades are considerably lower due to small number of
   blades used.

11.11 Specific Speed of Turbine

The specific speed of a turbine is defined as the speed at which the turbine runs developing one
B.H.P. under a head of one metre.

The equation for the specific speed of a turbine can be obtained by using the principle of similar-
ity.

(1) V =

60

πDN∝ H

∴ D ∝

H

N

, where D and N are diameter and speed of a turbine and H is the head acting on

the turbine.

(2) Q = πDB. Vf ∝ D^2 H

where B is the height of the blade and Vf is the velocity of flow.

Substituting the value of D in the above equation,

∴ Q ∝

3/ 2

2

()H

N

(3) P =

75

ρQH

where P is the power developed.