Understanding Pump Curves
Unstable Curve Geometries
The curves below are for real pumps commonly sold on the pump market. They were selected to illustrate each type of pump curve. Study them to make sure you agree with the description below each curve.
Graph #1 - Un-Stable Curve - Droop
This curve is un-steady because head increases as flow increases at the lower end of the flow range, from 0 gpm to about 200 gpm. At flow rates beyond 200 gpm the curve is steady.
Curve geometry for this pump is described as having a "droop".
This pump can be difficult to control, and because the droop can cause system oscillations where the system and pump interact in uncontrollable ways.
Consider the case of a pump operating at BEP and then flow rate begins to fall as system resistance increases. As the flow rate falls pump head increases until the droop is encountered. As flow rate continues to decrease system resistance also continues to decrease, resulting in a momentary increase in flow, and then system resistance increases again. The cycle may repeat itself again and again until something in the system changes.
Graph #2 - Un-Stable Curve - Midrange Instability
Beginning at 245 gpm the rate of head drop begins to change so that from 290 gpm to 330 gpm there is little head drop. When the flow rate reaches 330 gpm the rate of head drop increases steadily again.
There is nothing inherently wrong about using unstable curve pumps so long as system designers understand the pump characteristics including head flow instabilities.
Follow the links below to pages focusing on other types of information provided in the Pump Characteristic and more, including graphs and explanations.
