Pump Selection for Water Supply: Head, Discharge and Power

Start with the required discharge

The discharge a pump must deliver is set by the volume of water the scheme needs in a day divided by the pumping hours allowed for that day. Fewer pumping hours mean a higher flow rate and a larger pump, so this assumption is a genuine design decision, not a detail.

Choosing realistic pumping hours matters: a design built on sixteen hours that is actually run for eight will never meet demand, while an over-conservative figure buys a needlessly large pump and bigger rising main.

Work out the total dynamic head

The head a pump must overcome is the total dynamic head: the static lift from the pumping water level to the delivery point, plus the friction losses in the rising main and fittings, plus any required residual pressure at delivery.

Friction loss grows sharply with flow and falls with pipe diameter, so head and pipe size are linked. A cheaper, smaller rising main raises friction and therefore the head and the running energy cost — one of the classic trade-offs in scheme design.

Compute the power requirement

The hydraulic power is proportional to the product of discharge and total head; the actual power drawn at the shaft is that divided by the pump efficiency, and the motor rating adds a margin above it. A low-efficiency pump at the same duty simply costs more to run forever.

Because energy is the dominant lifetime cost of a pumping scheme, selecting a pump that runs near its best efficiency point at the design duty often saves far more over the years than the difference in its purchase price.

Match the pump to the system curve

A pump does not deliver a fixed flow; it delivers the flow at which its own head-discharge characteristic crosses the system's head-discharge curve. The design duty point should sit at or near the pump's best efficiency point so it is neither straining nor throttling.

Selecting a pump whose curve meets the system curve well away from its efficiency peak wastes energy and shortens life, even if it technically delivers the required flow. The curves, not the nameplate alone, decide the match.

Practical selection and redundancy

For reliability, schemes commonly provide a standby pump so supply continues during maintenance or breakdown, and multiple smaller pumps can give flexibility over a single large one. The choice between submersible and surface pumps depends on the source and the lift.

Record the design discharge, total dynamic head, chosen pump and its efficiency in the scheme design so the basis of selection is transparent. Future augmentation or energy audits all begin from these documented duty figures.

Reading the pump curve against the system curve

A pump does not deliver a fixed discharge — it delivers whatever point the pump curve and the system curve cross. The system curve is the static lift plus friction, which rises with the square of flow; the pump curve falls with flow. The operating point is their intersection, and a good selection places it at or near the pump's best-efficiency point so energy is not wasted and the pump does not run hot at shut-off or cavitate at run-out.

Total head must include static suction and delivery lift, friction in both suction and delivery mains, and residual head needed at the delivery. NPSH available at the site must comfortably exceed the pump's NPSH required, or the pump will cavitate regardless of how well the head and discharge are matched. Where demand varies, a sensibly staged set of pumps beats a single oversized unit throttled down.

Record the duty point, the selected pump model and the motor rating in the design so field replacement years later reproduces the hydraulics rather than guessing.