How to Select a Dosing System: Matching Tank, Mixer and Metering Pump

A lot of inquiries we get read like this: "Need a dosing system for PAC, please quote." That is not enough to work with. Strip any dosing system down and you have three main parts, a tank, a mixer and a metering pump, plus piping accessories and a control panel. The three parts depend on each other. In a dosing system, pump flow sets the tank size, and the tank size sets which mixer you need. Do it in the wrong order and you end up with a tank that runs dry by lunchtime, or a mixer that can't turn over a full batch.

Start the Dosing System With the Metering Pump

We always size the pump first. A metering pump delivers made-up solution, not neat chemical, so work out product demand per hour and then convert it into litres of solution at your chosen dilution.

Here is an illustration only, not a design figure. A plant treating 100 m³/h at a dose of 20 mg/L needs 2 kg of product per hour. Made up at 10%, that is roughly 20 L/h of solution. The NEWDOSE solenoid pump manual recommends about 20% operating margin, which takes you to around 24 L/h. If the injection point is close to atmospheric, a DFD-30-03-X (30 L/h, 3 bar) covers it. Injecting into a pressurized main, you would look at a mechanical pump such as the NDWS-33/1.0 (33 L/h, 10 bar).

Once the pump is fixed, your hourly solution demand is fixed, and every other part of the dosing system follows from that number.

Size the Tank From the Refill Interval

Bigger is not automatically better. A tank that is too small means operators are mixing chemical several times a shift. One that is too large leaves solution sitting for days, and some products, polymer in particular, lose strength as they age.

The quick method is solution demand × refill interval. With the 20 L/h example and a once-a-day batch, 20 × 24 gives about 480 L, so a 500 L tank fits. If the site runs two shifts and mixes once per shift, something around 250 L is enough. Dosing system tanks are usually PE, in common sizes such as 200 L, 500 L and 1000 L.

Leave room at the bottom for sediment. Dissolved powder products often leave residue, so keep the suction inlet off the tank floor and fit a foot valve strainer.

Match the Mixer to Tank Volume and Chemical

In any dosing system, mixer size follows the tank. Taking the NEWDOSE NDM range as an example, it covers tank volumes from 100 L to 2000 L. A 100 L tank takes a 150 mm impeller, 500 L takes 280 mm, and 1000 L and up use 330 mm, with shaft length increasing accordingly. Motor power is 40 W, with output speeds of 200, 150, 100 or 75 rpm.

Speed depends on the chemical:

• Easy-dissolving products like PAC or caustic can take a faster speed;
• Polymer (PAM) should be mixed slowly, because high shear breaks the long molecular chains and hurts performance;
• For aggressive chemicals, change the wetted materials. NDM comes standard with a PP impeller and PVC-coated cast iron shaft, with PVDF or SS304/316 impellers and PTFE-coated stainless shafts as options.

If you are only diluting a liquid product, you can sometimes skip the mixer and stir by hand at batching time.

Accessories on a Chemical Dosing Skid

A spec sheet for a chemical dosing skid usually lists a long row of accessories after the tank and pumps. Each one is there for a reason:

1. Safety valve: protects a mechanical pump and the line if a discharge valve is closed by mistake;
2. Back pressure valve: needed when the pressure difference across the pump is below 1 bar, otherwise you get siphoning and the flow wanders;
3. Pulsation dampener: calms the line on long runs and higher flows;
4. Calibration column: lets operators check actual pump output at any time;
5. Y-strainer and level switch: one keeps debris out of the check valves, the other alarms or stops the pump before the tank runs dry.

Duty and standby, or a single pump?

On a dosing system for a plant running around the clock, losing chemical feed shows up in water quality quickly. Use duty and standby pumps and rotate them. Where short interruptions are acceptable, a single pump with a spare diaphragm and valve kit on the shelf can do.

Small Flows: Integrated Unit. Larger Flows: Skid.

At small flows, think boiler makeup, cooling towers or a small wastewater plant, an integrated dosing system is often the neatest answer. The NEWDOSE TANK range includes the TANK-60-1-M (60 L tank, one solenoid pump) and the TANK-120-2-M (120 L, one or two pumps). Tank, pump mounting and a multi-function valve for calibration and draining come as one unit, with optional mixer, level sensor and controller. Connect water and power and it is ready.

As flow increases, you move to mechanical pumps, NDJ, NDWS, NDWL, or ND1000 and ND2000 for big volumes, on a skid with a larger tank, panel, piping and valves assembled at the factory. Plant flow is usually what decides which route your dosing system takes.

Decide the Control Method Early

With steady inflow, manual setting is fine. Where inflow swings, the dose should follow it. On the solenoid side, the DP accepts pulse signals and the DM accepts 4 to 20 mA. Mechanical pumps can take an electric stroke actuator driven by 4 to 20 mA. Settle this at the start, because changing it after the dosing system is built usually means touching both the panel and the pumps.

Conclusion

Work in one order. Calculate solution flow from dose and dilution, pick the metering pump with around 20% margin, size the tank from solution demand times refill interval, then match the mixer impeller, speed and materials to tank volume and chemical. Don't drop the safety valve, back pressure valve, calibration column, strainer or level switch to save a line on the spec sheet. Small flows suit an integrated unit, larger flows suit a skid, and the control method should be agreed before anything is built.

FAQ

Should the dosing system tank be PE or stainless steel?

For most water treatment chemicals a PE tank is fine: it resists corrosion and is light. Consider other materials only when the chemical doesn't suit PE or the site has fire or temperature requirements, and confirm with the supplier.

Can one dosing system tank feed two pumps?

Yes. Duty and standby pumps normally share a tank. Just don't run two pumps at once into two points at different pressures from one line, because the low-pressure point takes most of the chemical.

Does the mixer need to run continuously?

Not always. Once an easy-dissolving product is mixed it can stop. Products that settle or separate are better with intermittent mixing.

What does "2P" mean in a dosing system model code?

Many suppliers use it to show the number of pumps, so 2P means two. Coding isn't standardized, so ask the supplier rather than guess.

I already have pumps. Can I buy just the tank and mixer?

Yes. Send us the pump model, flow and pressure, and we will size the tank and mixer around them and check for missing accessories.

Send your plant flow, chemical, dose and injection pressure, and contact us to have the whole dosing system specified in one go.