The formula
The pump must deliver the chemical mass flow the water needs, diluted however the product arrives:
Pump rate (L/h) = water flow (m³/h) × dose (mg/L) ÷ chemical concentration (g/L)
Example: chlorinating 10 m³/h to 2 mg/L with commercial hypochlorite at ~150 g/L available chlorine: 10 × 2 ÷ 150 = 0.13 L/h. Antiscalant at 3 mg/L into a 20 m³/h RO feed, dosed neat at ~1 050 g/L: 20 × 3 ÷ 1050 = 0.06 L/h. The numbers are usually startlingly small — which is exactly why the next rule exists.
The 20–80% rule
A metering pump should run its duty point between roughly 20% and 80% of its maximum capacity. Below ~10% of capacity, stroke-to-stroke repeatability degrades and the tiny per-stroke volumes become vulnerable to any imperfection — a soft bubble, a slightly worn seat — so accuracy quietly evaporates. Above ~80 – 90%, there is no headroom for the day the dose must rise or the chemical arrives weaker.
The corollary surprises buyers: the smallest pump in the range is often still too big, and the correct fix runs backwards through the formula — dilute the chemical. Our 0.06 L/h antiscalant duty on a 5 L/h pump is 1.2% of capacity: hopeless. Diluted 10:1 in a day tank, the duty becomes 0.6 L/h — 12% — and 4:1 further, comfortably mid-range. Dilution rules of the road: use good-quality (ideally softened or RO) water, make what will be used within days (dilute hypochlorite decays; some polymers hydrolyse), and label the tank with the dilution so the next adjustment is arithmetic rather than archaeology.
Pressure: the second axis
The pump must inject against the real pressure at the injection point: line pressure at that point, plus static head if the injection sits above the pump, plus the injection valve's own spring. Two errors recur:
- Undersized on pressure — sized from the pump-room gauge, forgetting the injection point is downstream of a pump or up a hill. The symptom is a doser that "runs but doesn't dose": the solenoid clicks, the check valves never open against the pressure.
- The suction side ignored. Keep suction lift under ~1.5 m — or better, flood the suction from a tank above pump level. Long, lifting, gas-releasing suction lines are behind most "lost prime" callouts, hypochlorite being the standing offender (see the pump-types article for why peristaltics shrug at this).
Control mode, chosen at order time
Manual constant-rate dosing is only correct when the water flow is constant. Where flow varies, dose flow-proportionally — a pulse water meter or 4 – 20 mA flow signal pacing the pump — otherwise the water is overdosed at low flow and underdosed at peak, simultaneously wasteful and ineffective. Specify the meter and the pump's control option together; retrofitting is always messier.
Rule of thumb: run the formula, then check the percentage — if the duty point is under 20% of the pump's capacity, dilute the chemical rather than hunt a smaller pump. And size against the pressure at the injection quill, not the pressure where the pump happens to stand.