Book - School of Science and Technology

Pump speed controlPumps 211Variable flow pumped circuits, normally associated with two-way modulating valvecontrol (Chapter 22, p. 651), can give significant savings in electrical energy. Such controlthese days is achieved by changing the speed of rotation of the pump impeller usinginverter control on the power supply to the pump. An inverter functions by rectifying themains supply to direct current and converting this back to an alternating current with avariable amplitude and frequency. Inverter drives may be fitted to both single and threephasesupplies.Pump constructionCasings for centrifugal pumps are commonly manufactured in close-grained cast ironalthough a copper alloy may be used in particular circumstances. Impellers are of castgunmetal, machined and balanced to close tolerances, and are mounted on stainless steelshafts. A problem which is common to almost all such pumps, no matter how arranged, isthat the rotating shaft must pass through some form of gland or seal in the casing towhatever form of drive is to be used. This gland must, of course, allow for free rotation ofthe shaft with the minimum practicable leakage of the fluid pumped. Mechanical seals aregenerally preferred to packed glands for most applications but special designs are necessaryfor higher pressures and for temperatures above atmospheric boiling point. In thelatter instance, water cooling arrangements as in Figure 8.9 are required.Pump mountingsIn addition to the isolating valves mentioned previously, each pump, or pair of pumpswhere in duplicate, should be provided with facilities for establishing the suction anddelivery pressures. These may take the form of a pair of pressure gauges, a singledifferential gauge which is connected to both suction and delivery or, at the very least, apair of pressure tapping points. Pressure readings will allow the performance of the pumpto be checked against the characteristic curve when the system is commissioned and willprovide a means for monitoring any fall off in performance during the life of the plant.Cooling wateroutletCooling waterinletPumpendDriveendImpellorFigure 8.9 Centrifugal pumps. Water cooled bearing for high temperatures