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Most everyone who has installed, sold, or manufactured pumps, is familiar with the pump curve.

 

Basic understanding of the pump curve, which is a plotting flow vs. head, is that when we are given a desired flow, we do some basic calculations considering elevation, pressure of inlet and discharge, and piping losses. Then we come up with the total head associated with the given flow. (See Fig. 1)

 

Many times, piping loss is neglected, or not really needed to pick a pump based on it doing at/or better than the calculated head-flow design point.

 

Often, this is not a problem. A private well may pump more than required, but the extra is stored in a tank for future use. The same can be said for a public system with water towers. Many times, however, getting the flow and head near the pump curve is critical. The water source may not supply the extra water that the pump is producing. Likewise, the receiving system may not sustain the increased pressure created by throttling the flow to the designed point.

 

Remember, the pump curve is a plot of the pump’s performance. It cannot hit any point that is not on the plotted curve. So, if our design point is below the plotted curve and no additional controls are added, the pump will produce both more flow and more head (pressure) than we were looking for.

 

Producing more than needed can be critical when it is necessary for processing, long lengths of water lines, intricate piping, multiple valves, and so on. The system may create extra head or more suction issues due to increased flow for instance, and what seemed a safe bet is not, at this point. Plotting the system is the best way to determine the right pump for the job.

 

A system curve is a plot of the conditions required at each flow rate along the curve. Many pump selection programs, such as found in the Webtrol Intelliquip program, can add the system curve to a pump curve. With the click of a button and by adding in the static head, also know as how much head you have before the pump starts. (See Fig 2.)

 

The intersection of both curves determines exactly what the pump will produce. In the field or when no program is available, take a copy of the pump curve, mark your design point, then calculate the total head (including piping losses) at a flow 5% less than design as well as 5% more than design, and plot them on the page. Sketch a line through the 3 points. Where it crosses the pump curve is your actual pumping condition. If this point is satisfactory, you have determined what the pump will produce under the conditions. (See Fig. 3.)

 

Efficiency, product costs, and product lifetime costs are all items scrutinized as the cost of pumping increases. By following the steps to select the pump best fit for the curve, you will be able to sell your product as best value for the price. You can show your calculations, explain why this was the best value selection and be the expert in your market.