Calculation specific speed

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. 

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. 

A t)pical puuip selectiou cbait sucb as sbowu iu Fig, 10-36 calculates tbe specific speed for a giveu flow, bead, aucl speed recpiire-uieuts. Based ou tbe calculated S] iecific speed, tbe optiuial puuip clesigu is iudicated,... 

This operating window is quantified or rated by the term Suction Specific Speed, Nss . The Nss is calculated with three parameters, the speed, the flow rate, and the NPSHr. These numbers come from the pump s performance curve, discussed in Chapter 7. The formula is the following ... 

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... 

Sonic flow, safety relief, 438 Rupture disk, 460, 461 Sub-sonic flow, 461 Sonic or critical flow, 115, 125 Calculations, 125 Velocity, 126 Specific speed, 194-197 Impeller designs, 194 Upper limits, chart, 195-197 Specifications,... 

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 ... 

Annex A specifies calculations for specific speed and suction specific speed. 

The pump suction specific speed shall be calculated in accordance with Annex A and, if specified, limited as stated on the data sheet. 

Specific speed is calculated for the pump s performance at best efficiency point with the maximum diameter impeller. Specific speed is expressed mathematically by the following equation ... 

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) ... 

After consulting Ref. P3 (p.582 Figure 14.2), it is confirmed that the high suction pressure available in this application ensures that cavitation will not occur in the pump. For the calculated specific pump speed, there is a safety factor of over 500%. Cavitation should not be a problem provided that the column is only partially repressurised before turning on the pump (a usual start-up procedure on the absorption column). 

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 ... 

Related Calculations. Use the method given here for any type of pump whose variables are included in the Hydraulic Institute curves (Figs. 6.10 and 6.11) and in similar curves available from the same source. Operating specific speed, computed as above, is sometimes plotted on the performance curve of a centrifugal pump so that the characteristics of the unit can be better understood. Type specific speed is the operating specific speed giving maximum efficiency for a given pump and is a number used... 

The discontinuities diagrammed in Fig. 5.4.1 are termed kinematic shocks in that they represent discontinuities in density. Let us calculate the speed at which the top discontinuity moves down and the bottom one up. For specificity consider a downward-moving shock. With respect to a coordinate system moving down with the speed of the discontinuity u (Fig. 5.4.2A), the flow is steady and conservation of mass for the one-dimensional picture considered gives... 

Answer by Author A criterion for NPSH computation reliability is its repeatability when using a wealth of data. A three-thermocouple redundancy per measurement and the multiplicity of test runs pinpointed the temperatures selected. Then, by using equivalent vapor pressures, and by judiciously establishing pump inlet head losses, no erroneously-negative values of NPSH were realized. Since related suction specific speed values were neither imaginary nor infinite, and, because no hypothetical approaches were necessary to calculate their real values, the NPSH values were deemed reliable. 

To calculate the speed (rate) at which a solute will diffuse through a solution, we need to know the driving force for the diffusion, and the diffusion coefficient for the solute in the particular solvent. The diffusion coefficient depends upon the shape of the solute and the specific kinds of interacfions it has with the solvent. Further, the viscosity of the solvent itself affects the diffusion coefficienf. Table 3.5 shows several diffusion coefficients for different kinds of species in different solvents. In general, standard rate constants for diffusion of a solute through a solvent are on the order of 10 to 10 s h Therefore, diffusion controlled reactions occur on a timescale of ns. 

In order to make the correlation described in this paper, it is necessary to perform the calculation at a point where the curves become vertical, in other words, the point where the pump is fully cavitating. For this unit the difference in NPSH is 0,6 foot, Calculations using only the water data show that the unit should perform satisfacto rily with liquid oxygen requiring a NPSH which is 0,6 foot lower than that needed for water. It is noteworthy that the corresponding difference in Suction Specific Speed values is a factor of 2,... 

In addition to this information, specific sensors may be used in conjunction with DIGISAFE XME. These sensors ate most often used for calculations of speed and train position. There ate two kinds of sensors ... 

Based on above suction specific speed guidelines and Bq. (10), maximum allowable pump speed can be calculated. In general, a pump with higher speed will be smaller (less expensive). On the other hand, a lower speed pump is more reliable. Therefore, we need to consult pump vendor to select a high speed, but also reliable pump. 

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