And personally, I’ll never give up the unit of Horsepower. For example, let us not forget that an important benefit of Metric is its clearly defined unit of force and weight, the Newton, even though customary practice in commerce continues with the Kilogram as the Metric unit of weight. One could ask, “Why don’t we all just adopt dimensionless specific speed?” Well, all one has to do is look at the situation with US Customary Units and Metric. Except: customary usage practices have resulted in at least 5 sets of specific speed values, depending on the units used, only one of which is dimensionless. Specific speed is a dimensionless numerical value. Specific speed is the key pump performance parameter.īut additional commentary on this is in order. In actual practice, pump engineers and applications professionals use and “think with” specific speed values, based on actual pumps, when communicating about performance, hydraulic geometry and the type of pump and its arrangement under consideration. But in the abstract it is difficult to relate to. The pure definition of specific speed, as provided here, is theoretically correct. Read Theory Bites: Suction Specific Speed Post your questions in the comments section below! When you look at the chart you can see there is a gradual change from the radial flow impeller, which develops pressure principally by the action of centrifugal force, to the axial flow impeller, which develops most of its head by the propelling or lifting action of the vanes on the liquid. Pumps are traditionally divided into three types: radial flow, mixed flow, and axial flow. Once the desired specific speed is known, basic dimensions of the unit’s components can be easily calculated. It is this impeller’s “shape” that predicts its flow and head characteristics so that the designer can then select a pump most appropriate for a particular application. It predicts the general shape of a pumps impeller. Specific speed Ns, is used to characterize turbomachinery speed: It is an index used to predict desired pump performance. Specific speed is defined as “the speed of an ideal pump geometrically similar to the actual pump, which when running at this speed will raise a unit of volume, in a unit of time through a unit of head”. Theory bites is a collection of basic hydraulic theory and will touch upon pump design and other areas of pump industry knowledge.
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