The Specific Speed

Specific Speed. A review of the dimensionless analysis (qv) as related to pumps can be found in Reference 14. One of these nondimensional quantities is called the specific speed. The universal dimensionless specific speed, Q, is defined as in equation 9 ... 

Similar to the concept of the specific speed, a suction specific speed, S, is defined as... 

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. 

There are two main reasons why a pump should not operate below its MCSF (/) the radial force (radial thmst) is increased as a pump operates at reduced flow (44,45). Depending on the specific speed of a pump, this radial force can be as much as 10 times greater near the shut off, as compared to that near the BEP and (2) the low flow operation results in increased turbulence and internal flow separation from impeller blades. As a result, highly unstable axial and radical fluctuating forces take place. 

Figure 10-35 shows the schematic of specific-speed variation for different types of pumps. The figure clearly indicates that, as the specific speed increases, the ratio of the impeller outer diameter Di to inlet or eye diameter Do decreases, tending to become unity for pumps of axial-flow type. 

The specific speed compares the adiabatic head and flow rate in geometrically similar machines at various speeds. 

Size, rotating speed, and efficiency correlate well with the available isentropic head, the volumetric flow at discharge, and the expansion ratio across the turboexpander. The head and the volumetric flow and rotating speed are correlated by the specific speed. Figure 29-49 shows the efficiency at various specific speeds for various sizes of rotor. This figure presumes the expansion ratio to be less than 4 1. Above 4 1, certain supersonic losses come into the picture and there is an additional correction on efficiency, as shown in Fig. 29-50. 

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. 

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 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... 

To calculate the specific speed, N, it is necessary to select a reasonable shaft speed. First, calculate the approximate shaft power by assuming... 

For the calculated shaft horsepower. Figure 2-13 presents the speed trend of present advanced bearing technology. Any speed below the limiting line can be used for calculating the specific speed. With N... 

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

A typical operating specific speed curve is shown in Figure 3-50 and represents a technique for plotting the specific speed on the operating performance curve. Figure 3-50 represents a 6-inch pump operating at 1760 rpm, with maximum efficiency at 1480 GPM and 132 feet head [25]. The operating specific speed is zero at no flow and increases to infinity at the maximum flow of 2270 gpm and zero head. Stable operations beyond about 1600-1700 gpm cannot be planned from such a curve with a sharp cutoff drop for head capacity. 

Type specific speed is defined as that operating specific speed that gives the maximum efficiency for a specific pump and is the number that identifies the pump type [25]. This index number is independent of the rotative speed at which the pump is operating, because any change in speed creates a change in capacity in direct proportion and a change in head that varies as the square of the speed [25]. Practice is to true type the specific speed of the pump reasonably close to the conditions of maximum effi-... 

The specific speed of a given tv pe pump must not exceed the specific speed values presented by the Hydraulic Institute [17]. This is based on a known or... 

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. 

This is based on the flow and specific energy produced by the pump at its best efficiency point of performance following the approach stated by Wisclicenus Any fixed value of the specific speed describes a combination of operating conditions that permits similar flow conditions in geometrically similar hydrodynamic machines. ... 

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 mJ/s of water against a head of 20 m, Calculate the specific speed. What type of pump does this value suggest ... 

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... 

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 ... 

Another dimensionless group, analogous to the specific speed, that relates directly to the cavitation characteristics of the pump is the suction specific speed, Nss ... 

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. 

A typical pump selection chart such as shown in Fig. 10-46 calculates the specific speed for a given flow, head, and speed requirements. Based on the calculated specific speed, the optimal pump design is indicated. 

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. 

This concept is called the specific speed. It is commonly used in the... 

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.

Certain pumps will operate more efficiently in certain ranges of the specific speed, and an optimum pump speed range can be determined. The pump power is calculated using the formula (Ref. P2, p. 6.5) ... 

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 ... 

A test-model pump delivers, at its best efficiency point, 500 gal/min (0.03 m3/s) at a 350-ft (107-m) head with a required net positive suction head (NPSH) of 10 ft (3.05 m) and a power input of 55 hp (41 kW) at 3500 r/min, when using a 10.5-in-diameter impeller. Determine the performance of the model at 1750 r/min. What is the performance of a full-scale prototype pump with a 20-in impeller operating at 1170 r/min What are the specific speeds and the suction specific speeds of the test-model and prototype pumps ... 

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. 

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. 

Choose the best speed for the pump. Analyze the specific speed and suction specific speed at each of the various operating speeds using the data in Tables 6.20 and 6.21. These tables show that at 870 and 1160 r/min, the suction specific-speed rating is poor. At 1750 r/min, the suction specific-speed rating is excellent, and a turbine or mixed-flow type of pump will be suitable. Operation at 3500 r/min is unfeasible because a suction specific speed of 26,000 is beyond the range of conventional pumps. 

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