Specific speed

N is pump speed, in rpm Q equals total pumping rate, in gpm. (for double suction pump, in USA, total flow is used for specific spenl calculation, while in Europe, half flow is used.) [1] and H is head per stage, in ft. These values should always take at maximum pump impeller diametm and at Best Bftictency Point (BEP). 

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  

The principle of dynamical similarity expresses the fact thaL two pumps geometrically similar to each other will 

Francis-Screw Field Mixed-Flow Field Axial-Flow Field 

Applied Process Design for Chemical and Petrochemical Plants 

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Specific resistance Specific rotation Specific speed Specific viscosit Specimen contrast Speckle interferometry Spectazole... 

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 ... 

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. 

Fig. 4. Chart for efficiency estimates and curve shapes, where (a) represents curve shapes showing the relationship between efficiency (Eff), head (H), and power (P) as a function of flow (b) specific speed, where the numbers represent flow in nr /s and (c) impeller profiles.

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. 

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. 

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. 

Pump Selection One of the parameters that is extremely useful in selecting a pump for a particular apphcatiou is specific speed N,. Specific speea of a pump can be evaluated based on its design speed, flow, and head. 

Typically, for single-suction pumps, suction-specific speed above 11,000 is considered excellent. Below 7000 is poor and 7000-9000 is of an average design. Similarly, for double-suction pumps, suction-specific speed above 14,000 is considered excellent, below 7000 is poor, and 9000-11,000 is average. 

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. 

TABLE 10-8 Specific Speeds of Different Types of Pumps... 

FIG. 10-35 Specific speed variations of different types of pump. 

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,... 

Typical impeller proportion as a function of specific speed above... 

FIG. 10 36 Relationships lietween specific speed, rotative speed, and impeller proportions (Woiihim lon Pmiij) Itu ., Pump World voj. 4, no. 2, 197-Sj. 

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

In selec ting the machines of choice, the use of specific speed and diameter best describe the flow. Figure 10-67 shows the characteristics of the three types of compressors. Other considerations in chemical plant service such as problems with gases which may be corrosive or have abrasive solids in suspension must be dealt with. Gases at elevated temperatures may create a potential explosion hazard, while air at the same temperatures may be handled qmte normally minute amounts of lubricating oil or water may contaminate the process gas and so may not be permissible, and for continuous-process use, a high degree of equipment rehability is required, since frequent shutdowns for inspec tion or maintenance cannot be tolerated. 

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. 

Efficiency at various specific speeds for various sizes of rotor. 29-50 Loss of efficiency as a function of the pressure ratio. 

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). 

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 ... 

The operating window is the effective zone around the BH.P on the pump curve that must be respected by the process engineer and/or the operators of the pump. How far away from the BHP a pump can operate on its performance curve without damage is determined bv its impellers suction specific speed. 

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... 

SPECIFIC SPEED (Ns) AT FULL IMPELLER DIAMETER ATBEP... 

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