Correct Answer - Option 1 : Less than the P
vap
Explanation:
Cavitation:
- Cavitation is the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below the vapour pressure of the fluid and sudden collapsing of these bubbles in the region of higher pressure.
- The hydraulic machines subjected to cavitation are reaction turbines and centrifugal pumps.
- In order to determine whether cavitation will occur in any portion of the suction side of the pump, the critical value of Thoma’s cavitation factor is calculated.
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 cavitation may damage the pump.
If NPSH reaches zero then the liquid starts boiling and cavitation starts.
Cavitation in Centrifugal Pump:
- In centrifugal pumps, the cavitation may occur at the inlet of the impeller of the pump or at the suction side of the pumps, where the pressure is considerably reduced. So to avoid cavitation pressure on the suction side should be high.
Cavitation also occurs in reciprocating pumps if there is a high-velocity suction or discharge.
Effects of cavitation:
- The metallic surfaces are damaged and cavities are formed on the surfaces.
- Due to the sudden collapse of the vapour bubble, considerable noise and vibrations are produced.
- 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.
