A pump station has been designed to lift water out of a 6 metre deep pit (vented

Question

A pump station has been designed to lift water out of a 6 metre deep pit (vented to atmosphere) via a centrifigual pump mounted at ground level.

Liquid conditions 20 OC

Suction pipe work losses 2.0 metres

NPSH safety factor 5.0 kPa

Vapor pressure @ 20oC 0.25 metres

Calculate the Net Positive Suction Head (NPSH) for the system ?

In summer the liquid temperature rises to 60 oC, this changes the water vapor pressure to 2.05 metres. What is the revised NPSH for the higher water temperature ?

Pressure loss in the suction pipe work is directly proportional to water veloicty in the pipe work squared, which is inversely proportional to suction pipe work diameter squared.

Explanation / Answer

P1 + 1/2*p*v1^2 + p*g*z1 = Patm + 1/2*p*v2^2 + p*g*z2 + p*g*Hloss

z1 = 6 m

z2 = 0 m

v2 = 0 m/s

P1/(p*g) +1/2*v1^2/g + z1 = Patm/p*g + 0 +0 + Hloss

h1 + v1^2/(2*g) + 6 = 10^5/(1000*10) + 2

h1 + v1^2/(2*g) = 6

NPSH = h1 + v1^2/(2*g) = 6 m and

(h1 - hvap) > 0 to avoid cavitation

h1>0.25 m

In summer

NPSH = 6 m and

(h1-hvap)>0

h1>2.05 m

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