Whereas specific speed is appHcable primarily for centrifugal but not positive displacement pumps, the US value can be used as a guide. As a rule of thumb, the choice of a kinetic, such as centrifugal, or a positive displacement pump is made on the basis of the specific speed. Thus a final choice on a pump type is often a result of personal experience and usage history. Often more than one pump type can accomplish the required job. Selection of pump for a given appHcation is not a trivial task. Similar to the concept of the specific speed, a suction specific speed, S, is defined as. Chart for efficiency estimates and curve shapes, where (a) represents curve shapes showing the relationship between efficiency (Eff), head (H), and power (P) as a function of flow (b) specific speed, where the numbers represent flow in nr /s and (c) impeller profiles. 9), high head, low flow pumps have low specific speed low head, high flow pumps, such as turbine and propeller pumps, have high specific speed. For multistage pumps, the developed head must be taken per stage for the NS calculation. įor double suction pumps, using the HI convention, is taken as the total pump flow, although some pubHcations use half-flow, ie, flow per impeller eye.
The universal dimensionless specific speed, Q, is defined as in equation 9. One of these nondimensional quantities is called the specific speed. A review of the dimensionless analysis (qv) as related to pumps can be found in Reference 14. Specific resistance Specific rotation Specific speed Specific viscosit Specimen contrast Speckle interferometry Spectazole. The principle of dynamical similarity expresses the fact thaL two pumps geometrically similar to each other will įrancis-Screw Field Mixed-Flow Field Axial-Flow Field Īpplied Process Design for Chemical and Petrochemical Plants The flow rate, head, and impeller speed at the maximum or best efficiency point (BEP) of the pump characteristic can be used to define a dimensionless group called the specific speed These values should always take at maximum pump impeller diametm and at Best Bftictency Point (BEP). (for double suction pump, in USA, total flow is used for specific spenl calculation, while in Europe, half flow is used.) and H is head per stage, in ft. N is pump speed, in rpm Q equals total pumping rate, in gpm.
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