Automatic Dosing System Sizing: Make-Up, Aging Time and Feed Capacity

Automatic dosing system sizing rests on five numbers: the water flow you treat, the dose in mg/L, the strength you mix the solution at, the make-up and aging time, and how many injection points the pumps must feed. Flow and dose give you the pure chemical demand. Solution strength turns that into a liquid flow. Time sets the tank size, and the liquid flow plus line pressure sets the pump. This guide is written for process engineers, buyers and plant operators specifying a packaged unit for PAC, polymer or hypochlorite.

The five inputs behind automatic dosing system sizing

Many enquiries arrive as one line: "we need an automatic dosing unit." The supplier then guesses, and the result is a tank that has to be refilled three times a shift or a pump that runs at a sliver of its stroke. Before anyone draws a skid, collect these:

1. Plant flow Q in m³/h. Give the maximum, the normal value and the night-time low, because the low end decides whether the pump can turn down far enough.
2. Dose D in mg/L of pure product, ideally from a jar test or from records at a similar plant.
3. Solution strength c, the weight percentage you will make up. Coagulants are often fed fairly strong; polymer is fed much more dilute because it gets viscous fast.
4. Make-up and aging time. Aging (also called maturing) is the period a polymer needs to uncoil and fully hydrate. PAM needs it; PAC mostly just needs mixing.
5. Feed capacity: number of injection points, the peak demand at each, and duty/standby arrangement.

A worked approach to automatic dosing system sizing

No invented figures here. Plug your own site values into these automatic dosing system sizing steps.

Step 1: pure chemical demand

G (kg/h) = Q (m³/h) × D (mg/L) ÷ 1000. One cubic metre dosed at 1 mg/L uses one gram of product.

Step 2: solution flow

q (L/h) ≈ G ÷ c ÷ ρ, with c as a decimal and ρ the solution density in kg/L. For dilute polymer you can take ρ close to 1. For concentrated liquid coagulant, use the supplier's density figure or the error creeps in.

Step 3: make-up tank and aging tank

Make-up tank volume V ≥ q × T, where T is how long one batch should last. A fully automatic unit can use a shorter T because level interlocks trigger a new batch. For polymer, size the aging tank separately: effective volume ≥ q × t, where t is the aging time from the product sheet or your trials. An undersized aging tank lets half-dissolved polymer reach the pump, and flocculation suffers.

Step 4: pump rated flow

The rated pump flow must exceed q at peak conditions, with a sensible margin so normal operation sits in the middle of the pump range rather than at either end.

How automatic dosing system sizing inputs pull against each other

Solution strength is the input people forget to question. A strong solution shrinks the tank and pump, but strong polymer becomes thick, check valves seat poorly and the suction line has to be upsized. A weak solution flows easily yet needs a bigger tank, and the dosing room may not have the floor space.

Do not size on a single dose value either. When raw water turbidity jumps, D can double. Base automatic dosing system sizing on the upper dose and turn the pump down for normal days.

Averages are the trap in automatic dosing system sizing. An average produces a package that looks right on paper. The maximum and minimum cases are what decide whether it runs smoothly.

Site details that automatic dosing system sizing sheets often miss

Cold water slows polymer hydration, so aging time in winter can be longer than in summer. If the aging tank is sized on summer data, you will see fish-eyes, the gel lumps of undissolved polymer, and they block check valves and injection lines.

Injection point pressure matters too. High line pressure means the pump pressure rating must cover it. Near-atmospheric injection points need a back pressure valve to stop siphoning. Where several points share one header, check the split, or the nearest point takes most of the flow.

Make-up water is the last one. Automatic make-up relies on a solenoid valve on the water inlet. Unstable water pressure means unstable solution strength, so put the supply pressure on the automatic dosing system sizing sheet.

From automatic dosing system sizing to equipment

With the inputs settled, matching tank, mixer and pump becomes straightforward; see matching tank, mixer and metering pump in a dosing system. For polymer units, our note on three-tank PAM units vs batch make-down capacity covers throughput, and converting a jar test polymer dose to pump flow goes deeper on the arithmetic. Packaged options are listed under dosing skids, and pump choices under mechanical diaphragm metering pumps.

A practical order: confirm the chemical and whether it needs aging, calculate G and q from the upper Q and D, size tanks from the batch interval, then select the pump from q and injection pressure. Picking the pump first and fitting the tanks around it usually means rework.

Conclusion

Good automatic dosing system sizing links five inputs instead of choosing equipment one item at a time. Chemical demand drives solution flow, solution flow drives tanks and pumps, and aging time decides whether polymer is actually ready when it is fed. Gather maximum and minimum Q and D, the chemical type, the number of injection points and their pressures, then contact our engineers and we will check the automatic dosing system sizing calculation against your duty.

FAQ

Can I work back from how many bags of chemical we use per day?

Yes. Divide daily consumption by operating hours to get G, then convert with your solution strength. That only gives an average, so add data from your heaviest dosing days.

Does a PAC unit need an aging tank?

Usually not. PAC dissolves quickly and only needs mixing. Aging tanks are for high molecular weight polymers.

Can solution strength be changed later?

It can, but it changes the pump duty. A weaker solution means more litres per hour, so check the pump against the weakest strength you might run.

With duty and standby pumps, do I size each pump for the full load?

Yes. Each pump must carry the full peak demand of its injection point on its own. The standby only takes over; it does not share the load.

Is automatic dosing system sizing different for semi-automatic units?

The base calculation is the same. A fully automatic unit adds level interlocks, automatic water inlet and powder feeder control, which affect the control panel and instruments.