How to read a pump curve

1 Feb 2016

Here are the basics of reading the key points on a pump curve.  The pump curve used for this illustration is for a Grundfos in-line circulator pump, and the principles apply to most types of centrifugal pump.

 

A

The required ‘Head’ (output pressure) from the pump is set at 4 metres.  This is a combination of the vertical distance the pump needs to push the water, and the extra pressure put on the pump by ‘friction losses’ (the effort of getting the water through the pipework, round the bends, etc).

NB: in a ‘closed loop’ (where the water keeps circulating round a system) the water goes down as much as it goes up, so the only pressure is from friction losses.

 

B

The required ‘Flow’ (amount of water) from the pump is set at 16 cubic metres per hour.

 

C

This point, combining the flow and the pressure requirement, is called the ‘Duty point’.   The closer it is to the pump curve, at point ‘D’, the more efficient the pump is for the application.

 

D

The actual output of the pump differs from what is required.  This is a fixed speed pump, so it will run at full power when it is turned on.  Full power, in this case, will actually generate a flow of 16.6 cubic metres per hour.  As the flow increases, so does the pressure (it’s harder for the pump to move the water faster, because the friction losses increase), so the actual pressure from the discharge of the pump is 4.309 metres.

 

E

The pump curve illustrates the flow and pressure that will be generated at every point.  If this pump has to cope with an additional 1 metre of head, the flow is reduced to about 12.7 cubic metres per hour.

 

F

Most pumps need a certain minimum amount of flow to keep functioning properly, mainly because they require a consistent flow of water to keep the pump cool.  The paler line at the beginning of the pump curve shows what the pump would do at the higher pressures, but the pump should not consistently be used here.

 

G

This curve shows how efficient the pump will be when used at this flow and pressure.  Use the scale on the right hand side of the graph to read off the amount.  The figure in this example is 65%, which means that 65% of the pump’s energy will be transferred to the water.  Note that these points are directly under point ‘D’, as this is where the pump will actually be operating (rather than point ‘C’, which is the minimum output required).

 

H

Similar to point ‘G’, this curve measures the efficiency, but this time of both the pump and the motor combined.  A certain amount of energy is lost between the motor and the pump, as well as between the pump and the water, so the efficiency here will always be lower.  In this example it is 50.6%.

 

This guide is just a basic outline of how to read a pump curve.  For more in-depth knowledge, or to discuss your specific pump application, please don’t hesitate to contact us.  We supply, install, maintain and repair pumps and pump-related equipment, and we’re always glad to hear from you!

 

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