How to size a dewatering pump

Step 1: establish the flow you actually need

Flow is usually derived one of two ways. Either you have a volume and a deadline — an excavation holding a known quantity of water that has to be clear by a given time — or you have a rate of ingress that has to be matched or exceeded.

For a volume and a deadline, divide one by the other and add margin for the water still arriving while you pump. For an ingress rate, measure it if you can: a pump sized exactly to the inflow never gets ahead and runs continuously.

Round up to the model above your calculated figure rather than the one below. Capacity falls as head rises, so a pump quoted at your exact requirement will deliver less than that at your actual duty point.

Step 2: calculate total head, not vertical distance

Total head is static lift plus friction loss. Static lift is the vertical distance from the water surface to the discharge point. Friction loss is the pressure the pipework consumes, and it depends on pipe diameter, total run length, the number and type of fittings, and the flow rate itself.

Two things make this counter-intuitive. Friction loss rises sharply with flow rate — roughly with its square — so doubling the flow through the same pipe does not double the friction, it roughly quadruples it. And it falls sharply as diameter increases, so a larger bore over a long run is often cheaper than a bigger pump.

On short runs with generously sized pipe, friction may add only a few metres. On a long discharge line at high flow through undersized pipe, it can be the larger half of the total. If the pipework is not yet fixed, size the pipe and the pump together rather than in sequence.

Step 3: check what the water is carrying

Solids handling is the largest spherical particle the pump will pass without blocking. It is a physical clearance, not a filter rating: a pump rated at 74 mm passes a 74 mm stone and jams on a 90 mm one.

This constraint can override the flow and head calculation entirely. If the required duty is only met by a clean-water model and the water is not clean, the specification has to change — screen upstream, split the duty, or accept a different operating point. That is a cheaper problem to find before an order than after.

Where the material is fibrous rather than granular — sewage, rag, organics — a standard solids impeller will bind on material well under its nominal rating, and a sewage impeller is the correct specification.

Step 4: match the ports to the pipework

Port diameter varies with model size across the eBORA range, from 100 mm on the B100 to a 500 mm inlet on the B500. Connecting a large pump to undersized pipework throttles it at the connection and adds friction precisely where you are trying to avoid it.

Suction lift is not a differentiator here. Every model across the B, K and L series self-primes to 8.5 m, so the pump can sit above the water rather than in it, and no foot valve or vacuum assistance is needed to start or restart.

A worked example

A basement excavation is taking on groundwater carrying sand and gravel. Measured ingress is around 700 m³/hr. The discharge point is 12 m above the water surface, along a 150 m run.

Flow: 700 m³/hr measured, so size above it — call the requirement 800 m³/hr to stay ahead of the inflow.

Total head: 12 m static lift, plus friction loss for the run. Assume 18 m of friction for this example — the actual figure depends on the pipe diameter, length and fittings, and should be calculated for the specific layout rather than assumed. That gives roughly 30 m total head.

Solids: site spoil, so a solids-handling range is required. That rules out the K Series regardless of its performance.

Against those figures the B200 fits: 845 m³/hr capacity, 52 m total head, 74 mm solids handling, with 200 mm inlet and outlet. It clears the flow requirement, has head margin in hand, and passes the material. The B150 at 520 m³/hr would not meet the flow, and the B300 at 1,360 m³/hr is more pump than the duty needs.

Change one variable and the answer moves. If the same duty needed 85 m of head rather than 30 m, no B Series model would reach it, and the choice would move to the L Series — at the cost of maximum flow.

How much margin to leave

Some margin is necessary because conditions change: ingress rises after rain, pipework gets extended, strainers partially block and add friction.

Too much margin has its own costs. An oversized pump run well away from its best efficiency point wastes fuel, and on a variable duty it can cycle rather than run steadily. Sizing one model above the calculated requirement is usually right; sizing three above rarely is.