Most pumps that fail early were never wrong mechanically. They were the wrong pump. Selection starts with four numbers — flow, head, fluid and available power — and every other decision follows from them.
A pump is not a product you buy off a shelf; it is a component matched to a system. The same 1.1 kW motor can move 3 m³/h to the top of a hill or 15 m³/h across a flat yard, and the pump that does one will be a poor choice for the other. This guide walks through the numbers we ask for on every enquiry and why each one matters.
The duty point is the whole job
Engineers talk about the duty point: the combination of flow rate and total head the system actually demands. Plot flow on one axis and head on the other, and a pump’s performance curve shows every combination it can deliver. Where your system curve crosses that pump curve is where the pump will run — whether or not that is where you wanted it to run.
Get the duty point right and selection is straightforward. Get it wrong and you will either starve the system or run the pump far from its best efficiency point, which wastes energy and shortens its life.
Step 1: Establish the flow rate you actually need
Flow is measured in cubic metres per hour (m³/h) or litres per second (l/s). One l/s is 3.6 m³/h. Work from real demand, not from the size of the pipe or the pump you happen to be replacing.
| Application | Typical design flow | Notes |
|---|---|---|
| Household pressure system (3–4 bathrooms) | 1.8–3.6 m³/h | Based on simultaneous fixture use, not total fixtures |
| Borehole to storage tank | Limited by tested yield | Never exceed the borehole’s sustainable yield |
| Livestock watering (500 head cattle) | 2–3 m³/h | Allow for peak drinking periods |
| Centre pivot irrigation | Set by the pivot design | Obtain from the pivot supplier, not estimated |
| Commercial booster set | Calculated per SANS 10252 | Duty plus standby is normal practice |
| Construction dewatering | Highly variable | Size on inflow rate, then add margin |
For a borehole the flow is not a free choice at all. The tested yield of the hole is a hard ceiling, and pumping above it draws the water level down until the pump breaks suction and burns out. If you do not have a recent yield test, that is the first thing to arrange.
Step 2: Work out total head
Total head is the resistance the pump must overcome, expressed in metres. It has three parts: the vertical lift (static head), the energy lost to friction in the pipework, and any pressure that must be delivered at the outlet. Ten metres of head is roughly one bar.
This is the number most often estimated badly, and the one that most often produces a pump that will not reach the tank. We have written a full walk‑through with a worked example in How to Calculate Pump Head.
Pipe size is part of the duty point. Undersized pipe can add more head than the actual lift. Doubling the pipe diameter cuts friction loss by roughly 97 per cent for the same flow — often cheaper over the life of the system than the bigger pump and bigger motor you would otherwise need.
Step 3: Characterise the fluid
Clean water is the easy case. Anything else changes the pump type, the materials and sometimes the whole approach.
- Solids content. Sewage and effluent need open or vortex impellers, or a grinder pump where rag and fibre are present. A clean‑water impeller will block within days.
- Abrasion. Sand and slurry wear impellers and casings quickly. High‑chrome or rubber‑lined slurry pumps exist for a reason.
- Chemistry. pH, chlorides and dissolved chemicals decide whether cast iron, stainless steel or a plastic‑bodied pump is appropriate.
- Temperature. Hot water affects seal selection and reduces the net positive suction head available.
- Viscosity. Thicker fluids shift the pump curve downward and may call for a positive displacement pump instead of a centrifugal one.
Step 4: Confirm the power supply
The power available on site narrows the field before anything else does. A 380 V three‑phase supply opens up the whole range; a 220 V single‑phase supply caps practical motor sizes at roughly 2.2 kW for most domestic installations.
| Supply | Practical range | Typical use |
|---|---|---|
| 220 V single phase | Up to ~2.2 kW | Households, small farms, light commercial |
| 380 V three phase | 2.2 kW and above | Industry, municipal, irrigation, boreholes |
| Diesel or petrol drive | Any | Sites with no grid, mobile dewatering, standby |
| Solar / off‑grid | Sized to daily volume | Remote boreholes, livestock, irrigation |
Where grid power is absent, unreliable or expensive, a solar system can be the lower‑cost option over its life — see Solar Water Pumping for how those systems are sized.
Matching the duty to a pump type
| Pump type | Best suited to | Typical head range |
|---|---|---|
| Jet / peripheral | Domestic pressure, shallow wells | 10–50 m |
| Multistage surface | Pressure boosting, transfer | 30–150 m |
| End‑suction centrifugal | High flow, moderate head | 5–80 m |
| Submersible borehole | Groundwater abstraction | 20–300 m+ |
| Submersible drainage / sewage | Wastewater, drainage pits | 5–40 m |
| Self‑priming dewatering | Site water with debris | 10–35 m |
Browse the full range by category on the Products page.
Read the curve before you commit
Every pump has a best efficiency point (BEP) — the flow at which it converts the most electrical energy into useful water movement. Running well to the left of BEP causes recirculation, vibration and premature bearing and seal wear. Running well to the right risks cavitation and motor overload.
Aim to land the duty point between roughly 70 and 120 per cent of BEP flow. A pump that is technically capable but runs at 40 per cent of BEP will cost more to own than the correct pump cost to buy.
Four mistakes we see repeatedly
- Oversizing “to be safe”. An oversized pump throttled back by a valve wastes the energy it never needed to use, and runs away from BEP.
- Ignoring NPSH. If the available net positive suction head is below what the pump requires, it will cavitate — and cavitation destroys impellers.
- Matching the old pump’s nameplate. The previous pump may have been wrong, or the system may have changed since it was installed.
- Leaving out the fittings. Bends, valves and non‑return valves add real head. On a short run with many fittings they can dominate the friction loss.
A worked selection
A guesthouse outside Kempton Park needs to fill a 10 000 litre tank on a 6 m stand from a borehole with a tested yield of 1.2 l/s. The pumping water level sits 38 m below the collar and the pipe run is 45 m of 50 mm class 10 HDPE.
- Flow: keep below the tested yield. Design at 3.6 m³/h (1.0 l/s).
- Static head: 38 m to the collar plus 6 m to the tank inlet = 44 m.
- Friction: roughly 2.5 m over that run and its fittings at this flow.
- Pressure head: none required — it discharges into an open tank.
- Total head: about 47 m.
The selection is therefore a borehole pump delivering 3.6 m³/h at 47 m, with the duty point sitting near BEP, a motor suited to the available supply, and a control box with dry‑run protection. That last item is not optional on a borehole.
What we need from you
Send us the flow, the head or the measurements to calculate it, the fluid and the power supply, and we will come back with equipment options and pricing. If you are not sure of the numbers, the site details are enough to start — approximate values are fine and we confirm the duty point with you before anything is quoted.
Ready to size a pump? Use the technical enquiry form or call +27 (15) 516-4991. Every enquiry is reviewed by a specialist before equipment is recommended.