Under-dosing fails consent directly — and the frustrating part is that the pump often looks like it is running normally. The set-point is right, the stroke counter ticks, yet the chemical isn't reaching the water. This page covers why dosing pumps under-deliver — gassing and vapour lock, valve fouling, suction problems, calibration drift and control faults — and how to find and fix each.
The water tells you before the pump does.
The controlled variable creeps the wrong way — residual falling, pH not reaching target, turbidity climbing — despite an unchanged set-point.
A feedback loop pegs the pump at 100% trying to reach set-point and can't — a clear under-delivery signal.
Reagent consumption lower than the calculated demand — the day-tank lasts “too long”. A simple, powerful check.
Visible bubbles in the pump head or suction line — the pump strokes but compresses gas instead of moving liquid.
The dose is calculated right — the chemical just isn't arriving.
| Root cause | Mechanism | Fix |
|---|---|---|
| Gassing / off-gassing reagent | Hypochlorite, peroxide release gas that fills the head — pump compresses vapour | Auto-degassing dosing head; flooded suction; keep reagent cool/fresh |
| Vapour lock / loss of prime | Suction lift too high or warm volatile reagent flashes | Flooded suction, shorten/cool suction line, foot valve, priming aid |
| Fouled / worn check valves | Crystallised reagent or debris holds a valve ball off seat — back-leakage every stroke | Clean/replace valves; fit strainer; flush line; correct material |
| Suction-side restriction | Blocked foot valve/strainer, collapsed hose, closed valve | Clear restriction; check NPSH available vs required |
| Calibration drift | Diaphragm wear / back-pressure change lowers real output below the set % | Re-calibrate against the calibration column; replace diaphragm |
| Back-pressure too low/high | Without a loading valve, siphoning or back-pressure changes shift delivery | Fit/adjust back-pressure (loading) valve |
| Crystallisation / scaling | Lime, polymer or saturated reagent blocks lines & quill | Dilute at injection, flush programme, heat-trace where needed |
| Control/signal fault | Pacing signal lost, analyser reading high → loop cuts dose | Check 4–20 mA pacing, analyser calibration, no-flow interlock state |
Prove delivery first, then work back up the line.
Run the pump against the calibration column for a timed draw-down. Compare actual mL/min to the set point — this separates a delivery fault from a control fault.
Look for gas, check suction/discharge valves for fouling and back-leak, confirm the diaphragm is intact.
Strainer, foot valve, hose condition, suction lift and NPSH. Most under-delivery is a suction-side problem.
Confirm pacing input, analyser calibration and that no interlock (no-flow, low-level) is silently inhibiting the dose.
A timed draw-down against the calibration column. If real output matches the set-point, the problem is upstream (control/analyser); if it doesn't, the problem is the pump or its suction/discharge train. Do this before changing anything.
If a loop is pegged at maximum or a tank isn't emptying, we will run the delivery checks, find the suction, valve, calibration or control fault, and restore reliable dosing — with the preventive fixes to keep it that way.
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