PAC Dosing System Operating Procedure: Startup, Running and Shutdown

Many water and wastewater plants hang a laminated instruction sheet on the dosing room wall, yet the text is so generic that a new operator still has to guess which valve to open first. A practical PAC dosing system operating procedure should read like a cockpit checklist: what to inspect before energizing the panel, how to dissolve solid polyaluminum chloride (PAC) without stalling the mixer, how to prime and calibrate the metering pump, what numbers to log on rounds, and how to flush the wet end before a shutdown. Shift supervisors can post this PAC dosing system operating procedure directly beside the skid for daily handovers. For jar-test dosing ratios and floc evaluation, pair this guide with how to dose PAC step by step.

1. Pre-start checklist in a PAC dosing system operating procedure

Before pressing any start button under the PAC dosing system operating procedure, walk around the skid frame and verify five mechanical and electrical conditions:

1. Tank drain valves and liquid level: confirm that bottom drain valves on both the dissolving tank and storage tank are closed, and that the storage tank holds enough solution to sit well above the low-level switch.
2. Suction and discharge valve line-up: verify that the storage tank outlet valve, Y-strainer isolation valves, duty metering pump suction and discharge ball valves, and point-of-injection quill valve are fully open. Confirm that standby pump valves and the calibration column fill valve are in their normal standby positions.
3. Gearbox oil and mechanical guards: on motor-driven mechanical diaphragm pumps and geared agitators, check the oil sight glass level and confirm coupling guards are secure.
4. Control panel status: check that incoming power is normal, no fault indicators are latched, and the Hand/Off/Auto selector switch matches your intended startup mode.
5. Personal protective equipment (PPE): put on chemical goggles, acid-resistant gloves and a dust mask before handling dry PAC bags or opening wet-end unions.

2. Batch make-up: always start water and mixer before adding PAC

The fastest way to trip a mixer overload is dumping two bags of dry PAC into an empty tank and turning on the water afterward; the powder cements into a sticky slab on the tank floor around the impeller. Follow this sequence in the PAC dosing system operating procedure:

1. Open the make-up water valve (or trigger the auto-fill sequence) and fill the dissolving tank until water covers the lower mixer impeller blades, then start the mixer.
2. While water continues filling and the mixer runs, pour dry PAC slowly and steadily through the charging hopper. Break up any damp lumps beforehand, and keep bag ties and plastic strings out of the tank.
3. Close the water inlet when the level reaches the target batch mark, then keep the mixer running for 15 to 30 minutes until the liquid is uniform and free of undissolved granules.
4. On two-tank systems, open the transfer valve (or transfer pump) to move the finished batch into the storage tank once mixing is complete.

3. Metering pump startup and priming in the PAC dosing system operating procedure

Once the storage tank is ready, the PAC dosing system operating procedure brings the duty metering pump online step by step:

1. First start or post-maintenance start: set the stroke length knob low (around 20%–30%), open the pump head bleed valve (air purge valve) slightly, and start the pump in Manual mode. Once a steady, bubble-free stream of PAC returns through the clear bleed tubing, close the bleed valve gently.
2. Routine daily start: confirm the discharge line is open, start the pump, and smoothly bring stroke length or frequency up to the target setting.
3. Calibration column check: isolate the tank outlet valve briefly and time a 60-second drawdown in the calibration column to verify real L/h flow before switching the panel to Auto mode. Detailed pump-side checks are covered in our guide on how to write a dosing pump SOP.

4. Shift inspection points during continuous operation

During normal running, the PAC dosing system operating procedure requires an operator walk-through every 2 to 4 hours to log four operating signs:

Discharge pressure gauge. The needle should pulse gently near the back pressure valve setpoint. A pressure spike accompanied by flow returning through the relief valve means the discharge pipe or injection quill is scaling up; a drop to zero points to an empty tank, a plugged Y-strainer, or a fouled check valve ball.

Storage level vs pump rate. Check that tank level is dropping at a rate consistent with pump output, and prepare a fresh batch when the low-level alarm sounds so the pump never runs dry.

Leaks and temperature. Inspect pump head bolts, union nuts and the skid containment pan for yellow-brown drips, and touch motor and gearbox housings to check for abnormal heat or vibration. Logging these checks keeps the PAC dosing system operating procedure auditable across shifts.

5. Normal shutdown and clean-water flushing procedure

Walking away after pressing Stop leaves concentrated PAC sitting inside the check valves; as water evaporates around the stems, hard crystals lock the valve balls. A sound PAC dosing system operating procedure distinguishes between brief pauses and extended stops:

1. Short pause (restarting within a few hours): switch the panel to Manual, stop the metering pump and mixer, and keep tank lids closed.
2. Extended shutdown or duty/standby changeover (over 24 hours): before stopping the pump, close the storage tank outlet valve and open the clean-water flushing valve on the suction manifold. Run the pump on clean water for 5 to 10 minutes to purge all PAC solution from the pump chamber, check valves, back pressure valve and injection line, then stop the pump and close the valves. In unheated rooms where winter temperatures drop below 0 °C, drain the pump head and piping after flushing. If crystals have already fouled a valve, follow the steps in PAC dosing pump crystallization troubleshooting.

Conclusion

A reliable PAC dosing system operating procedure comes down to four non-negotiable habits: verifying open discharge valves and closed drains before power-up, covering the mixer blades with water before charging dry PAC, priming air and checking the calibration column at startup, and flushing the pump head with clean water before any shutdown longer than a day. Standardizing this PAC dosing system operating procedure across shifts cuts check-valve blockages and keeps effluent turbidity steady. Browse complete coagulant skids in our dosing skid catalog and replacement pumps in our metering pump range, or contact our engineers to configure a package for your plant.

FAQ

Can we start the metering pump first and open the discharge valve afterward?

Never start a positive-displacement metering pump against a closed discharge valve. Dead-heading builds pressure on the very first stroke; even with a relief valve installed, repeated pressure spikes stress the diaphragm and PVC unions.

What should we do if a woven bag string falls into the PAC tank?

Fish it out immediately before transferring solution. Polypropylene fibers pass through coarse strainers and wrap around check valve balls and seats, preventing the valves from sealing and stopping pump delivery.

What safety steps are required before cleaning the suction Y-strainer?

Under any PAC dosing system operating procedure, stop the metering pump first, close the isolation valves upstream and downstream of the strainer, place a catch tray underneath, and wear chemical goggles and gloves before cracking the strainer cap loose to release trapped pressure.

How often should we switch between duty and standby PAC pumps?

Include a weekly or biweekly duty/standby rotation in your PAC dosing system operating procedure. Flush the outgoing pump with clean water for five minutes before parking it on standby so its check valves stay clean and ready.

How often should insoluble mud be drained from the bottom of the PAC tank?

Depending on commercial PAC purity, open the bottom drain valve and hose out settled mud every one to three months when the tank is drawn down low, keeping sludge well below the pump suction nozzle.