Pumps suction specific speed

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. 

When specified by the purchaser, the pump suction specific speed shall be limited as stated on the data sheet. 

Typically for single-suction pumps, suction-specific speed above... 

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. 

Hallam,J. L., Centrifugal Pumps Which Suction Specific Speeds are Acceptable , Hydrocarbon. Processing, 61, No. 4, 1982. 

Table 3-26 gives the specific speeds for various centrifugal pump types. Table 3-27 gives the suction specific speed ratings for single-suction and double-suction centrifugal pumps. These tables are for pumps handling clear water. 

For the centrifugal pump, two terms are in common use, the Thoma cavitation number cr and the suction specific speed Sn ... 

Another dimensionless group, analogous to the specific speed, that relates directly to the cavitation characteristics of the pump is the suction specific speed, Nss ... 

Centrifugal potting, 16 18 Centrifugal pumps, 19 513 affinity laws related to, 21 63 costs associated with, 21 87 efficiency of, 21 60 nonmetallic, 21 76 suction specific speed of, 21 63 Centrifugal sedimentation, 18 142, 143-144 Centrifugal separation(s), 5 505-551 ... 

Another parameter that helps in evaluating the pump suction limitations, such as cavitation, is suction-specific speed. 

Recommendations also are made by the Hydraulic Institute of suction specific speeds for multistage boiler feed pumps, with S = 7900 for single suction and 5 = 6660 for double suction. Thus the required NPSH can be found by rearrangement of Eq. (7.14) as... 

A test-model pump delivers, at its best efficiency point, 500 gal/min (0.03 m3/s) at a 350-ft (107-m) head with a required net positive suction head (NPSH) of 10 ft (3.05 m) and a power input of 55 hp (41 kW) at 3500 r/min, when using a 10.5-in-diameter impeller. Determine the performance of the model at 1750 r/min. What is the performance of a full-scale prototype pump with a 20-in impeller operating at 1170 r/min What are the specific speeds and the suction specific speeds of the test-model and prototype pumps ... 

Compute the specific speed and suction specific speed. The specific speed or, as Horwitz [2] says, more correctly, discharge specific speed, Ns = N(Q)0 5/(H)075, while the suction specific speed S = Al(g)° 5/NPSH0 7, where all values are taken at the best efficiency point of the pump. 

For the model, Ns = 3500(500)as/350a75 = 965 S = 3500(500)°-5/10°-75 = 13,900. For the prototype, Ns = 1170(1158)a5/142.5a75 = 965 5 = 1170(1156)a5/4.06a75 = 13,900. The specific speed and suction specific speed of the model and prototype are equal because these units are geometrically similar or homologous pumps and both speeds are mathematically derived from the similarity laws. 

Choose the best speed for the pump. Analyze the specific speed and suction specific speed at each of the various operating speeds using the data in Tables 6.20 and 6.21. These tables show that at 870 and 1160 r/min, the suction specific-speed rating is poor. At 1750 r/min, the suction specific-speed rating is excellent, and a turbine or mixed-flow type of pump will be suitable. Operation at 3500 r/min is unfeasible because a suction specific speed of 26,000 is beyond the range of conventional pumps. 

Figures 5 and 6 are photographs of an inducer portion of a high head pump operating on water during various phases of cavitation. This pump was designed to flow 6 gpm at a shaft speed of 30,000 rpm. Figure 5 shows the inducer operating at approximately 95 % noncavitating head which corresponds to a suction specific speed of 18,450. The same...

Hydraulic reliability is reflected in the pump s ability to maintain steady discharge pressures and corresponding flow rates in the range of outputs desired. Relatively high flow rates and impeller speeds are conducive to the production of a cavitating flow. Such conditions become critical and hydraulic-flow deterioration occurs when the inlet pressure becomes inadequate for vapor suppression. A criterion for establishing the vulnerability of the pump to incipient cavitation is the well-known dimensionless parameter called suction specific speed. 

Answer by Author A criterion for NPSH computation reliability is its repeatability when using a wealth of data. A three-thermocouple redundancy per measurement and the multiplicity of test runs pinpointed the temperatures selected. Then, by using equivalent vapor pressures, and by judiciously establishing pump inlet head losses, no erroneously-negative values of NPSH were realized. Since related suction specific speed values were neither imaginary nor infinite, and, because no hypothetical approaches were necessary to calculate their real values, the NPSH values were deemed reliable. 

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