Shear sensitivity

Shear is the stress that develops when one layer of the medium slides past another at a different speed. In a pump that happens at the impeller tip, in the clearance between a rotor and the casing, and anywhere flow is forced through a narrow gap at high velocity.

Most media do not care. Water, thin oils and clean chemicals behave as water hydraulically, and no amount of shear changes what arrives at the far end. Shear-sensitive media are the ones where some fragile structure is doing the useful job. Break that structure and you have delivered a different product from the one you picked up.

The damage is usually permanent. A separated emulsion does not re-emulsify in the tank, and sludge whose floc has been torn apart will not re-flocculate on its own. That makes this a selection problem rather than an operating one. You get it right when you select the pump type, or you live with the result for the life of the installation.

Emulsions, flocculated sludge, polymer solutions, structured food, and suspensions of latex or live cells are the shear-sensitive media you meet most often. The test for anything not on that list is two questions: what is holding the medium together, and is that structure doing a job downstream? Two yeses and you treat the duty as shear sensitive.

• Emulsions, creams and cosmetics. Two phases held apart by a stabiliser and an interfacial film. Shear tears the droplets down and disrupts that film, so you get one of two failures: a formulated product that destabilises and separates on standing, or an oily water that comes out as a tighter emulsion the downstream separator can no longer break. Both come from the same damage — droplets broken faster than the stabiliser can cover the new surface — and which one you see depends on the medium.
• Flocculated sludge. Polymer has been dosed to bind fine solids into settleable flocs. A harsh pump undoes the dosing you have just paid for, and the dewatering downstream gets worse while the polymer bill goes up.
• Polymer solutions themselves. Long molecules can be cut. The solution loses viscosity and does not recover it.
• Food with structure. Yoghurt, cream, and fruit preparations carrying whole pieces. Both the texture and the particulates suffer.
• Latex, paints and fermentation broths. Suspended solids and live cells that a high tip speed will damage.

One distinction is worth holding onto, because the words look alike and mean opposite things. A shear-thinning medium drops in viscosity while it is being sheared and gets it back when the shearing stops, either at once or over a few minutes if it is thixotropic as well. Shear sensitivity is permanent degradation. A medium can be both, so the fact that something thins in the pipe tells you nothing about whether it survives the pump.

A centrifugal pump works by accelerating the medium to a high velocity and converting that velocity into head. The mechanism and the damage are the same thing, so you cannot have the first without some of the second. Tip speed does most of it, and internal recirculation in the casing does the rest.

Two common installation choices make it considerably worse. Throttling a discharge valve to trim flow forces the medium through a small gap at high velocity, which is a shearing device by any other name. And a pump running well off its best efficiency point recirculates more inside the casing, so an oversized centrifugal throttled back along its performance curve shears harder than a correctly sized one.

This is not an argument that a centrifugal can never move a shear-sensitive medium. On a short duty at low head, a slow-running pump with a large open impeller is often acceptable, and it is cheaper to buy and to maintain. The trouble is that the typical installation is oversized and throttled, which is the worst case rather than the tolerable one.

The low-shear families are positive displacement. They carry a fixed volume from inlet to outlet at low velocity rather than accelerating it, so the medium sees far less stress for the same duty. Each one costs you something, and the cost is the part worth knowing before you commit.

• Progressing cavity. A rotor turning in an elastomer stator moves the medium along at near-constant velocity, which is about as gentle as pumped transport gets. It handles sludge and viscous media well. You pay for it in stator wear, and it will not tolerate running dry: take the liquid away and the stator is finished in minutes.
• Peristaltic and hose pumps. The medium touches only the inside of the hose, so there is no seal to fail and no clearance to shear across. Gentle, and it will run dry without complaint. The hose is a consumable with a finite life, the flow pulses, and flow and pressure are both limited.
• Rotary lobe. A large pumping chamber and a low tip speed. Good on sludge and on food duties where strip-down for cleaning matters. The clearances that make it gentle also let thin media slip back, so it suits a medium with some body to it.
• Air-operated diaphragm. Gentle in the chamber and simple to maintain. Compressed air is expensive to produce, so the running cost sits above an electric drive. The ball check valves are also a shear point in their own right: the medium is squeezed past the seat twice per stroke, which on a flocculated sludge undoes some of what the gentle chamber saved you. The stroke reversal pulses the flow as well.

Where the duty is flocculated sludge feeding a dewatering process, a progressing cavity or a hose pump running slowly is the right answer, and we would say so even where a centrifugal is cheaper on the quote. The polymer you stop destroying pays the difference back.

Shear rate is the velocity across a gap divided by the size of that gap. Slow the pump and you cut it directly, and the damage falls faster still. So the single most effective thing you can do is slow the pump down and make up the flow with displacement. A larger pump turning slowly moves the same duty with markedly less stress than a small one turning fast. It wins on both halves of that relationship, because the bigger unit has wider clearances as well as lower velocity. It costs more to buy and it takes more floor space, which is the trade you are making.

The rest of the levers sit in the system rather than the pump:

• Size against the real duty point so the pump runs near its best efficiency point instead of being throttled back along its performance curve.
• Control flow with a variable speed drive rather than a valve.
• Use a generous pipe bore and long-radius bends. Velocity in the pipework shears the medium too, and it is cheap to fix at design stage.
• Cut recirculation loops where you can. Every pass through the pump is another dose of shear, so a medium that circulates all shift gets a far larger total dose than the single transfer you sized for.

If you are specifying for one of these duties, send us the medium, the flow, the head and what happens to the product downstream. The low-shear duty pages cover how we work the selection.