# Which of the following statements are TRUE, when the cavitation parameter σ = 0? i. the local pressure is reduced to vapor pressure ii. cavitation sta

151 views
in General
closed

Which of the following statements are TRUE, when the cavitation parameter σ = 0?

i. the local pressure is reduced to vapor pressure

ii. cavitation starts

iii. boiling of liquid starts

iv cavitation stops

1. i, ii and iv
2. only ii and iii
3. only i and iii
4. i, ii and iii

by (37.3k points)
selected

Correct Answer - Option 4 : i, ii and iii

Explanation:

Thoma's cavitation parameter (σ): It is the ratio of Net Positive Suction Head (NPSH) to the total head.

$\sigma = \frac{{NPSH}}{H} = \frac{{\left( {\frac{{{P_A}}}{{\rho g}} - \frac{{{P_V}}}{{\rho g}} - z - {h_f}} \right)}}{H}$

NPSH: It is defined as the net head developed at the suction port of the pump, in excess of the head due to the vapour pressure of the liquid at the temperature in the pump. NPSH must be positive for preventing the liquid from boiling. Boiling or cavitations may damage the pump. If NPSH reaches zero then the liquid starts boiling and cavitation starts.

Cavitation: It is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure and the sudden collapsing of these vapour bubbles in a region of higher pressure. When the vapour pressure collapse, very high pressure is created.

Effects of cavitation:

(i) The metallic surfaces are damaged and cavities are formed on the surfaces.

(ii) Due to the sudden collapse of the vapour bubble, considerable noise and vibrations are produced.

(iii) Due to cavitation, the metal of the runner vanes and the draft tube is gradually eaten away, which results in lowering the efficiency of the turbine.