Specific speed, Ns

Another distinction in impellers is the way the liquid traverses and leaves the impeller blades. This is called the Specific Speed, Ns. It is another index used by pump designers to describe the geometry of the impeller and to classify impellers according to their clesign type and application. By definition, the Specific Speed, Ns is the revolutions per minute (rpm) at which a geometrically similar impeller would run if it were of such a size as to discharge one gallon per minute at one foot of head. 

The equation for determining the Ns is similar to equation for the Nss, except that it substitutes the NPSHr in the denominator with the pump s discharge head  

Where N = the speed of the pump/motor in revolutions per minute Q = the sc]uare root of the flow in gallons per minute at the Best Efficiency Point BKP. Eor double suction impellers, use I/2 BEP flow. H = the discharge head of the pump at the BHP. 

The Specific Speed is a dimensionless number using the formula above. Pump design engineers consider the Ns a valuable tool in the development of impellers. It is also a key index in determining if the pump 

SPECIFIC SPEED (Ns) AT FULL IMPELLER DIAMETER ATBEP 

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Surface finish of internal surfaces - Kffieicney inerea,ses from better surface finishes are mostly attributable to the specific speed Ns (discussed in Chapter 6) of the pump. Generally, the improvements in surface finishes are economically justifiable in pumps with low specific speeds. 

Wear ring tolerance - Close tolerances on the wear rings have a tremendous effect on the pump s efficiency, particularly for pumps with a low specific speed (Ns < 1500). 

The majority of centrifugal pumps have performance curves with the aforementioned profiles. Of course, special design pumps have curves with variations. Eor example, positive displacement pumps, multi-stage pumps, regenerative turbine type pumps, and pumps with a high specific speed (Ns) fall outside the norm. But you ll find that the standard pump curve profiles are applicable to about 95% of all pumps in the majority of industrial plants. The important thing is to become familiar with pump curves and know how to interpret the information. 

The constant in equation 4.21 is known as the Specific Speed Ns of the pump. Although commonly used, this definition of the specific speed is unsatisfactory because, following from equation 4.20, the value of Ns depends on the units used. Moreover, manufacturers sometimes mix the units. When using specific speed data it is essential to know the definition of Ns and the units of N, Q and h employed. 

Specific speed Ns = (rpm)(gpm)0 5/(head in ft)0-75. Pump may be damaged if certain limits of Ns are exceeded, and efficiency is best in some ranges. 

Compute the specific speed and suction specific speed. The specific speed or, as Horwitz [2] says, more correctly, discharge specific speed, Ns = N(Q)0 5/(H)075, while the suction specific speed S = Al(g)° 5/NPSH0 7, where all values are taken at the best efficiency point of the pump. 

Figure 3 Impeller profiles versus pump specific speed (Ns). [Courtesy of Hydraulic Institute, Parsippany, NJ, www.Pumps.org]...

Specific speed (Ns) is used to describe pump impeller shape and type, see Figure 3. It is defined as pump speed in rpm that will pump LO gpm liquid against 1.0 ft of liquid head. It is calculated by following equation. 

For 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. For multistage pumps, the developed head must be taken per stage for the NS calculation. By definition (eq. 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. 

Selection of pump for a given appHcation is not a trivial task. Often more than one pump type can accomplish the required job. Thus a final choice on a pump type is often a result of personal experience and usage history. 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. Whereas specific speed is appHcable primarily for centrifugal but not positive displacement pumps, the US value can be used as a guide. Generally, for calculated values of specific speed, eg, nS > 10 [NS > 500), kinetic-type pumps are usually selected. For nS < 10 [NS < 500), positive displacement pumps are typically appHed. 

Note Specific speed derived using cubic meters per second and meters multiplied by a factor of 51.6 is equal to specific speed derived using U.S. gallons per minute and feet. The usual symbol for specific speed in U.S. units is Ns. 

Standards for upper limits of specific speeds have been established, like those shown in Figure 7.6 for four kinds of pumps. When these values are exceeded, cavitation and resultant damage to the pump may occur. Characteristic curves corresponding to widely different values of Ns are shown in Figure 7.3 for several kinds of pumps handling clear water. The concept of specific speed is utilized in Example 7.3. Further data are in Figure 7.6. 

Figure 7.3. Performance curves of single-suction impellers corresponding to two values of the specific speed, (a) Ns - 1550, centrifugal pump, (b) Ns = 10,000, mixed and axial flow pumps.

Figure 7.25. Efficiency and head coefficient qad as functions of specific speeds and specific diameters of various kinds of impellers (Evans, 1979). Example An axial propeller has an efficiency of 70% at A, = 200 and Ds = 1.5 and 85% at Ns = 400 and Ds = 0.8. See Table 7.4 for definitions of qad, Ns, and Ds.

Specific speed is discussed in Section 8.2.3 of Volume 1, where it is shown to be Ns = /V( l/2/(g/i)3/4. This expression is dimensionless providing that the pump speed, throughput, and head are expressed in consistent units. 

For the model, Ns = 3500(500)as/350a75 = 965 S = 3500(500)°-5/10°-75 = 13,900. For the prototype, Ns = 1170(1158)a5/142.5a75 = 965 5 = 1170(1156)a5/4.06a75 = 13,900. The specific speed and suction specific speed of the model and prototype are equal because these units are geometrically similar or homologous pumps and both speeds are mathematically derived from the similarity laws. 

Determine the pump efficiency and impeller shape. Figure 6.12 shows the general relation between impeller shape, specific speed, pump capacity, efficiency, and characteristic curves. At Ns — 2000, efficiency = 87 percent. The impeller, as shown in Fig. 6.12, is moderately short and has a relatively large discharge area. A cross section of the impeller appears directly under the Ns = 2000 ordinate. 

If the dimensions of are substituted, it will be found dimensionless and because it is dimensionless, it can be used as a general characterization for a whole variety of pumps without reference to their sizes. Thus, a certain range of the value of Ns would be a particular type of pump such as axial (no size considered), and another range would be another particular type of pump such as radial (no size considered). is called specific speed. 

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