PAC Dosing System: How It Works From Make-Up Tank to Injection Point

A PAC dosing system takes polyaluminium chloride, makes it into a solution of known strength and feeds it into the water at a controlled rate. A typical unit includes a make-up tank with a mixer, a day tank, duty and standby metering pumps, discharge accessories and a control panel. The working principle is three steps: dissolve solid PAC or dilute liquid PAC, hold the solution in a day tank, and let the metering pump inject it at the coagulation point in proportion to flow. Unlike a polymer unit, PAC dissolves quickly and needs no ageing, so the process is much shorter.

This guide is for operators new to a chemical room and for engineers and buyers putting a specification together.

What is inside a PAC dosing system

Following the liquid, a common PAC dosing system is laid out like this:

1. Make-up tank and mixer: solid PAC is added to water and stirred until dissolved.
2. Day tank: holds the prepared solution, with a level gauge or level switches.
3. Metering pumps: usually one duty and one standby, drawing from the day tank.
4. Discharge accessories: pressure relief valve, pulsation dampener, pressure gauge, back pressure valve.
5. Control panel: manual start and stop, or automatic control from a flow or turbidity signal.

Small systems often combine make-up and storage in one tank that lasts a day or two. Larger plants use two tanks in rotation, one being made up while the other feeds, so dosing never stops.

Solid vs liquid PAC: the make-up stage

Solid PAC must be dissolved; liquid PAC usually only needs dilution. That is the biggest difference between PAC dosing system designs.

Solid PAC comes as powder or granules. Operators add a weighed amount to the make-up tank with the mixer running. Sprinkle it in gradually; dumping a whole bag at once leaves lumps on the bottom. PAC solution always carries some insoluble matter, so it is better to let it settle before transferring to the day tank, and to keep the pump suction a little above the tank floor.

Liquid PAC is usually delivered by tanker into a bulk tank, then transferred by a small pump or gravity into a dilution tank and diluted to the working strength. Large water plants tend to go this way because it removes manual bag handling.

Solution strength affects pump size and reliability. Too strong and crystals form in valves; too weak and you need bigger pumps and tanks.

Storage and metered injection: the core of a PAC dosing system

In a PAC dosing system the day tank feeds the pump. Fit it with a level gauge or high and low level switches: low level raises an alarm, low-low stops the pump so it never runs dry. Tanks and pipework are normally PE or UPVC; see PAC dosing line material.

The metering pump is the heart of the PAC dosing system. Each stroke displaces a fixed volume, and adjusting stroke length or frequency changes output, so the dose is stable and adjustable. PAC is corrosive, so head and valve materials must suit it, and the pump should be sized for maximum dose plus margin, with normal duty mid-range. For details, see PAC dosing pump selection. Solenoid pumps cover small flows; larger or higher-pressure duties use mechanical diaphragm metering pumps.

Discharge accessories: where each one goes

A metering pump is a positive displacement pump, so on any PAC dosing system the discharge accessories decide both accuracy and safety:

• Pressure relief valve: close to the pump outlet, with no valve between pump and relief valve. Set it roughly 0.1 MPa above system pressure so a blocked line relieves back to the tank. Mechanical diaphragm pumps must have one.
• Pulsation dampener: absorbs pump pulsation, steadies the gauge reading and reduces pipe vibration.
• Back pressure valve: as close to the injection point as possible. If the injection point is at atmospheric or negative pressure, the chemical can siphon through without it and accuracy is lost.
• Pressure gauge: between the pump and the back pressure valve, for setting back pressure and spotting blockages.
• Suction Y-strainer: catches insoluble matter; clean it on a schedule.

Control options: manual, flow-paced and feedback

The simplest PAC dosing system runs manually: an operator sets stroke and frequency and the pump runs at that rate. That suits small plants with steady flow.

Where flow varies, the PAC dosing system pump is commonly paced by a 4–20 mA flow signal: more water, more chemical. A further step trims dose from settled or filtered turbidity. The more automatic the control, the more important pump calibration becomes, because an accurate signal cannot fix a pump that delivers the wrong volume.

Common PAC dosing system mistakes on site

The principle is simple; problems come from details:

• Pump suction sitting on the tank floor, drawing in sediment that jams valve balls.
• Leaving the system idle for days without flushing, so PAC crystallises in the pump head and lines. After a long stop, flush heads and lines with clean water.
• Low injection pressure with no back pressure valve, so the dose siphons through at night when flow is low.

For fixes, see PAC dosing pump not delivering.

Safety: stop the pump and relieve pressure before opening a pump head or strainer. Wear gloves and eye protection, and flush any splash on skin or eyes with plenty of clean water.

Conclusion

A PAC dosing system works in three stages: make-up, storage and metered injection. Solid PAC is dissolved or liquid PAC is diluted, the solution waits in a day tank, and a metering pump injects it at a set rate or in proportion to flow, protected by a relief valve, kept accurate by a back pressure valve and smoothed by a dampener. Fix plant flow, dose, solution strength and injection pressure first, and the pumps and accessories follow. Complete units are listed under dosing skids; send us those figures and contact our engineers for a configuration check.

FAQ

Does a PAC dosing system need a mixer?

Yes if you use solid PAC; the make-up tank needs one. For liquid PAC dilution a smaller mixer is enough, but some mixing keeps concentration even.

Is one pump enough, or do I need a standby?

For continuous duty, fit one duty and one standby pump so dosing continues during maintenance.

How big should the PAC dosing system day tank be?

Size it for how long one batch should last; a day or two is common. Much larger and the solution sits long enough to settle.

Can I fit only a relief valve or only a back pressure valve?

No. The relief valve protects against overpressure; the back pressure valve prevents siphoning and steadies accuracy. They do different jobs.

Can a PAC dosing system be installed outdoors?

Yes, with sun and frost protection. UV ages tanks and pipes, and cold weather thickens the solution, so insulate exposed lines.