How To Read A Centrifugal Pump Curve (2024)

How To Read A Centrifugal Pump Curve (1)

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. Just when you think you understand curves, you realize that different types of pumps (centrifugal, positive displacement, air-operated diaphragm, etc.) have different types of curves. In this post, we'll break down the anatomy of a centrifugal pump curve.

Here are some topics we will cover:

  • How a centrifugal pump works
  • A review of Best Efficient Point (and why it matters)
  • Why it is important to know how to read centrifugal pump curves
  • Basic features of a centrifugal pump curve
  • How to use centrifugal pump curves to troubleshoot a problem

How A Centrifugal Pump Works

A centrifugal pump imparts energy on a liquid, and based on the system, has flow and head characteristics. The amount of required pressure the pump must overcome dictates where the performance point will be on the curve and how much flow is produced. As pressure increases, the flow decreases moving your performance point to the left of the curve. As pressure decreases, the performance point runs out to the right of the curve and flow increases.

What is ‘Best Efficiency Point’?

When looking at curves for a new application, there are many factors to consider. But in most cases, a selection should be made as close to the Best Efficiency Point, or BEP, as possible. The BEP is an operating point along the performance curve that shows the highest efficiency point for the impeller diameter of the pump.

Ideally, pumps run at this point for their entire lives. But in real-world applications, system upsets and fluctuating demand cause pumps to operate outside ideal conditions. Here are some common terms you might hear when describing a pump running off its best efficiency point:

  1. Operating away from BEP - the pump is operating at flows greater than or less than the best efficiency point
  2. Operating to the right of BEP - operating at a higher flow rate than the best efficiency point
  3. Operating to the left of BEP - the pump is operating at flows less than the best efficiency point

Why does it matter? Running a pump off its best efficiency point, one way or another can add, energy costs, but the biggest consequence of running off the BEP is an increased chance for failure.

Operating the pump to the left or right of BEP means the door is open to challenges like:

  • Cavitation
  • Excessive vibration, leading to premature wear of seals, shafts, impellers, and bearings
  • Increased operating temperature caused by suction or discharge recirculation, vibration, or breakdown of lubricants
  • Repeated failures that cost money and time to repair

Now that we understand how a centrifugal pump operates and how to maintain efficiency, let’s review the basics of reading the curve.

Understanding the Basics of a Centrifugal Pump Curve

Knowing how to read a centrifugal 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.

How To Read A Centrifugal Pump Curve (2)
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.

How To Read A Centrifugal Pump Curve (3)

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

How To Read A Centrifugal Pump Curve (4)
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.

How To Read A Centrifugal Pump Curve (5)

Reading a Centrifugal Pump Curve

This video will walk you through the process of reading a centrifugal pump curve, with more detailed information included below.

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.

How To Read A Centrifugal Pump Curve (6)

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.

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 if you are in Wisconsin, Minnesota, Michigan, Iowa, North Dakota, and South Dakota. Crane Engineering also builds and designs customized skid systems nationwide.

Let us know how we can help you, request a quote today!

How To Read A Centrifugal Pump Curve (2024)
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