Technical perspective
A hydraulic system can transmit power while contamination is already shortening component life. Servo and proportional valves are especially sensitive because small clearances and control orifices can be affected by particles that are not obvious in a bottle. The useful question is not whether the oil looks clean; it is whether the particle population and fluid condition are appropriate for the component, pressure and reliability target.
Chem-Tech supports cleanliness programmes that connect a defined sampling point with ISO 4406 or an agreed coding system, lubricant condition and maintenance history. The programme helps plants verify new fills, monitor working systems, investigate recurring valve faults and confirm that flushing or filtration achieved the intended result.
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Why one cleanliness code is not a complete diagnosis
An ISO 4406 code reports particle counts in defined size channels. It does not identify particle chemistry, hardness, origin or the condition of the base oil. Air bubbles, free water and poor sampling can distort some measurements, while a good code from an unrepresentative sample can provide false reassurance.
Interpret the result against the equipment manufacturer's target, the same sampling point and the operating state. A sudden change after maintenance may indicate ingress or dirty fill oil; a gradual rise may point to filtration performance, reservoir breathing or wear. Repeated valve trouble with stable counts may require debris characterisation and fluid-condition testing.
- Servo or proportional valves stick, hunt or respond intermittently.
- Filters reach differential-pressure limits sooner than expected.
- New oil, hose replacement, cylinder work or flushing has recently occurred.
- Particle codes vary widely between samples drawn from different locations or techniques.
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Connect particle count with source and consequence
Automatic particle counting provides a repeatable cleanliness trend when the sample is suitable. ISO 4406 coding makes counts easier to communicate, while NAS 1638 may still appear in legacy plant specifications. The two systems should not be treated as a simple one-to-one conversion.
Wear metals by ICP, PQ Index and analytical ferrography cover different particle populations. Water, viscosity, acid number and FTIR add fluid condition. Together they help distinguish external contamination, active wear, water ingress and oil degradation.
Count
ISO 4406 or agreed cleanliness coding establishes the particulate trend at the selected point.
Characterise
Ferrography and PQ evidence can show whether larger ferrous wear particles are present.
Locate
Matched upstream, downstream and reservoir samples help assess filter and system zones.
Confirm fluid health
Viscosity, water, TAN and FTIR prevent a particle-only interpretation.
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Indicative hydraulic cleanliness packages
The target code and panel must be agreed for the specific servo, pump, valve and oil. Chem-Tech reports the measured evidence; the equipment owner sets the acceptance target.
| Package | Use when | Indicative tests and methods | Decision supported |
|---|---|---|---|
| Cleanliness verification | New fill, transfer cart or post-filtration check | Particle count/cleanliness — ISO 4406 or NAS 1638; water — ASTM D6304 | Verify fill or filtration condition |
| Routine hydraulic health | Operating trend programme | Particle count; kinematic viscosity — ASTM D445/D7042; ICP — ASTM D5185; TAN — ASTM D664 | Trend contamination, wear and lubricant condition |
| Servo fault investigation | Recurring sticking or unexplained filter debris | PQ Index — ASTM D8184; analytical ferrography — ASTM D7690; FTIR — ASTM E2412; water separability — ASTM D1401 where relevant | Separate wear, ingress and degradation causes |
Particle-count sample suitability is reviewed before analysis. Samples with free water, heavy opacity or other interferences may need an alternative preparation or a different evidence path.
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Sample a live zone, not convenient stagnant oil
Cleanliness targets can only be trended when the sampling point and procedure are controlled. A reservoir drain or open container often carries settled debris that does not represent the circulating oil.
- Use a permanently identified live-zone valve downstream of turbulent flow where practical.
- Flush the valve and sampling hardware using a documented volume before filling the bottle.
- Use certified-clean or laboratory-supplied containers and keep caps protected.
- Record operating pressure, temperature, filtration state and recent maintenance.
- For filter-performance studies, collect matched samples at defined upstream and downstream points.
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Use cleanliness trends to control the contamination pathway
If the code exceeds the agreed target, first confirm the sample and result. Then examine fill-oil cleanliness, breathers, seals, maintenance practices, filtration and component wear. Replacing a filter without addressing the ingress route can produce a repeating cycle.
After flushing or offline filtration, verify from the same sampling point under comparable operation. Maintain the code with disciplined transfers and component work; cleanliness is a system practice, not a laboratory result alone.
- Set component-specific targets with the OEM or reliability standard.
- Pre-filter new oil and verify transfer-container cleanliness.
- Track particle code alongside filter life and valve events.
- Escalate to debris characterisation when the source remains unclear.
Apply this guidance
