Dosing Pump Not Delivering Flow? 9 Common Causes and Fixes

A dosing pump that runs but does not deliver chemical is one of the most common plant complaints. The motor hums, the stroke knob moves, but the calibration column does not drop or the dose reading stays low. Work through the causes below in order, from the quickest to check to the ones that need a strip-down.

Safety first: isolate the power, close the suction and discharge valves, release pressure and wear the right PPE before you open any part of the liquid end. Many dosing chemicals are corrosive or toxic.

1. Air or gas in the suction line

Air leaking in through a loose suction joint, or gas from the chemical itself (sodium hypochlorite and hydrogen peroxide release gas), collects in the pump head. The plunger or diaphragm then compresses gas instead of pushing liquid.

Fix: tighten suction joints, prime the pump, and keep the suction flooded (tank above the pump). For gassing chemicals, fit a degassing or vent valve and keep the suction line short.

2. Dirty or worn check valves

Suction and discharge check valves (ball and seat) must seal on every stroke. Crystals, sludge or a worn ball stop them sealing, so chemical flows back and the net output drops.

Fix: remove the valves, clean them, and check balls and seats for pitting or wear. Replace worn parts as a set.

3. Blocked foot valve or suction strainer

Strainers clog with sediment, especially with lime, alum or polymer. The pump starves and runs noisily.

Fix: clean the strainer and foot valve, and keep the foot valve off the tank bottom.

4. Suction lift too high

If the pump sits above the tank, or the suction line is long or narrow, the pump may not be able to draw the chemical, especially viscous ones.

Fix: lower the pump or raise the tank so the suction is flooded, and use a larger, shorter suction pipe. See our guide on sizing a dosing pump for the full selection checklist.

5. Stroke length set too low

A stroke knob that has been turned down, or has slipped, gives a lower flow than expected. At very low stroke settings, small leaks have a larger effect.

Fix: check the stroke setting and confirm the actual flow with a calibration column. If you always run at a very low setting, the pump may be oversized.

6. Worn plunger packing (plunger pumps)

Packing seals the plunger. When it wears, chemical leaks at the gland and the pump loses output. A scored plunger wears new packing quickly.

Fix: replace the packing and inspect the plunger surface. A scored plunger needs regrinding, coating or replacement.

7. Damaged diaphragm or low hydraulic oil (diaphragm pumps)

A torn diaphragm or a low oil level in a hydraulic diaphragm pump reduces or stops the output. Chemical in the oil, or oil in the chemical, is a sign of diaphragm failure.

Fix: check the oil level and condition, and replace the diaphragm if it is damaged. Find out why it failed, such as overpressure or the wrong material, before fitting a new one.

8. Relief valve opening or a blocked injection point

If the injection point is blocked or a discharge valve is partly closed, pressure rises and the pressure relief valve opens, sending chemical back to the tank instead of the process.

Fix: check the relief valve setting and the return line, and clean the injection point or quill.

9. Motor or gearbox problem

Reverse rotation after rewiring, a slipping coupling or a worn worm gear means the stroke does not reach its full length.

Fix: check the motor rotation and coupling, and listen for gearbox noise. Gearbox wear usually needs a workshop overhaul.

Quick troubleshooting table

When to send the pump for repair

If cleaning and new soft parts do not restore the flow, or the plunger, gearbox or liquid end is worn, a workshop overhaul is usually cheaper than a new pump. Our pump repair and overhaul service covers JEW and other makes, with testing before dispatch. Regular servicing under an annual maintenance contract catches most of these problems before they stop your dosing.

Frequently asked questions

Why does my hypochlorite dosing pump keep losing prime?

Hypochlorite releases gas, which collects in the pump head. Keep the suction flooded and short, avoid high points in the suction line, and fit a degassing valve.

How do I check the actual flow of a dosing pump?

Fit a calibration column on the suction side. Time how much chemical the pump draws from the column over one minute, then convert to litres per hour.

Do you repair pumps from other manufacturers?

Yes. We repair and overhaul most makes of metering pumps at our Mumbai works and supply compatible spares.