Self Priming Metering Pump: What Diaphragm Pumps Can Lift and When to Use Flooded Suction

Is a diaphragm dosing pump a self priming metering pump? Yes, but only within tight limits. When the diaphragm retracts, the pump chamber expands, pressure drops, the suction check valve opens and liquid is drawn up from a tank below the pump. That is what makes it a self priming metering pump. The catch is that rated suction lift is short, typically one to a few meters, and a pocket of air in the head can stop the pump from priming at all. Our advice is simple: use flooded suction wherever you can, and treat suction lift as the fallback when space leaves no choice. This guide is for design engineers and the technicians who install and commission dosing pumps.

How a self priming metering pump draws liquid

Every stroke of a self priming metering pump is a suction half and a discharge half. On suction, the vacuum in the chamber lifts the liquid; the stronger the vacuum, the higher the column it can raise.

The idea that explains most priming trouble is compression ratio: the stroke volume compared with the dead volume left in the chamber. With the chamber full of liquid, which barely compresses, vacuum builds the moment the diaphragm moves. With air inside, the air simply compresses and re-expands. The diaphragm completes its stroke, the pressure swing is absorbed by the air, and neither check valve opens. That is vapor lock, or air lock.

Short strokes and large dead volume make any self priming metering pump more prone to it. IWAKI offers a high compression ratio version of its EHN series, described as optimizing the static volume in the pump chamber to raise the compression ratio, and recommended for sodium hypochlorite for exactly this reason.

Rated suction lift of a self priming metering pump

There is no universal figure. Take it from the manual for each model and treat it as a ceiling. Examples from the documentation we hold:

• NEWDOSE solenoid and mechanical diaphragm pump manuals: 2 m water column.
• Cheonsei KD series: maximum self priming height of 1 m at full stroke.
• LMI B and D series: 1.5 m water column.
• Milton Roy MAXROY B: 2 m water column, except viscous fluids; Milton Roy G series: suction lift up to 3 m water column.

All of these assume clean water at ambient temperature. Viscous, gassing or warm chemicals and short stroke settings all cut the real figure. Measure from the lowest tank level; our article on diaphragm metering pump suction lift covers how to measure it and what to do when you are over.

When a self priming metering pump will not prime on startup

The most common complaint on a new install is that the self priming metering pump "will not prime". Usually the discharge side is under pressure. The head is full of air, there is line pressure behind the discharge valve, the air cannot be pushed out, and so no liquid comes up.

Vent the discharge to atmosphere first. Open the bleed valve on the head, or route the outlet temporarily to a return line, run at full stroke length and full speed, and close the valve once liquid flows from the bleed port without bubbles. Step-by-step procedures are in how to prime a diaphragm metering pump.

The other usual cause is an air leak on the suction line. A loose fitting or aged tubing means the pump is drawing air, not chemical.

When a self priming metering pump should be on flooded suction

Flooded suction means the pump inlet sits below the lowest liquid level in the tank, so liquid flows into the head by gravity. The head always holds liquid, and the chance of vapor lock drops sharply.

We recommend flooded suction in these cases:

1. Gassing chemicals such as sodium hypochlorite and hydrogen peroxide; the NEWDOSE solenoid pump manual recommends this layout for them.
2. Viscous chemicals, where suction losses make lift unreliable.
3. Pumps that run at short stroke settings for long periods.
4. Dose points that cannot tolerate an interruption from a single air bubble.

The NEWDOSE mechanical pump manual also states that the pump suction should sit below the lowest tank level so liquid can flow straight in.

How to install flooded suction correctly

Short, straight, full-bore suction line

Keep the pump close to the tank and the suction line short and straight, with no loops or high points. Do not reduce the line below the pump inlet size.

Isolation valve and strainer ahead of the pump

A flooded inlet is always under head. Without a valve, liquid keeps draining out when you open the pump. Close the valve, relieve pressure and wear goggles and chemical gloves before maintenance.

Check maximum inlet pressure

A tall tank raises inlet pressure. The Milton Roy G series and MAXROY B data sheets both state a maximum inlet pressure of 2 bar, so check it when a large storage tank feeds the pump directly.

Prevent siphoning

If the tank level sits above the injection point, liquid can flow through a stopped pump. Fit a back pressure valve on the discharge; the NEWDOSE manual also calls for one when the pressure difference across the pump is under 1 bar. See why a metering pump needs a back pressure valve.

Conclusion

A diaphragm pump is a self priming metering pump, but a modest one. Rated suction lift in our documentation runs from about 1 to 2 m, with one model listed at 3 m, and air in the head can cause vapor lock. Take lift only from the manual and measure from the lowest tank level. For gassing or viscous chemicals, short strokes or dose points that cannot stop, use flooded suction with an isolation valve, strainer, inlet pressure check and siphon protection. Compare options in our diaphragm metering pump range and the NEWDOSE product line. Send the chemical, tank height and injection pressure and contact our engineers to confirm whether a suction lift layout will work.

FAQ

Does a self priming metering pump need filling before the first start?

Yes. Filling the head and suction line with water or chemical removes the air and makes self priming far easier. A flooded installation does not need it.

The pump primes fine but loses prime overnight. Why?

Liquid is probably draining back to the tank and letting air into the head. Check whether the foot valve or suction check valve is sealing, or move to flooded suction.

Does flooded suction make the pump overdose?

Not by itself. If the tank level is above the injection point, though, siphoning can push extra chemical through, and a back pressure valve fixes that.

Is a solenoid pump a better self priming metering pump than a motor-driven one?

It depends on the model, not the drive type. Compare rated suction lift and compression ratio, and consider auto-degassing or high compression heads for gassing chemicals.

Can a booster pump solve a lift problem?

It can, but it adds equipment and another failure point. Moving a self priming metering pump next to the tank on flooded suction is usually simpler.