Solenoid Metering Pump Selection: Flow Range, Back Pressure and Control Signal

Solenoid metering pump selection comes down to three groups of numbers: the flow and pressure duty point printed on the data sheet, the real back pressure and suction lift at site, and the control signal the pump has to accept. Get those three right and the model code more or less writes itself. It is written for engineers laying out a feed point and buyers preparing an inquiry, and covers small pumps where a solenoid drives a PTFE diaphragm and stroke rate sets flow.

Solenoid metering pump selection starts with the flow column

Flow is usually the first column, in L/h or GPH. Take the NEWDOSE DFD series as an example. DFD-02-16 is rated 2 L/h at 16 bar, DFD-12-07 is 12 L/h at 7 bar, and DFD-50-02 is 50 L/h at 2 bar. The first number is capacity, the second is pressure.

The catch is the footnote. NEWDOSE states that the table shows maximum flow at maximum pressure, measured on clean water, that real flow may deviate by up to 5%, and that you should keep about 20% operating margin. If your calculation says 8 L/h, step up one size. That margin is the first rule of solenoid metering pump selection.

The bottom of the range matters in solenoid metering pump selection too

Flow is adjusted by stroke rate, meaning the number of strokes per minute. The DFD panel adjusts from 0 to 100%. Run a pump too slowly and it delivers a few shots a minute instead of a steady feed. For a small, continuous dose, a smaller pump running mid-range beats a big pump turned right down. Pumps such as the LMI P+, with both stroke length (30 to 100%) and stroke rate (1 to 240 SPM) adjustment, give finer control at the low end.

Pressure rating and back pressure in solenoid metering pump selection

On a solenoid pump, flow and pressure trade against each other. Within the LMI P+ range, output code 1 is 0.9 L/h at 17.2 bar while output code 9 is 15.1 L/h at only 1.0 bar. The magnet has a fixed amount of force, so a higher pressure rating means a smaller stroke volume.

That is why the pressure column must be compared with real back pressure, not just the pressure at the injection point. Back pressure is the total the pump must push against: process pressure, line friction and the opening pressure of the injection valve. An open tank discharge can still mean real pressure if the line is long and climbs a floor; in solenoid metering pump selection that hidden line loss is the usual miss.

Too little back pressure is also a problem. If the chemical tank level sits above the injection point, liquid can siphon through the pump even when it is stopped, and the pump will overfeed when it runs. NEWDOSE recommends a back pressure valve on the discharge, adjusted until siphoning stops. The Iwaki EHN catalog likewise says discharge pressure should be at least 0.12 MPa to prevent overfeeding. See why a metering pump needs a back pressure valve for the details.

Suction lift and mounting height for solenoid metering pump selection

Suction lift decides whether the pump will prime at all. The NEWDOSE DFD, DP, DM and DC series are rated for 2 m suction lift; above that height difference between the pump head and liquid level, the pump will not work properly. LMI B and D series data sheets quote 1.5 m. These figures are for water. Sodium hypochlorite and hydrogen peroxide release gas, and viscous chemicals pull harder, so expect less in practice.

The safer layout is flooded suction, with the pump mounted below the tank level so liquid flows into the head by gravity. If the pump must sit above the tank, keep the suction line short with few bends, and keep the foot valve clear of sludge at the bottom of the tank.

Choosing the control signal during solenoid metering pump selection

The NEWDOSE solenoid range is split by control mode, and the series name tells you which one you have:

1. DFD: manual stroke rate set on the panel, for duties where flow barely changes.
2. DP: external pulse input, typically from a pulse water meter for flow-proportional dosing; pulse voltage must not exceed 24 V.
3. DM: 0/4-20mA analog input from a PLC or analyzer, with direct or reverse proportional control.
4. DC: RS485 input for running several pumps from one touch screen. It uses a proprietary protocol, not Modbus or Profibus, so check before you plan a DCS connection.

All four also have a dry-contact remote start/stop input. Add a low-level switch on the tank as well; a pump that runs dry wears its diaphragm and valves. Our guide to 4-20mA vs pulse control for dosing pumps covers the wiring side.

A common site problem: the analyzer outputs 4-20mA but the pump was ordered with pulse input, and nobody notices until commissioning. State the controller output on the inquiry and this solenoid metering pump selection mistake disappears.

Materials, power and enclosure

NEWDOSE material codes PPV, PVT, PVDF, SST and PTFE map to PP, PVC, PVDF, SS316 and PTFE pump heads, all with PTFE diaphragms. Standard power is 230 V 50 Hz with a 115 V option, the enclosure is IP55, and ambient temperature should stay between 0 and 40 ℃. Isolate power before wiring or opening the pump head.

A simple order for solenoid metering pump selection

Run through these steps:

1. Take the maximum dose rate and add roughly 20% margin.
2. Add up injection pressure and line losses; the rated pressure must be higher. If back pressure is too low, add a back pressure valve.
3. Check mounting height against suction lift; switch to flooded suction if needed.
4. Pick the signal: manual for steady flow, pulse for flow pacing, 4-20mA for analyzer control, RS485 for multi-pump panels.
5. Choose pump head material, then power supply.

If the duty exceeds solenoid pump flow or pressure, look at motor-driven mechanical diaphragm pumps instead. Browse our solenoid dosing pump range, compare the NEWDOSE DFD, DP, DM and DC series, or read more about the NEWDOSE brand.

Conclusion

Good solenoid metering pump selection means reading the data sheet against site conditions: flow with margin, pressure against real back pressure, suction lift discounted for the chemical, and the signal matched to your controller. Send us your maximum dose, line pressure, mounting height, chemical and control method, and contact our engineers to confirm the model.

FAQ

What does "07" mean in a model code during solenoid metering pump selection?

In NEWDOSE codes it is pressure in bar, so DFD-12-07 means 12 L/h at 7 bar. Other brands use different codes, so check the model key in their catalog.

Will flow be higher if back pressure is far below the rating?

Usually yes, and the pump may siphon. Calibrate in place and fit a back pressure valve when discharge pressure is very low.

Can I run a solenoid dosing pump on a VFD?

No. There is no motor; the pump strokes electrically. For remote adjustment, choose a pulse, 4-20mA or RS485 model.

Can one pump switch between manual and automatic?

Yes. NEWDOSE DP and DM pumps switch between manual and external control, but the manual says to change mode only while the pump is stopped.

What matters most in solenoid metering pump selection for sodium hypochlorite?

Suction lift and gas venting. Hypochlorite gasses off, so use flooded suction and a pump head with a bleed valve.