Specific speed, pump

SINGLE SUCTION MIXED FLOW AND AXIAL FLOW PUMPS SPECIFIC SPEED, N. = PPMv/G-M... 

Pumping is a unit operation that is used to move fluid from one point to another. This chapter discusses various topics of this important unit operation relevant to the physical treatment of water and wastewater. These topics include pumping stations and various types of pumps total developed head pump scaling laws pump characteristics best operating efficiency pump specific speed pumping station heads net positive suction head and deep-well pumps and pumping station head analysis. 

The flow rate, head, and impeller speed at the best efficiency point (BEP) of the pump characteristic are used to dehne the pump specific speed ... 

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

Figure 11 Centrifugal pump efficiency as a function of pump specific speed and pumping rate. [15J liU.7 Pump speed...

Pump affinity law is used to estimate pump performance, when its speed or impeller diameter is chmiged. It is derived based on pump specific speed doesn t change. The first set of pump affinity law is based on fixed impeller diameter and varied pump speed. It is used to estimate pump performance at different pump speed. The new pumping rale (Q2), pump head (H2), and required power (BHP2) are estimated from the known pumping rate (Ql), pump head (HI), and power (BHPl) by the following equations ... 

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. 

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. 

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

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... 

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. 

This available value of NPSHa (of the system) must always be greater b) a minimum of two feet and preferably three or more feet than the required NPSH stated by the pump manufacturer or shown on the pump curves in order to overcome the pump s internal hydraulic loss and the point of lowest pressure in the eye of the impeller. The NPSH required by the pump is a function of the physical dimensions of casing, speed, specific speed, and type of impeller, and must be satisfied for proper pump performance. The pump manufacturer must ahvays be given complete Suction conditions if he is to be expected to recommend a pump to give long and trouble-free service. 

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

The principle significance of specific speed for the process engineer is to evaluate the expected performance of a second pump in a particular manufacturer s series while basing it on the known performance (or curve) at the point of optimum efficiency of a first and different size pump. In effect the performance of any impeller of a manufacturer s homologous series can be estimated from the known performance of any other impeller in the series, at the point of optimum efficiency. Figures 3-48 and 3-49 represent the standardized conditions of essentially all pump manufacturers. 

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