Joe Evans, Ph.D

http://www.pumped101.com

THE PERFORMANCE CURVE

Once a pump has been designed and is ready for production, it is given a complete and thorough test. Calibrated instruments are used to gather accurate data on flow, head, horsepower, and net positive suction head required. During the test, data is recorded at shut off (no flow), full flow, and 5 to 10 points between. These points are plotted on a graph with flow along the X axis (abscissa) and head along the Y axis (ordinate). The efficiency, brake horsepower, and Net Positive Suction Head Required (NPSHR) scales are also plotted as ordinates. Therefore, all values of head, efficiency, brake horsepower, and NPSHR are plotted versus capacity and smooth curves are drawn through the points. This curve is called the Characteristic Curve because it shows all of the operating characteristics of a given pump. The curve below is an example.

Figure 3

For publication purposes, it is much more convenient to draw several curves on a single graph. This presentation method shows a number of Head-Capacity curves for one speed and several impeller diameters or one impeller diameter and several different speeds for the same pump. This type of curve is called an Iso-Efficiency or Composite Characteristic Curve. You will note that the efficiency and horsepower curves are represented as contour lines. Also the NPSHR curve applies only to the Head-Capacity curve for the full size impeller. NPSHR will increase somewhat for smaller diameters. The curve below is a typical Composite Curve.

Figure 4

The method of reading a performance curve is the same as for any other graph. For example; in Figure 3, to find the head, horsepower and efficiency at 170 GPM, you locate 170 GPM on the abscissa and read the corresponding data on the ordinate. The point on the head / capacity curve that aligns with the highest point on the efficiency curve is known as the Best Efficiency Point or BEP. In Figure 3, it occurs at approximately 170 GPM @ 125’TDH.

The composite curve shown in Figure 4 is read in much the same manner. Head is read exactly the same way; however, efficiency must be interpolated since the Head-Capacity curve seldom falls exactly on an efficiency contour line. Horsepower can also be interpolated as long as it falls below a horsepower contour line. If the Head-Capacity curve intersects or is slightly above the horsepower contour line, brake horsepower may need to be calculated. NPSHR is found in the same manner as head. We will discuss NPSHR in detail a little later.

Calculating Brake Horsepower (BHP)

Usually, if a specific Head-Capacity point falls between two horsepower contour lines, the higher horsepower motor is selected. Sometimes, however, we may need to know the exact horsepower requirement for that point of operation. If so Brake BHP for a centrifugal pump can be calculated as follows:

BHP = GPM X Head / 3960 X Efficiency

For example, in Figure 4, the BHP required at 170 GPM for the 5-1/2" impeller is:

BHP = 170 X 90 / 3960 X .74

BHP = 15300 / 2860.4

BHP = 5.22

Many operating points will fall between the various curves, so it is important that you understand a composite curve well enough to interpolate and find the approximate values.

A pump is typically designed for one specific condition but its efficiency is usually high enough on either side of the design point to accommodate a considerable capacity range. Often, the middle one third of the curve is suitable for application use.

Different pumps, although designed for similar head and capacity can vary widely in the shape of their Characteristic Curves. For instance, if two pumps are designed for 200 GPM at 100' TDH, one may develop a shut off head of 110' while the other may develop a shut off head of 135'. The first pump is said to have a flat curve while the second is said to have a steep curve. The steepness of the curve is judged by the ratio of the head at shut off to that at the best efficiency point. Each type of curve has certain applications for which it is best suited.

Operation in Series (Booster Service)

When a centrifugal pump is operated with a positive suction pressure, the resulting discharge pressure will be the sum of the suction pressure and the pressure normally developed by the pump when operating at zero suction pressure. It is this quality of a centrifugal pump that makes it ideally suited for use as a booster pump. This quality also makes it practical to build multi-stage (multiple impeller) pumps. A booster pump takes existing pressure, whether it be from an elevated tank or the discharge of another pump, and boosts it to some higher pressure.

Two or more pumps can be used in series to achieve the same effect. The figure below shows the curves for two identical pumps operated in series. Both the head and horsepower at any given point on the capacity curve are additive. Capacity, however, remains the same as that of either one of the pumps.

Figure 5

Parallel Operation

Two or more pumps may also be operated in parallel. The curves developed during parallel operation are illustrated below.

Figure 6

Pumps operating in parallel take their suction from a common header or supply and discharge into a common discharge. As Figure 6 illustrates, the flows and horsepower are additive. You will notice that, while head does not change, flow is almost doubled at any given point.

About the Author

Joe Evans lives in beautiful Rhododendron Oregon and retired from PumpTech Inc on 12/31/15. Since entering graduate school, a continuing interest has been one of computer control of mechanical and electronic systems. It began with the introduction of the minicomputer, in the late sixties, and continued with the advent of the PC and PLC in the eighties and nineties. He accidentally entered the pump industry in 1986, and has been trapped there since. He is passionate about the sharing of knowledge and its ability to replace memorization with understanding.