PAC Dosing System Troubleshooting: Tank, Mixer, Level, Pump, Line

Good PAC dosing system troubleshooting starts away from the pump. On a packaged skid, most "no dosing" calls trace back to the tank, the mixer, the level switch or the discharge line, not the metering pump itself. We split the skid into five zones and follow the liquid: tank, mixer, level, pump, line. Confirm each zone before moving to the next. This PAC dosing system troubleshooting guide is for operators at water and wastewater plants and for service engineers who look after polyaluminium chloride (PAC) skids.

PAC dosing system troubleshooting: three readings before you open anything

A PAC skid usually combines a make-up tank, a mixer, a level device, one or two metering pumps, a control panel and the valves and piping on one frame. When any part fails, the symptom looks the same: no chemical, or an alarm and a stopped pump.

Before touching tools, take three readings:

• the alarm history on the control panel;
• the real liquid level in the tank, checked by eye;
• the discharge pressure gauge while the pump runs.

Together these tell you whether the fault is upstream of the pump, in the pump, or downstream. That step saves many needless strip-downs.

Before opening any wetted part, stop the pump, close the isolation valves, relieve line pressure and wear gloves and goggles. PAC solution is acidic and you do not want it in your eyes.

Zone 1 of PAC dosing system troubleshooting: tank and solution strength

The most common tank problem is the wrong solution strength. Too much solid PAC makes a thick solution that the pump struggles to draw and that glues the check balls. Too little gives poor coagulation, and operators blame the pump. Solution strength is usually set from a jar test and the process design.

Sludge in the tank bottom is the second issue. Commercial PAC contains insolubles. If nobody drains the tank, the sludge gets pulled into the foot valve and chokes the suction. Older tanks with the outlet close to the floor suffer most, so PAC dosing system troubleshooting on those units should always include a look at the tank floor.

Zone 2: why the mixer stops on a PAC skid

In PAC dosing system troubleshooting, a stopped mixer usually comes down to one of these:

1. Motor overload trip, often after a large charge of powder formed lumps around the impeller.
2. A gearbox that is low on oil or has taken in water.
3. A timer in the panel that stops the mixer on schedule, which looks like a fault to a new operator.
4. A low-level interlock that stops the mixer when liquid falls below the impeller. That is protection, not a failure.

Once PAC has fully dissolved it rarely needs continuous mixing. See the PAC solution tank mixer guide for when intermittent mixing is enough.

Zone 3: false level alarms in PAC dosing system troubleshooting

Level faults are frequent on integrated skids. A float switch crusted with dried PAC can report low level in a full tank, so the interlock stops the pump. The opposite also happens: the float sticks high and the pump runs dry on an empty tank.

The test is simple. Move the float up and down by hand and watch the panel indicator. If the indicator does not change, check the wiring and the switch. If it changes but the alarm stays, look at the delay and interlock settings. Ultrasonic level sensors need a clean, dry face and should not point straight into the mixer vortex.

Should you keep the low-level interlock?

Yes. It stops the pump from running dry and drawing air. Clean the float and adjust the delay instead of bridging the contacts. Wiring and delay setup are covered in our low level alarm setup guide.

Zone 4: PAC dosing unit not pumping

Only once the tank and level are confirmed does PAC dosing system troubleshooting reach the pump. For a solenoid or mechanical diaphragm pump, check in this order:

• air in the pump head, and whether liquid appears when you open the bleed valve;
• suction and discharge check balls stuck by crystals, and swollen or damaged seat O-rings;
• stroke length and frequency settings turned down by someone;
• a ruptured diaphragm, shown by liquid at the leak hole behind the head.

Crystallization is the classic PAC problem. After a long stop, residue in the head and valves dries into a hard crust and the balls cannot lift. Cleaning methods and the effect of dilution ratio are explained in PAC pump not delivering: crystals and dilution.

Zone 5: line blockage and leaks

In the last stage of PAC dosing system troubleshooting, downstream faults usually show on the pressure gauge. A reading much higher than normal points to a half-closed valve before the injection point or scale inside the line. A reading near zero means a leaking joint or a failed back pressure valve.

Old solvent-welded PVC joints often weep. Follow the white crystal trail on the floor and you will find the leak. If the injection point sits on a pressurized main, a crusted injection valve makes the pump build pressure until the relief valve lifts. In winter, outdoor lines need insulation because colder solution gets thicker and output drops.

Faults that stack up

A lot of PAC dosing system troubleshooting goes wrong because two faults overlap. A stuck float lets the pump run dry for hours, the head fills with air, and cleaning the float alone does not fix it. You still need to bleed the pump.

Another one: the solution was diluted but nobody changed the pump setting. The dose falls and the skid gets blamed. For the wider interlock logic, see integrated dosing system troubleshooting. Replacement skids are listed under dosing skids.

Conclusion

Effective PAC dosing system troubleshooting means working through tank, mixer, level, pump and line in order instead of pulling the pump apart first. Crystallization is common, but false level signals, wrong solution strength and leaking joints are just as frequent, and they often happen together. Regular tank draining, flushing the pump head before long stops and testing the float will prevent most callouts. If the cause is still unclear, send us the skid model, the symptom and your solution strength, and contact our engineers for a second opinion.

FAQ

Why does the PAC pump deliver nothing after a few days off?

Usually crystals in the head and check valves. Isolate, relieve pressure, soak the valve parts in warm water, refit and bleed.

Can I bridge a float switch that keeps giving false low-level alarms?

No. The pump will run dry on an empty tank, draw air and may damage the diaphragm. Clean the float and set a delay.

Does the PAC mixer need to run around the clock?

Generally not. Mix until fully dissolved, then run it on a timer if the solution tends to settle.

What does a zero reading on the discharge gauge mean in PAC dosing system troubleshooting?

Either the pump is not delivering or the line is leaking. Open the bleed valve first, then walk the line and check the back pressure valve.

Does regular tank cleaning reduce PAC dosing system troubleshooting calls?

Yes, noticeably. The interval depends on how dirty the product is and how much you use; when sludge reaches the outlet height, drain and clean it.