Slurry Viscosity & Pumps

The subject of fluid dynamics can soon get very complex, perplexing and challenging even the best scientists and mathematicians. As Sir Horace Lamb, the celebrated fluid mechanist and author of the widely acclaimed “Hydrodynamics” book expressed,

“when I die and go to Heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics and the other is the turbulent motion of fluids - and about the former I am rather more optimistic.” – Sir Horace Lamb

Before we go any further, I must disclose that this blog isn’t going to enlighten any student of hydrodynamic formulae - the author of this blog’s biggest mathematical breakthrough was scraping through GSCE maths, with A LOT of tutor and parental patience!

However, I’ll try to cover off some practical aspects of pumping challenging slurries, with a light-touch on the science. 

The non-scientific explanation of Newtonian and Non-Newtonian fluids 


With Newtonian fluids (you know, the ‘normal’ ones that behave how Newton said to), the viscosity (resistance to flow) remains constant regardless of velocity. Clean water and most oils are Newtonian fluids. Nice and easy.

However, many slurries can have a high concentration of very small fines. Although the carrying fluid (water) is a well-behaving Newtonian fluid, the influence of these suspended fines can convert this slurry into a non-Newtonian fluid! Breaking Newton’s rule of viscosity one way or another, working with these fluids gets rather unpredictable, especially during periods of high stress. How relatable is that


Whilst we all get stressed at different things, fluids experience shear-stress when they are forced to run alongside surfaces or other bodies moving at a different velocity. Think of dashing through a busy station to catch the train in rush hour, when everyone else is going a different speed or direction!

Scientifically, shear-stress is the “force per unit area acting tangentially to the fluid's surface due to its continuous relative motion” – thanks ChatGPT!

As Study Smarter (one of ChatGPT’s sources in my cheat above) puts it “Shear Stress refers to the force per unit area that acts parallel to the surface, causing deformation. In fluid mechanics, it characterizes the internal frictional resistance in a fluid due to its layers moving at different velocities.” 

Fluid Shear-stress in pumping

Shear-stress is increased when fluid is pumped at high velocity through a narrow pipe; the pipe surface being stationary, friction occurs against the flow. Another example within fluid transfer is a pipe bend, where the velocity (and therefore friction) at the outside of the curve is higher than the inside.

A pump works by energising the fluid to create flow; this of course creates a shear stress between the stationary chamber and rotating parts.

Fast-spinning impellers, like those found in centrifugal pumps, produce the highest shear stress, so positive displacement pumps are often specified for shear-sensitive fluids. Progressive cavity (or progressing cavity) pumps and peristaltic pumps belong in this class, and have a slower rotation for the same amount of pressure.

Again, just like people, fluids react differently to stress depending on their natural characteristics.

The different types of Non-Newtonian slurries can be broadly classified into those whose viscosity increases (thickens) and those whose decreases, when under increased shear stress. 

Dilatant fluids

Dilatants are fluids whose viscosities increase when under shear stress, they are known as shear-thickening fluids (STF). Care must be taken when engineering a pumping system for such fluids as the shear stress of long or narrow pipe systems can thicken up the slurry.  This can cause blockages and possibly make it un-pumpable – at least for that particular pump.

Such fluids benefit from slower flow rates, and/or wider pipe diameters to keep the shear stress below the fluid’s critical shear rate - the point at which it becomes dilatant. Of course, when we talk about flow rates it's worth mentioning that slurries need to be kept above their ‘settling velocity’ – the point below which silt will start to settle out and potentially block the pipe.

Common dilatant fluids in the mineral processing industry are cement, sand, lime slurries, and thickener underflow.

Some dilatant substances can react so fast that they are used for the production of stab and impact protective clothing.

Anti-thixotropic Fluids – a dilatant type with a longer fuse

Anti-thixotrophic (Rheopectic) fluids don’t ‘snap under stress’ as quickly as dilatant liquids.

Fluids with Anti-thixotropy (or Rheopexy) characteristics have a similar effect to dilatant fluids, the difference being that they have a time-dependent factor in the reaction to shear stress changes.

While they might not cause any issues passing through a pump and a short length of pipe, spending time in a mixing tank, or traveling for a time in a long pipe system will tend to thicken them up.

Ways to pump anti-thixotrophic fluids are similar to dilatants; reducing shear stress by slowing the flow, reducing bends and valves if possible, or increasing pipe diameter. For long-distance pumping, either a low-geared pump or multiple ‘tank and pump stations’ can be used. 

Thixotropic Fluid

Thixotropy is the shear-thinning of a liquid, where stress applied causes a decrease in viscosity. A well-known example of a thixotropic fluid is tomato ketchup – a quick shock to the bottle helps it run freely.

These fluids need to be kept in motion to avoid them thickening up, which could cause a low flow-rate or even a blockage or strain on the pump.

Mixing paddles on a separate motor, or the Audex pump AW models that are fitted with an agitator can enable a regular pump to transfer thixotropic fluids. However, problems can arise with intermittent pumping as the stationery fluid will thicken in the pipework, leading to blockages. In these cases, the LSM pumps range can be a good choice, as they can deal with thick pastes to thin fluids.

Pumping is very much about understanding the characteristics of the fluid, the capabilities of the pump, and the design of the whole system.

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