Gel That Won’t Pump in February: Stiffness, Ice Lenses and Discharge Head

On the fourth of February a transfer station in Hebei tried to empty a 12 cubic metre gelled waste tank into a tanker and got 3 cubic metres in 90 minutes before the progressive cavity pump tripped on over-pressure. In July the same tank emptied in 25 minutes. The gel had not changed recipe. The tank had been standing in an unheated bay at minus 6 C for three days, and the material had gone from a pumpable paste to something closer to cold butter with ice in it.

Winter is when gel handling failures cluster, and they are almost always predicted by two numbers nobody records: the gel’s apparent viscosity at the actual tank temperature, and the fraction of free water that has frozen. This article covers both, plus the pump and line changes that buy back the lost hours.

From 8,000 to 40,000 cP Between 20 C and 2 C

A 1.5 percent polyacrylate gel at 20 C typically shows an apparent viscosity of 6,000 to 10,000 cP at a shear rate of 10 per second. Cool it to 2 C and the same gel reads 30,000 to 45,000 cP, because the polymer network contracts and the water between the chains becomes more ordered. That is a fivefold rise for a 18 degree drop, and it happens gradually enough that nobody notices until the pump does.

Design your line for the cold number, not the summer number. A 50 mm line that carries gel at 1.2 bar in July will need 4 to 5 bar at 2 C for the same flow. If the pump curve tops out at 6 bar, you have almost no margin, and a single ice lump will stall the transfer.

Ice Lenses in a Half-Frozen Drum

Water in a gel freezes at a lower temperature than free water, typically between minus 1 and minus 4 C depending on the salt content, but it does freeze. What forms is not a solid block: it is a matrix of ice lenses inside a concentrated gel, and the lenses are hard, abrasive and much denser than the surrounding material. A pump impeller or a rotor that meets a lens stalls instantly.

Partial freezing is worse than full freezing because it is invisible from outside. A drum that has been at minus 2 C for two days will feel firm and look uniform, but a probe will find layers. Push a 10 mm steel rod in and feel the resistance: steady resistance is cold gel, a sudden hard stop is a lens. If you find lenses, the material needs to be warmed above 5 C before pumping, not hammered.

Progressive Cavity Versus Diaphragm at Cold Viscosity

A progressive cavity pump handles viscous gel well because the flow is positive displacement at low shear, but its pressure capability falls as viscosity rises and the stator heats. At 40,000 cP, expect 60 to 70 percent of the rated flow and watch the stator temperature: a dry run of 30 seconds will destroy a nitrile stator at that viscosity.

A diaphragm or peristaltic pump is gentler and self-priming but delivers lower pressure, which matters on a 40 m line. For cold transfer, the most reliable arrangement is a progressive cavity pump with a 1.5 times oversized motor, a 75 mm line kept as short as possible, and a bypass loop back to the tank so the pump never dead-heads against a blockage.

Line Heat, Insulation and a 12-Hour Discharge Rule

The cheapest fix is a rule: no gelled load sits longer than 12 hours in an unheated tank below 5 C. That single scheduling change removes most winter failures. Where the rule cannot be kept, three measures work in combination: trace heating on the discharge line at 30 W per metre, 50 mm of closed-cell insulation on the tank and line, and a 2 kW immersion heater set to 8 C that runs only when the tank is full.

Do not heat above 15 C. Warm gel releases water, and the free water migrates to the top of the tank and freezes first next time. A tank held at 8 to 12 C gives pumpable gel without syneresis, and the difference in energy cost between 8 C and 20 C is about 40 percent of the heating bill.

If your winter problem is stiffness rather than freezing, a lower crosslink density grade with a slightly coarser granule will pump better at low temperature. The polyacrylate granule options for cold-climate dosing are worth reviewing before next season.