How to understand a Pump Flow Curve

When considering a new pump, you will invariably come across a graph which shows the pump’s performance characteristics. This will normally be a curve (or straight line) of flow rate against head.

Defining Pump Head

Head, usually given in units of meters or feet, is really just a measure of the pressure a pump can produce. It tells you the maximum vertical height to which a pump can deliver water (and not the horizontal distance or maximum hose length it will pump).

Take a look at the curve below for one of our most popular submersible pumps, the RSD-400 Puddle Pump.
RSD-400 Submersible Puddle Pump Flow Curve
You can see that the maximum delivery head for this pump is 11m when the delivery volume (or flow rate) is 0 (zero) l/m. What does this mean? Quite simply that the pump can lift water to a vertical height of 11m, but will not deliver any flow – the weight of the water will equal the amount of “push” the pump can produce (or more correctly the pressure it can generate).

The pump will need to generate 1 psi of pressure to “push” a column of water vertically up by 2.307 ft, therefore our RSD-400 will produce maximum pressure of:

11 m = 36.01 ft

36.01 ft / 2.307 ft = 15.6 psi

So, what if the height you’re pumping to is less than the maximum delivery head? The “left-over” pressure will be used to actually pump the water. For instance, at just under 7 m head, the pump will deliver approximately 100 l/m of water and with zero head the pump will deliver its maximum flow rate, in this case 180 l/m.

Another way to visualise this is to imagine a pump which has a pressure gauge and valve connected on the outlet. With the pump running and the valve closed, the pressure gauge will display the maximum pressure (or head) it can produce. If you slowly start to open the valve and let the water flow out, the pressure will drop until it reads zero when the valve is fully open (maximum flow rate).

This however, doesn’t tell the whole story – you also have to factor in the friction losses in the delivery hose. Using our on-line friction loss calculator, you can determine the pressure (or head) loss for flow through a length of hose.

Let’s consider a real-world example for a submersible “puddle sucker” pump

A cellar measuring 4 m x  4 m has been flooded to a depth of 300 mm. Using the RSD-400 and a 10 m length of 32 mm layflat hose we shall pump the water out through the coal shoot (a total height of 3 m).

RSD-400 Submersible Puddle Pump Pumping Out Flooded Cellar

Referring back to the flow curve, you can see that a 3 m head would give us roughly 150 l/m of flow at the pump outlet. Using our pressure drop calculator, we can determine that the friction losses in the layflat hose would be equivalent to an extra 2.8 m head, making our total head 5.8 m. Again, referring back to the flow curve this gives approximately 112 l/m of flow.

The volume of water in the cellar is as follows:

4m x 4m x 0.3 m = 4.8 m³ = 4800 litres

Therefore, assuming there is no further ingress of water and at a flow rate of 112 l/m, our pump would take 43 minutes to drain the cellar.