Choosing Between 300mm, 350mm and 400mm at the Same Duty Point

A fair number of large municipal projects land on a duty point that two different bore sizes can both technically hit. Take roughly 1100 m³/h at 10 m head: a 300mm unit reaches it at 55 kW running near the top of its own catalogue range, and a 400mm unit at a similar point (1500 m³/h at 10 m) does it at 75 kW while barely working. Both numbers pass the tender spec. They’re not the same decision.

The 300mm option costs less up front, and if this flow is genuinely the ceiling of what the site will ever see, running a smaller pump near its rated capacity is not a problem; that’s what it’s built for. The case against it is entirely about the future: municipal flow tends to grow with population and infiltration, rarely shrinks, and a pump already living near its top-end performance has no room to absorb five or ten years of gradual increase without an early replacement or a second pump added later.

The 400mm option is the opposite trade. Paying for headroom you don’t strictly need today, in exchange for a pump running comfortably under its ceiling, with lower relative wear at the same flow and margin to absorb growth without a capital project. Whether that’s worth the upfront difference depends on how confident anyone actually is in the flow projection, a number that, on most municipal projects, was estimated years before construction started and rarely gets revisited once the contract is signed.

There’s a middle path worth mentioning: some agencies deliberately spec the larger bore and then throttle or trim the impeller to match current flow, planning to remachine it back to full performance if demand grows. It adds a line item to the maintenance record but avoids a full pump swap later. Whether that’s sensible depends on how the project’s engineers weigh a known present cost against an uncertain future one, which is really the whole decision in miniature.