Question

If atmospheric pressure head is 9 m, vapour pressure head (maximum) is 1 m, failure head is 40 m and cavitation coefficient \( \sigma \) is 0.15, then height at which the turbine can be set above the tailrace level is ...........

• A. 6 m
• B. 4 m
• C. 3 m
• D. 2 m

Hint:

Always consider cavitation when determining turbine elevation — ensure pressure doesn't fall below vapor pressure.

Answer 1

The Correct Option is D

Given:
Atmospheric pressure head \( H_a = 9 \, \text{m} \)
Vapour pressure head \( H_v = 1 \, \text{m} \)
Failure (total) head \( H = 40 \, \text{m} \)
Cavitation coefficient \( \sigma = 0.15 \)
The safe height \( h \) above tailrace level is given by:
\[ \sigma = \frac{H_a - H_v - h}{H} \]
Substitute known values:
\[ 0.15 = \frac{9 - 1 - h}{40} \]
\[ 0.15 \times 40 = 8 - h \]
\[ 6 = 8 - h ⇒ h = 8 - 6 = 2 \, \text{m} \]
\[ \boxed{h = 2 \, \text{m}} \]