Understanding Pump Curves
System Resistance
&
Pump Capacity
Have you ever wondered much fluid a pump will deliver when connected to a pipe system? The answer is often simple, if you have the right information about the system the pump is connected to. If you do not have the information about the system that you need, then you cannot know that answer, no one can know that answer.
The graph below has two curves plotted. The black curve is the head capacity curve for a pump. The red curve is a System Resistance curve showing how much a system will resist any given flow rate. Both curves are calibrated to the same two scales: Feet of Head on the left side of the graph, Flow Rate in GPM on the bottom of the graph.
Looking at the graph with just common sense you might think that the pump would deliver 180 gpm @ 145 Feet TDH if connected to the system represented by the red curve line. And you would be correct. So we conclude that if you have a system resistance curve, and then place that curve on the same graph with the Head Flow curve for a pump, where the two curves intersect is where the pump will operate.
We then conclude that the pump does not determine how much fluid will be pumped, and the pump does not determine what the pressure will be on that fluid. But rather, the determining factor will be how much resistance is placed against that pump.
We can state it this way,
A constant speed pump operates on its Head Capacity curve line at the point where System Resistance at a specific flow rate equals the pump's Head at that same flow rate.
For the pump and system plotted on the graph above,
System Resistance at 180 gpm is 145 Feet TDH.
The pump delivers 180 gpm at 145 Feet TDH.
Therefore, the pump will operate at that exact point, 180 gpm @ 145 Feet TDH.
The system determines where the pump will operate on its head capacity curve, not the pump itself.
Follow the links below to pages focusing on other types of information provided in the Pump Characteristic and more, including graphs and explanations.
