Hydraulic uptime is largely a contamination-control exercise. The practical question is not “centrifuge or filter,” but which failure mode you are targeting: (1) water, (2) abrasive solids, and (3) oxidation byproducts that behave like “soft contaminants.” In a well-designed program, filtration provides the precision particle barrier, while bypass centrifugation does the heavy lifting on water and dense solids—often with a meaningful impact on oil life and corrosion risk.
The contamination types that matter (and why they fail equipment)
1) Particulate contamination (hard solids)
Dirt/silica, wear metals, rust, and debris cause the classic hydraulic problems: pump wear, valve sticking, and servo instability.
How it’s tracked: ISO 4406 cleanliness coding assigns a three-number code based on particle counts at >4 μm(c), >6 μm(c), and >14 μm(c).
2) Water contamination
Water drives corrosion, additive depletion, and accelerated oxidation. It shows up as:
Free water (separate phase)
Emulsified water (haze, suspended droplets)
Dissolved water (molecularly in the oil; invisible)
3) Oxidation byproducts (“soft contaminants”)
Sludge/resins/varnish-like deposits can create sticky valves and deposits even when particle counts look acceptable. (This often requires dedicated varnish mitigation; neither “standard filters” nor “standard centrifuges” are universal varnish cures.)
What bypass centrifugation removes (and why it is often underused in hydraulics)
Bypass (kidney-loop) centrifugation uses centrifugal force to separate contaminants based on density differences and separation physics—not pore size.
Removes very well
1) Free water and water that can be separated as a distinct phase/droplets
Alfa Laval’s own guidance for oil cleaning emphasizes centrifugal separation for oil contaminated with particles and/or water, with water and sludge separated from the recirculating oil.
2) Dense solids (wear metals, dirt, rust)
Centrifuges preferentially drive higher-density solids outward, forming a sludge phase that is physically removed, rather than captured in a porous element. Alfa Laval describes centrifugal oil cleaning as separating particles of higher density than oil under high G-force.
3) Smaller solids improve with recirculation (turnover)
With recirculation through disc-stack separator modules, Alfa Laval notes that cleanliness improves over time and can remove progressively smaller particles (their discussion explicitly references improved removal of smaller particles with repeated treatment).
Removes conditionally / depends on the system
Emulsified water can be partially removable depending on emulsion stability, additives, temperature, and residence time. Expect variability—this is not a “one size fits all” claim.
Does not remove well
Dissolved water (molecularly in solution) generally requires dehydration strategies (e.g., vacuum dehydration) rather than phase separation.
Low-density, soluble oxidation products that remain dissolved will not reliably separate by density alone.
What filtration removes (and what it does not)
Filtration is a barrier technology: you force oil through media and retain particles above a given performance threshold.
Removes very well
Hard particles at and above the element’s effective performance size
Filter efficiency is commonly expressed as beta ratio from standardized multi-pass testing (ISO 16889).
In practical terms: filters are the tool that most directly drives ISO 4406 numbers down in a controlled, spec-driven way.
Removes only with special media (or not at all)
Water: standard particulate elements do not reliably remove water unless you use water-absorbing/coalescing technology.
Varnish/soft contaminants: standard elements may capture insoluble agglomerates but do not reliably address dissolved precursors.
“What each removes” — a program-level view
Bypass centrifugation (Alfa Laval-style oil cleaning focus)
Best at:
Water risk reduction (especially free water and separable water)
Dense solids removal without rapidly plugging media
Extending oil service life by continuously pulling out contamination that accelerates corrosion and oxidation
Limitations:
Dissolved water and fully soluble degradation products generally persist.
Filtration
Best at:
Precision particle control for tight-clearance components (pumps, servo valves)
Delivering performance backed by standardized test methods (ISO 16889 beta ratio)
Limitations:
Water and certain soft contaminants require specialized approaches.
Why centrifugation is often the “money step” in hydraulic oil programs
If your customer’s pain is any of the following, centrifugation tends to produce more visible operational ROI than “more filtration”:
Chronic water ingress (condensation, humid breathers, washdown environments, cooler leaks)
Corrosion evidence (rust staining, pitting, frequent component replacement)
High dirt load that drives frequent filter change-outs and bypass events
Oil life problems (oxidation/depletion accelerated by water and solids)
Alfa Laval explicitly positions centrifugal separation as an alternative to filtration in oil cleaning contexts because it separates both solids and water, highlighting water’s damaging effect on equipment.
The recommended architecture (centrifuge-forward, but technically correct)
A robust, centrifuge-forward design typically looks like:
Primary system filtration (pressure and/or return)
Protects components continuously during operation.
Sized and specified by beta ratio/multi-pass performance.
Bypass centrifuge (kidney loop)
Runs continuously or on a duty cycle to remove water + dense solids from the reservoir.
Creates a “conditioning backbone” that prevents water-driven corrosion and reduces the solids burden that would otherwise load filters.
Breather control and ingression management
The best centrifuge program still needs ingress control; otherwise you are “processing the same mistake” indefinitely.
KPIs and how to prove it is working
Minimum monitoring set:
ISO 4406 cleanliness (trend at a consistent sampling point)
Water (ppm or saturation; use consistent test method and trend it)
Filter ΔP and element life
Centrifuge sludge/water removal rate (basic operational KPI that correlates with ingress severity)
Common errors that sabotage centrifuge programs
Under-sizing bypass flow so reservoir turnover is too slow to impact water and particle trends.
No baseline sampling, so improvements are anecdotal rather than measurable.
Assuming centrifugation eliminates the need for filtration (it typically reduces filter burden; it does not replace precision filtration for all component protection cases).
Ignoring ingress sources (breathers, seals, maintenance practice, cooler leaks).
Practical takeaway
If you sell Alfa Laval centrifuges, the correct positioning is:
Bypass centrifugation is the primary tool for water and dense solids management—the contamination modes most associated with corrosion, additive depletion, and shortened oil life.
Filtration remains the precision particle barrier used to meet ISO cleanliness targets and protect tight-clearance components with performance defined through ISO multi-pass testing.
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