Page 232 - Chemical engineering design

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CHEMICAL ENGINEERING
tanks are nitrogen blanketed and maintained at a pressure of 1.05 bar. The storage tank
has a floating roof, which exerts a pressure of 1.1 bar on the liquid.
The ship must unload 1000 tonne within 5 hours to avoid demurrage charges. Estimate
the power required by the pump. Take the pump efficiency as 70 per cent.
3
Physical properties of toluene: density 874 kg/m , viscosity 0.62 mNm 2 s.
Solution
3 2 2
Cross-sectional area of pipe D 225 ð 10 D 0.0398 m
4
1000 ð 10 3 1 1
Minimum fluid velocity D ð ð D 1.6m/s
5 ð 3600 0.0398 874
3
Reynolds number D 874 ð 1.6 ð 225 ð 10 /0.62 ð 10 3
D 507,484 D 5.1 ð 10 5 5.4
Absolute roughness commercial steel pipe, Table 5.2 D 0.046 mm
Relative roughness D 0.046/225 D 0.0002
Friction factor from Figure 5.7, f D 0.0019
Total length of pipeline, including miscellaneous losses,
D 900 C 600 ð 225 ð 10 3 D 1035 m
1035 1.62
2
Friction loss in pipeline,P f D 8 ð 0.0019 ð ð 874 ð
225 ð 10 3 2
D 78,221 N/m 2 5.3

Maximum difference in elevation, z 1 z 2 D 0 30 D 30 m
3
5
Pressure difference, (P 1 P 2 D 1.05 1.1 10 D 5 ð 10 N/m 2
Energy balance
9.8 30 C 5 ð 103 /874 78,221 /874 W D 0 5.5
W D 389.2 J/kg,

Power D 389.2 ð 55.56 /0.7 D 30,981 W, say 31 kW . 5.6a

5.4.4. Characteristic curves for centrifugal pumps
The performance of a centrifugal pump is characterised by plotting the head developed
against the flow-rate. The pump efficiency can be shown on the same curve. A typical
plot is shown in Figure 5.10. The head developed by the pump falls as the flow-rate is
increased. The efficiency rises to a maximum and then falls.
For a given type and design of pump, the performance will depend on the impeller
diameter, the pump speed, and the number of stages. Pump manufacturers publish families
of operating curves for the range of pumps they sell. These can be used to select the best
pump for a given duty. A typical set of curves is shown in Figure 5.11.