Page 270 - Chemical process engineering design and economics

250 Chapter 5

ferent fluids are the same, the pressure will differ. The denser fluid will exert the
greater pressure. The power consumed will also be greater for the denser fluid.

Head

Flowrate, gal/min

Figure 5.28 Characteristic curve for a centrifugal pump.

Figure 5.28 shows typical performance curves for a centrifugal pump. The pump
manufacturer supplies these curves for water. When a control valve in the pump
discharge opens or closes, the pump will follow these performance curves.
Pumps wear and the curve will change with time. In addition, friction fac-
tors will generally increase with time because of corrosion and deposits. For
these reasons, pumps are usually oversized and thus will initially deliver larger
flow rates than required. A control valve installed on the discharge side of the
pump will bring the pump to the desired operating point on the curve.

Peripheral Pumps

A peripheral pump, shown in Figure 5.27, is sometimes referred to as a regen-
erative pump or a turbine pump because of the shape of the impeller. This pump
employs a combination of mechanical impulse and centrifugal force to produce
heads of several hundred feet at low flow rates. The impeller, which rotates at
high speed with small clearances, has many short radial passages milled on each
side at the periphery of the impeller. Similar passages are milled in the mating
surfaces of the casing. Upon entering, the liquid flows into the impeller pas-

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