Reading a pump curve: how to choose a pump at the right duty point

The pump curve shows how much water a pump moves (flow rate) against a given back pressure (head). If you choose a pump based only on its maximum flow rate or maximum head, you will almost always get it wrong in practice, because at those extremes the pump delivers no pressure or no water respectively. In this article you will learn to read the curve the way an installer does and determine the real duty point of your pump.

What does a pump curve show?

The vertical axis (Y) shows the head in metres, the horizontal axis (X) the flow rate in m³/h or litres per minute. The descending line is the pump curve: the higher you need to pump, the less flow rate remains. The two extremes are Hmax (maximum head at zero flow) and Qmax (maximum flow rate at zero head). Neither of them is your duty point; that always lies somewhere in between.

The duty point: where system and pump intersect

Your installation has its own system curve: the resistance the pump has to overcome. It consists of the static head (the fixed height difference) plus the pipe friction loss, which increases as you pump more water. Lay the system curve over the pump curve and the intersection is the duty point: the flow rate and head you actually achieve in practice. The flow rate is not determined by the pump alone, but by the combination of pump and installation.

Static head and pipe friction loss

The static head is the vertical height difference between the water level and the discharge point; it does not change with the flow rate. Pipe friction loss (friction in hose, pipe and fittings), on the other hand, rises roughly with the square of the flow rate: double the flow and the resistance roughly quadruples. Long or narrow pipes therefore shift your duty point considerably to the left, meaning less flow rate.

Common mistakes when reading a pump curve

• Choosing by Qmax. You only reach the maximum flow rate at zero back pressure, never in a real installation.
• Forgetting pipe friction loss. If you do not plot the system curve as well, you will consistently choose a pump that is too light.
• Looking only at height or depth. Also include the pressure you need at the outlet: 1 bar is roughly 10 metres of head.
• Combining two pumps the wrong way. Connecting in parallel increases the flow rate, connecting in series increases the head, not the other way round.

How to use the curve when choosing a pump

First determine your required duty point (flow rate and head together), then look for a pump whose curve runs comfortably above it. You will find the relevant specifications on every product page in our shop. Not sure? Browse our range of submersible pumps or let our customer service work out the duty point with you.

Frequently asked questions

What is a pump curve?
A pump curve is the graph that shows how much flow rate a pump delivers at each head. The line descends: the higher the back pressure, the less water the pump moves.

What is the duty point of a pump?
The duty point is the intersection of the pump curve and the system curve of your installation. That is where you find the actual flow rate and head you get in practice.

Why don't I reach the stated maximum flow rate?
Qmax only applies at zero head. As soon as height and pipe friction loss come into play, the duty point shifts and the flow rate drops.