PAC Dosing System Specifications: How to Size the Tank, Mixer and Pump

Good PAC dosing system specifications come down to three linked numbers: how big the solution tank is, what mixer drives it, and what flow and pressure the metering pump must deliver. None of these should be picked on its own. They all fall out of plant flow, coagulant dose and solution strength, and if you write them in that order, every supplier quotes the same machine. This guide is for consulting engineers drafting a tender, buyers preparing an RFQ, and plant engineers checking a vendor datasheet. We skip how the system works and focus on what belongs in PAC dosing system specifications.

What data do PAC dosing system specifications start from?

A supplier who receives "please quote one PAC dosing unit" has to guess, and guesses are where price gaps and disputes begin. Put these five inputs at the top of your PAC dosing system specifications:

1. Plant flow Q in m³/h, both normal and peak.
2. PAC dose D in mg/L, ideally from a jar test, otherwise a design range.
3. Solution strength C, the mass fraction of product in the day tank. For powder PAC, follow the chemical supplier's make-up recommendation; for liquid PAC, state the delivered strength and whether it is diluted again.
4. Batches per day, which sets how many hours one tank must last.
5. Number of injection points and the line pressure at each one.

If you still need to turn a jar test result into a feed rate, our guide on PAC dosage calculation from jar test to pump flow walks through it.

Sizing the solution tank in your PAC dosing system specifications

Pure product demand is Q × D ÷ 1000 in kg/h. Divide by C (as a decimal) to get solution mass. PAC solution is close enough to water density that kilograms can be read as liters, so solution flow q ≈ Q × D ÷ (1000 × C) in L/h.

Working volume is V = q × T, where T is the number of hours one batch must cover. One batch per day means T equals daily run hours; one batch per shift cuts it accordingly. The vessel you buy must be larger than V, because there is dead volume below the outlet and freeboard above the liquid for mixing turbulence and the overflow.

Specify two tanks, one mixing while the other feeds. With a single tank, dosing stops during make-up and settled water turbidity swings. For material, PE or PP is the usual choice. PAC solution carries chloride, so unlined carbon steel is out, and standard stainless grades can pit over time. A line in your PAC dosing system specifications saying "unlined carbon steel tanks not accepted" saves an argument later.

Mixer power, speed and shaft details

Many PAC dosing system specifications say "mixer: 1 set" and nothing more. Then the impeller arrives too short to reach the tank floor, and powder PAC settles into a crust.

Spell out:

• Motor power and output speed after the gearbox, matched to tank volume by the vendor
• Impeller type and diameter, enough to lift solids off the bottom
• Wetted material for shaft and impeller, confirmed by the vendor as compatible with PAC
• Motor enclosure rating, because dosing rooms are damp and terminal boxes take on water
• Mounting: top-entry on the tank lid or a separate bridge

PAC dissolves quickly, so it does not need the slow, long ageing that polymer make-up demands. Whether it needs continuous stirring is covered in our article on mixer selection for PAC dosing tanks.

PAC metering pump sizing in PAC dosing system specifications

Pump flow comes straight from q. Calculate q at peak flow and peak dose, add a margin, then choose a pump whose rated flow sits one size above that figure. Normal operation should land in the middle of the pump's range, so stroke length is not parked at a low setting where accuracy suffers.

Pressure depends on the injection point. Feeding into a pressurized pipe or static mixer, discharge pressure must exceed line pressure plus friction loss. Feeding into an open flash-mix basin, pressure is low, but siphoning becomes the risk, so fit a back pressure valve.

Typical wetted parts for PAC are PVC or PVDF heads, PTFE-faced diaphragms and ceramic or glass balls. PAC dosing system specifications should also state the duty/standby arrangement per injection point. You can compare pump types on our metering pump product range.

Accessories and controls belong in PAC dosing system specifications too

Two quotes for "the same skid" can differ widely because one includes accessories and the other does not. List them: Y-strainer, back pressure valve, pressure relief valve, pulsation dampener, calibration column, pressure gauge, level gauge or level switches, fill-water solenoid valve, isolation valves, plus pipe material and size.

For control, state whether the pumps run on manual start/stop, level interlock, or a 4-20 mA flow-paced signal, where the control panel sits, and whether status goes to SCADA. Packaged layouts are shown on our dosing skid product page, and the parts list in PAC dosing system components makes a handy checklist.

Two field problems that specs often miss

Cold weather slows dissolution. In an unheated building, allow longer batch times and do not undersize the mixer. Crystallization is the other one: concentrated liquid PAC in long lines, left idle, plugs discharge tubing and check valves. Add a flush water connection and valve to the skid scope in your PAC dosing system specifications.

Conclusion

Write PAC dosing system specifications in this order: process data, tank volume from q × T, mixer to suit the tank, pump flow from peak q plus margin, then accessories and controls, with every wetted material chosen for chloride service. Do that and vendor bids become directly comparable. Send us your flow, dose and injection pressure and contact our engineers to have each line of the datasheet checked.

FAQ

We have no jar test yet. What dose should go on the sheet?

Give a design range. Ask the vendor to size the pump on the upper value and the tank on the middle value, then trim settings after commissioning.

How do specs differ for powder and liquid PAC?

Powder needs a make-up tank, mixer, solution strength and a loading hatch. Liquid product may skip mixing but needs bulk storage volume, a transfer pump and a note on inline dilution.

Can I round tank volume up to one day of use?

Yes. Good PAC dosing system specifications simply add dead volume and freeboard on top of the working volume. Otherwise a full tank will not quite last the day.

Can one pump serve two injection points?

Avoid it. The two points rarely see the same pressure, so most chemical goes to the lower-pressure side. One pump per point is easier to control.

Should PAC dosing system specifications name a pump brand?

If you name one, add "or approved equal" and fix flow, pressure and wetted materials so a substitute cannot slip in undersized.