When you need to choose a centrifugal pump, your selection process can become slightly more complicated by the array of centrifugal pump styles that are on the market. But one of the most important tools for making this selection is a pump curve, also known as a pump performance curve. Learning how to read a pump curve is essential in making the right choice for an HVAC or water system pump in an industrial building.
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The importance of getting the right size pump the first time around can't be overstated. A poorly sized pump will fail if it's too small, and if it's too big it may incur unnecessary costs. A pump curve can help you avoid making these mistakes. And if you understand the basic elements of a pump curve and know the fundamentals of reading one, you can make smart decisions about pump type, size, speed, horsepower and other efficiencies required for the system you're working on.
What is a Pump Curve?
A pump curve shows a pump's performance in the form of a graph. The information shown is based on the pump manufacturer's testing conditions, which are specified by international standards. Every pump has its own curve, based on pump horsepower and the size and shape of the impeller.
Flow and Head: Key Elements of the Pump Curve
Two important pump performance factors are shown on a pump curve: flow capacity (horizontal axis) and head flow (vertical axis). Flow capacity is measured in gallons per minute (GPM), which is the rate at which water must be moved to carry heat through a hydraulic system. Head flow is liquid pressure measured in head feet and is measured as the total mechanical energy content of a fluid at a specific point in a piping system. On a pump curve, you can trace the relationship between how much flow a pump can deliver against the corresponding head.
Steps for Reading a Simple Pump Curve
While other factors come into play when you're considering which pump to use, finding the intersection where points that represent head and flow meet on the pump curve is straightforward. Once you've mastered this important part of reading a pump curve, you are on your way. Keep in mind that the first step of selecting the proper pump is to ensure that the required system flow and head fall on or just below the pump performance curve.
- To read the curve, find a point on the horizontal axis, which represents pump flow capacity in GPM (gallons per minute). We'll choose 2 GPM for this example.
- Draw an imaginary vertical line from the 2 GPM dash straight up to where the 2 GPM line meets the pump curve.
- At the intersection of the 2 GPM line and the curve, draw an imaginary horizontal line directly over to the vertical axis to identify the corresponding head. Because the vertical line falls between 2 and 3 head in feet, we can conclude that the pump can deliver 2 GPM at 2.5 feet of head.
- To read from the vertical axis, find a point on the vertical axis. In this example, we'll select 3 feet of head. To find the flow rate, draw an imaginary horizontal line from the 3-foot mark to the curve.
- At the curve, draw a vertical line down to the flow capacity's horizontal axis. Because the vertical line falls between 1 and 2 GPM, you can see that, at 3 feet of head, the pump delivers 1.5 GPM.
Other Essential Elements of a Pump Curve
In addition to showing the relationship between flow and head, a pump curve has other elements that contain useful information for pump selection. While each manufacturer may present some of the details of these elements slightly differently, you can typically find at least some of the following elements on a simple pump curve.
Specific impeller diameters, also known as impeller trims, produce flow and head rates that are shown as multiple performance curves. Most centrifugal pumps are available with a range of impeller sizes, so be aware that changing the impeller trim will give you a new flow/head relationship and thus change the line of the pump curve.
Brake horsepower (BHP) indicates the amount of horsepower required to run the pump at any given point along the performance curve. Brake horsepower lines are the straight and occasionally broken lines that typically slope downward from left to right. Brake horsepower changes along with the impeller trim.
Pump Efficiency Curve
Pump efficiency is measured as the ratio of the liquid horsepower that the pump delivers and the brake horsepower delivered to the pump shaft. These efficiency curves intersect with the head-flow curves and are labeled with percentages. The efficiency varies throughout the operating range of the pump.
NPSHr stands for Required Net Positive Suction Head, which is the minimum head required at the suction port of the pump to prevent the pump from cavitating. Cavitation is caused when the NPSHa (Net Positive Suction Head Available) is lower than the NPSHr required by the pump and can cause severe damage to the impeller. You can find the NPSHr line at the bottom of the pump curve.
Mader Electric offers a wide range of industrial pumps, from small chemical pumps to centrifugal, close coupled, turbine, and large sewage pumps. We have over 30 years of experience in pump repair and can offer the expertise you need in selecting a pump solution. For more information, contact us today.