CHEMICAL ENGINEERING

PUMPING OF FLUIDS 117

The losses in the suction pipeD 1 .5m,andui^2 / 2 gDhfD 1. 5
The net positive suction head (NPSH) is discussed in Section 8.2.3 where it is
shown that:

NPSHDhiPv/g

wherePvis the vapour pressure of the liquid being pumped. The minimum heightxis
then obtained from:

3 DP 0 /gCx 1. 5 Pv/g

P 0 D 760  640 D120 mm HgD 16 ,000 N/m^2

PvD 760  710 D50 mm HgD6670 N/m^2

D1000 kg/m^3 ,gD 9 .81 m/s^2

∴ xD 3 C 1. 5  16 , 000 C 6670 / 1000 ð 9. 81 D 3 .55 m

PROBLEM 8.13

What is meant by the Net Positive Suction Head (NPSH) required by a pump? Explain
why it exists and how it can be made as low as possible. What happens if the necessary
NPSH is not provided?
A centrifugal pump is to be used to circulate liquid of density 800 kg/m^3 and viscosity
0 .5mNs/m^2 from the reboiler of a distillation column through a vaporiser at the rate of
400 cm^3 /s, and to introduce the superheated liquid above the vapour space in the reboiler
which contains liquid to a depth of 0.7 m. Suggest a suitable layout if a smooth-bore
25 mm pipe is to be used. The pressure of the vapour in the reboiler is 1 kN/m^2 and the
NPSH required by the pump is 2 m of liquid.

Reboiler

P = l kN / m^2

0.7 m

h 0

Vaporiser

Figure 8d.

Solution

See Volume 1, Example 8.2