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

1 : NPSH for the system = total head required prior to starting the pump;

NPSH = Pipe loss + Vapour pressure loss + depth;

at 20 C

NPSH = 2.0 + 0.25 + 6 ;

NPSH = 8.25 m;

2. NPSH at 60 C ;

NPSH = 2.0 + 2.05 + 6;

NPSH = 10.05 m;

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