PAC Dosing Pump Selection: Head Material, Flow Rate and Dilution Ratio

Polyaluminium chloride (PAC) is one of the most widely used coagulants in water treatment, and almost every wastewater plant or waterworks runs at least one PAC dosing pump. Plenty of buyers treat selection as "find a pump with roughly the right flow". Then the dilution changes and the pump can't keep up, or six months in the valve seats are crusted with deposits and won't close. Here is the order we work through with customers: head material, flow, and dilution ratio.

Start With the Liquid End Material

PAC solution is acidic and contains chloride. NEWDOSE solenoid pumps offer five liquid end sets: PPV, PVT, PVDF, SST and PTFE. For a PAC dosing pump the choice is normally between PVC and PVDF. Stainless steel SS316 heads are generally not recommended for chloride-bearing chemicals, and it's worth stating that on your inquiry.

The sets break down like this. PVT is a PVC head with PTFE diaphragm and valve seats, ceramic balls and FKM seals. PVDF swaps only the head for PVDF and keeps the rest. PVC is the usual pick for normal PAC strengths; if the solution runs warm or you aren't sure of the concentration, stepping up to PVDF is the safer call.

Don't look at the head alone. Seats, balls and seals sit in the chemical too. If your PAC is blended with other products, or it's an industrial grade with more impurities, send the product name, concentration and temperature to the manufacturer for a compatibility check rather than guessing.

On mechanical pumps, check the series. NDJ, NDWS and NDWL come with PVC, PVDF, PTFE or SS316 heads. For the ND1000 and ND2000 the documentation lists only PVC, PTFE and SS316, with no PVDF, so a large PAC dosing pump from those ranges will normally have a PVC head.

Working Out Flow: Don't Size on Neat Chemical

A metering pump moves made-up solution, not pure product. Sizing a PAC dosing pump takes two steps: work out how much product you need per hour, then convert that into solution volume at your dilution.

A worked example

Suppose the plant treats 100 m³/h and jar testing gives a dose of 20 mg/L. That figure is only for illustration; your own water decides the real number.

1. Product per hour: 100 m³/h × 20 g/m³ = 2000 g/h, or 2 kg/h;
2. Made up as a 10% solution: 2 kg/h ÷ 10% = 20 kg/h. The solution is slightly denser than water, so call it roughly 20 L/h;
3. Add margin: the NEWDOSE solenoid manual recommends about 20% operating margin, so 20 L/h × 1.2 = 24 L/h.

If the injection point is at low pressure, a DFD-30-03-X (30 L/h, 3 bar) covers it. If you are injecting into a pressurized main, look at a mechanical PAC dosing pump such as the NDWS-33/1.0 (33 L/h, 10 bar).

Change the Dilution and the Pump Changes Too

Take the same example. If operators prefer a 5% solution, the volume doubles to about 40 L/h, or 48 L/h with margin. The only solenoid model that reaches 50 L/h is the DFD-50-02-X, and that's at just 2 bar, leaving almost no headroom. At that point it's time for a mechanical pump, for example the NDJS-60/0.5 (60 L/h, 5 bar) or NDWS-68/1.0 (68 L/h, 10 bar).

Go the other way and mix it stronger, and the flow drops so the PAC dosing pump gets smaller. But concentrated PAC crystallizes more readily in check valves and at the injection valve tip, and after a stop the pump may not deliver. Agree the dilution ratio with the operators during selection and then leave it alone. If it does change, recalculate flow and recalibrate the PAC dosing pump.

Piping and Accessories for a PAC Dosing Pump

PAC often goes into a flash mix tank or a low-pressure line, so the pump sees almost no back pressure. The NEWDOSE mechanical pump manual is clear: when the pressure difference across the pump is under 1 bar, fit a back pressure valve to create discharge pressure. Without one you get siphoning and the flow wanders.

A few other common traps:

• Dissolved solid PAC usually leaves insoluble residue in the tank bottom, so fit a strainer on the suction side and clean the foot valve screen regularly;
• Keep the suction line short and mount the pump below the tank level where possible; on mechanical pumps keep suction lift within 2 m;
• Don't let one PAC dosing pump feed several points, because the lowest-pressure point takes most of the chemical;
• Put a safety valve on the outlet of a mechanical pump so a valve closed by mistake can't damage the pump or piping.

If the make-up tank needs mixing, NEWDOSE NDM agitators are available for tank sizes from 100 L to 2000 L.

Control: Match It to How Steady the Inflow Is

For a small station with steady inflow, manual setting is enough, either a DFD or a mechanical pump on manual stroke. Where inflow swings, the dose should follow it. With a pulse-output flow meter, use a DP; with a 4 to 20 mA signal, use a DM; on a mechanical PAC dosing pump, add the electric stroke actuator, which takes 4 to 20 mA and changes stroke length in real time.

When inflow varies a lot, don't let the pump spend its life at the bottom of its range. A solenoid pump reduces output by slowing down, and at very low stroke rates each pulse is distinct, so the chemical mixes unevenly. In that case dilute the solution a bit more and keep the pump in mid range.

Conclusion

Three things decide a PAC dosing pump: a PVC or PVDF head with stainless avoided, a flow figure based on product demand converted to solution volume plus about 20% margin, and a dilution ratio that you fix and stick to, since it decides between solenoid and mechanical. Add a back pressure valve, strainer and safety valve, and the whole PAC dosing pump setup is usually stable.

FAQ

Can liquid PAC be dosed neat, without dilution?

Yes, but the flow becomes very small and neat product crystallizes more easily at the valves. If you dose neat, clean the check valves and injection valve more often.

How often should the check valves on a PAC dosing pump be cleaned?

There is no fixed interval; it depends on the product and the water. Open them up periodically during the first weeks of service, see how fast deposits build, and set your own schedule.

The pump is running but coagulation is up and down. Is the pump at fault?

Not necessarily. Check that the tank concentration is consistent and that there is a back pressure valve, then calibrate flow with a measuring cylinder under real conditions. Once those match, adjust the dose.

Do I need a standby PAC dosing pump?

If PAC stops, effluent quality drops fast. For plants running around the clock we recommend duty and standby, rotating both pumps regularly.

Why does my PAC dosing pump lose flow in winter?

Cold solution gets thicker and suction resistance rises. This matters most in unheated dosing rooms in cold regions. Insulate the tank and lines and recalibrate if needed.

Send us your plant flow, dose and injection pressure, and contact us to get the pump and accessories specified together.