Will It Pump? A Discharge Test the Receiving Site Will Accept

A 24 m³ tanker arrived at a receiving facility, connected to the 150 mm discharge, and ran the pump for twenty minutes without moving more than a third of its load. The gel had passed the producer’s paint filter test and met the moisture specification. It simply would not flow. Yield stress at 28 kg/m³ was high enough that the material held its shape in the tank, and the facility sent it back with a note that has since been copied into three other sites’ acceptance conditions.

Pumpability is the acceptance criterion nobody writes down until it fails. A gel can meet every chemical and moisture requirement and still be undischargeable, and the difference is rheology rather than dose. This article covers what actually controls flow, the two field tests that predict it, how to design a gel that pumps at a workable dose, and how to agree the test before the first load.

Yield Stress, Not Viscosity, Decides Whether It Moves

Viscosity describes how a fluid resists flow once it is flowing. Yield stress describes the force needed to start it moving at all. A leachate gel is a yield-stress material: below a certain stress it behaves like a solid, and above it, it flows like a very thick liquid. A tanker discharge applies stress at the tank wall and around the outlet, and if that stress is below the yield value, nothing moves no matter how long the pump runs.

Yield stress rises steeply with dose. In a typical leachate, going from 20 to 26 kg/m³ can take yield stress from 400 Pa to 1,800 Pa, a fourfold increase for a 30 percent dose change. It also rises with time as the gel continues to hydrate, which is why a load that pumped easily at 10:00 can be immovable at 16:00.

Outlet size and tank geometry decide how much stress the discharge can apply. A 150 mm outlet on a cylindrical tank with a sloped floor and a 1.5 bar pump will move a gel up to roughly 900 Pa. The same gel through a 100 mm outlet needs 2.5 bar. Most rejected loads sit between 900 and 1,500 Pa, which is precisely the range a 26 to 28 kg/m³ dose produces.

Two Field Tests That Predict Flow

The slump test is the simplest and it is borrowed from concrete. Fill a 100 mm diameter, 150 mm high open cylinder on a flat tray, lift it vertically, and measure the height of the patty after 60 seconds. A spread of 160 to 190 mm indicates a gel that will pump. Below 150 mm it will hold its shape in a tank. Above 210 mm it is probably too soft to pass a paint filter.

The incline test adds time. Place a 300 mm cube of gel on a board, raise one end to 20 degrees, and measure how far the leading edge travels in five minutes. Less than 50 mm of travel indicates a material that will bridge in a tanker. More than 200 mm indicates it will flow out under its own weight, which is what you want.

Run both at the dose you intend to ship, on a sample that has stood for the same length of time the load will stand before discharge. A test on fresh gel at 09:00 says nothing about a load discharged at 17:00.

Designing a Gel That Pumps at 20 kg/m³

Three levers bring yield stress down without dropping below the paint filter threshold. Use a coarser mesh: a 1.0 to 2.0 mm grade produces a gel with larger, more deformable domains and a yield stress 20 to 30 percent lower than a 0.2 to 0.5 mm grade at the same dose. Reduce mixing energy: a batch sheared at 1,400 rpm for twelve minutes develops a tighter network than one mixed at 800 rpm for eight, and the gentler batch pumps better and passes the filter just as reliably.

Add water at the point of discharge. A 3 to 5 percent addition at the tank outlet, mixed by the discharge itself, drops yield stress by 25 to 35 percent and costs nothing except the volume, which is far cheaper than a rejected load. Some sites fit a small dosing pump on the discharge line for exactly this. Time the discharge to the load: ship a 20 kg/m³ gel and discharge within four hours rather than holding it overnight, because the network continues to build and a load that was pumpable at loading may not be at the gate.

Agreeing the Test Before the First Load

Ask the receiving site two questions in writing before dispatch. What is the minimum outlet diameter and pump pressure at the discharge point, and what field test will they use to decide whether a load is acceptable? Most facilities have an informal test that nobody has documented, usually watching whether the load leaves the tanker within a set time.

Then agree a number: discharge of 90 percent of the load within 30 minutes at the facility’s own pump pressure. Put it in the transfer agreement, test it once at your own site with a similar pump, and keep the slump result on the batch sheet. A load that arrives with a documented slump of 175 mm and a note of the discharge time is very hard to reject on rheological grounds.

Pumpability is cheap to test and expensive to discover at the gate. Specifying SAP by mesh range gives a site that control, and it is the mesh, more than the absorption figure, that decides whether a 20 kg/m³ gel will flow out of a tanker at four in the afternoon.