Water Well Journal
Issue link: http://read.dmtmag.com/i/728580
An example of an application using suction specific speed follows here. Nominal rotational speed: 1800 RPM Capacity at BEP: 1500 GPM NPSH R at BEP: 12 feet S = 1800 RPM × 1500 GPM .50 = 1800 × 38.73 = 10,813 12′ (.75) (NPSH R ) 6.447 Or conversely, another application where the suction specific speed from a pump is provided: S = 9000 Nominal speed: 3600 RPM Capacity at BEP = 125 GPM To find approximate NPSH R of pump: 9000 = 3600 RPM × 125 GPM .50 = 40,249 NPSH R .75 9000(NPSH R .75 ) = 1.0 — NPSH R = ~7.368 feet (7.37 feet) Approximate total dynamic suction lift at BEP (at sea level) = 32 feet – 7.37 feet = 24.63 feet Typical Specific Speed Ranges It is important to note there are no universal or established categories for low, moderate, or high head or capacity pumps. Each individual who works with pumps has their own per- sonal definition of what constitutes a low or high head or capacity pump. Table 3 has been prepared with the author's general inter- pretation of these classes based on the industry's generally ac- cepted ranges. The most critical element of using this table is to divide pumps based on the relative head or flow rates used to determine the specific speed, not an arbitrary classification. Most standard well pumps of 6 inches or greater diameter fall into a given range of specific speed (N S ) between 2000–5000. These ranges can help determine the type of impeller and class of pump you are working with or intend to buy. Obviously, the information contained in the table should be used as an approximation only, but in many cases this data can be extremely helpful during an analysis of a potential pump purchase. Specific Speed Impact on Developing Pump Curves and Performance The use of specific speed is not only helpful when examin- ing a particular pump, it is also a valuable tool often used when developing a pump curve. When using this value to compare or evaluate pumps, it must be remembered the spe- cific speed is a number calculated at the pump's best effi- ciency point (BEP) at maximum diameter for both head and capacity. This is a singular value that applies to all pumps, and try- ing to adjust or shift the value to retain it for trimmed impeller diameters will not work. This value defines the basic geomet- ric path of the impeller with lower specific speed pumps, usually with larger outside diameters and narrower radial (right angle) flow passages, and higher specific speed pumps equipped with smaller outside diameters and larger more axial (straight through) flow passages. Pumps of the same specific speed can be scaled up or down for greater or less flow and head requirements. Specific speed also dictates the shape of the head-capacity curve with low specific speed pumps having flatter or droop- ing head-capacity curves, and higher specific speed pumps having steep curves (Figure 2). The pump's shutoff flow brake horsepower also increases with increasing specific speed val- ues, exceeding the best efficiency flow input power require- ments at N S values above 4500 to 5000. N S also dictates the maximum obtainable efficiency of a pump with the maximum obtainable efficiencies occurring at specific speed values between nearly 2000 and 3000 (Figure 3). This is the area where single stage pumps begin to meet their effective limit for head and multistage units start to occur for smaller diameter (< 8-inch) pumps. If the application calls for a low, single stage specific speed value of 500–2500 or lower, such as those often found on 4- inch and 6-inch submersible pumps, the specific speed value of the overall pump is increased by adding these additional impeller stages, which reduces the head produced by each stage. This results in an increase in the efficiency of the pump. This is one of the advantages of vertical turbine or sub- mersible pumps, which can vary up or down the number of impeller stages and also helps to explain why efficiency de- ductions are often needed for some multistage vertical turbine pumps with only one to three stages. ENGINEERING continues on page 52 Figure 2. Specific speed and pump performance curves. Courtesy of www.pumpfundamentals.com WWJ October 2016 51 Twitter @WaterWellJournl