Not all fluids survive being pumped the same way. An emulsion, a biological broth, a polymer solution, or a flocculated suspension can be destroyed by passing through the wrong pump — not chemically, but mechanically. The shear forces generated inside a centrifugal impeller or at an AODD check valve seat are sufficient to break droplet structures, cleave polymer chains, rupture cell membranes, or destroy floc that took hours to build. The result arrives at the destination in a chemically intact but functionally ruined state.
Peristaltic pumps — the Fluimac Helios in Autoflo’s range — handle shear-sensitive fluids because of something they do not do: they do not subject the fluid to rotating impellers, close-clearance passages, or impact events. The fluid contacts only the hose bore, and the pumping action is a gentle, progressive compression wave moving along the hose. Understanding when this matters — and when it does not — is the practical guide to peristaltic specification.
What Shear Damage Actually Is
Shear is a velocity gradient — a difference in fluid velocity across a short distance. When one layer of fluid moves faster than an adjacent layer, the interface experiences shear stress. At low shear rates, most fluids tolerate this without structural damage. At high shear rates — the kind generated at a centrifugal impeller tip moving at 20–40 m/s through a tight casing clearance — the mechanical stress is sufficient to damage structures held together by relatively weak intermolecular or interfacial forces.
Emulsions: droplets in a stable emulsion are held at a particular size by the balance between interfacial tension and the emulsifier system. High shear can either break larger droplets into smaller ones (sometimes acceptable) or coalesce fine droplets into larger ones by disrupting the emulsifier film (usually not acceptable). The result is phase separation or visible creaming that indicates the emulsion structure has been destroyed.
Biological fluids: cell suspensions, fermentation broths, and enzyme solutions contain structures — cells, cell fragments, protein complexes — held together by membranes and tertiary structure. High shear at pump impellers or valve seats ruptures membranes, denatures proteins, and reduces cell viability. In pharmaceutical bioprocess applications, this is a critical quality parameter.
Polymer solutions: long-chain polymer molecules in solution can be mechanically cleaved at high shear rates. The effect is irreversible — the molecular weight of the polymer drops, and with it, the viscosity and functional performance of the solution. Polyacrylamide used in water treatment flocculation and drag-reduction applications is particularly susceptible to shear degradation.
Flocculated suspensions: wastewater treatment and mineral processing regularly produce flocculated slurries where fine particles have been aggregated into larger floc by the addition of polymer flocculants. Floc is fragile — it is held together by polymer bridging and electrostatic attraction, not by covalent bonds. High-shear pumping destroys the floc back to primary particles, eliminating the settling or filtration characteristics the flocculation was intended to produce.
Why Peristaltic Pumps Avoid Shear Damage
In a peristaltic pump, the rotor shoes or rollers progressively compress the hose, pushing a discrete plug of fluid forward through the tube bore. The fluid plug moves as a relatively undisturbed body. The velocity profile across the bore is not the high-gradient shear profile of a centrifugal pump — it is a slug flow where the entire cross-section moves at roughly the same velocity. The shear rate at the compression zone is a function of how fast the rotor shoe moves along the hose, which at typical peristaltic operating speeds is orders of magnitude lower than centrifugal impeller tip speeds.
There are also no check valves, no ball seats, and no high-velocity jet injection points where concentrated shear events occur. The fluid enters the hose, moves through, and exits. The wetted path is entirely the hose bore, and the mechanical interaction between the pump and the fluid is compression rather than shear.
When Peristaltic Is Not the Right Answer
Peristaltic pumps are not universally superior. At flow rates above approximately 100–200 L/min, the hose and rotor sizes required become expensive and the pump’s physical footprint large. For high-flow continuous transfer duties, a centrifugal pump or AODD pump — even with some shear — may be more practical if the fluid can tolerate moderate shear.
For very abrasive slurries, the hose is the wear component. Abrasive solids abrade the hose bore from the inside, shortening hose life significantly and making the peristaltic uneconomical compared to a pump with harder-wearing wetted components.
For continuous high-pressure duty above 8–10 bar, the repeated compression cycling of the hose at high pressure accelerates fatigue failure. The hose has a finite flex life that is shortened at higher pressures and higher RPM. Sustained high-pressure applications are better served by other pump types.
For fluids where any hose material contact would contaminate the product or react with the fluid, the limited range of hose materials (natural rubber, EPDM, NBR, Santoprene, silicone) may not provide a suitable option. PTFE-lined peristaltic hose exists but is significantly more expensive than standard hose materials.
If you are handling a shear-sensitive fluid and need guidance on whether a peristaltic pump is the correct specification, contact Autoflo at info@autoflotechnology.com.