How to Read a Deep Well Submersible Pump Performance Curve？

A catalogue may list flow, head, efficiency, power and several models together. These values describe performance at different operating points, not independent maximums.

Reading the chart correctly helps prevent a common selection mistake: combining the maximum flow with the maximum head even though the pump cannot deliver both at the same time.

Before opening the pump catalogue, calculate the required operating condition:

• Required flow rate

• Total dynamic head

• Minimum and maximum expected pumping water levels

• Required outlet pressure

• Pipe and fitting losses

• Water temperature and properties

• Supply frequency and intended pump speed

Required flow and total dynamic head form the duty point to locate on the chart.

Do not use well depth as pump head. Total dynamic head normally includes vertical lift from the pumping water level, discharge elevation, pipeline resistance and required outlet pressure.

Flow is normally shown on the horizontal axis and may be expressed in:

• Cubic metres per hour (m³/h)

• Litres per minute (L/min)

• Litres per second (L/s)

• US gallons per minute (US gpm)

Always confirm the unit before comparing the catalogue with the project requirement. Confusing litres per minute with cubic metres per hour can produce a major selection error.

Flow is determined by the actual operating point, not the model’s largest published capacity.

Head is normally shown on the vertical axis in metres or feet. It represents the energy the pump adds to the water.

For a centrifugal borehole pump, head usually decreases as flow increases. At very low flow, the pump produces a higher head. As the discharge flow rises, the available head falls.

The left end is near shutoff, where flow approaches zero and head is highest. The right end represents high flow and lower head. Neither extreme is automatically suitable for continuous duty.

Locate the required flow on the horizontal axis. Move vertically until reaching the required head. The point where these two values meet is the duty point.

Identify the model curve passing near that point. It should fall within the recommended range, not merely touch an extreme.

If the point lies between models, compare efficiency, power, motor size and water-level variation.

Oversizing can create excessive flow, high current, valve throttling and frequent starting. Undersizing can result in inadequate flow or failure to reach the required pressure.

The pump curve shows what the pump can produce. The system curve shows the head the installation requires at different flow rates.

The real operating point occurs where these two curves intersect. It is not permanently fixed by the model name.

Pipe, valve and fitting losses rise with flow, so installation changes move the operating point:

• A lower dynamic water level increases the required head.

• A partially closed valve increases system resistance.

• A smaller pipe increases friction loss.

• Opening additional outlets can increase flow demand.

• A blocked filter increases pressure loss.

• A leaking pipeline changes the delivered flow.

This is why a pump may perform differently after the well level, pipeline or valve arrangement changes.

The best efficiency point, or BEP, is the operating point where the pump reaches its highest hydraulic efficiency at the stated speed and impeller configuration.

Operation near this region generally gives smoother flow and lower energy use. Continuous operation far away can increase recirculation, vibration, bearing load or wear.

Do not assume the entire published curve is equally suitable for continuous service. Use the preferred or allowable operating range supplied for the exact model. If that range is not shown, request it from the manufacturer.

Efficiency may be shown as a separate curve, contour lines or a percentage at selected operating points. Confirm whether the value represents pump efficiency, motor efficiency or total unit efficiency.

The power curve shows how much shaft or input power is required as flow changes. Depending on the catalogue, it may be labelled P, P2, absorbed power, shaft power or input power.

These terms are not always interchangeable. Select the motor by the manufacturer’s method for the approved operating range.

Do not select motor power only at the design point if the pump may operate at a higher-flow condition that requires more power. Review the complete expected range, including high water levels, open valves and VFD operation.

Motor nameplate rating is not actual consumption; efficiency, power factor, voltage and load affect electrical input.

Some SLAPK SP borehole pump catalogues provide performance tables instead of drawn curves. Each row identifies a pump model and motor power. Several flow values are shown across the top, with the corresponding head values below them.

Read each flow and head as a pair in the same column. For example, the head beneath one stated flow belongs only to that flow. Never combine the head from the low-flow column with the capacity from the high-flow column.

If the duty lies between listed values, request the detailed curve and confirmed data rather than relying on rough interpolation.

Multistage borehole pumps increase head by passing water through several impeller and diffuser stages. Models in the same hydraulic family may have similar flow ranges but different head because their stage counts differ.

Adding stages mainly increases head, while also increasing pump length, weight and required power.

Model-number formats differ between manufacturers. Confirm the meaning of every number instead of assuming that it always represents flow, head, stage count or motor size.

A curve is valid for its stated speed, frequency, impeller and liquid. Do not apply a 50 Hz curve directly at 60 Hz.

A VFD changes pump speed and therefore changes flow, head and power. The approximate affinity relationships can help with preliminary estimates, but final selection must remain within the pump and motor manufacturers’ permitted speed, cooling and power limits.

Never increase frequency simply to correct an undersized pump without checking motor load, cable requirements and hydraulic limits.

A borehole system rarely operates at only one water level. When the dynamic water level falls, static lift and total head increase, usually moving the operating point toward lower flow. When the water level rises, required head falls and flow may increase.

Plot or check both expected conditions:

1. Lowest water level and highest required head

2. Highest water level and lowest required head

The selected pump and motor must operate safely at both points. Also check minimum submergence, motor cooling and sand conditions.

• Combining maximum flow and maximum head

• Using well depth as total dynamic head

• Reading the wrong flow or head unit

• Treating shutoff head as a normal duty point

• Ignoring efficiency and absorbed power

• Assuming every point on the curve is suitable continuously

• Using a 50 Hz curve for a 60 Hz project

• Ignoring changes in dynamic water level

• Selecting a larger motor without checking the hydraulic mismatch

• Reading values from different columns of a performance table

Provide the required flow, total dynamic head, static and dynamic water levels, well diameter, installation depth, pipe details, required pressure, voltage, frequency, water temperature and water analysis.

Ask the supplier to mark the proposed duty point on the exact model curve and confirm efficiency, absorbed power, motor rating, operating range and performance at the expected minimum and maximum water levels.

A pump performance curve connects flow and head at a stated speed. The correct model is not the one with the largest published numbers, but the one whose approved operating range covers the real system duty.

Read flow and head as a pair, locate the duty point, review efficiency and power, and then check how changing water levels and system resistance move the operating point.

Contact SLAPK for a marked pump curve and model recommendation