The selected rate of discharge from the pump station determines the number and size of pumps required for the facility. However, pump selection is a matter of economic analysis by the designer. To continue the example above using an allowable discharge of 100 cfs, Table 11-2 lists imaginary pumps data. For a real design, the designer must consult manufacturer's technical data to select a pump or combination of pumps to achieve the allowable discharge.

Table 11-2: Imaginary Pump Data

Pump Designation	Pump Capacity
Type AA	5,000 gpm = 11.1 cfs
Type BB	6,000 gpm = 13.4 cfs
Type CC	7,000 gpm = 15.6 cfs
Type DD	8,000 gpm = 17.8 cfs
Type EE	9,000 gpm = 20.0 cfs
Type FF	10,000 gpm = 22.3 cfs

From Table 11-2, five Type EE pumps (20 cfs x 5) will match the allowable pump rate of 100 cfs. Alternatively, four type FF pumps (22.3 cfs x 4) will yield a pump rate of 89.2 cfs, which is less than the allowable but still a significant discharge. However, the lower pump rate will require a larger wet well volume.

The designer must also consider the cost of construction and physical restrictions for the wet well. Enlarging the wet well and using fewer pumps might be a reasonable alternative to a larger wet well. In situations where one pump may be able to supply the entire discharge necessary, a minimum of two smaller pumps is recommended for reliability and maintenance. Multiple pumps also offer the opportunity for a staggered startup of pumps. Manufacturer's printed technical data and a sales or technical representative can be invaluable sources at this stage of the design in selecting the right pumps. The final design and pump selection must be based on all the considerations together.