MICROFICHE REFERENCE LIBFUUW - Cd3wd.com

106making this type pump useful for high pressure requirements. At the outletport, the casing narrows to a clearance just sufficient to pass the impeller,thus forcingthe water out.4.2.5 CharacteristicsSince the operation principle of radial, mixed, and axial flow pumpsis the same, a discussion of their basic characteristics can be madej oint ly . Because of their unique operation, regenerative pumps areincluded only where specifically mentioned.Along with its performance curve, several other parameters can beused to describe a pump’s operation. The first such parameter is called(nominal) specific speed, or more properly, kientic specific speed sinceanother term, dynamic specific speed, is also used in design work. It isdefinedasm- Ns = -H3/4where N is shaft speed in rpm, Q is discharge in gpm, and H is headin feet. Specific speed is used as a V%ype’f number which is constantfor similar pumps. It is indicative of impeller shape and varies uniformlywith changes in the impeller diameter ratio, D2/D1 , another characteristicparameter. Figure 4.1 shows this relationship between Ns, D2/Dl , andimpeller type. By utilizing Ns as a selection tool, optimum installationconditionscan be designed.