BLOG | Pump performance curves explained

As a manufacturer of pumps, we are regularly asked questions about how to understand pump performance curves. In this blog we explain why pump speed is a key factor. Step by step, we explain the four performance curves charts of a BBA Pumps centrifugal pump. We also share a video where we briefly present the main information shown in the pump curve.

Pump Speed

The pump speed is very important information and is always shown in the performance curves chart. Manufacturers test their pumps at different speeds and show the corresponding curves for each. In doing this, users gain an understanding of how the pump could perform at different speeds. The curves always include a maximum speed, operating above this maximum speed would reduce the lifespan of the pump. Running at a reduced speed is usually no problem, however you can see from the curves that this would mean a reduction in pressure.

Explanation of pump performance curves in four charts

1. Pump curve: In the top chart you can see the pump flow along the horizontal axis. The pump head is shown on the vertical axis. For example, at 250 m³/h we draw a line upwards to the curve. From that point we draw the line to the left and we know the pressure that the pump delivers at that capacity, which is 18 mwc. This way you can easily determine the duty point at which the pump runs. 

• The pump curve changes when the speed changes
• On the left side of the pump curve the pressure is high and the capacity is low
• On the right side of the curve, the pump delivers a lot of capacity and low pressure
• The lines come together at 250 m³/h at 18 mwc, that point we call the duty point 

2. Pump curve power: The second chart shows the pump power requirements on the vertical axis. Depending on the duty point in which the pump is operating at that moment, the pump requires a certain amount of power. We continue with the duty point and draw the line at 250 m³/h vertically to the second graph. At the intersection of the power curve, we draw a line to the left and read the pump power required.

Tip: Always make sure you have enough engine power to allow the pump to run at any duty point in the pump curve. 

3. Pump curve NPSH: This curve can be simply understood as follows: In the NPSH chart we can see the maximum achievable suction head of the pump. This depends on the operating point at which the pump is running and the presently atmospheric pressure.

Theoretically, the maximum suction lift of a pump is equal to the prevailing atmospheric air pressure. At sea level the air pressure varies at around 1000 mbar, at that value the theoretically achievable suction head is 10 mwc. However, each pump has a certain internal loss of suction lift. This loss of suction we can see in the graph with the NPSH curve.

We continue with the operating point and draw the line at 250 m³/h vertically to the third graph. At the intersection of the NPSH curve, we draw a line to the left and read the internal loss of suction lift of the pump. In this example you can see that the loss at the operating point is 2 mwc.
 
The theoretically achievable suction head was 10 mwc, the internal loss in the pump is 2 mwc leaving a suction head of 8 mwc. However, in practice we never select the maximum NPSH value because this could lead to a risk of pump damage. Additional points to keep in mind:

• Locations above sea level will have lower pressure
• Take into consideration the pipe resistance in the suction pipe
• The temperature of the liquid pumped can also affect the NPSH curve
• The thickness and density of the liquid can lead to air bubbles forming in the liquid

4. Efficiency curve: With energy prices being so high, the efficiency curve of the centrifugal pump is very important. By understanding this curve, you can understand the efficiency of the pump when in use at the operating point. We continue the 250 m³/h line to the last graph. At the intersection of the efficiency curve, we draw a line to the left and read the pump efficiency.

Tip: Pump professionals always try to operate the pump with the duty point at BEP – best efficiency point. That’s the area of where the pump is operating close to its peak efficiency. 

Video pump performance curves explained

To summarize this information, we have made a short pump curve video (2 minutes). Herein we visualize the reading of a pump performance curves chart in a simple way. If you would like to know more about pump performance, pump power consumption, NPSH or pump efficiency, please contact one of our pump specialists.
Written by Henno Schothorst
Video voice-over Maas-Jan Koudijs