HYDRO-ELECTRIC POWER PLANTS 373Cavitation and Limitation of Turbine Height above Tailrace Level. The formation of water
vapour and air bubbles on the water surface due to the reduction of pressure is known as "Cavitation".
When the pressure on the water reduces below the saturation pressure corresponding to the temperature
of the water, the rapid formation of water vapour and air bubbles starts. The bubbles suddenly collapse
with the violent action and collapsing pressure will be very high. The rapid formation and collapsing of
the bubbles causes the pitting of the metallic surface. It also reduces the efficiency of the hydraulic
prime mover causing honeycombing of runner and blade contours which reduces the power output.
Referring to Fig. 11.27, we can write
Ec = Ed22cc
dVp
hh
g
++ +
ρ- hf =
22Vpdd
g
+
ρ∴22Vc
g+ h + hd + cp
ρ- hf =
22Va
g+ ap
ρ+ hdas dp
ρ=pa
ρ + hd for pressure equilibrium∴22Vc
g+ h + cp
ρ- hf =
22Vd
g+ ap
ρ∴ pc
ρ= ap
ρ222cd
f
hhVV
g−
+−
h = acpp−
ρ
222VVcd
g−
+ hf= acpp−
ρ
22Vc
g+22d
fV
h
g
+
The equation shows that the pressure at point c (at exit of the turbine) is below atmospheric
pressure. The pressure p; should not be below the cavitation pressure which is the saturation pressure of
water at the water temperature to avoid the cavitation in turbine.
An increase in height of the draft tube also increases the height of the turbine (h) above tailrace
level and reduces the pressure p, and increases the danger of cavitation. The height of the turbine above
tailrace level to avoid the flooding of superstructure is also controlled by the occurrence of cavitation
danger.
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 byσc =[(HH hav) ]
H−−