TABLE 4. Type of Pump for Highest Energy Efficiency*
Area 1: Single-stage, 3500 r/min
Area 2: Single-stage, 1750 r/min or lower
Area 3: Single-stage, above 3500 r/min, or multistage, 3500 r/min
Area 4: Multistage
Area 5: Multistage
Area 6: Reciprocating*Includes ANSI B73.1 standards, see area number in Fig. 14.is that improved impeller inlet design allows operation at 3500 r/min to capacities of 5000
gal/min (19,000 L/min) and higher.
Choice of operating speed also may be indirectly limited by specifications pertaining
to suction performance, such as that fixing the top suction specific speed S directly or
indirectly by choice of the sigma constant or by reliance on Hydraulic Institute charts.
Values of S below 8000 to 10,000 have long been accepted for avoiding cavitation.
However, since the development of the inducer, S values in the range of 20,000 to
25,000 have become commonplace, and values as high as 50,000 have become practi-
cal.
The sigma constant, which relates NPSH to total head, is little used today, and Hy-
draulic Institute charts (which are being revised) are conservative.
In light of today's designs and materials, past restrictions resulting from suction per-
formance limitations should be reevaluated or eliminated entirely.
Even if the most efficient pump has been selected, there are a number of circum-
stances in which it may not operate at peak efficiency. Today's cost of energy has made
these considerations more important.
A centrifugal pump, being a hydrodynamic machine, is designed for a single peak op-
erating-point capacity and total head. Operation at other than this best efficiency point
(bep) reduces efficiency. Specifications now should account for such factors as these:
- A need for a larger number of smaller pumps. When a process operates over a wide
range of capacities, as many do, pumps will often work at less than full capacity,
hence at lower efficiency. This can be avoided by installing two or three pumps in par-
allel, in place of a single large one, so that one of the smaller pumps can handle the
flow when operations are at a low rate. - Allowance for present capacity. Pump systems are frequently designed for full flow at
some time in the future. Before this time arrives, the pumps will operate far from their
best efficiency points. Even if this interim period lasts only 2 or 3 years, it may be
more economical to install a smaller pump initially and to replace it later with a full-
capacity one. - Inefficient impeller size. Some specifications call for pump impeller diameter to be no
larger than 90 or 95 percent of the size that a pump could take, so as to provide reserve
head. If this reserve is used only 5 percent of the time, all such pumps will be operat-
ing at less than full efficiency most of the time. - Advantages of allowing operation to the right of the best efficiency point. Some speci-
fications, the result of such thinking as that which provides reserve head, prohibit the
selection of pumps that would operate to the right of the best efficiency point. This