Water quality for filling packs: what hard water does in a month

By the twenty-third day after a filler was commissioned, the operator was adding a quarter-turn to the dosing valve every shift to keep fill weights on target. Nothing had changed in the formulation. Nothing had changed in the SAP. What had changed was the inside of the line: a pale, hard film had built up on the piston, the valve seat and the inside of the supply pipe, and the machine was quietly delivering less water per stroke than it had three weeks earlier.

Water is the cheapest ingredient in a gel pack and the one nobody specifications. Plants test the SAP, argue about the film, and then fill with whatever comes out of the tap. Over a month, that tap water decides how often the line stops.

The first month is a slow calibration drift

Hard water carries calcium and magnesium, and when it is heated or simply left standing in a line with any residual alkalinity, those ions come out of solution as carbonate scale. Scale on a piston rod changes the seal’s fit. Scale in a valve seat changes the flow profile. Neither failure is sudden, which is why it gets blamed on the SAP when packs start coming out 15 grams light.

The tell is the shape of the drift, not the size. A formulation problem tends to appear as a step change after a new batch. A water problem appears as a ramp, a little worse every week, and it resets when the line is stripped and cleaned. If your fill weights recover after a clean-in-place and then walk away again over three weeks, you are looking at water, not polymer.

What it does to the gel itself

Absorbency in a polyacrylate is sensitive to the ionic content of the water it is asked to absorb. Distilled water will give you the number on the datasheet. Hard water will not. Calcium and magnesium compete for the charged sites on the polymer chain, and the practical result is a gel that holds less water, feels firmer at the same dose, and releases water more readily under pressure.

That last effect is the one that reaches the customer. A pack made with hard water can pass a squeeze test at packing and still weep under a 20 kg carton in a trailer, because the water it is holding is held less tightly. The pack is not leaking through the film; it is bleeding through the gel.

Three measurements worth the effort

Get a water analysis once, from the actual tap the filler uses, not from the building’s incoming main. You want hardness as calcium carbonate, alkalinity, chloride, sulphate and conductivity. Compare it with the water used in your formulation trials. If they differ, your trial results describe a product you are not making.

Then put a conductivity meter on the line and log it every shift. Conductivity is a crude number, but it moves before anything else does, and a jump of 40 microsiemens tells you a softener has gone into bypass long before the fill weights tell you. Finally, weigh ten packs at the start and end of every shift. Ten packs, not one, because the spread matters more than the mean.

The fixes, in order of cost

The cheapest fix is a softener on the filler supply, sized for peak draw and maintained on a schedule that is written on the wall rather than remembered. The second is a simple in-line filter to catch the particulate that comes with a municipal supply after main works. The third, and the one that changes formulations rather than equipment, is to run your dosing trials on the plant’s real water so the dose accounts for the ions it will actually meet.

Do not skip the third because the first two seem to solve it. A softener that fails on a Friday night will run the line on hard water until Monday, and a dose validated only on softened water will produce packs that are 8 to 12% short of their hold time during that window. We have watched a plant lose a week of output to exactly this, with a softener alarm that nobody had wired to anything.

If you are qualifying a new SAP grade against your own water rather than lab water, the absorption figures in our SAP powder specifications are quoted for deionised water and for 300 mg/L hard water side by side, which makes the gap visible before you commit to a dose.

Book the water test this week, put the result next to your last three months of fill weight records, and see whether the ramp is there. Most plants find it in an afternoon, and most of them then wonder why they spent a quarter arguing about polymer.