Onsite additive depletion monitoring in turbine oils by FTIR spectroscopy

Importance of the Topic

Monitoring antioxidant levels in turbine oils is critical to prevent accelerated oxidation, which can lead to varnish formation, loss of lubrication properties and unexpected turbine shutdowns. Rapid, onsite measurement of both phenolic and aminic antioxidants helps maintenance teams take timely corrective actions to extend oil life and ensure reliable turbine operation.

Objectives and Overview

This application note aims to demonstrate how the Agilent 5500t FTIR spectrometer enables fast, easy, and accurate onsite monitoring of phenolic and aminic antioxidant depletion in ISO 32 turbine oil. Key objectives include:

• Simultaneous measurement of phenolic and aminic antioxidants
• Real-time tracking of oxidation and water contamination levels
• Comparison with voltammetric methods

Methodology and Instrumentation

The Agilent 5500t FTIR system uses characteristic infrared absorbance bands (fingerprint bands) of the phenolic and aminic antioxidants to quantify their concentration in weight %. Analysis requires only a drop of neat oil with no sample prep, calibration, or electrode maintenance.

Instrumentation

• Agilent 5500t FTIR spectrometer
• Pre-set software methods for turbine oil

Main Results and Discussion

Thermal aging tests at 135 °C with iron and copper catalysts over 26 days revealed distinct depletion patterns:

1. Phenolic antioxidant drops rapidly to ~40 % within early service life, partly due to evaporation of simpler phenolics.
2. Aminic antioxidant remains > 80 % until mid-life, then depletes in two stages—first a 25 % drop, followed by rapid decline once phenolic falls below 30 %.
3. Oxidation products increase exponentially when both antioxidants approach critical depletion, highlighting the need for timely intervention.

These trends confirm the synergistic role of phenolic antioxidants in protecting the rechargeable aminic species and demonstrate the FTIR system’s ability to predict imminent oxidation before reaching a critical point.

Benefits and Practical Applications

The 5500t FTIR approach offers several advantages over cyclic voltammetry:

• No extraction step or electrolyte solutions—analysis on neat oil
• Direct weight % quantification of each antioxidant
• Fast, on-site results with minimal operator training
• Resistance to interference from water, metals or contaminant fluids
• Integrated warnings (monitor frequently/change immediately) for antioxidants, oxidation and water

This enables maintenance teams to optimize top-offs, bleed-and-feed procedures, filtration and dehydration strategies in real time.

Future Trends and Applications

Advances in portable FTIR technology will likely expand onsite monitoring to a wider range of lubricant types, including synthetic and high-performance esters. Integration with online data management and predictive analytics could further enhance condition-based maintenance programs. Real-time trend analysis across multiple turbines could enable fleet-wide optimization and extended service intervals.

Conclusion

The Agilent 5500t FTIR spectrometer provides a rapid, easy-to-use solution for independent measurement of phenolic and aminic antioxidants in turbine oil, along with oxidation and water levels. Its onsite capability, no-prep workflow and clear warning system support proactive maintenance decisions, reducing downtime and extending oil life.

References

1. Higgins F. Onsite additive depletion monitoring in turbine oils by FTIR spectroscopy. Agilent Technologies Application Note, Publication Number 5990-7801EN, May 1, 2011.