Consider NPSH and BEP when choosing a pump

How do I use the information on a pump curve to select a pump for my system?

A pump curve (sometimes called a performance curve) is a graph that shows the total head, power, efficiency and net positive suction head (NPSH) where a 3 percent head loss occurs (NPSH3) plotted against rate of flow.

These curves contain extremely important data that pump users need to analyze and interpret for proper pump selection and efficient operation.

Image 1. Pump family selection chartImage 1. Pump family selection chart (Images courtesy of HI)

There are three main types of pump curves supplied by the pump manufacturer: the selection chart shown in Image 1, the published curve shown in Image 2, the certified curve

Image 2. Published pump curveImage 2. Published pump curve

The certified curve is different from the selection chart and published curve because it is for the specific pump and impeller trim purchased and not the general product line. Often it will include the acceptance test standard and acceptance grade that the pump was tested against.

The selection chart shows the various pump sizes available for a given manufacturer’s pump line and speed. The desired head and flow rates are entered on the curve, and the pumps that overlap the area are valid choices to consider for selection. The selection chart is useful in developing a short list of pumps for consideration. For example, if the application called for a pump running at a nominal 1,800 revolutions per minute (rpm), that could provide 1,000 gallons per minute (gpm) at 100 feet of total head, the chart shows that 5 × 6 × 11 and 6 × 8 × 11 size pumps overlap on the selection chart and will likely be the two best sizes to evaluate further.

Although the published curve may seem confusing, a lot of critical information can be extracted from this pump curve. If you understand the charts, you will benefit from the data they offer. Remember:

  • The Y axis (vertical) on this curve is the head in feet and meters, and the X axis (horizontal) is the capacity (flow rate) in m3/h and gpm.
  • Each downward sloping blue line is called a head capacity curve.
  • Each number above the head capacity curves to the right of the Y axis represent different impeller diameters. Total head is reduced when the impeller diameter is reduced.
  • The numbers in the circles above the topmost head capacity curve are the pump efficiency, and the lines stemming from these circles are lines of constant efficiency. The triangles that contain a number and word “NPSH” are constant lines of NPSH (in feet) that the system must supply for the pump to operate with a 3 percent head loss. NPSH margin above this value is required for the pump to operate at the published head.
  • The diagonal lines that run through the head capacity curves signify lines of constant pump input power.

Using the selection chart to narrow down the appropriate pump’s size for the duty point of 1,000 gpm and 100 feet of head, the manufacturer’s published curves can be referenced to help determine the best pump for an application. Image 1 shows the published curve for a 5 × 6 × 11 pump running at 1,770 rpm. Information can be derived from the manufacturer’s pump curve for this application, including the following:

  • The impeller diameter that meets the duty point falls between 10 and 10.5 inches.
  • The pump is 85 percent efficient at the rated point and 86 percent efficient at the best efficiency point (BEP).
  • At the rated point, the shaft power will be between 25 horsepower (hp) and 30 hp. To ensure a non-overloading condition at the end of the curve, a 40-hp motor may be required.
  • NPSH3 is between 9 and 10 feet at the duty point.

Note that data displayed on a manufacturer’s pump curve is based on 68 F or 20 C water. If a liquid other than water will be pumped, information on the manufacturer’s published curve must be adjusted for the liquid density and viscosity, which affects the head, flow, efficiency and pump input power.

See other HI Pump FAQs articles here.