Book - School of Science and Technology

204 Pumps and other auxiliary equipmentCentrifugal pumps (general)Some form of centrifugal pump, be it single-stage or multi-stage, is now used to meet mostduties including that of boiler feed against pressure. In principle, such pumps consist of animpeller, having backward curved blades, which rotates within a scroll casing. Thiscasing, more properly called a volute, has a profile which is so formed that the flow,discharged at high velocity by the impeller to the circumference, passes smoothly throughan increasing area. As a result, when the point of discharge is reached, most of the velocityenergy has been converted into pressure. Multi-stage units consist of a grouping of suchvolutes, arranged in series such that the discharge from the first passes to the inlet of thesecond and so on.A number of simple relationships shows how, for a given diameter of impeller, theperformance of a centrifugal pump will change as the speed is varied, as follows:. volume varies directly with speed. pressure varies with the square of the speed. power absorbed varies with the cube of the speed.To these may be added a further series of similar relationships which show that, for a givenspeed, the performance of a pump will alter as the diameter of the impeller is changed. Toexpand further upon these and other similar matters is outside the scope of this book.The performance of a typical centrifugal pump may be represented by characteristiccurves as illustrated in Figure 8.1. Part (a) of this diagram shows alternative relationshipsbetween the volume flow and the pressure developed. It will be seen that, dependent uponthe pump design, the shape of the curve may be either steep or shallow to suit theparticular application. Part (b) of the diagram repeats the steep curve and adds furthercharacteristics which illustrate how efficiency varies over the operating range and, consequently,the power absorbed. It will be noted that, in this instance, near-to-peak efficiencyis retained over a relatively wide range of volumes.The manner in which the volume/pressure characteristic may be adjusted either byvarying the speed or fitting an impeller of a different diameter is shown in Figure 8.2(a).Centrifugal pumps may be arranged to operate either in series or in parallel and theoutputs which result from such arrangements are shown in Figure 8.2(b), taking the sameShallow60 100EfficiencyPer cent50PressurekPaSteepPowerVolumeVolume0kW321(a)0 (b)0Litre/s10Figure 8.1 Centrifugal pumps. Typical characteristic curves