A customer may receive several quotations for a deep-well project and find that every supplier recommends a different pump model.
One quotation may show a 30 kW pump, another may show 37 kW, and a third may only provide maximum flow and maximum head. Without checking the performance curves, it is difficult to know whether the proposed pumps are technically equivalent.
A submersible well pump curve shows how the pump performs at different combinations of flow and head. It also helps buyers compare efficiency, motor power, operating range, and whether the selected model can meet the actual project duty point.
Before reading the curve, the required flow and head should already be known. You can first review how to calculate the required well pump flow rate and how to calculate Total Dynamic Head.
What Information Does a Pump Curve Show?
The main performance curve normally shows the relationship between flow and head. Flow is usually shown on the horizontal axis, while head is shown on the vertical axis.
As the flow increases, the head produced by a centrifugal submersible pump normally decreases. This means that a pump does not produce one fixed flow and one fixed head under every condition. Its actual performance depends on the resistance of the complete water system.
A technical pump data sheet may also include curves for efficiency, shaft power, motor power, and sometimes NPSH. For submersible well pump selection, the most important values are usually:
① Flow
② Head
③ Efficiency
④ Power
⑤ Recommended operating range
These values should be reviewed together rather than separately.
Start with the Required Duty Point
The duty point is the combination of flow and head required by the project.
For example:
Required duty point: 60 m³/h at 120 m TDH
To check this point on the curve, locate 60 m³/h on the horizontal axis and move upward until it reaches the pump curve. Then read the corresponding head on the vertical axis.
If the curve shows approximately 120 m at 60 m³/h, the model may be suitable. If the curve shows only 100 m, the pump cannot meet the required pressure. If it produces 155 m, the model may be oversized or may require additional control.
A supplier should always mark the proposed duty point on the performance curve. A quotation that only provides a model number and horsepower does not give the buyer enough information to verify the selection.
How to Read the Flow and Head Curve
The main pump curve is often called the H-Q curve, where H represents head and Q represents flow.
The left side of the curve represents low-flow, high-head operation. The right side represents high-flow, low-head operation. The middle section is usually closer to the pump’s preferred operating area.
Suppose a pump curve shows:
① 30 m³/h at 145 m head
② 50 m³/h at 125 m head
③ 70 m³/h at 95 m head
The same pump can operate at each of these points, depending on the system resistance. However, not every point is equally efficient or suitable for continuous operation.
This is why a buyer should not ask only, “What is the maximum flow?” The more useful question is, “What flow does this pump deliver at my required Total Dynamic Head?”
Maximum Head Is Not the Rated Head
Maximum head is normally shown at the far-left side of the curve, where the flow is close to zero. It is also called shutoff head.
A pump with a maximum head of 180 m does not necessarily deliver useful flow at 180 m. At the required flow, the available head may be much lower.
For example, a pump catalogue may show:
① Maximum head: 180 m
② Maximum flow: 100 m³/h
These two values occur at different ends of the curve. The pump does not deliver 100 m³/h at 180 m head.
When comparing a high-head submersible well pump, buyers should ask the manufacturer to provide the head at the required flow rather than relying on the maximum value printed in the catalogue.
Maximum Flow Is Not the Recommended Flow
Maximum flow is normally located near the far-right end of the curve. At this point, the pump produces relatively low head and may be operating outside the preferred range.
Continuous operation near maximum flow can result in higher motor load, lower efficiency, greater hydraulic forces, and faster wear. It may also reduce the pressure available at the discharge point.
The selected operating point should normally remain within the manufacturer’s recommended flow range. A professional submersible well pump supplier should identify this range clearly on the curve or technical data sheet.
If a quotation promotes only maximum flow, ask for the rated flow, recommended range, and actual performance at the project duty point.
Check the Pump Efficiency Curve
Pump efficiency shows how effectively the pump converts mechanical power into hydraulic power.
The highest point on the efficiency curve is often called the Best Efficiency Point, or BEP. Pumps generally operate most efficiently when the duty point is reasonably close to this area.
A pump operating far away from the efficient range may consume more electricity and experience higher internal loads. For projects running many hours each day, even a small efficiency difference can have a significant effect on annual energy cost.
When comparing two submersible well pump quotations, do not compare only purchase price. Also compare the efficiency at the actual duty point. A slightly more expensive pump may have a lower lifetime cost if it operates more efficiently.
Check the Power Curve
The power curve shows how much shaft power or motor input is required at different flow rates.
As flow changes, the power requirement may also change. A motor that appears large enough at one point may become overloaded if the pump operates farther to the right of the curve.
The selected motor should provide sufficient power across the intended operating range. The supplier should confirm:
① Motor rated power
② Rated current
③ Power required at the duty point
④ Power margin
⑤ Voltage and frequency
A larger motor does not automatically mean a better pump. The hydraulic curve, efficiency, and motor should all match the project conditions.
Stay Within the Recommended Operating Range
Most pumps have a recommended operating range between a minimum and maximum continuous flow.
Operating too far to the left may cause internal recirculation, heat generation, vibration, and unstable hydraulic forces. Operating too far to the right may cause excessive flow, higher motor load, low discharge pressure, and accelerated wear.
A correctly selected pump should normally operate near the middle of its recommended range rather than at an extreme end of the curve.
This becomes especially important when water levels change seasonally. As the dynamic water level falls, the operating point may shift along the curve. The supplier should check that the pump remains within an acceptable range under both high-water and low-water conditions.
Read more about this relationship in Static Water Level vs. Dynamic Water Level.
How the System Determines the Actual Operating Point
The pump curve shows what the pump can produce. The system curve shows what the pipeline and water system require.
The actual operating point occurs where the pump curve and system curve intersect. If pipeline resistance increases, the operating point moves toward lower flow and higher head. If resistance decreases, the operating point moves toward higher flow and lower head.
Changes that can shift the operating point include:
① Falling dynamic water level
② Partially closed valves
③ Dirty filters
④ Smaller pipe diameter
⑤ Longer pipeline
⑥ Changes in outlet pressure
This explains why the same pump may deliver different flow rates in different projects.
A Practical Pump Curve Example
Suppose a project requires:
50 m³/h at 110 m TDH
A supplier proposes a pump with the following performance:
① 40 m³/h at 125 m
② 50 m³/h at 112 m
③ 60 m³/h at 95 m
At 50 m³/h, the pump produces approximately 112 m head. This is close to the required 110 m, so the hydraulic performance appears suitable.
The buyer should then check the efficiency, motor power, rated current, and recommended operating range at 50 m³/h. If the duty point is close to the efficient region and the motor has sufficient power, the model may be a good selection.
The final decision should also consider pump diameter, material, sand content, voltage, installation depth, and daily operating hours. These factors are explained in our submersible well pump selection guide.
How to Compare Curves from Different Manufacturers
Two pump models with the same horsepower may have very different curves.
One model may be designed for higher flow and lower head, while another is designed for lower flow and higher head. Comparing only motor power can therefore be misleading.
When comparing submersible well pump manufacturers, use the same duty point for every quotation. Check:
① Flow and head at the project duty point
② Efficiency at the duty point
③ Motor power and rated current
④ Recommended operating range
⑤ Pump materials
⑥ Number of stages
⑦ Maximum allowable sand content
⑧ Voltage and frequency
⑨ Warranty and spare parts
A low-priced quotation is not a better offer if the pump cannot deliver the required flow at the required head.
What Should Be Included in a Pump Quotation?
A professional submersible pump quotation should contain enough technical information for the buyer to verify the selection.
The quotation should normally include:
① Pump model
② Required duty point
③ Performance curve
④ Efficiency
⑤ Motor power
⑥ Rated current
⑦ Pump dimensions
⑧ Pump and impeller materials
⑨ Cable specification
⑩ Voltage and frequency
⑪ Included accessories
⑫ Warranty terms
For important industrial, irrigation, mining, or municipal projects, buyers should also ask whether spare parts, technical drawings, control panels, and engineering support are available.
A quotation showing only model, horsepower, and price is difficult to evaluate properly.
Commercial Factors Beyond the Pump Curve
The performance curve confirms whether the pump can meet the hydraulic requirement, but purchasing decisions also involve product quality and supplier capability.
Before buying a submersible well pump, consider whether the manufacturer can provide:
① Model selection based on project data
② Custom voltage and frequency
③ Material options
④ Motor and control-panel matching
⑤ Inspection and test reports
⑥ Spare parts
⑦ Installation guidance
⑧ After-sales technical support
For export projects, buyers may also need packing specifications, shipping dimensions, certificates, and documentation.
A suitable curve is essential, but the manufacturer must also be able to supply a pump that matches the complete operating and commercial requirements.
Common Pump Curve Reading Mistakes
The first common mistake is assuming that maximum flow and maximum head occur at the same point.
Another mistake is selecting a pump because the required duty point touches the curve, even though the point is outside the recommended operating range.
Buyers may also ignore the efficiency and power curves, compare different quotations at different duty points, or assume that a larger motor always provides better performance.
The correct method is to compare each model at the same required flow and head, then review efficiency, power, materials, operating range, and supplier support.
Request a Submersible Well Pump Selection from SLAPK
To select a pump from a performance curve, SLAPK normally needs:
① Required flow
② Total Dynamic Head
③ Total well depth
④ Static and dynamic water levels
⑤ Well casing diameter
⑥ Water quality and sand content
⑦ Voltage and frequency
⑧ Daily operating hours
⑨ Pipeline information
⑩ Required accessories
Our engineering team can review the project duty point and recommend a suitable model from the SLAPK submersible well pump range.
The recommendation can include the pump curve, motor power, materials, dimensions, and operating data needed to compare the model with other technical quotations.
For a full overview of pump construction, selection, installation, and maintenance, read our complete guide to submersible well pumps.
