Centrifugal 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. 

From the definition of specific speed (eqs. 9 and 10), it follows that reciprocating pumps operate at high pressures and low flow rates. Conversely, centrifugal pumps are appHed at lower pressures and higher flow rates. Many rotary pumps are selected for viscous Hquids having pressures equal to or less than, and capacities lower than, centrifugal pumps. However, these limits are relative and a gray area exists as some pump types cross boundaries into the domain of other types. 

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. 

Centrifugal compressors in general are used for higher-pressure ratios and lower-flow rates compared to lower-stage pressure ratios and higher-flow rates in axial compressors. Figure 6-3 is a map for centrifugal compressors that shows the effect of specific speed (A ) and specific diameter (Z) ) on their... 

The specific speed of a centrifugal pump correlates the basic impeller types as shown in Figure 3-47. 

Figure 3-48. Upper limits of specific speeds for single suction overhung impeller pumps handling dear water at 85°F at sea level. (By permission, Hydraulic Institute Standards for Centrifugal, Rotary, and Reciprocating Pumps, Hydrauiic Institute, 13th ed., 1975.)...

Figure 3-50. Typical centrifugal pump characteristic curve vrith auxiliary specific speed curve. Double-suction, single-stage, 6-in. pump, operating at 1760 rpm constant speed. (By permission, Karassik, I. and Carter, B., Centrifugal Pumps, McGraw-Hill Book Co., inc., 1360, p. 197.)...

Hallam,J. L., Centrifugal Pumps Which Suction Specific Speeds are Acceptable , Hydrocarbon. Processing, 61, No. 4, 1982. 

Centrifugal compressors usually have specific speeds of 1,500-3,000 rpm at the high efficiency point. The axial flow fans and blowers are high specific-speed wheels mixed flow units are lower and the centrifugal compressor wheels are the lowest in specific speed range because they have narrow impellers. 

Figure 3-65 gives the upper limits of specific speeds of single-stage, single and double-suction centrifugal pumps handling clear water at 85°F at sea level [22]. 

Figure 3-67 gives the approximate relative impeller shapes and efficiency variations for various specific speeds of centrifugal pumps. 

Table 3-26 gives the specific speeds for various centrifugal pump types. Table 3-27 gives the suction specific speed ratings for single-suction and double-suction centrifugal pumps. These tables are for pumps handling clear water. 

For the centrifugal pump, two terms are in common use, the Thoma cavitation number cr and the suction specific speed Sn ... 

A set of curves for h, rj, and P as a function of Q are shown in Figure 8.25 from which it is seen that when die pump is operating near optimum conditions, its efficiency remains reasonably constant over a wide range of flowrates. A more general indication of the variation of efficiency with specific speed is shown in Figure 8.26 for different types of centrifugal pumps. The power developed by a pump is proportional to Qghp ... 

Explain briefly the significance of the specific speed of a centrifugal or axial-flow pump. 

Centrifugal pumps are characterised by their specific speed (see Volume 1, Chapter 8). In the dimensionless form, specific speed is given by ... 

The specific speed for centrifugal pumps (equation 5.2) usually lies between 400 and 10,000, depending on the type of impeller. Generally, pump impellers are classified as radial for specific speeds between 400 and 1000, mixed flow between 1500 and 7000, and... 

Centrifugal potting, 16 18 Centrifugal pumps, 19 513 affinity laws related to, 21 63 costs associated with, 21 87 efficiency of, 21 60 nonmetallic, 21 76 suction specific speed of, 21 63 Centrifugal sedimentation, 18 142, 143-144 Centrifugal separation(s), 5 505-551 ... 

Centrifugal pumps require appropriately large circumferential speed of the impellers and a number of serially arranged stages, in order to obtain the high-pressure differences. For efficient and economic operation the specific speed, nq, of the individual pump stages (n<, = n V0,5 / H0 75 V m3/s H, m n, min 1) should stay above 10 to 20. Too small a capacity or hydraulic power transmission will not provide favourable operating conditions and it is then recommended to focus on positive-displacement pumps alternatively. 

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.

Following the recommendations of Ref. P4, single-stage centrifugal pumps should be operated at specific speeds greater than 0.07. A conservative estimate suggests an optimum pump speed (N) of 3000 rpm. This figure corresponds to a specific pump speed of about 0.09. 

Explain briefly the significance of the specific speed of a centrifugal or axial-flow pump. A pump is designed to be driven at 10 Hz and to operate at a maximum efficiency when delivering 0.4 m3/s of water against a head of 20 m. Calculate the specific speed. What type of pump does this value suggest A pump built for these operating conditions has a measured overall efficiency of 70%. The same pump is now required to deliver water at 30 m head. At what speed should the pump be driven if it is to operate at maximum efficiency What will be the new rate of delivery and the power required ... 

Reference should be made to specialist texts on pumps where classifications of pump types as a function of specific speed are presented. A centrifugal pump is suggested here. 

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