Dosing pump will not prime — seven fixes

Short answer: The pump is running but no chemical is moving — and there are three usual causes: air in the suction line, dirt or crystals in a valve, or the stroke set too low. All three are fixed at no cost.

Work through these seven checks before buying a new pump or calling for service.

First, confirm the chemical really is not moving

The simplest check is the tank level. Mark it and look again 24 hours later. If the level has not dropped, nothing is being dosed.

A second method: take the suction pipe out of the tank into a measuring jar, run for 10 minutes and see how many millilitres are drawn. This also confirms your actual dosing rate.

Cause 1: Air in the suction line

The most common cause. A diaphragm pump struggles to clear air, particularly when the chemical level in the tank is low.

Fix:

1. Set the pump to 100 percent stroke
2. Open the air release or bleed valve on the discharge side
3. Run the pump until chemical flows without bubbles
4. Close the bleed valve, then return the stroke to its setting

Cause 2: Dirt or crystals in the foot valve

The foot valve sits at the end of the suction pipe. Antiscalant or hypo can dry and crystallise on it, or debris from the bottom of the tank can lodge in it.

Fix: remove the foot valve and soak it in warm water for 15 minutes, then clean it. For crystals use a mild acidic solution. Replace the strainer if it is torn.

Cause 3: A pump valve stuck

The pump head has two valves, suction and discharge, each containing a small ball or cone that moves back and forth. Dried chemical can seize them.

Fix: undo the pump head nuts, remove the valves and clean them in water or a mild acid. When refitting, note the orientation — fitted the wrong way round the pump will not work at all.

Cause 4: Stroke set too low

Below about 5 percent stroke a pump may not be able to prime at all. This often happens when the solution has been made up too concentrated.

Fix: prime at 100 percent first, then return to your setting. If your normal setting genuinely is below 5 percent, dilute the solution — the full method is in our dosing calculation guide.

Cause 5: Diaphragm torn or swollen

Signs of this: chemical weeping from under the pump, or a change in the sound it makes.

The cause is usually the wrong material — a PP pump on an acid line. See our PVDF vs PP guide.

Fix: the diaphragm has to be replaced. If it is an acid line, move to a PVDF model instead, or it will keep happening.

Cause 6: Discharge pressure too high

If the line pressure exceeds the pump rating, nothing will go in. For example a 4 kg/cm² pump on an 8 kg line.

Fix: check the line pressure with a gauge. If it is too high you need the Edose HP (10 kg/cm²), or move the injection point to a lower pressure location.

Cause 7: Suction line too long or too narrow

A diaphragm pump has limited suction lift. As a general rule:

• Suction lift no more than 1.5 metres
• Keep the suction pipe short and straight
• Do not reduce the pipe size — a narrow pipe restricts suction

Quick checklist

Four ways to prevent it

1. Mount the tank above the pump — the single most effective step
2. Fit a level switch (₹1,050) — the pump stops before the tank empties, so air never enters
3. Check the tank level weekly — problems are caught early
4. Do not keep solution beyond 15 to 20 days — old solution forms crystals and deposits

If you are buying new, the NEO PRO models include a level switch, which removes most of this trouble.

Frequently asked questions