The performance curve — also known as the characteristic curve — of an industrial pump is a special operational performance chart. Most industrial pumps come with a performance curve. It illustrates the variation in head and other parameters, as well as the operating flow rate of the pump under various operating conditions. Typically, each characteristic curve chart represents the operating state under a specific set of conditions.

Performance curve charts (characteristic curves) are typically provided by the manufacturer. Their purpose is to inform customers that the pump genuinely operates in accordance with the data, parameters, and standards established by the manufacturer.

Looking for equipment? Need to cross-reference performance curves to select the correct operating point? Browse our industrial centrifugal pumps — H–Q–NPSH specifications by pump series.

What Is a Pump Performance Curve

To explain the pump performance curve, let us refer to the illustrative chart above. This is essentially a diagram that shows the characteristics of flow rate, head, pressure, efficiency, and more relative to the pump’s rotational speed. In addition, the pump’s characteristic curve also indicates the allowable operating range of the pump.

The pump performance curve is plotted based on experimental test results conducted in a laboratory. This is an important piece of information for selecting and operating a pump.

What Is Pump Head

Pump head can be understood in simple terms as the height to which a pump can lift liquid. This is an important parameter when selecting the installation position of a pump within a system.

Typically, when selecting a pump, this parameter is chosen to match the system design drawings.

NPSH Parameter of a Pump

The NPSH parameter of a pump refers to the net positive suction head. It is a very important factor when selecting a pump — from submersible borehole pumps to centrifugal pumps.

Industrial pump

How does NPSH relate to pump operation?

A pump operates by creating a pressure differential between the inlet and the outlet — higher pressure at the inlet causes the liquid to flow toward the lower pressure at the outlet. As liquid flows through the pump, it undergoes a gradual pressure reduction along the flow path.

If the pressure at the inlet falls below the vapor pressure of the liquid, bubbles will form at the inlet.

These bubbles collapse rapidly inside the pump as they move toward the outlet. This cavitation causes the pump to operate noisily, producing a sound similar to gravel in a concrete mixer. The bubbles in the liquid also reduce the pump’s output capacity. Most importantly, cavitation gradually erodes the surfaces of the impeller, pump casing, and causes permanent damage to the pump.

How to Calculate Head

Calculating NPSH is essential to prevent cavitation, improve efficiency, and ensure optimum pump performance. When selecting the most suitable pump for a specific application, it is important to ensure that NPSHa is greater than NPSHr to avoid cavitation. In other words, the system must have a suction-side pressure margin greater than the pump’s required level.

NPSHa is calculated using the following formula:

NPSHa = Ha ± Hz – Hf + Hv – Hvp

– Ha: Pressure acting on the liquid surface, typically atmospheric pressure

– Hz: Distance between the liquid surface in the tank and the pump centerline

– Hf: Friction losses in the suction pipe

– Hv: Velocity head at the pump suction port

– Hvp: Vapor pressure of the liquid at the pumping temperature

Explaining the Characteristic Curve of an Industrial Pump

When a large volume of liquid is pushed into a pump, the pump produces a smaller differential head. Although the pump may be limited by its construction and power capacity, this can be observed from the chart. For any given impeller:

The pump generates maximum differential pressure or maximum differential head near the shut-off point — the point at which very little liquid is passing through the pump.

Chart showing the shut-off point of a pump

Continuing to follow the chart below, we can see that for the same impeller diameter, once the flow rate starts to increase, the differential head begins to decrease.

Chart showing increasing flow rate of a pump

Even as the differential head decreases, the output result — flow rate multiplied by the differential head — still increases, because the flow rate values have risen. As shown by these results, there is an optimum efficiency point at which the pump can operate with high performance at both the inlet and outlet. This is called the operating point, and it is clearly displayed on the characteristic curve chart of all pumps.

Chart showing the operating point of a pump

Selecting a Pump Based on NPSH Parameters

As mentioned, the most important factor to consider when selecting a pump is NPSH. The safety margin between the calculated NPSHa and the manufacturer-supplied NPSHr must be 10% or more to ensure cavitation is avoided. Other factors to consider when selecting a pump include:

– Temperature: The higher the liquid temperature, the greater its vapor pressure. This has a significant effect on NPSH, causing NPSHa to decrease. It is important to take this into account, especially when the pump is being used in high-temperature applications.

– Specific Suction Speed (Nss): This unique value is calculated from three factors: the head generated by a pump, the pump’s operating speed, and the amount of NPSH required for pump operation. A high Nss value may indicate that the pump is susceptible to cavitation.

– Distance between the pump and the suction source: Because it affects NPSHa, the distance between the pump and the suction source must be taken into consideration.

– Pump design: The pump design, age, and rotational speed of the impellers will affect the NPSH required for normal pump operation.

The Significance of the Pump Characteristic Curve

The characteristic curve chart is extremely important and useful for selecting, testing, operating, and maintaining pumps.

When selecting a pump for a specific application or process, reviewing the characteristic curve charts is essential. The chart not only shows performance characteristics but also provides flow rate values during operation and NPSH requirements. From this, engineers can make informed decisions to select a pump suited to each specific application requirement.
Once a pump has been selected and purchased, the characteristic curves are used to verify pump performance — to confirm that it meets the standards and specifications set by the pump manufacturer.
Throughout operation at industrial facilities, process conditions and application requirements may change significantly, along with the corresponding flow rate and head values. It is therefore necessary to check the characteristic curve to determine whether the current pump can meet the changed operating conditions.

We hope the information above helps you in reading and understanding the performance curve / technical characteristic curve of various types of pumps.
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