Metering Pump Suction Lift: How High Above the Liquid Can a Diaphragm Pump Sit?

How far above the liquid can a diaphragm dosing pump be mounted? There is no universal number: metering pump suction lift is whatever the manual rates it at, and that rating is for clean water at room temperature. In the manuals we work from, NEWDOSE DFD, DP, DM and DC solenoid diaphragm pumps are rated at 2 m, the NEWDOSE mechanical diaphragm pump manuals give a maximum suction height of 2 m water column, and the Cheonsei KD series states a maximum self-priming height of 1 m at full stroke. Real chemicals usually do worse. Our standing advice: use flooded suction whenever you can, and if the pump must sit above the liquid, stay well inside the rated lift.

This article is for installers and project engineers working with solenoid and mechanical diaphragm pumps. Hydraulic diaphragm and plunger pumps follow the same physics, but check their own figures.

What Metering Pump Suction Lift Actually Means

On the suction stroke, the pump creates a partial vacuum in the head and atmospheric pressure pushes liquid up the suction line. The height it can raise liquid this way is the metering pump suction lift. It depends on the head design, the check valves and the stroke length.

Rated figures are measured under easy conditions. The Cheonsei performance curves, for instance, are tested at 1 m lift with clean water. On site, several things cut the usable lift:

• Viscous chemicals, such as concentrated PAC or polymer solutions, add friction.
• Gassing chemicals, such as sodium hypochlorite and hydrogen peroxide, release bubbles under vacuum.
• Warm liquids get close to their vapour pressure and cavitate.
• Long suction lines, many fittings, and undersized pipe all add loss.
• A very short stroke length produces a weaker vacuum each cycle.

The last point surprises people. The 1 m figure from Cheonsei is for full stroke. Turn the stroke right down and self-priming gets noticeably worse.

Measure Metering Pump Suction Lift From the Lowest Level

Measure from the lowest liquid level in the tank to the pump inlet valve, not from today's level and not from the tank lid. A classic complaint is "it pumped fine all week, then stopped when the tank got low." The pump did not fail; the metering pump suction lift grew as the level dropped until it passed the limit.

Mounting a solenoid pump on top of a PE tank is common and tidy. Just remember that the taller the tank, the bigger the lift when it is nearly empty. Either account for it when choosing the tank, or set the low-level alarm higher.

When to Switch to Diaphragm Pump Flooded Suction

Flooded suction means the pump inlet sits below the lowest tank level, so chemical reaches the head by gravity. The NEWDOSE mechanical pump manual recommends this arrangement, and the NEWDOSE solenoid manual specifically advises it for gassing liquids such as hypochlorite and peroxide, and for viscous liquids.

Our decision order on site:

1. Is the chemical gassy or viscous? If yes, go flooded. No calculation needed.
2. Is the height from lowest level to pump inlet clearly below the rated metering pump suction lift? If not, lower the pump.
3. Can the suction line be short, straight and at least as large as the pump inlet? If not, move the pump closer to the tank.

Running at the very edge of the rated lift is the most common mistake. It looks fine on paper. Add a strainer and a few elbows and the pump becomes unreliable.

Two Things to Handle After Going Flooded

The first is siphoning. When the tank level is higher than the injection point, chemical can flow through the pump even when it is stopped. The NEWDOSE solenoid manual's answer is a back pressure valve on the discharge line, set high enough that free flow stops.

The second is maintenance. A flooded pump always has liquid pressure at the inlet, so an isolation valve before the pump is essential. Without it, removing the head drains the tank onto the floor. Close the valve, relieve pressure, and wear goggles and gloves, especially with hypochlorite, acids and caustic.

If the Pump Has to Sit Above the Liquid

Sometimes the layout leaves no choice. In that case:

• Fit a foot valve with a strainer at the bottom of the suction line, so the line stays full between starts.
• Before first start, fill the head with liquid, or run full stroke with the bleed valve open until air is gone.
• Use clear suction tubing where possible so air pockets are visible.

Leak-test every suction joint. A small air leak on the vacuum side looks exactly like insufficient metering pump suction lift and sends people chasing the wrong fault.

Conclusion

The height a diaphragm pump can sit above the liquid is the rated metering pump suction lift in its manual, measured from the lowest tank level and reduced for viscous, gassy or warm chemicals. In our documents that is 2 m for NEWDOSE solenoid and mechanical diaphragm pumps and 1 m at full stroke for Cheonsei KD, all on clean water, so leave margin. For difficult chemicals go flooded, then add a back pressure valve and an inlet isolation valve. See our diaphragm metering pumps and the NEWDOSE range, or send tank size, levels and chemical and contact our engineers to check your metering pump suction lift.

FAQ

The pump stops dosing when the tank is nearly empty. Is it faulty?

Probably not. The lift increases as the level drops and passes the rated metering pump suction lift. Lowering the pump or going flooded usually fixes it. Other causes are covered in mechanical diaphragm pump not dosing.

Will a pump below the liquid level over-dose?

It can. With high inlet pressure and low outlet pressure, liquid pushes through the check valves. A back pressure valve that keeps outlet pressure above inlet pressure fixes it.

Will a bigger pump lift higher?

Not necessarily. Metering pump suction lift depends on head and valve design, and bigger pumps do not always lift more. Try lowering the pump first.

The pump will not prime on first start. What now?

Fill the head, set full stroke and open the bleed valve until air clears. Step-by-step instructions are in how to prime a diaphragm metering pump.

Lift is within spec, but flow is still low. Why?

Look for suction leaks, a blocked strainer, or a stroke set too low. Our guide to metering pump low flow causes gives the full checking order.