Introduction to borehole pump sizing and selection
Selecting the correct borehole pump is one of the most important decisions in designing a reliable groundwater supply system. A pump that is too small may fail to deliver sufficient water pressure or flow, while an oversized pump can consume unnecessary electricity, increase operating costs, and place excessive demand on the borehole.
Borehole pump sizing requires an understanding of water demand, borehole construction, static and pumping water levels, total dynamic head, pump performance curves, motor characteristics, electrical protection, pipework, and storage arrangements.
For homeowners, property developers, farmers, institutions, commercial facilities, and industrial users, the objective is to select a pumping system that delivers the required water volume at the required pressure without exceeding the sustainable capacity of the borehole.
Pro-Logic Technologies Limited provides technical support for borehole pump assessment, pump selection, installation planning, electrical controls, testing, troubleshooting, and maintenance.
Website: https://prologictecnologies.co.ke
Phone: 0723763173
1. Understanding what borehole pump sizing means
Borehole pump sizing is the process of determining the pump's required hydraulic performance and motor capacity for a particular installation.
The selection must account for two main hydraulic requirements:
- Flow rate: The quantity of water the pump must deliver per unit of time.
- Total dynamic head: The total energy per unit weight that the pump must supply to move water through the system at the required flow.
The pump must also fit the borehole casing, operate at an appropriate submergence depth, tolerate the water conditions, and match the electrical supply and control system.
Selecting a pump based only on horsepower or borehole depth is insufficient. Two boreholes of equal depth can require different pumps because their water levels, delivery pressures, pipework, and flow requirements may differ considerably.
A proper selection process starts with reliable site information and a clearly defined water demand.
2. Determining the required water demand
The first stage is estimating how much water the property needs.
Water demand varies according to the number of users, the type of property, operating hours, irrigation requirements, and the equipment connected to the water supply.
A household may require water for bathing, cooking, cleaning, laundry, and sanitation. A farm may need substantial volumes during irrigation periods. An apartment development may experience high demand at particular times of the day.
Daily demand and peak flow should be considered separately.
Daily water demand
Daily demand is the total water volume expected to be consumed during a typical day. It is usually expressed in litres per day or cubic metres per day.
For example, if a property uses 6,000 litres per day, its estimated daily demand is 6 cubic metres.
This figure helps determine the required pumping schedule and storage capacity. It does not, by itself, determine the pump's flow rate.
Peak demand
Peak demand describes the rate at which water is needed during busy periods.
A property may use relatively little water overnight but experience substantial demand in the morning. The pump and storage system should be designed to accommodate this pattern.
Where a storage tank is available, the borehole pump may replenish the tank over several hours while the tank supplies the property's peak demand.
This arrangement can help prevent the borehole pump from needing to match every short-term fluctuation in household consumption.
Establishing a realistic demand estimate
The estimate should be based on actual usage records where available. For new developments, expected occupancy, plumbing fixtures, irrigation requirements, and operational schedules should be considered.
Avoid selecting a pump solely because a larger motor appears more powerful. Excessive pump capacity does not create additional groundwater.
3. Understanding borehole depth and water levels
Borehole depth is an important installation measurement, but it is not the same as the vertical lift the pump must overcome.
Three measurements are particularly useful.
Total borehole depth: The distance from the borehole reference point to the bottom of the borehole.
Static water level: The depth to the water surface when the borehole has recovered and is not being pumped.
Pumping water level: The depth to the water surface while the borehole is being pumped under specified conditions.
The pumping water level is especially important when calculating the operating head.
For example, a borehole may be 120 metres deep, but the pumping water level might be 55 metres below the reference point. If the water is delivered into a ground-level tank, the static vertical lift at that operating condition is related to the pumping water level, not automatically to the full 120-metre borehole depth.
The final pump selection must still account for the required discharge pressure, pipe friction, valves, and other losses.
Why water levels change
Water levels can change with pumping rate, duration of pumping, seasonal groundwater conditions, nearby abstraction, and the characteristics of the aquifer.
If the pumping water level falls substantially during operation, the pump may need to overcome a greater vertical lift. This can reduce flow and change the operating point.
A suitable borehole assessment should establish whether the water level remains compatible with the proposed pumping rate.
4. Understanding borehole yield and sustainable pumping
Borehole yield is the rate at which water can be abstracted under specified conditions. It should not be confused with the pump's maximum rated flow.
A pump may be capable of delivering a high flow rate, but the borehole may not be able to sustain that rate without excessive drawdown.
When extraction exceeds the available supply, the pumping water level may fall, dry-run protection may activate, and water delivery may become unreliable.
The sustainable pumping rate should be informed by appropriate yield testing, water-level observations, and the hydrogeological characteristics of the borehole.
The relationship between yield and storage
Storage tanks can help manage differences between pumping rate and demand.
For example, a property may require 6,000 litres of water each day. If the borehole can sustainably supply that volume over a suitable pumping period, a storage tank can collect the water before it is distributed to users.
However, storage cannot compensate indefinitely for a borehole whose long-term yield is below the property's consumption. The water balance must remain sustainable.
Why oversized pumps can cause problems
A pump that extracts water faster than the borehole can replenish it may cause excessive drawdown or frequent low-water shutdowns.
Increasing the motor rating does not solve this limitation. The correct response may involve adjusting the pumping rate, improving storage management, investigating the borehole, or reassessing the property's water demand.
5. Calculating total dynamic head
Total dynamic head, commonly abbreviated as TDH, is one of the most important values in pump selection.
It represents the total head the pump must provide to deliver water at the required flow through the complete system.
For a typical borehole pumping installation, the calculation considers:
- The vertical lift from the pumping water level to the discharge point.
- The pressure required at the delivery point.
- Friction losses in pipes.
- Losses through fittings, valves, and other components.
A simplified calculation is:
[ H_{\mathrm{TDH}}=H_{\mathrm{static}}+H_{\mathrm{pressure}}+H_{\mathrm{friction}} ]
Where the discharge point is above the borehole reference level, the static component is generally measured from the pumping water level to that discharge elevation.
The pressure component is the head required to maintain the specified delivery pressure. Friction losses are determined for the expected flow rate and the actual pipe system.
Additional components may be needed where the system includes filters, treatment equipment, pressure-reducing devices, or other restrictions.
Example of a simplified head calculation
Assume a borehole system has the following illustrative design values:
- Pumping water level: 50 metres below the reference point.
- Delivery point: 5 metres above the reference point.
- Required pressure at the delivery point: equivalent to 15 metres of water head.
- Estimated pipe and fitting losses: 8 metres.
The static lift is:
[ H_{\mathrm{static}}=50+5=55\text{ m} ]
The simplified total dynamic head is:
[ H_{\mathrm{TDH}}=55+15+8=78\text{ m} ]
The preliminary design point is therefore the required flow rate at approximately 78 metres of total dynamic head.
These figures are illustrative, not a specification for a particular borehole. Actual values must be established from site measurements and hydraulic calculations.
The selected pump must be capable of delivering the required flow at the calculated head, with appropriate consideration of operating conditions and the manufacturer's recommendations.
6. Converting pressure into water head
Pressure and head are related, but they are not interchangeable without conversion.
For water under ordinary conditions, approximately 10.2 metres of water head corresponds to 1 bar of pressure.
A required pressure of 2 bar at a delivery point therefore represents approximately 20.4 metres of water head, before accounting for elevation differences and friction losses elsewhere in the system.
This relationship is useful when selecting pumps for systems that must deliver water at a specified pressure.
For example, if a building requires a minimum pressure at its highest outlet, the pump must provide enough head to overcome the elevation difference and system losses while maintaining that residual pressure.
Pressure should be evaluated at the relevant point in the system rather than assumed to be the same everywhere.
7. Selecting a pump from its performance curve
A pump performance curve shows how the pump behaves at different operating conditions.
The curve commonly relates flow rate to head. Other curves may show efficiency, power consumption, or required net positive suction head, depending on the pump type and manufacturer's documentation.
The operating point is determined by the interaction between the pump curve and the system curve.
As flow increases through a typical centrifugal pumping system, the head available from the pump generally decreases. At the same time, friction losses in the system usually increase with flow.
The correct pump is one that can meet the required flow and head while operating within its permitted range.
Why the performance curve matters
A pump advertised as capable of delivering a particular maximum flow may only achieve that flow at a relatively low head. At a much higher head, its actual flow may be considerably lower.
Similarly, a pump with a high maximum head may deliver little or no useful flow at that maximum-head condition.
This is why the maximum flow and maximum head values should not be treated as if they are available simultaneously.
Checking the operating point
When reviewing a pump curve, confirm:
- The flow required at the design head.
- Whether the operating point lies within the manufacturer's recommended range.
- The expected motor power demand.
- The efficiency at the selected operating point.
- The effect of changes in pumping water level.
- Compatibility with the borehole and delivery pipework.
The selected pump should be checked against the complete system requirements, not only one headline specification.
8. Matching motor power to hydraulic requirements
Motor power depends on the flow rate, total head, water density, gravitational acceleration, and overall pump efficiency.
A simplified hydraulic power relationship is:
[ P_{\mathrm{hyd}}=\rho gQH ]
Where:
- (\rho) is the water density.
- (g) is gravitational acceleration.
- (Q) is the flow rate in cubic metres per second.
- (H) is total dynamic head in metres.
The required input power is higher than the hydraulic power because energy is lost through the pump and motor.
For a preliminary estimate:
[ P_{\mathrm{input}}\approx \frac{\rho gQH}{\eta_{\mathrm{overall}}} ]
Here, overall efficiency represents the combined efficiency of the relevant power-conversion stages, as defined for the calculation.
The motor must be suitable for the pump's actual operating range and the manufacturer's specified power requirements. Final selection should use the pump curves and motor documentation rather than relying solely on a simplified equation.
An oversized motor is not a substitute for correct pump hydraulics. A motor can have a high power rating while the pump remains unsuitable for the required flow and head.
9. Choosing the correct pump diameter
Submersible borehole pumps are manufactured in different diameters to suit different borehole casings and installation conditions.
The pump must fit within the available casing with the clearances required by the manufacturer.
Insufficient clearance can complicate installation and removal. It may also affect cooling arrangements where the pump and motor rely on water movement along the motor surface.
Before selecting a pump, verify the internal casing diameter rather than relying only on the nominal casing size.
Also consider the pump's length, cable arrangement, discharge connection, lifting requirements, and the space needed for safe installation.
10. Choosing the correct motor and electrical configuration
Pump sizing includes the electrical characteristics of the motor.
Important specifications include rated voltage, phase, frequency, current, power rating, starting method, insulation requirements, and compatible protection devices.
Single-phase and three-phase motors require different electrical arrangements. Some single-phase motors use capacitors or starting components, while three-phase motors require appropriate phase conditions.
The cable must be sized according to current, length, installation conditions, allowable voltage drop, and applicable electrical requirements.
Motor protection should be coordinated with the manufacturer's recommendations and the installation's protective devices.
The final design should also consider the electrical supply's ability to support motor starting and continuous operation.
11. Understanding pump installation depth
The pump's installation depth must be selected with reference to the borehole construction, water levels, required submergence, pump specifications, and maintenance considerations.
Installing the pump too close to the pumping water level may risk insufficient submergence when the water level falls during operation.
Installing it excessively deep may create other problems, including unnecessary lifting head and more difficult retrieval, depending on the borehole arrangement.
The installation should also avoid placing the pump where it is likely to draw excessive sediment from the bottom of the borehole.
The appropriate depth cannot be determined from borehole depth alone. It requires information about water levels, pump dimensions, casing, and the manufacturer's installation requirements.
12. Preparing for the next stage of pump selection
Before purchasing a pump, collect the information required for the final selection:
- Borehole depth and construction details.
- Static and pumping water levels.
- Sustainable yield or suitable pumping-test results.
- Required flow rate and daily demand.
- Delivery elevation and required outlet pressure.
- Pipe diameter, length, and fitting details.
- Available electrical supply.
- Tank capacity and operating schedule.
- Water quality and sediment conditions.
- Pump manufacturer's performance data.
This information forms the basis for a defensible pump selection and helps reduce the risk of buying equipment that cannot meet the site's requirements.
The next part of this guide will examine pump sizing examples, pipe friction, flow-rate conversions, storage calculations, energy efficiency, installation checks, commissioning, maintenance, and common selection mistakes.