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Why is a minimum of Net Positive Suction Head required for a hydraulic pump?
1. To prevent cavitation
2. To increase discharge
3. To increase suction head
4. To increase efficiency 

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Correct Answer - Option 1 : To prevent cavitation

Explanation:

Net Positive Suction Head (NPSH):

  • The Net Positive Suction Head (NPSH) is defined as the absolute pressure head at the inlet to the pump, minus the vapour pressure head (in absolute units) plus the velocity head.

\({\rm{NPSH}} = \frac{{{P_1}}}{{\rho g}} - \frac{{{P_v}}}{{\rho g}} + \frac{{{v_s}^2}}{{2g}}\)

where,

P1 = Absolute pressure at the inlet of the pump, Pv = Vapour pressure at the inlet, v= Velocity of the fluid in the suction pipe.

  • Vapour pressure is the amount of pressure required for a specific liquid to stay in liquid form.
  • When the pressure of the fluid falls below the vapour pressure then cavitation occurs.
  • NPSH must be positive to prevent cavitation.
  • As the temperature of the liquid increases, the vapour pressure increases, decreasing the amount of NPSH.

​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.

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.

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