5 mistakes when selecting a borehole pump

The selection of a borehole pump in building systems is often approached as a “standard” choice based on three main parameters: borehole depth, required flow rate and nominal head.

In practice, however, this is precisely the stage where many of the issues that appear after installation originate: performance drops, high energy consumption, premature failures or the need for costly corrective interventions.

For installers and distributors, preventing these mistakes means reducing service calls, improving system reliability and ensuring operational continuity for the end customer.

Below, we analyse the 5 most common mistakes when selecting borehole pumps for building applications and how to avoid them through a more informed design approach.

1. Underestimating water quality

One of the most common mistakes is considering borehole water as a “standard” fluid. In reality, the presence of sand and abrasive particles can have a direct impact on the pump’s service life.

Abrasion accelerates the wear of impellers, diffusers and internal surfaces. The result is often a progressive reduction in performance and the need for premature replacement.

In practice, this mistake comes from an incomplete assessment of the geological and hydrogeological conditions of the borehole.

Effective solutions include:

• Assessing the number of operating hours the pump will be required to handle
• Checking the suspended solids content
• Selecting pumps specifically designed to operate in the presence of sand

Ranges such as Lowara borehole pumps are designed with different construction solutions and materials to adapt to varying operating conditions, including applications with significant sand content in the water: up to 150 g/m³ of water for e-GS pumps and up to 100 g/m³ for 6”-10”-12” electric submersible pumps.

2. Selecting the pump based on the borehole dynamic water level

A typical design mistake is sizing the pump based only on the water level measured at the time of assessment (static water level).

However, it is essential to consider the dynamic water level, i.e. the stabilised fluid level during pumping, which may vary due to seasonal fluctuations or intensive groundwater extraction.

Underestimating this difference in level can compromise the system for several reasons:

• Risk of dry running: If the water level drops below the expected value, the pump may start drawing in air, causing mechanical damage to the hydraulic components.
• Insufficient performance: The calculated head may prove inadequate because it does not take into account the increased static lifting height caused by the dynamic reduction in water level.
• Overpressure: Conversely, if sizing is carried out based on an exceptionally low groundwater level, a subsequent rise in the static level will reduce the lifting height, generating excessive pressure at the point of use and causing the pump to operate “off curve” (with a risk of motor overload).

Understanding the behaviour of the dynamic water level is a key requirement for correct pump selection. If this data is not available, the following precautions should be adopted:

• Safety margins: Calculate the pump head by applying a conservative margin to the geometric suction height.
• Use of a variable frequency drive (VFD): Install a frequency converter to modulate pump performance and automatically compensate for groundwater fluctuations, maintaining the required constant pressure.
• Dry-run protection: Always provide level probes to stop the pump promptly and prevent irreversible damage.

3. Selecting the correct cable according to motor power and voltage

Submersible borehole pumps are often supplied through electrical cables of considerable length. It is therefore essential that the cable cross-section (expressed in mm²) is correctly sized according to the absorbed current (expressed in Amps), which is itself closely linked to the supply voltage (single-phase 230 V or three-phase 400 V).

Failure to correctly size the cable can lead to serious risks for the system:

• Voltage drop: Using a cable with an insufficient cross-section creates high electrical resistance, causing voltage loss along the line. As a result, the motor receives a lower voltage than its rated value.
• Overheating and failures: To compensate for the voltage drop and maintain the required power output, the motor tends to draw more current, overheating both the cable and the motor windings. This creates a real risk of insulation damage or motor burnout.

4. Neglecting installation conditions and motor cooling

Another frequently underestimated factor is the analysis of the physical environment in which the pump is installed.

Submersible pump motors require specific thermal conditions to operate correctly. Cooling is not guaranteed simply by static immersion in the fluid; it depends on the dynamic flow of water passing along the external motor casing.

To ensure correct heat dissipation, the fluid must flow along the motor surface at a minimum specific velocity, generally between 0.15 and 0.5 m/s (depending on the manufacturer and motor diameter).

The risk of insufficient cooling and consequent motor overheating occurs in specific installation conditions:

• Oversized boreholes (or installation in tanks/reservoirs): If the space between the motor and the borehole or tank wall is too large, the water drawn by the hydraulic section will not flow closely along the motor surface but will move too slowly to remove heat effectively.
• Boreholes that are too narrow: Excessively limited clearance may cause high local pressure losses or restrict correct water flow, reducing the circulation required.
• Pump installed below the borehole filter level: If the pump is positioned below the water inlet area (the borehole filters), water will flow down towards the hydraulic section without passing along the motor located below, creating a thermal stagnation area.
• Low flow conditions: Prolonged operation at low flow rates proportionally reduces the fluid velocity around the motor.

To prevent early motor insulation failures and ensure maximum service life, it is essential to adopt the following measures:

• Use of a cooling sleeve: In installations with tanks, oversized boreholes or pumps positioned below the filters, the use of a flow sleeve (cooling sleeve) is mandatory. This accessory forces water to flow from the bottom upwards, passing along the motor before entering the pump suction.
• Check operating limits: Always refer to the manufacturer’s technical documentation to verify the minimum required flow velocity and the maximum allowable liquid temperature for the motor.

5. Operation with a variable frequency drive using incorrect parameters

Variable frequency drives (VFDs) are widely used together with submersible pumps, offering significant advantages in terms of energy savings, pressure stabilisation and reduction of water hammer effects.

However, many premature motor failures are caused by incorrect configuration of the operating parameters on the device.

The critical factors requiring particular attention during commissioning include:

• Minimum operating frequency set too low: This is the most common and serious issue. If the VFD is configured to operate at excessively low frequencies, hydraulic performance decreases significantly and, as a result, motor speed may no longer be sufficient to ensure the minimum water flow required to cool the motor casing.
• Incorrect acceleration and deceleration ramp times: Borehole pumps often use thrust bearings that need to quickly reach a certain rotational speed to create the lubricating water film. Excessively long acceleration ramps can cause premature wear of these components.

To protect the integrity of the submersible motor, VFD programming must respect the following safety limits:

• Minimum frequency threshold: A cautious setting widely recommended by manufacturers is never to operate below 30 Hz (or below the minimum frequency specified on the motor nameplate). This value ensures sufficient rotational speed and minimum flow to dissipate heat.
• Fast ramps in the critical range: Configure a fast acceleration ramp (usually below 2–3 seconds) when moving from 0 Hz up to the minimum operating frequency (30 Hz), then slow down the ramp during subsequent modulation phases, protecting the thrust bearings.

Conclusion: the right choice is a system choice

Selecting a borehole pump should never be considered an isolated process, but rather an integral part of designing the building’s entire water system.

Correctly considering water quality, actual sizing requirements, control integration, installation conditions and lifecycle performance makes it possible to avoid most of the operational issues that arise after commissioning.

In this context, Lowara borehole pumps are designed to provide application flexibility, long-term reliability and compatibility with the main system configurations used in the building sector, ensuring continuity of service even under complex operating conditions.

Giulio Zavan – Borehole Pumps Product Manager