Borehole pump sizing, selection and installation

Flow rate is one of the main parameters used when selecting a borehole pump. It describes the volume of water delivered during a specified period and is commonly measured in litres per second, litres per minute, or cubic metres per hour.

The required flow rate depends on water demand, the available pumping period, storage capacity, and the borehole's sustainable yield.

A common mistake is selecting a pump according to the maximum possible flow rather than the flow needed at the actual operating head.

Converting between flow-rate units

The following conversions are useful during pump selection:

1 litre per second equals 60 litres per minute.
1 litre per second equals 3.6 cubic metres per hour.
1 cubic metre equals 1,000 litres.
1 cubic metre per hour equals approximately 16.67 litres per minute.

For example, a pump delivering 2.5 litres per second supplies:

[ 2.5\times60=150\text{ L/min} ]

Its hourly delivery is:

[ 2.5\times3.6=9\text{ m}^3/\text{h} ]

These are unit conversions, not guarantees of actual pump performance. The delivered flow must be confirmed against the pump curve and operating conditions.

Calculating the flow required to fill a tank

Suppose a property needs to transfer 8,000 litres into a storage tank over four hours.

The average required flow is:

[ Q=\frac{8,000}{4}=2,000\text{ L/h} ]

Converting to litres per minute gives:

[ Q=\frac{2,000}{60}=33.33\text{ L/min} ]

The preliminary requirement is approximately 33.3 litres per minute during the planned pumping period.

The final pump must deliver that flow at the calculated total dynamic head, while remaining within the borehole's sustainable pumping capacity.

14. Calculating daily pumping time

Daily pumping time helps establish whether a selected flow rate can meet the property's expected consumption.

The simplified relationship is:

[ t=\frac{V}{Q} ]

Where:

(t) is pumping time.
(V) is the required water volume.
(Q) is the actual delivered flow rate.

Units must be consistent. If volume is expressed in litres and flow in litres per minute, the result is in minutes.

For example, a property requiring 6,000 litres daily and receiving an actual flow of 30 litres per minute would need:

[ t=\frac{6,000}{30}=200\text{ minutes} ]

This equals 3 hours and 20 minutes of pumping, assuming the stated flow remains available and all delivered water can be used or stored.

The estimate should also allow for actual operating conditions, control interruptions, maintenance, and the borehole's recovery characteristics.

15. Understanding static head and friction head

A borehole pump must overcome both the vertical elevation difference and the losses associated with moving water through the system.

Static head relates to the elevation difference between the pumping water level and the discharge point. Friction head accounts for energy lost as water moves through pipes, bends, valves, filters, and other components.

Friction losses depend on flow rate, pipe diameter, pipe length, internal roughness, and the geometry of the fittings.

A system with a long, narrow delivery pipe may require substantially more head than a system with a larger pipe carrying the same flow.

This means that pump sizing and pipe sizing must be considered together.

16. Selecting the correct delivery pipe diameter

Pipe diameter affects water velocity, friction losses, pressure at the delivery point, and the amount of energy the pump must supply.

A smaller pipe may cost less initially but can increase friction losses. A larger pipe may reduce friction but increase material and installation costs.

The appropriate diameter depends on the design flow rate, pipe material, route, operating pressure, and applicable installation requirements.

Estimating water velocity

Water velocity can be estimated using:

[ v=\frac{Q}{A} ]

Where (v) is velocity in metres per second, (Q) is flow in cubic metres per second, and (A) is the pipe's internal cross-sectional area in square metres.

The area of a circular pipe is:

[ A=\frac{\pi D^2}{4} ]

Here, (D) represents the internal pipe diameter.

These equations help estimate velocity, but the acceptable design range depends on the system and relevant engineering guidance. The internal diameter must be used rather than assuming the nominal pipe size is the exact bore.

Why excessive velocity matters

High water velocity can increase friction losses and may contribute to pressure fluctuations when valves close or pumps stop.

The pipe should therefore be selected to provide an appropriate balance between installation cost, flow capacity, friction losses, and operating reliability.

17. Calculating pipe friction losses

Friction losses can be estimated using recognised hydraulic methods. One commonly used method for pressurised water pipes is the Darcy–Weisbach equation:

[ h_f=f\frac{L}{D}\frac{v^2}{2g} ]

Where:

(h_f) is friction head loss in metres.
(f) is the Darcy friction factor.
(L) is pipe length in metres.
(D) is internal pipe diameter in metres.
(v) is average water velocity in metres per second.
(g) is gravitational acceleration.

The friction factor depends on the flow regime, pipe roughness, and Reynolds number.

Additional losses through bends, valves, and fittings can be calculated separately or included using an appropriate equivalent-length or loss-coefficient method.

For practical pump selection, the calculation should reflect the actual pipe material, dimensions, flow rate, and route. Rough estimates may be useful during preliminary planning, but final sizing should use suitable engineering data.

18. Understanding the system curve

The system curve represents the head required by the water distribution system at different flow rates.

It includes the static elevation difference, any required delivery pressure, and flow-dependent losses.

In a typical system, friction losses rise as flow increases. Consequently, a pump that delivers the required pressure at a lower flow may not maintain that pressure when several outlets operate simultaneously.

The intersection of the pump curve and system curve determines the operating point under the relevant conditions.

When reviewing a proposed pump, check that the operating point meets the demand and remains within the manufacturer's recommended range.

A pump should not be selected solely because its maximum head appears greater than the borehole depth.

19. Storage tank sizing for borehole systems

Storage tanks can separate the borehole pumping schedule from the property's immediate water consumption.

Tank sizing should consider daily demand, peak consumption, pumping hours, available borehole yield, and the desired reserve during supply interruptions.

A simple initial water balance is:

[ V_{\text{required}}= V_{\text{demand}}-V_{\text{pumped}} ]

This relationship is useful for estimating a storage deficit over a specified period, provided both volumes refer to the same period and system boundary.

A complete tank design must also account for usable tank volume, minimum operating level, control settings, reserve requirements, and expected variations in supply and demand.

Example of a daily water balance

Suppose a property consumes 7,000 litres per day and the borehole supplies 5,500 litres during the same period.

The shortfall is:

[ 7,000-5,500=1,500\text{ litres} ]

If that difference persists over time, a larger tank alone cannot solve the long-term water deficit. The source, sustainable pumping rate, consumption, and any supplementary supply must be assessed.

Storage is most useful when it manages timing differences between available supply and water demand.

20. Direct pumping versus pumping into storage

Two common arrangements are direct pumping to the distribution system and pumping into a storage tank.

Direct pumping

In a direct arrangement, the pump supplies the distribution network according to the system's pressure and flow requirements.

This may be appropriate where the pump, borehole, controls, and water demand can be coordinated safely. The design must account for varying demand and ensure the pump does not operate outside its intended conditions.

Pumping into a storage tank

In this arrangement, the borehole pump replenishes a tank. A separate distribution pump may supply the building if gravity pressure is insufficient.

This arrangement can provide a reserve and allow the borehole pump to operate according to a controlled schedule.

The best option depends on the site, borehole yield, demand pattern, required pressure, energy use, and maintenance considerations.

21. Pressure requirements in multi-storey buildings

The required pump head increases when water must be delivered to a higher elevation or at a specified residual pressure.

For a building with several floors, calculate the vertical difference between the pumping water level and the most demanding delivery point. Add the required outlet pressure and relevant friction losses.

Pressure should also remain within the permitted range for pipes, fixtures, valves, and connected equipment.

In some buildings, pressure zoning or pressure-reducing devices may be necessary to provide suitable pressure across different floors.

The pump should be selected for the actual design condition rather than using a general rule based only on the number of storeys.

22. Evaluating energy consumption during pump selection

The energy required to pump water depends on flow, total head, overall efficiency, and operating time.

For an electric pumping system, energy consumption is commonly expressed in kilowatt-hours.

A preliminary energy estimate can be made by multiplying the electrical input power by the operating time:

[ E=P_{\text{input}}\times t ]

Where power is measured in kilowatts and time in hours.

For example, if a pump system draws 1.5 kW while operating for four hours, the estimated energy use is:

[ E=1.5\times4=6\text{ kWh} ]

Actual consumption should be measured where possible and may differ because of changing operating conditions, motor loading, and control behaviour.

Comparing suitable pumps at their intended operating points can help identify an energy-efficient option without compromising water supply requirements.

23. Common hydraulic sizing mistakes

Common mistakes include:

Using total borehole depth as the only head calculation.
Selecting a pump based on horsepower alone.
Ignoring pumping water-level drawdown.
Using an unrealistic peak-flow estimate.
Selecting pipes without calculating friction losses.
Assuming maximum pump flow and maximum pump head occur together.
Ignoring storage and the daily water balance.
Overlooking required pressure at the delivery point.
Failing to check the borehole's sustainable yield.
Ignoring manufacturer performance curves.

Avoiding these errors improves the reliability of the final design and reduces the risk of purchasing equipment that cannot meet the site's requirements.

24. Preparing the hydraulic design for final pump selection

Before finalising the pump, compile a design sheet containing the estimated daily demand, required flow, pumping water level, discharge elevation, delivery pressure, pipe dimensions, friction losses, and expected operating hours.

Use those values to establish the design total dynamic head and the required operating point.

Then compare suitable pump curves and verify motor power, electrical compatibility, borehole casing clearance, and manufacturer operating limits.

Where the site information is incomplete, obtain the necessary measurements or testing results before committing to a pump purchase.

A well-documented design makes installation, commissioning, troubleshooting, and future replacement easier.

Part 2 complete. The next part will cover pump types, submersible pump construction, motor selection, cable sizing, electrical protection, installation depth, and control systems.