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Pump Suction Head and NPSH Available

Mediumpumpscavitationnpsh
A pump draws 20 °C water from a reservoir whose surface is 3.0 m below the pump centreline. Friction loss in the suction line is 1.2 m. Atmospheric pressure is 101.3 kPa, the vapour pressure of water at 20 °C is 2.34 kPa, and the specific weight of water is 9.79 kN/m3. The net positive suction head available is most nearly:
  1. A4.7 m
  2. B5.9 m
  3. C6.2 m
  4. D11.9 m
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Answer: B. 5.9 m

NPSH available is the absolute head at the pump suction above vapour pressure: NPSHA = (patm − pvapour)/γ − zlift − hf = (101.3 − 2.34)/9.79 − 3.0 − 1.2 = 10.11 − 3.0 − 1.2 ≈ 5.9 m. Because the reservoir is below the pump, the static lift is subtracted. Ignoring the vapour pressure term gives 6.2 m, and treating the lift as positive suction head gives 11.9 m.
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