How to read a pump curve?
How to read a pump curve?
Learn how to read a pump curve in this short module.
Being able to read a pump curve is essential for anyone working with pumps, as it makes it easier for you to always select the right pump for a particular hydraulic system.
In this module, we’re going to show you how to read a pump curve. Let’s get started.
First of all, in order to understand a pump curve, you need to know your system’s characteristics.
The system characteristics show the pressure losses in the system as a function of the flow.
And when there’s a cross point between the system’s characteristics and the pump’s performance,
you have the specific duty point.
Take this pump performance curve of specific Grundfos pump, for instance.
If your system requires a flow rate of 140 m3/h and a head of 6.3 metres, the performance curve shows that this particular pump is perfect for these requirements.
Technically speaking, it will also meet the system requirements of any other intersection of flow rate and head in the area below the curve.
However, oversizing and undersizing a pump can have performance consequences, so you ought to stay as close to the duty point as possible.
Now, let’s take a look at a couple of common pump curves, namely:
- The QH curve
- The ŋ-curve
- The P2-curve
- And the NPSH curve.
Together, these four curves provide a good overview of the performance of a specific pump,
so we recommend including them in the same data sheet.
In a QH-curve, which measures the flow and head, you can see that a low flow results in a high head and a high flow results in a low head.
Once you have identified the requirements for your application, the Flow (Q) and Head (H) will help you determine the overall dimensions of the pump. In this example, you can see that a flow of 71 m3/h equals a head of 42m.
An ŋ-curve highlights a pump’s efficiency. The total efficiency is the ratio between hydraulic power and supplied power. This figure shows the efficiency curves for just the pump and for a complete pump unit respectively.
It is worth noting that the efficiency is always below 100%, as the supplied power is always larger than the hydraulic power due to losses in motor and pump components.
This is also why the efficiency of a complete pump unit is lower than the efficiency of just of a pump.
Next, there’s the P2-curve, measuring the relation between a pump’s power consumption
and flow. The pump’s power consumption determines the size of the electrical installations which must supply the pump with energy.
Power consumption also depends on the density of the fluid. If the fluid’s density is higher than that of, say, water, motors with a correspondingly higher output must be used.
As you can see here, the P2 value increases when the flow increases.
Finally, there’s the net positive suction head – or the NPSH-curve. Since the NPSH appears in metres and not kW, it is not necessary to take the density of different fluids into account.
NSPH describes conditions related to damaging cavitation. And that’s where the NSPH-curve comes in handy.
To determine if a pump can operate safely in a system, the NPSH must be known for the largest flow and temperature within the operating range.
The recommended minimum safety margin is 0.5m, but you may need a higher safety margin depending on your application.
Pump design can also affect what the pump performance curve will look like.
There are both flat and steep pump curves. Flat curves occur when there is a small change in head resulting in a big change in flow, as you can see here.
Steep curves, on the other hand, occur when there is both a small change in head and the flow, as shown here.
That covers our module on how to read pump curves. We hope you enjoyed it.