ISO Cleanliness Is Still Good—So Why Is the Filter Plugging? When the Blockage May Not Be Hard Particles

A good-looking Particle Count does not answer every question about Soft Contamination and Oil Degradation Products.

Technical Explanation

Particle Count is generally used to assess the number of particles within size ranges detectable by the instrument. It is highly useful for controlling Hard Particle Contamination, but a Filter can become blocked by other materials that are not reflected in the same particle count.

Certain Soft Contaminants, Gel-like Material, Oxidation Products, or Varnish Precursors can coat and block the pores of Filter Media even when Hard Particle levels in the oil are not high. Material that remains dissolved in the Oil Phase may also not appear in Particle Count, but can change state when Temperature or Solubility changes.

This is why Filter Plugging must be analyzed together with MPC/varnish indicators, Filter Inspection, Oil Temperature, and Operating History, rather than trying to explain it with the ISO Code alone.

The distinction between “oil that is clean of Hard Particles” and “oil that contains no Deposit-forming material” is important, especially in Servo/Hydraulic Control Systems with low Clearance.

Impact on Equipment

Filter Differential Pressure increases even though the ISO Code remains good.
Filter Replacement Frequency increases without clear evidence of Hard Contamination.
Servo Valves may experience Stiction from Soft/Sticky Deposits.
The Maintenance Team may increase Filter Rating without correcting the cause.
A false sense of confidence may develop that the system has no problem because Particle Count passes.

Recommended Approach

When Filter Life and Particle Count do not correlate, change the question from “Is the oil clean?” to “What is plugging the Filter?” and inspect the Filter Media and Oil Degradation Indicators further.

KPI setting should separate Hard Cleanliness, Water, Oil Degradation, and Filter Performance, then examine their relationship as a Trend so that problems can be seen before they affect Control Reliability.

What Can SUTAIYO & Lubefix Do?SUTAIYO can design a Test Package that addresses the actual system symptoms, such as separating Hard Particle Contamination from Soft/Varnish-related Contamination and interpreting the results together with Filter DP.Lubefix can perform Filtration or Varnish-related Treatment according to the confirmed type of Contaminant, rather than automatically increasing Filter fineness.

Expected Customer Outcomes• Understand the cause of Filter Plugging more accurately.• Reduce repeated Filter replacement without correcting the root cause.• Maintain Valve Reliability by controlling both Hard and Soft Contamination.