Define the duty point — the flow and head you actually need — then pick a pump whose best efficiency point (BEP) sits at or just above that flow. Aim to operate within the preferred operating region (~70–120% of BEP flow).
Oversizing is the most common and most expensive mistake. A pump bought "with margin" runs left of its BEP, where efficiency falls and recirculation, vibration and seal failures rise — then gets throttled, burning the surplus energy as heat.
Parallel pumps add flow (at the same head); series pumps add head (at the same flow). But the system curve always bites back: two pumps in parallel give noticeably less than double the flow.
1 · Start with the duty point
Everything in selection begins with one specified condition: the duty point — the flow rate you must deliver and the head required to deliver it. The flow comes from the process. The head comes from the system, exactly as we built it in Part 1:
The pump you select must produce exactly Hduty at Qduty — that is, its curve must pass through the duty point. Many pumps can do that. The art is choosing the one that does it efficiently and reliably, which means choosing where the duty point lands relative to the pump's BEP.
2 · Read the curve around the BEP
The best efficiency point is the flow at which the pump converts the most shaft power into useful work. It is also, and not by coincidence, the flow at which the pump is hydraulically quietest: flow enters and leaves the impeller cleanly, with minimal recirculation and the lowest radial thrust on the shaft. Run far from the BEP in either direction and reliability suffers:
- Left of BEP (low flow): internal recirculation, pressure pulsations, rising temperature, increased radial thrust → bearing and seal damage. Below the minimum continuous flow the pump must not run at all.
- Right of BEP (high flow / run-out): rising NPSH-required (cavitation risk), rising power (possible motor overload), and again high radial thrust.
Manufacturers and the Hydraulic Institute define regions around the BEP:
| Region | Typical range (% of BEP flow) | Meaning |
|---|---|---|
| BEP | 100% | Peak efficiency and lowest loads — the target |
| Preferred operating region (POR) | ~70–120% | Reliable continuous operation; select the duty here |
| Allowable operating region (AOR) | ~50–120%+ | Acceptable but with reduced life; set by the maker |
The interactive below lets you choose a pump for a fixed duty and see immediately where the duty lands — and what it costs you when you choose badly.
Interactive 1 — Select for the duty, not for the margin
Live modelThe duty is fixed at 150 m³/h & 30 m (the red point). Slide the selected pump's size up and down and watch where the duty falls relative to that pump's BEP and its preferred operating region (green band).
Selected pump curve vs the duty
3 · The oversizing trap
Slide that pump well above the duty in the model and watch the duty point drift left of the BEP into the low-flow region. This is exactly what happens when a pump is bought "with margin": every added safety factor on flow or head pushes the real operating point further from where the pump wants to run.
The damage is rarely dramatic enough to notice at commissioning, which is why it persists:
- Wasted energy — an oversized pump is throttled back, and the throttle valve destroys the surplus head as heat (see the work argument in Part 1). Over a pump's life, energy dwarfs its purchase price.
- Reduced reliability — low-flow recirculation and high radial thrust shorten seal and bearing life. Oversized pumps are over-represented in bad-actor lists.
- Higher capital cost — bigger pump, bigger motor, bigger starter, bigger everything, to do less well what a correctly sized unit would do.
Margin belongs on data you don't trust, not on the pump. If the system head is uncertain, tighten the calculation — don't bolt on 20% "to be safe." A variable-speed drive (the subject of the next guide) is the honest way to keep margin in reserve without dragging the operating point off the BEP.
Impeller trimming — the fine adjustment
Pumps come in discrete sizes, so the catalogue curve rarely lands exactly on the duty. The standard fix is impeller trimming: machining the impeller to a smaller diameter lowers the curve. Within a modest trim range it follows affinity-like laws:
4 · Pumps in parallel & series
Sometimes one pump can't (or shouldn't) do the whole job. Two arrangements extend the envelope, and they are mirror images:
- Parallel — both pumps share the same suction and discharge headers. At any given head, their flows add, so the combined curve stretches to the right. Used for high or variable flow, and for sparing (run one, two, or three as demand changes).
- Series — the discharge of one feeds the suction of the next. At any given flow, their heads add, so the combined curve stretches upward. Used for high head (a multistage pump is series staging in one casing).
The catch — and the single most misunderstood thing about parallel pumps — is the system curve. Adding a second pump in parallel raises the flow, but more flow means more friction, which raises the system head, which pushes both pumps back up their curves to a lower individual flow. Two pumps in parallel deliver well under twice the flow — and on a steep (friction-dominated) system, barely more than one.
Interactive 2 — Parallel & series
Live modelCombined curve × system curve
H = 50 − 0.0009·Q² each. Parallel adds flow at equal head; series adds head at equal flow; operating point is the intersection with H = Hstat + R·Q². Watch the flow gain as you steepen the system.When to choose which. Parallel suits flat (static-dominated) systems and variable demand — and gives natural sparing. Series suits high-head duties. If your system curve is steep, a single larger pump or a series arrangement usually beats throwing pumps in parallel.
5 · A note on specific speed
One number ties selection back to the pump type from Part 1: the specific speed, computed from the duty (speed, flow and head). Low specific speed points to a radial impeller (high head, low flow); high specific speed to a mixed- or axial-flow impeller (low head, high flow). It is the bridge from "what do I need" to "what kind of machine delivers it" — and it sets the very shape of the curves you have been reading.
6 · Selection checklist
- Pin down an honest duty point — real flow, real system head, no padded margins.
- Select so the duty lands at 80–110% of BEP flow, comfortably inside the preferred operating region.
- Trim the impeller to land on the duty rather than throttling for life.
- Check NPSH at the duty (and at run-out) using the method in Part 1.
- Use parallel for variable/flat-system flow and sparing; series for high head — and remember parallel never doubles flow.
- Reach for variable speed, not oversizing, when the future duty is uncertain.