Please look at the illustration. You will note that I have plotted the head of the pump against its capacity. The Head of a pump is read in feet or meters. The Capacity units will be either gallons per minute or cubic meters per hour.

According to the illustration this pump will pump a 40 capacity to a 110 head, or a 70 capacity to a 85 head (you can substitute the proper metric or imperial units as you see fit)

The maximum head of this pump is 115 units. This is called the Maximum Shutoff Head of the pump. Also note that the best efficiency point (B.E.P). of this impeller is between 80% and 85% of the shutoff head. This 80% to 85% is typical of centrifugal pumps, but if you want to know the exact best efficiency point (B.E.P) you must refer to the manufacturers pump curve.

Ideally a pump would run at its best efficiency point all of the time, but we seldom hit ideal conditions. As you move away from the best efficiency point (B.E.P). the shaft will bend and the pump will experience some vibration. You will have to check with your pump manufacturer to see how far you can deviate from the best efficiency point (B.E.P). It is a function of the shaft (a maximum of 10% either side is typical).

Now look at the following illustration:

Note that I have added some additional curves to the original illustration. These curves show what happens when you change the diameter of the impeller.

Impeller diameter is measured in either inches or millimeters. If we wanted to pump at the best efficiency point with an 11.5 impeller we would have to pump a capacity of 50 to a 75 head.

The bottom half of the illustration shows the power consumption at various capacities and impeller diameters. I have labeled the power consumption Horsepower, but in the metric system it would be called Kilowatts

Each of the lines represents an impeller diameter. The top line would be for the 13 impeller the second for the 12.5 etc. If we were pumping a capacity of 70 with a 13 impeller it would take about 35 horsepower. A capacity of 60 with the 12 impeller would take about 20 horsepower

Most pump curves would show you the percent of efficiency at the best efficiency point. The number varies with impeller design, but numbers from 60% to 80% are considered normal.

If you find that your pump curve dos not match your system cure, please keep in mind that you will have to change valve positioning, or modify the piping system. A VFD (variable frequency drive) changes only the pump cure