A system curve is a plot of your pumping system for various flow. The system curve calculates your static conditions, those that exist without pumping product such as line pressures or elevation, and then adds conditions that vary with flow such as piping friction loss or equipment in the system that causes more TDH (Total Dynamic Head) as flow increases.
A pump curve is a representation of flow vs. TDH. Typically, we search for a curve on which the design point is on or below a chosen curve. When the point is below the curve some interpolation is needed to determine what the pump will do under the design conditions.
If we read straight up on a curve from the design point, we would see that we can achieve more TDH at the given flow and if we read straight to the right, we see we can achieve more flow at the given TDH. Neither is the case unless restraints are added to the system to create different conditions.
The system curve, however, can take the difference between static and flow conditions and extend the system curve to the point where flow and TDH meet the pump curve.
Engineers often plot a system curve against a pump curve to determine exactly what the flow will be when placed in the pump application. These curves are not always placed in specifications, but the results are given as flow vs. TDH in the final specifications.
In the curve above, the design point is marked at 40 gpm at 65psi TDH. If we extend up we see we can achieve 40 gpm at 78 psi or we extend to the right and get 56 gpm at 65 psi. Neither condition will occur without modifying the system. The system curve adds in the piping losses for the additional flow to give a true flow for the system. In this case 46 gpm at 74 psi where the system curve meets the pump curve.
Sizing software programs available now offer this feature when searching for the best pump curve. At Webtrol, we have installed our curves on the Intelliquip software where you can view them. If you would like more information on this topic, contact our Webtrol customer support team at 800-769-7867.