Book - School of Science and Technology

Pumps 207Dealing with these in turn, the split casing type, as in Figure 8.3(a), is used only for thelargest installations where access to the impeller or casing is necessary for cleaningpurposes. End suction is the most common arrangement as shown in Figure 8.3(b).A close-coupled drive, where the pump is attached to a flange on the motor casing andthe impeller is mounted on an extension to the motor shaft, is a compact arrangement wellsuited to industrial use or pressure development but is often too noisy for use as acirculating pump. Direct drive, where motor and pump are mounted on a commonbaseplate, with drives coaxial one to the other but joined only by a flexible coupling, isa convenient arrangement and will provide long service without problems provided thatthe pump and motor shafts are aligned at manufacturers' works. Overall, however, driveswhich employ generously proportioned vee belts and pulleys, as in Figure 8.4, are thosepreferred by many designers. Here, the pump shaft runs at a speed which is quiteindependent of that of the motor and water-borne noise is reduced by the separation ofthe two components. Adjustments to the duty of the pump may be achieved by a simplechange of pulleys and belts.The axis of the driving shaft between motor and pump is most commonly arranged tobe horizontal and supported by intermediate bearings: there is in consequence much tocommend this arrangement. As an alternative, which may be more economic of spacethan the horizontal direct drive arrangement, pumps having vertical shafts (sometimesknown in this context as spindles) are available, as in Figure 8.5. Some, as part (a) of thediagram, are provided with stools or feet for floor mounting but smaller sizes, as part (b),are sold as being suitable for pipeline installation. It is wise to establish that the latter havethrust bearings and shaft seals which have been designed to suit a vertical configuration.The types and styles noted above do not include that which is probably best known: theglandless submerged (canned) rotor pattern as fitted in most domestic systems. This iseffectively an end suction type, and has the advantage of being without a gland or sealsince the driving shaft does not pass through the casing. Figure 8.6 provides a crosssectionthrough two pumps of this pattern. The penalty paid for this considerableadvantage is that the clearances between the stator and the rotor can are necessarily smalland, in consequence, any foreign matter in the water circulated may lead to seizure. Mostsuch pumps incorporate devices whereby output may be varied: by an electrical adjustmentto motor speed; by use of a hydraulic spoiler by-pass or by some combination ofPipelinemountingSupport stoolSide elevation(a)(b)Figure 8.5 Centrifugal pumps. Vertical spindle types