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