TABLE 1. Pump Types Listed by Specific
Speed*
Specific speed range Type of pump
Below 2,000 Volute, diffuser
2,000-5,000 Turbine
4,000-10,000 Mixed-flow
9,000-15,000 Axial-flow
*Peerless Pump Division, FMC Corporation.
Calculation Procedure:
- Determine the specific speed and suction specific speed
Ac motors can operate at a variety of speeds, depending on the number of poles. Assume
that the motor driving this pump might operate at 870, 1160, 1750, or 3500 r/min. Com-
pute the specific speed Ns = N(Q)^0 -^5 /(H)^0 -^15 = A^(10,000)°-^5 /(100)°-^75 - 3.147V and the suc-
tion specific speed S = 7V(0°-
5
/(NPSH)°
75
= 7V(10,000)°
5
/(32)°
75
= 1A3N for each of the
assumed speeds. Tabulate the results as follows:
Operating speed, Required specific Required suction
r/min speed specific speed
870 2,740 6,460
1,160 3,640 8,620
1,750 5,500 13,000
3,500 11,000 26,000
- 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 1 and 2. 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 pump will be suitable. Operation at 3500
r/min is unfeasible because a suction specific speed of 26,000 is beyond the range of con-
ventional pumps.
Related Calculations. Use this procedure for any type of centrifugal pump
handling water for plant services, cooling, process, fire protection, and similar require-
TABLE 2. Suction Specific-Speed Ratings*
Single-suction pump Double-suction pump Rating
Above 11,000 Above 14,000 Excellent
9,000-11,000 11,000-14,000 Good
7,000-9,000 9,000-11,000 Average
5,000-7,000 7,000-9,000 Poor
Below 5,000 Below 7,000 Very poor
*Peerless Pump Division, FMC Corporation.
