Your Guide to Understanding Centrifugal Pump Curves
Selecting the right pump for your sewer pump station or stormwater pump station can be challenging, especially if you're not familiar with submersible pump curves. A common pump performance curve is a graph that shows the relationship between the flow rate produced by the pump (i.e. how much fluid is moved) vs the system head (pressure) encountered by the pump. Understanding pump curves and how they relate to system curves can help you make informed decisions. The system curve represents the resistance within the piping system and its impact on the pump. This guide explains how to read pump curves and their interaction with system curves for both sewer and stormwater applications.
What Is a Pump Curve?
A pump curve is a graphical representation of a pump's performance showing the relationship between the pump’s affected flow rate vs the different levels of head resistance. On a typical submersible pump curve chart, the flow rate is plotted along the horizontal axis, and the head is plotted along the vertical axis. By analysing this curve, you can see how much flow the pump should expect at various levels of resistance.
How to Read a Pump Curve
Although pump curves may seem complex they are straightforward once you know what to look for:
- Flow Rate: Measured in litres per second(L/sec), the flow rate indicates the volume of fluid the pump can move. As you move right along the horizontal axis, the flow rate increases. Learn about calculating the flow rate of a pump.
- Head (Pressure): Measured in metres, the head represents the pressure generated by the pump. Moving up the vertical axis indicates an increase in pressure.
This means that as the flow rate increases, the head decreases. Typically, a centrifugal pump curve slopes downward from left to right. This inverse relationship reflects the pump’s energy limitations.
Pump Performance Curves Explained
A pump performance curve provides additional details, such as efficiency, power consumption, and net positive suction head (NPSH). These curves often display multiple lines representing different impeller diameters or speeds, helping you determine which configuration best suits your stormwater or sewage pump station needs. NPSH is less relevant for submersible pumps because as the name suggests, submersible pumps sit in the liquid they are pumping. This means they are mainly required to push the water, not suck. For surface mounted pumps, NPSH is very relevant as they are ‘sucking’ the liquid as well as pushing.
Understanding Pump and System Curves
While a pump curve shows the pump’s capabilities, the system curve illustrates the same relationship but for the piping system. Factors like pipe length, diameter, and fittings contribute to the system curve, which typically slopes upward - indicating that as flow increases, the resistance (and required head) also increases due to friction losses. The amount of head differs depending on what kind of hydraulic system the pump is installed in. Factors such as pipe length, vertical lift, pipe diameter and pipe material are all factors of system head.
While the pump curve is a set relationship for the pump and doesn’t change, the system curve differentiates depending on the site it’s being installed in. The intersection of the pump curve and system curve, known as the ‘duty point’, is where the selected pump would perform in that particular system. Ideally, this point should be near the pump's Best Efficiency Point (BEP) for optimal performance.
Pump Curve vs. System Curve: Finding the Balance
Comparing pump and system curves helps assess how well the pump aligns with your system’s requirements. If the curves don’t intersect at a suitable point, the pump may operate inefficiently, leading to excessive wear, vibration, or even system failure.
How to Choose the Right Pump
To choose the right pump for your sewer or stormwater pumping station, overlay the pump curve with your system curve as closely as possible. System curves can be hard to generate manually - but our selection tool can help. By inputting parameters such as pipe size, length, and elevation changes, our pump station calculator generates a system curve, and also selects the best pumps from our range helping you to find the best pump match.
Interpreting Pump Curves: Pressure vs. Flow Rate
Understanding the relationship between head (pressure) and flow rate is critical, as an inverse relationship exists: increasing the flow rate decreases the head and vice versa. Finding the right balance ensures the pump meets the system’s required flow rate outcome without undue stress.
How to Read Key Indicators on a Pump Curve Chart
When reading a pump curve chart, look for these indicators:
- Best Efficiency Point (BEP): A pump’s preferred duty where the pump operates most efficiently.
- Maximum Flow Rate: The highest flow rate the pump can achieve aka. Qmax and Hmax
- Shut-off Head: The maximum head the pump can produce when the flow rate is zero.
- Efficiency Curves: Lines indicating the pump's efficiency at various points.
Need Help Calculating your Pump Curve?
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