Solenoid Metering Pump Selection: When Low-Flow Dosing Calls for One

If the job is a few liters per hour, a lot of buyers still ask whether a motor-driven pump would be the safer bet. Most of the time it isn't needed. A solenoid metering pump is compact, easy to wire, and you change the flow with a couple of button presses. The catch is that it has a clear operating envelope, and once flow and pressure push past it, trouble starts. Picking one well comes down to knowing where that line sits and choosing a model inside it.

How a Solenoid Metering Pump Moves Liquid

When the coil is energized, a push rod drives a PTFE diaphragm forward and liquid leaves through the discharge valve. When power drops, a spring pulls the diaphragm back and fresh liquid is drawn in through the suction valve. That is one stroke. The volume per stroke is essentially fixed, so flow is set by changing the stroke rate, anywhere from 0 to 100% on the front panel.

That design has a plus and a limit. The plus: no motor, no gearbox, a small body you can hang on a wall or set next to the tank. The limit: the coil only pushes so hard on each stroke, so as flow goes up, available pressure comes down. The NEWDOSE data shows it plainly. The DFD-02-20-HX gives 2 L/h at 20 bar, while the DFD-50-02-X gives 50 L/h but only 2 bar.

First Check: Flow and Pressure as a Pair

The NEWDOSE DFD solenoid metering pump range covers roughly 1–55 L/h, with a maximum of 20 bar. Those two extremes never sit on the same pump. High-pressure models are low-flow, and high-flow models are low-pressure. So when you size a solenoid metering pump, always look up flow and pressure together, never one at a time.

Our rough sorting goes like this:

1. Peak dose within a few tens of liters per hour and modest injection pressure: start with a solenoid metering pump.
2. Twenty or thirty liters per hour and 7 bar or more: there is rarely a solenoid model that fits, so go straight to a mechanical diaphragm pump.
3. Above 20 bar: a solenoid metering pump is off the table.

Here's an example. Say the duty is 20 L/h at 10 bar. Go through the solenoid tables and the 20 L/h size only reaches 3 bar, while the 06-16-GX that reaches 16 bar only delivers 6 L/h. Neither end covers it. A mechanical pump such as the NDWS-24/1.6, rated 24 L/h at 16 bar, handles it with room to spare.

Reading Solenoid Metering Pump Model Codes and Specs

A NEWDOSE solenoid model code has six parts: control type, capacity, pressure, frame, wetted material set, and power supply. DFD-06-05-M-PPV-A, for example, means manual control, 6 L/h, 5 bar, M frame, PP pump head, 230 V.

The frame letter matters most for suction lift. The manual lists 3 m for the M, HX, NX and GX frames, and 2 m for the classic X and the LM. If the pump sits above the tank on a long suction line, that one meter can decide whether it primes at all.

Read the footnote on the spec sheet too. Figures are maximum flow at maximum pressure, measured on clean water, real conditions may differ by up to 5%, and the maker recommends a 20% operating margin. Plenty of people size a solenoid metering pump right at the catalog number and then find it can't keep up once installed.

Margin and Back Pressure: Where Sites Get Caught Out

The actual output of a solenoid metering pump shifts with discharge back pressure. The manual gives a quick estimate: current flow is about rated flow divided by maximum stroke rate, times the current stroke rate. Take the DFD-06-07-X, rated 6 L/h at 120 strokes per minute. Set to 80 strokes, it gives about 4 L/h. That is only an estimate, so calibrate on site if accuracy matters.

Too little back pressure causes its own problem. If the tank level is above the injection point, liquid keeps flowing after the pump stops. That's siphoning. The manual's fix is a back pressure valve on the discharge line, set until the siphon stops. For dosing into open basins or low-pressure lines, fit one from day one.

Suction Lift, Gas and Dry Running

A solenoid metering pump is happiest mounted below the tank level, so liquid floods the head on its own. The manual recommends this flooded layout for gassing chemicals like sodium hypochlorite and hydrogen peroxide, and for thicker liquids. If the head has a degassing valve, open it half a turn on first start and close it once liquid appears.

Dry running is less scary than people think. The manual says it won't tear the diaphragm or burn out the pump, but frequent or long dry runs heat the unit and age the electronics faster. A level switch in the tank that stops the pump near empty saves a lot of grief. DP, DM and DC models all have a level switch input; among DFD pumps, only the NX, GX and X frames do.

A solenoid metering pump of this type is rated for 0–40 ℃ ambient. Keep the pump and tubing out of direct sun.

Match the Control Type to the Signal on Site

• No signal, set by hand: DFD manual control.
• A water meter or flowmeter sending pulses: DP, using division when pulses arrive faster than the pump's top rate and multiplication when slower.
• A pH or chlorine analyzer or PLC sending 4–20 mA: DM, with direct or reverse proportional control.
• A host system that needs remote start, stop and rate changes: DC over RS485.

Lock this down at the selection stage. Buy a manual unit and decide later you want signal control, and you usually end up buying another pump.

Bottom Line

A solenoid metering pump suits low-flow, low-to-medium pressure dosing. In the NEWDOSE range that means about 1–55 L/h and up to 20 bar, with pressure falling as flow rises. Size it in four steps: check flow and pressure as a pair, leave around 20% margin, pick a frame with enough suction lift for the mounting height, and choose the control type from the signal you have. Once you need tens of liters per hour at higher pressure, stop forcing a solenoid metering pump and move to a mechanical diaphragm pump.

FAQ

My calculation says 5 L/h. Is a 6 L/h pump enough?

It's right on the line. 5 L/h plus 20% margin is exactly 6 L/h, and if injection pressure is close to the pump's rating, real output drops further. We usually suggest going one size up.

Can a solenoid metering pump handle sodium hypochlorite?

Yes, it's one of the most common uses. The key is avoiding gas lock: mount the pump below the liquid level, choose a head with a degassing valve, and check the wetted materials against concentration and temperature.

The pump keeps clicking but nothing comes out. Where do I start?

Check whether suction lift is too high and whether the suction fittings leak air, then open the degassing valve. If that fails, pull the check valves and look for debris. Suspect the diaphragm last.

How often does it need servicing?

The manual says to check the head bolts and all seals at least every six months, more often with corrosive chemicals. Clean the check valves and foot filter regularly, at an interval that suits your chemical.

Can it connect straight to a PLC?

Yes. For a 4–20 mA signal choose DM; for communication choose DC. Before wiring a DC unit, ask the factory for the communication sheet for your batch and confirm the protocol and register addresses.

Send us your dose rate, injection pressure and mounting height through our contact page, and we'll match a solenoid metering pump to your duty.