Page 236 - Chemical engineering design

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To find the system curve the calculations were repeated for the velocities shown in the
table below: CHEMICAL ENGINEERING
velocity flow-rate static head dynamic head total head
3
m/s m /h m m m
1 18.1 9.4 2.0 11.4
1.5 27.2 9.4 4.3 14.0
2.0 36.2 9.4 6.8 16.2
2.5 45.3 9.4 10.7 20.1
3.0 54.3 9.4 15.2 24.6

Plotting these values on the pump characteristic gives the operating point as 18.5 m at
3
40.0 m /h and the pump efficiency as 79 per cent.

5.4.6. Net positive suction head (NPSH)
The pressure at the inlet to a pump must be high enough to prevent cavitation occurring
in the pump. Cavitation occurs when bubbles of vapour, or gas, form in the pump casing.
Vapour bubbles will form if the pressure falls below the vapour pressure of the liquid.
The net positive suction head available NPSH avail is the pressure at the pump suction,
above the vapour pressure of the liquid, expressed as head of liquid.
The net positive head required NPSH reqd is a function of the design parameters of
the pump, and will be specified by the pump manufacturer. As a general guide, the NPSH
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should be above 3 m for pump capacities up to 100 m /h, and 6 m above this capacity.
Special impeller designs can be used to overcome problems of low suction head; see
Doolin (1977).
The net positive head available is given by the following equation:
NPSH avail D P/ C H P f / P v / 5.7
where NPSH avail D net positive suction head available at the pump suction, m,
2
P D the pressure above the liquid in the feed vessel, N/m ,
H D the height of liquid above the pump suction, m,
2
P f D the pressure loss in the suction piping, N/m ,
2
P v D the vapour pressure of the liquid at the pump suction, N/m ,
3
D the density of the liquid at the pump suction temperature, kg/m .
The inlet piping arrangement must be designed to ensure that NPSH avail exceeds NPSH reqd
under all operating conditions.
The calculation of NPSH avail is illustrated in Example 5.4.

Example 5.4
Liquid chlorine is unloaded from rail tankers into a storage vessel. To provide the
necessary NPSH, the transfer pump is placed in a pit below ground level. Given the
following information, calculate the NPSH available at the inlet to the pump, at a maximum
flow-rate of 16,000 kg/h.