Power Plant Engineering

HYDRO-ELECTRIC POWER PLANTS 375

Table 11.7. (NsVsσσσσσs)

Francis Kaplan

Ns sc Ns σσσσσc

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 02 – Vd^2 )/g is considerably large as men-
   tioned earlier. In order to keep the value of pc 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.

11.12.2 Types of Draft Tubes

(1) Conical Draft Tube. This is known as tapered draft tube and used in all reaction turbines
where conditions permit. It is preferred for low specific speed and vertical shaft Francis turbine. The
maximum cone angle of this draft tube is limited to 8° (a = 4°) for the cause mentioned earlier. The
hydraulic efficiency of such type of draft tube is 90%.

In any event, the draft tube should be made as to secure a gradual reduction of velocity (uniform
decease towards the exit of draft tube) from the runner to the mouth. A form that is theoretically good is
“Trumpet Shaped”.