Reliability guide for hydraulic system fluids, sampling locations, common failure modes and evidence-led test selection.
Equipment overview
Hydraulic System condition monitoring must connect the fluid system to component metallurgy, operating regime, filtration, environmental exposure and maintenance history. The same concentration can represent different risk when oil age, sump volume or make-up rate changes.
Lubrication points and fluids
- Primary lubricated components
- Reservoir or sump
- Filters and debris sources
- Hydraulic fluid
Common failure modes
Use the following as hypotheses to test, not conclusions from one result.
Correlate fluid evidence with inspection, vibration, temperature, filter condition and operating history.
Correlate fluid evidence with inspection, vibration, temperature, filter condition and operating history.
Correlate fluid evidence with inspection, vibration, temperature, filter condition and operating history.
Correlate fluid evidence with inspection, vibration, temperature, filter condition and operating history.
Which tests are required—and why
| Test | Panel role | Question answered | What to do if abnormal |
|---|---|---|---|
| particle count | Routine trend | Quantify fluid cleanliness at defined size channels. | Confirm bottle/port cleanliness, inspect filtration and identify particle source. |
| karl fischer water | Routine trend | Quantify water that threatens film strength and corrosion control. | Protect sensitive equipment and locate condensation, cooler, seal or washdown ingress. |
| kinematic viscosity | Routine trend | Confirm the lubricant remains within its intended flow behaviour. | Check grade, dilution, shear, oxidation and temperature context. |
| acid number | Routine trend | Track oxidation and acidic contamination. | Compare with new oil and review heat, contamination and varnish evidence. |
| elemental analysis icp oes | Routine trend | Trend fine wear metals, contaminants and additive elements. | Map elements to metallurgy; corroborate rapid rises with particle-sensitive methods. |
| analytical ferrography | Triggered / advanced | Examine wear-particle size, shape and morphology. | Use to localise a wear mechanism after an abnormal routine trend. |
| pq index | Triggered / advanced | Detect bulk ferromagnetic debris including larger particles. | Inspect magnets, filters and gear/bearing surfaces when rising with iron. |
| ftir oil condition monitoring | Triggered / advanced | Trend molecular degradation and selected contaminants. | Correlate oxidation, nitration, soot, water or glycol indicators with companion tests. |
How often should this equipment be sampled?
Set frequency from asset criticality, failure-development time, operating severity and historical stability. Increase frequency after abnormal trends, maintenance or contamination events.
| Condition | Sampling response | Reason |
|---|---|---|
| Stable critical asset | Maintain a fixed interval and operating state | Builds a statistically and operationally comparable trend |
| New asset, rebuild or oil change | Create an early baseline and one or more shorter follow-ups | Break-in, flush residue and maintenance discontinuities can change results |
| Abnormal but unconfirmed result | Confirm promptly according to criticality and failure-development time | Separates persistent change from sampling or analytical variation |
| Rapid change or corroborated distress | Escalate through site procedures; collect oil, filter and debris evidence | Preserves evidence and supports a focused inspection |
Sampling locations and mistakes
| Location | Suitability | Advantage | Risk |
|---|---|---|---|
| Upstream of return filter | Preferred when safely engineered | Repeatable live-system evidence | Requires flushing and a documented procedure |
| Pressure-line minimess port where safe | Preferred when safely engineered | Repeatable live-system evidence | Requires flushing and a documented procedure |
Common mistakes
- Sampling a cold, stagnant drain
- Changing locations between rounds
- Failing to flush dead volume
- Omitting top-up and maintenance history
Common abnormal patterns
- Abrasive wear: requires correlation with the relevant fluid properties, companion signals and inspection evidence.
- Servo sticking: requires correlation with the relevant fluid properties, companion signals and inspection evidence.
- Oxidation: requires correlation with the relevant fluid properties, companion signals and inspection evidence.
- Water contamination: requires correlation with the relevant fluid properties, companion signals and inspection evidence.
References and method context
References identify authoritative source families; they do not reproduce protected procedures or acceptance tables. Verify the current official edition before procedural or conformity use.