How To Read A Centrifugal Pump Curve

Today we're going to go over how to read a pump curve. First, we're going to say why it is important to know how to read a pump curve. Second, we're going to go over some basic features of a pump curve. And third, we're going to discuss a common example of how to use a pump curve to troubleshoot a problem. It's important to know how to read a pump curve in order to correctly size a pump. It is also important to know how to read a pump curve when you're troubleshooting problems that arise.

Let's go through some basic features of a pump curve. First, we have the title box. The title box provides information about the pump model, size, speed, and other identifying criteria specific to the pump. Next, we'll look at the flow. Flow is on the x-axis of the pump curve. In this example we'll use 300 gallons per minute as our flow. Third, we'll look at the head. The head is the vertical line on the y-axis. Usually this is measured in feet. For this example, we'll use 100 feet of head. Fourth, we have the impeller trim. Impeller trim is listed as nine inches, ten inches, 11 inches, 12 inches in the pump curve shown. For this example, when we line the three hundred gallons per minute flow with our hundred feet of head, we see that we are roughly ten and a half inches for our impeller diameter. Fifth, we have the horsepower. 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 estimate the selection will require 12 horsepower. Sixth, we have 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. Seventh, we have efficiency. 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. Eighth, we have the minimum flow line. A centrifugal pump requires a minimum amount of flow moving through the pump to dissipate heat created. On the left side of the curve, minimum flow is indicated by a bold vertical line. Operation to the left of this line is not recommended and can significantly decrease the life of the pump. Now that we know the pieces and parts of a pump curve, let's go through an example.

One common issue, but having a pump curve and understanding how they operate can solve, 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 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 or 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 likely something wrong with the pump.

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