Power Plant Engineering

Cavitation Factor. Prof. D. Thowa (Germany) suggested a cavitation factor to determine the
zone where the turbine can work without any danger of cavitation.
The critical value of cavitation factor is given by
σ
c
= 
[(
)
]
a
v
H
H
h
H
−
−

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374
POWER PLANT ENGINEERING
where
H
a
= Atmospheric pressure head in meter of water
H
v
= Vapour pressure in metre of water corresponding to the water temperature
 H = Working head of turbine (difference between headrace and tailrace level in meters)
h = Height of turbine outlet above tailrace level in meters.
The values of H
a
with respect to the altitudes above sea level and the values of H,, with respect to
the water temperatures are tabulated in the tables (11.5) and (11.6). The values of critical factor depend
upon the specific speed of the turbine. The critical values of cavitation factors with respect to specific
speed are tabulated in tables.
Table 11.5. Altitude V
s
Atmospheric Pressure
Altitude above sea level in metres
0
1000
2000
3000
4000
Baromatric
mm of Hg
760
676
595
528
463
Pressure
Metres of water
10.35
9.2
8.1
7.2
6.3
Table 11.6. Saturation Pressure as Function of Temperature
Temp °C
Pressure in kg/cm
2
Pressure in mm of Hg
Pressure in metres of
water
0
0.0012
4.60
0.062
2
0.00716
5.30
0.072
4
0.00829
6.30
0.083
6
0.00953
7.04
0.095
8
0.01093
8.05
0.1095
10
0.01251
9.23
0.1251
12
0.01429
10.50
0.1430
14
0.01629
12.05
0.1630
16
0.01853
13.70
0.1855
18
0.02103
15.50
0.2105
20
0.12383
17.60
0.2385
22
0.02700
19.95
0.2700
24
0.00040
22.40
0.3040
26
0.03430
25.30
0.3430
28
0.03850
28.40
0.3850
30
0.04330
32.00
0.4330
32
0.04850
35.80
0.4850
34
0.05420
40.00
0.5420
36
0.06050
44.70
0.6060
38
0.06760
50.70
0.6760
40
0.07520
55.50
0.7520
42
0.08360
61.70
0.8360
44
0.09280
68.50
0.9280
46
0.10280
76.00
1.0280
48
0.11380
84.00
1.1310
50
0.12580
93.00
1.2580

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HYDRO-ELECTRIC POWER PLANTS
375
Table 11.7. (N
s
V
s
σσσσσ
s
)
Francis
Kaplan
N
s
s
c
N
s
σσσσσ
c
50
0.04
300 to 450
0.35 to0.40
100
0.05
450 to 550
0.40 to 0.45
150
0.07
550 to 600
0.46 to 0.60
200
0.1
650 to 700
0.85
250
0.14
700 to 800
1.05
300
0.2
—
—
350
0.27
—
—
11.12.1 METHODS TO AVOID CAVITATION
1. Installation of Turbine below Tailrace Level. The danger of cavitation increases in case of
low head and high speed propeller runner as the value of (V
0
2
– V
d
2
)/g is considerably large as men-
tioned earlier. In order to keep the value of p
c
within the cavitation limit, the value of h is made negative
keeping the runner below tailrace level. For such installations, the turbines remain always under water.
It is riot advisable as the inspection and repair of the turbine is difficult. The other method to avoid
cavitation zone without keeping the runner under water is to use the runner of low specific speed as
mentioned earlier.
2. Cavitation Free Runner. The cavitation free runner can be designed to fulfill the given con-
ditions with extensive research. The shape of the blade, the angle of the blade, the thickness of the blade
can be changed and experiments can be conducted to find out the best dimensions of the blade (shape,
size, angle. etc.
3. Use of Material. The cavitation effect can be reduced by selecting materials which can resist
better the cavitation effect. The cast steel is better than cast iron and stainless steel or alloy steel is still
better than cast steel. The pitting effect of cavitation on cast steel can be repaired more economically by
ordinary welding. It has been observed that the welded parts are more resistant to cavitation than ordi-
nary ones.
4. Polishing. The cavitation effect is less on polished surfaces than ordinary one. Mat, is why the
cast steel runners and blades are coated with stainless steel.
5. Selection of Specific Speed. By selecting a runner of proper specific speed for the given head
from equation (c) and from Tables (11.5) and (11.6), it is possible to avoid the cavitation.