Insights on How to Read a Centrifugal Pump Curve

A Centrifugal pump curve can be informative and important for your pump's health. However, fully understanding a pump curve can be overwhelming and challenging. Here are some topics we will cover in this blog post: 

• Why it is important to know how to read a pump curve
• Basic features of a pump curve
• How to use a pump curve to troubleshoot a problem

Understanding the Basics

Knowing how to read a pump curve can help you correctly size a pump and troubleshoot any problems that arise. Let's go through some basic features of a pump curve. The title box is located at the top of the graph and provides information about the pump model, size, speed, and other identifying criteria specific to the pump. See the title box highlighted on the graph below.  

When you take a look at the graph itself, you will see the flow rate (GPM) is on the x-axis of the pump curve.

The head is the vertical line on the y-axis. Usually, this is measured in feet.

The impeller trim (in) can be found at the beginning of the four horizontal curves, reading 9 in, 10 in, 11 in, 12 in, and 13 in.

Reading a Centrifugal Pump Curve

Let's walk through an example of how to read the centrifugal pump curve. If the flow rate is 300 gallons per minute and there are 100 feet of head, we can see where the points intercept to determine the impeller diameter. In this example, our impeller diameter is roughly 10.5 inches. Now that we know our performance point, we can determine the amount of horsepower required. Horsepower is indicated across the curve as a dotted line, in this case at a downward angle. Our performance point is between the 10 and the 15 horsepower lines. We can estimate the selection will require 12 horsepower.

Calculation Net Positive Suction Head Required

Next, we can figure out the net positive suction head required (NPSHr). This is important because you need to know the minimum amount of pressure on the suction side of the pump to overcome pump entrance losses. If sufficient NPSH is not met the pump will cavitate, causing many performance problems. When selecting the best pump for an application, efficiency many times is an important factor. The higher the efficiency, the less energy required to operate for a specific performance point. In this scenario, the NPSHr would be about 5.5 ft. 

Calculating Minimum Flow Line

Last, but certainly not least, we need to figure out the minimum flow line. A centrifugal pump requires a minimum amount of flow moving through the pump to dissipate the heat created. On the left side of the curve, minimum flow is indicated by a bold vertical line. Operating to the left of this line is not recommended and can significantly decrease the life of the pump. 

Determining the Impeller Trim Without a Nameplate

One common issue is determining the impeller trim when a nameplate has been lost or misplaced. To do this you will need a pressure gauge on the suction and discharge side of the pump. Close a valve on the discharge side of the pump and quickly measure the suction and discharge pressure. Reopen the discharge valve to return the pump to normal operating conditions. For this example, we will assume the suction pressure was 15 feet or 6.5 psi, and the discharge pressure was 165 feet and 71.5 psi. We can take the difference between the suction and the discharge pressure to get the dead head or no flow condition. Our result is 150 feet and 65 psi.

Looking at our pump curve we see that at 0 gallons per minute and 150 feet, we have a 12-inch diameter impeller. This test can also be used to determine if your pump is acting properly. If you know the diameter of your impeller and it's 13 inches you know that pump isn't performing to the capabilities. If we see the pump is not operating at the point it should be there is likely something wrong with the pump.

This video will walk you through the process of reading a pump curve one more time. 

Expert Centrifugal Pump Insights from Crane Engineering

Engineers and experts rely on Crane Engineering for insight and help with centrifugal pumps and positive displacement pumps. Our in-house team of engineers can answer questions related to not only pumps but valves and skid systems. We provide a complete service and repair team who will fix pumps back to OEM standards. We are ready to assist you, contact us, today!