Water Well Journal

October 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/728580

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Page 52 of 90

T his edition of "Engineering Your Business" will return to one of the fundamental subjects it has always concen- trated on—pumps and pump theory. On numerous occasions I have discussed many of the pri- mary and vital pump performance characteristics such as capacity, head, horsepower, and NPSH. One factor, however, just as important to the design and selection of a pump but often ignored or overlooked is the specific speed. Pump Impeller Types Before we can gain an understanding of specific speed, it is important to fully comprehend the types of impellers specific speed applies to. Specific speed is a method used by pump people to classify velocity pumps according to the type of impeller used plus the flow rate, head, and rotational speed the impeller has been designed for or is capable of. Velocity pumps function by imparting kinetic energy (en- ergy in motion, water being pumped) from potential energy (energy at rest, water from a static condition). The medium used to convert this energy is usually a liquid, such as water. Three classifications of pumps we are concerned with in this column—standard centrifugal, vertical turbine, and sub- mersible—are examples of velocity pumps. Velocity pumps can be further broken down into three pump types: radial flow, axial flow, and mixed flow. Radial flow pump: Often referred to as a volute or dif- fuser pump, this is the most popular and common style of cen- trifugal pump in use today. The direction of fluid flow through the impeller is 90° normal to the shaft. Simply stated, this means the flow enters the impeller parallel to the pump shaft through an entrance opening called the impeller eye. More simply, the eye travels through the impeller and is then dis- charged from the impeller at a perpendicular right angle, or 90°, from the pump shaft. Axial flow pump: This is more commonly referred to as a propeller pump. Flow through the impeller is routed straight through the impeller, or at an angle of 180° normal to the shaft. The pump's primary use is in high capacity and low head applications, such as high volume fluid transfer for dewatering, stormwater pumping, and tailwater recovery systems. Mixed flow pump: The mixed flow impeller is a com- promise of design between the radial and axial flow impeller where the flow through the impeller is oriented at an angle be- tween 90° and 180° normal to the pump shaft. This impeller combines the design characteristics of the two other impellers in order to achieve a system both efficient and capable of de- livering a high output of flow and head. This style of impeller is commonly used for applications in vertical turbine and sub- mersible pumps. Specific Speed Introduction "Specific speed" is a pump term often thrown around as a general term or applying to an actual single speed of a pump- ing unit. The term is actually quite important when defining a class of pump—or more specifically, the impeller in the pump. Specific speed is a term used to describe the geometry (shape) and nomenclature of a pump impeller. Individuals re- sponsible for the selection or design of a pump can use this specific speed information to: • Select the shape of the pump curve • Determine the efficiency range of the pump • Anticipate motor overloading problems • Predict NPSH requirements • Select the lowest cost pump for the specific application. Since I often deal in various pump types (standard centrifu- gal, axial flow, and mixed flow), I occasionally work with specific speed (1) as a factor for specifying a pump for pur- chase, (2) as a general descriptor when working with a pump manufacturer or supplier, or (3) to provide a specificity to a needed pump characteristic such as capacity or head. Understanding the meaning of specific speed and the dif- ferent pumps classified with differing numbers can be a real benefit when trying to work across the various types of dy- namic pump classes and the extreme differences that can be encountered in pump flow or head. A knowledge of specific speed is also important when trying to understand the subtle and sometimes not so subtle variances in pump impellers and shapes. Table 1 provides general guidance for classifying most velocity pumps (Figure 1). Specific Speed Definition Two design factors associated with impellers frequently misunderstood and misapplied are specific speed and suction specific speed. The specific speed (N S ) of an impeller or pump is a dimen- sionless index number used to relate the hydraulic perform- ance of a centrifugal pump to the shape and physical proportions of its impeller. Where the specific speed rating of ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS SPECIFIC SPEED It's more important to pump design than you think. ENGINEERING continues on page 50 waterwelljournal.com 48 October 2016 WWJ

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