System curve and operating point
The pump doesn’t choose its own flow — the pipes do. Find where the two curves meet.
⏱ 6 min
- Everyday comparisonStep 1 of 5
A cyclist on a hill
A cyclist rides up a hill. They don’t pick their speed from nowhere: they end up at the speed where their legs’ push exactly matches the hill plus the wind.
A pump is the same. Its flow settles where what the pump can give exactly matches what the pipes need.
- ExplainedStep 2 of 5
What the pipes need
To push water through a system, you need two things. First, the height to lift it: the . That doesn’t change with flow.
Second, enough push to overcome in pipes, bends and valves. Friction grows fast: twice the flow needs about four times the friction head.
Plot both together and you get the : it starts at the static head and bends upward.
- Try itStep 3 of 5
Find the operating point
The black line is the pump; the blue line is the system. The pump runs exactly where they cross: the .
Change the height to lift and the pipe friction, and watch the point move.
Challenge: Raise the height to lift until the flow stops. What is the highest height this pump can reach?
- ExplainedStep 4 of 5
What moves the point
Closing a valve or adding a long, narrow pipe makes the system curve steeper: the point slides left, less flow.
A higher tank makes the system curve start higher: again less flow. If the static head is above the pump’s shut-off head, the pump can’t deliver anything at all.
- The impeller connectionStep 5 of 5
Choosing a pump
Engineers first calculate the system curve of the pipes, then pick a pump whose curve crosses it at the flow they need.
A pump that is too big is not “extra safe” — it ends up running far from where it was designed to, wasting energy and wearing faster.
📌 Remember this
The system curve is what the pipes need: static height plus friction that grows with flow². The pump runs at the operating point, where its curve crosses the system curve.