PAC Dosing Pump Not Delivering: Crystals and Dilution Ratio

The PAC dosing pump ran fine in the morning; by afternoon the calibration column stopped moving. The motor sounds normal and the piping shows no obvious leak. Open a check valve and you find a chalky crust on the seat. Most recoveries on this duty start with crystals. The rest usually involve dilution ratio, strainers or suction lift.

PAC solutions are acidic. After solid PAC dissolves, insoluble grit often settles in the tank. Restarts after a stop are when a PAC dosing pump plugs hardest. Use the sequence below as a field checklist.

PAC Dosing Pump: Blockage Versus No Suction

Zero delivery points to plugged valves, a blinded strainer or air on the suction hose. A thin, unstable flow can mean a soft-seating valve, missing back pressure with siphoning, or a tank strength that swings. Time a draw into a cylinder before you strip the unit so you do not replace parts blind.

Do not start with the diaphragm. Sudden diaphragm failure is uncommon on PAC duty. Crystals and grit drive most PAC dosing pump tickets.

Crystals in Checks, Quills and the Head

Strong PAC or poorly mixed slurry dries at valve ports and forms a hard shell. The ball cannot lift, or it cannot seat, and the PAC dosing pump strokes without net flow. Clear it like this:

1. Stop the drive, isolate power and depressurize the discharge.
2. Remove suction and discharge checks and the injection valve; soak deposits soft in clean water and brush them off. Do not dig into sealing faces with metal tools.
3. Inspect balls and seats for scratches; renew as pairs when damaged.
4. Refit in the correct direction with seals seated; do not wrap tape under face seals.
5. Prime on clean water, confirm liquid at the outlet, then return to chemical.

Before a long idle, circulate clean water through the PAC dosing pump and lines for about ten minutes. Set cleaning intervals from what you see in the first weeks; dirty industrial PAC needs shorter gaps.

A half-blocked quill may still move the column slowly while flocculation at the dose point looks weak. Walk to the mixer and look. Catching a PAC dosing pump in that half-blocked state saves seat wear that washing alone will not fix later.

Strainers and the Make-Down Tank

Solid PAC leaves residue at the tank bottom. Without a suction strainer, or with a mat that never gets washed, the foot valve blinds and the PAC dosing pump cannot draw. Put strainer cleaning on the round sheet, not on the emergency list.

Weak mixing leaves a thin upper layer and a thick lower slurry. Drawing the heavy layer plugs valves faster. Size the mixer to the tank and finish dissolving before you dose. Scrape tank walls and floors on a schedule so old cake does not keep feeding the PAC dosing pump.

Dilution Changes Rewrite the Flow Number

A metering pump moves solution, not neat chemical. Example for orientation only: 100 m³/h of water at a jar-test dose of 20 mg/L needs about 2 kg/h of PAC. At 10% solution that is roughly 20 L/h of liquor; with about 20% operating margin you size near 24 L/h. Drop to 5% solution and the liquor rate doubles. The same PAC dosing pump can suddenly look “too small” even though nothing is broken.

So some PAC dosing pump complaints are not faults at all. Someone changed make-down strength, never recomputed solution flow and never recalibrated. Lock the dilution once operations agree; if it must change, recalculate and recalibrate the PAC dosing pump the same day.

Over-concentrated liquor reduces volumetric flow but crystallizes harder. Neat feed is possible, yet valve cleaning must be more frequent. Match plant habit to what the PAC dosing pump can sustain.

Operators sometimes chase pump speed when the real gap is make-down consistency. A five-minute stir check and a look at undissolved pockets in the tank often restore steady feed faster than changing stroke length again.

Suction Lift, Back Pressure and Materials

Keep suction lift on a mechanical unit near or under about 2 m. PAC is a bit viscous, so shorten the suction run, cut elbows and prefer flooded suction for the PAC dosing pump. When suction-to-discharge differential is under 1 bar, add a back-pressure valve so siphoning does not swing the dose.

Head materials for PAC are usually PVC or PVDF. Chloride-bearing acidic mixes are a poor match for SS316. Wrong materials age seals early and mimic a PAC dosing pump that will not deliver. Keep a discharge relief valve so a closed isolation valve cannot overstress the skid.

In cold sheds, PAC liquor thickens and suction suffers. Simple tank and hose insulation helps; if that is impossible, shorten suction further or confirm whether the PAC dosing pump needs a larger capacity step after winter calibration.

Keep a short log: valve cleaning dates, strainer condition, current dilution and column readings. When a PAC dosing pump fails, that sheet beats memory.

Conclusion

To restore a PAC dosing pump, clean crystals from checks and the quill, wash the strainer, check mixing and tank residue, then verify whether dilution changed without a new flow calculation and calibration. Recheck suction lift, back pressure and head materials while you are there. Crystals and dilution explain most PAC dosing pump stoppages on site.

FAQ

Water primes fine but PAC will not. Why?

Crystals or a too-strong liquor are likely. Clean the valves again and calibrate on the real solution for that PAC dosing pump.

Must we strip valves after every stop?

No. Flush with water before shutdown and open valves on the interval you learned. Constant hard dismantling damages seals on the liquid end.

How often should the strainer be washed?

It depends on raw PAC quality. Inspect often at start-up, then wash when the mesh discolors or suction weakens at the foot valve.

What dilution is “right”?

There is no single answer. Aim for mid-range stroke on the PAC dosing pump without running so strong that crystals dominate.

Will a standby pump plug if it never runs?

Yes. Rotate or flush the standby PAC dosing pump on a schedule so it is ready when duty fails.

If you want a pump size checked against water rate and make-down strength, contact us with the duty data.