Water Well Journal
Issue link: http://read.dmtmag.com/i/728580
a pump is used to help define the discharge characteristics of a pump, the suction specific speed is used to define the suction characteristics of the same pump. Although often mistaken as a sole speed factor, the index is often used to classify pumps within a group related to its rota- tional speed, head, and capacity and includes virtually all classes and types of commonly used centrifugal pumps— starting at 400–500 for the relatively low capacity radial flow style of impeller up to 15,000 for high capacity, axial flow units. The formula used to calculate specific speed is: N S = N × Q 1/2 H 3/4 where N S = Specific speed, dimensionless N = Rotational speed of impeller in revolutions per minute (RPM) Q = Capacity of pump at best efficiency point (BEP) in gallons per minute (GPM) (Q 1/2 = √Q) H = Total head developed by the maximum diameter impeller at the BEP in feet An application of specific speed for a typical 4-inch- diameter submersible pump can be found in the following example: Pump speed: 3600 RPM (nominal) Design capacity at BEP: 34 GPM Design head at BEP: 17.7′/stage N S = 3600 RPM × 34 GPM (.5) = 3600 RPM × 5.83 = 2432 17.7′ (.75) 8.629 Suction Specific Speed Defined There are actually two different types of specific speed in reality: the version first defined and suction specific speed. The suction specific speed (S) is also a dimensionless rat- ing number used to evaluate the relative ability of a centrifu- gal pump to operate under conditions of low net positive suction head–available (NPSH A ). Depending on the impeller design, suction specific speeds range from below 4000 to more than 11,000 with the higher values indicating lower net positive suction head–required (NPSH R ). The equation used to calculate this factor is: S = N × Q 1/2 NPSH R 3/4 where S = Suction specific speed, dimensionless N = Rotational speed of impeller in revolutions per minute (RPM) Q = Capacity at the best efficiency point (BEP) in gallons per minute (GPM) NPSH R = Net positive suction head–required by the maximum diameter impeller at the BEP in feet Unlike specific speed, a suction specific speed is not a type of number to simply rate a pump's performance, but a crite- rion of a pump's performance with regard to cavitation poten- tial. The suction specific speed (S) rating of a pump indicates how capable the pump's impeller inlet design is (i.e., how low is the NPSH R for a given pump speed and BEP flow rate). Higher "S" values mean lower NPSH R and therefore greater NPSH margins. The range in Table 2 shows how pumps are rated according to their suction specific speeds. ENGINEERING from page 48 Figure 1. Specific speed and pump impeller shapes. Courtesy of www.pumpfundamentals.com waterwelljournal.com 50 October 2016 WWJ Table 1. General Specific Speed Classifications Specific speed range Pump type Below 2000 Process (centrifugal) pumps 2000–5000 Turbine and submersible pumps 4000–10,000 Mixed flow pumps 9000–15,000 Axial flow pumps Table 2. Suction Specific Speed Range Single suction pumps Cavitation rating Above 11,000 Excellent 9000–11,000 Good 7000–9000 Average 5000–7000 Poor Below 5000 Very Poor