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Choosing a pump by creating a system curve first will ensure the pump you choose will operate properly and within the preferred operating range for that pump.

 

Creating the system curve uses the basic steps in creating TDH for various flows. By plotting several points on a pump curve, you can see what the pump performance will be under the conditions given. An example of a system curve is shown as the green line in Figure 1 and resembles ½ of a parabola.

 

The points on a system curve consist of static head and piping losses. Static head is anything that is pumped against when it starts such as discharge pressure or elevation change. The piping losses are pipe friction and other losses such as valves, fittings, etc.

 

In doing this by hand, Figure 2, it is not necessary to plot the entire curve. Just plot a few points on both sides of the design point and sketch through these points. In Figure 2 we see the intersection with the pump curve is at 100 GPM - 275 ft TDH. This is what the production of the pump will be in this system pumping at nominal full speed.

 

In many systems there will be a secondary system curve that, for instance, considers an additional 40 ft. of head at near shutoff as shown in Figure 3. The curve in the example starts out 40 ft. higher at 0 flow and the gap slightly increases towards the right side of the design point. As with the first system curve, the intersection with the pump curve is the performance at the higher head. In determining what a pump will produce in a system, plotting the system curve is the best means of pinpointing the performance.

 

As you can see on the curves in the example, the pump is overperforming the design point at the lower static head. There are ways to compensate if these points are not acceptable. One is trimming impellers to decrease the performance. Another is the use of Variable Speed Drives to slow the rotation of the pump. Both methods affect the pump curve in the same way.

 

With available software, the speeds can be shown on the same curve to see where you are at reduced speeds. On the example in Figure 4, at 3200 RPM, the pump is meeting the design point and with an analog pressure sensor, can maintain the desired head by changing speed as needed.

 

Whether you use software such as shown in the examples, or sketch your own, plotting the system curve will help size a pump to the best part of the operating range.

 

Webtrol curves are available on Intelliquip for those who would like to use them. If software is not an option, the curve can be plotted by calculating static head and piping loss at a few points as was done in figure 2. Many tools are available electronically for this convenience.

 

If you are working with Webtrol curves on Intelliquip and have questions regarding how to plot some of this information, please contact customer service or your salesperson.

 

Click on any image below to enlarge.