specific speed n pump rotational speed (rpm). A parameter that is often used in describing pump performance is the specific speed, Ns cu, given by N s c u N ( r p m ) Q ( g p m ) 1 / 2 H ( f t ). For simplicity, the velocity head components and gauge elevation adjustments were not considered because the correction would be insignificant for this test. 3,960 is a units conversion to state fluid power in terms of horsepower. The raw data were taken as discharge and suction pressures at several flow points, and the pump head was calculated from these data. It can be made unitless, though, by converting the speed to radians/second, the capacity to cubic feet/second and by multiplying the head by the gravitational constant g. In the U.S., with the above units, it is not unitless. In the PVA #6 video, one example illustrates how drastically the values of specific speed can change depending on which point on a head-flow curve it is calculated. Making It Unitless It is commonly said that specific speed is unitless, but it normally is not. Note that this video does not address at which flow point (Q) a pump’s specific speed is calculated. Industry experience has shown that a pump with a suction specific speed less than 120 (SI units) or 6000 (in US units) is in danger of recirculation and cavitation. In countries which use the metric system, the factor is omitted, and quoted. where n is the wheel speed in rpm P is the power in kilowatts h is the water head in meters The factor 0.2626 is only required when the specific speed is to be adjusted to English units. The specific revolution of the pump is numerically equal to the. Expressed in metric units, the 'specific speed' is n s 0.2626 n P /h 5/4. The specific speed is derived from a dimensionless analysis and is used to compare pumps with similar geometry but different parameters. The English name is Specific Speed, which means ns in the Chinese symbol. SPECIFIC SPEED NS IN ENGLISH UNITS HOW TOThe values of specific speed can change drastically, depending on the point on the head-flow curve at which it is calculated.Ī video experiment illustrates how to conduct pump testing and how to produce a head-flow curve. The specific speed is also called the specific speed or specific speed. While the rpm is included in the formula, the calculation’s main purpose is to establish the impeller’s shape and contour and to link this to other pump parameters-such as head rise to shutoff or the maximum achievable theoretical efficiency. Despite its name, pump specific speed (NS) does not have much to do with the actual speed of a pump.
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