MICROFICHE REFERENCE LIBFUUW - Cd3wd.com

109speed 1 ines , it can be seen that increasing or decreasing the speed ata constant head will increase or decrease, respectively, the capacity.However, two interdependent changes will also result from such speedchanges. Depending on the type pump, the power consumed may increase ordecrease, thus providing additonsl savings or cost. Also, as in mostmechanical devices, higher speeds will increase wear. When pumping abrasivefluids, such as most irrigation and drainage water, this wear can be quitesevere. Walker (1972) states that doubling the speed of rotodynamic pumpsecan resultin four times the wear.As indicated by the affinity laws, a change in impeller size willvary capacity. Except for permanent demand changes, disassembling thepump and exchanging or altering the impeller(s) is not usually practical.However, in propeller pumps, blades with adjustable pitch are available andin centrifugal and mixed flow pumps, adjustable diffusers or guide vanescan be utilized. These adjustable devices have the effect of changingimpeller size and capacity by permitting a broader high-efficiency-range(Rouse, 1950).Capacity regulation can also be accomplished by throttling the discharge.(Throttling the suction line is not recommended since this decreasesthe NPSHA and thus increases the probability of cavitation.) This willincrease the total head against which the pump must operate by increasingfriction and therefore lower the discharge.The second feature of rotodynamic pumps yet to be considered is theirneed to be primed. Unlike positive displacement pumps, when first started,they cannot displace air to create the pressure differential necessary to“suck” water. Also, since rotodynamic pumps utilize the water they lift