How to Read a Pump Curve

Do you want to learn how to read a pump curve? A pump curve is a graphical representation of a pump’s performance characteristics.

Understanding how to interpret a pump curve is essential for selecting the right pump for your application and ensuring optimal performance.

In this blog post, we’ll walk you through the steps to read and understand a pump curve effectively.

What Are the Pump Curves

Pump curves are graphical representations of a centrifugal pump’s performance characteristics, depicting the relationship between flow rate, head pressure, and efficiency.

Pump manufacturers typically provide a set of curves for each pump model, showcasing its performance at various impeller sizes and speeds. The most common pump curves include the head curve, efficiency curve, and power curve, which are plotted against the flow rate on the horizontal axis.

Typical Pump Curve

Flow Rate (X-Axis)

The x-axis of a pump curve represents the flow rate, typically measured in gallons per minute (GPM) or liters per minute (LPM). This axis shows the volume of fluid that the pump can move over a given period.

Head (Y-Axis)

The head, measured in feet or meters, is represented on the y-axis of a pump curve. It indicates the total dynamic head (TDH) the pump can generate at various flow rates.

The TDH consists of the static head, friction losses, and pressure head. Pump curves typically display multiple lines, each corresponding to a specific impeller diameter or pump speed.

As the flow rate increases, the head decreases, forming a downward-sloping curve.

Efficiency Curves

Efficiency curves display the pump’s efficiency at various flow rates. These curves help determine the optimal operating range for a pump, ensuring it runs at peak performance while minimizing power consumption.

The efficiency curve typically appears as a separate line on the pump performance curve, with efficiency percentages plotted against the flow rate. Pump efficiency is the ratio of hydraulic power output to the power input required to drive the pump, expressed as a percentage.

Power Curves

Power curves in pump performance charts illustrate the relationship between the power consumption and the flow rate of a centrifugal pump.

The power consumption is usually expressed in terms of brake horsepower (BHP) or kilowatts (kW) and is directly related to the pump’s flow rate, head, and efficiency. As the flow rate increases, the power consumption also rises, following the shape of the power curve.

The power curve helps in selecting the appropriate motor size for the pump, ensuring that the motor can handle the maximum power requirements at the desired operating point.

NPSHR Curve

The NPSHR (Net Positive Suction Head Required) curve represents the minimum pressure required at the pump inlet to prevent cavitation, which can cause damage to the pump impeller and reduce efficiency.

The NPSHR curve is typically plotted on the same graph as the pump head curve, with the flow rate on the x-axis and the NPSHR on the y-axis. As the flow rate increases, the NPSHR also increases, indicating that a higher suction pressure is needed to maintain proper pump operation.

It is essential to ensure that the available NPSH (Net Positive Suction Head) in the system is always greater than the NPSHR of the pump at the desired flow rate. Failure to meet this requirement can lead to cavitation, reduced pump performance, and potential damage to the pump components.

Pump Speed

Pump Speed curves usually measured in revolutions per minute (RPM). As the pump speed increases, the flow rate and head pressure also increase, resulting in a shift of the pump curve upwards and to the right.

Conversely, reducing the pump speed leads to a decrease in flow rate and head pressure, causing the pump curve to shift downwards and to the left.

Impeller Diameters

Pump impeller diameters curves illustrate how the pump’s performance, such as flow rate and head pressure, varies with different impeller sizes.

When analyzing a pump curve, pay attention to the impeller diameter that best meets your flow rate and head pressure needs while considering efficiency and power consumption. Opting for an impeller size that is too small or too large can lead to suboptimal performance, premature wear, and increased energy costs.

The Best Efficiency Point (BEP)

The Best Efficiency Point (BEP) is a crucial aspect of centrifugal pump curves, representing the flow rate at which the pump operates with maximum efficiency. This point is determined by the intersection of the pump performance curve and the efficiency curve, indicating the optimal balance between head pressure and flow rate for a specific pump model.

Operating a pump near its BEP ensures optimal performance, reduced power consumption, and minimized wear on pump components. Deviating significantly from the BEP can lead to decreased efficiency, increased energy costs, and potential damage to the pump, such as premature wear on bearings and seals.

Advanced Pump Curve

Capacity

The capacity of a centrifugal pump, measured in gallons per minute (GPM) or liters per minute (LPM), represents the volume of fluid the pump can move over a given time. Pump curves display the relationship between flow rate and head, with the flow rate typically shown on the horizontal axis.

Viscosity

Viscosity is a crucial factor to consider when reading pump curves. Centrifugal pumps are designed to handle fluids with specific viscosities, and deviations from these values can significantly impact pump performance.

Higher viscosity fluids require more power to pump and can reduce flow rates, leading to decreased pump efficiency. Pump curves typically assume the pumped fluid has a viscosity similar to water, so it’s essential to make adjustments when dealing with more viscous fluids.

Dynamic Viscosity

Dynamic viscosity is a critical factor to consider when reading pump curves for fluids other than water.

Pump curves are typically based on water, which has a lower viscosity compared to many other fluids. When pumping high-viscosity fluids, the pump’s performance will deviate from the pump curve, resulting in reduced flow rates and increased power consumption.

Density

Pump curves are typically based on water, which has a density of 1 g/cm³ (or 1000 kg/m³).

If the fluid being pumped has a different density than water, the pump’s performance will be affected. For example, if the fluid is more dense than water, the pump will require more power to achieve the same flow rate and head.

Shear

Pump curves typically do not include information about shear, as shear is more relevant to the fluid being pumped rather than the pump itself. However, it’s important to consider the effects of shear when selecting a pump, especially when dealing with sensitive or viscous fluids.

High shear rates can cause damage to certain types of fluids, such as emulsions or suspensions, leading to changes in their properties and potentially affecting the pump’s performance. When pumping viscous fluids, shear can also impact the pump’s efficiency and power consumption.

To minimize the effects of shear, it may be necessary to choose a pump with a lower operating speed or a different type of impeller design.

Brake Horsepower

The brake horsepower (BHP) curve on a pump performance chart indicates the power required by the pump to operate at various flow rates. It is typically displayed as a separate curve alongside the head and efficiency curves.

The BHP curve helps in selecting the appropriate motor size for the pump, ensuring that the motor can provide sufficient power to meet the pump’s requirements across its operating range. To determine the required motor horsepower, locate the point on the BHP curve corresponding to the desired flow rate and head, and select a motor with a power rating equal to or slightly higher than the BHP value.

Work Horsepower

The work horsepower curve on a pump performance chart represents the power consumption of the pump at various flow rates. It is typically displayed as a separate curve alongside the head and efficiency curves.

The work horsepower increases as the flow rate increases, indicating higher power consumption at higher flow rates. The horsepower required by the pump is determined by the intersection of the flow rate and the work horsepower curve.

Pump manufacturers provide work horsepower curves to help users select the appropriate motor size for their pumping applications. Oversizing the motor can lead to inefficiencies and increased energy costs, while undersizing can result in inadequate performance and potential damage to the pump.

Viscous Horsepower

Viscous horsepower is a critical factor to consider when selecting a pump for fluids with higher viscosity than water. The viscosity of the fluid directly impacts the power consumption and efficiency of the pump.

Pump curves typically assume the pumped fluid has the same viscosity as water, but many industrial applications involve fluids with higher viscosity. As viscosity increases, the pump requires more power to maintain the same flow rate and head, leading to reduced efficiency and increased energy consumption.

To account for the additional power required, pump manufacturers often provide correction factors or separate curves for different viscosity levels.

Using Pump Curves for Pump Selection

Plotting the System Curve

To select the appropriate pump, plot the system curve on the pump curve graph. The system curve represents the relationship between the required flow rate and the total dynamic head of the system, taking into account factors like pipe size, length, and fittings.

Selecting Pump Curve That Meets Flow and Head at or near BEP

Identify the pump curve that intersects the system curve at or near the Best Efficiency Point (BEP). Operating the pump at or near its BEP ensures optimal performance, efficiency, and longevity, reducing energy consumption and maintenance costs.

Checking Selected Pump’s NPSHR against System NPSHA

Compare the Net Positive Suction Head Required (NPSHR) of the selected pump with the Net Positive Suction Head Available (NPSHA) of the system. Ensure that the NPSHA is greater than the NPSHR to prevent cavitation and maintain proper pump performance.

Considering Efficiency, Power, Speed and Impeller Diameter

Evaluate the efficiency, power consumption, speed, and impeller diameter of the selected pump. Higher efficiency pumps reduce energy costs, while the power curve indicates the required motor size. Consider the pump’s speed and impeller diameter to ensure compatibility with the system and optimize performance.

Pump Curves for Troubleshooting

Using Curve to Check If Pump Is Operating at Design Point

Pump curves provide a graphical representation of a pump’s performance, allowing users to determine if the pump is operating at its designed flow rate and head. By plotting the system curve and finding the intersection with the pump curve, one can verify if the pump is running at its optimal design point.

Identifying Issues like Cavitation, Wear, and Recirculation

Pump curves can help identify potential problems such as cavitation, wear, and recirculation. Cavitation occurs when the pressure drops below the vapor pressure of the liquid, causing bubbles to form and collapse, leading to damage. Wear can be detected by comparing the current pump curve to the original curve, while recirculation can be identified by a dip in the pump curve at low flow rates.

Determining If Pump Is Oversized or Undersized for System

By comparing the system curve with the pump curve, users can determine if the pump is oversized or undersized for the application. An oversized pump will operate at a higher flow rate and lower head than the design point, while an undersized pump will operate at a lower flow rate and higher head.

Making Corrections through Impeller Trimming, Speed Changes, Etc.

If the pump is not operating at its design point, corrections can be made by trimming the impeller, changing the pump speed, or adjusting the system curve. Impeller trimming reduces the impeller diameter, which shifts the pump curve to the left, while increasing the pump speed shifts the curve to the right. Adjusting the system curve can be achieved by modifying the piping, valves, or other system components.

In Conclusion

In conclusion, understanding how to read a pump curve is essential for selecting and operating pumps efficiently. By following the steps outlined in this blog post, you can effectively interpret pump curves and make informed decisions.

Take action now and start applying this knowledge to optimize your pumping systems.