Onsite additive depletion monitoring in turbine oils by FTIR spectroscopy

Importance of the Topic

Monitoring antioxidant depletion in turbine oils is essential to maintain turbine reliability and prevent unplanned outages. Phenolic and aminic antioxidants inhibit oil oxidation and varnish formation, preserving viscosity and lubrication performance under high temperature, pressure and contamination conditions. Real-time, onsite analysis of these additives allows timely maintenance decisions before rapid oxidation escalation.

Objectives and Study Overview

This study demonstrates the use of the Agilent 5500t FTIR spectrometer for fast, direct measurement of phenolic and aminic antioxidant levels in turbine oil. It aims to provide actionable, time-sensitive data to prevent critical oxidation cycles and avoid non-scheduled turbine shutdowns. The application also compares FTIR-based monitoring with conventional voltammetric methods.

Methodology and Instrumentation

The Agilent 5500t FTIR spectrometer measures characteristic infrared fingerprint bands of phenolic and aminic antioxidants without sample preparation or calibration. A reference spectrum of new oil is stored in the software, and used oil spectra are overlaid to quantify additive weight percent and oxidation products. Accelerated aging tests with iron and copper catalysts at 135°C in air evaluated depletion kinetics over 26 days, sampling every 2-3 days.

• Instrumentation used: Agilent 5500t FTIR spectrometer

Main Results and Discussion

Accelerated stress tests showed a rapid decline of phenolic antioxidants to 40% of initial concentration early in the oil's life, while aminic antioxidants remained above 80% until mid-life. Beyond this point, aminic levels dropped sharply as phenolic antioxidants fell below 30%, coinciding with exponential oxidation product increases. FTIR alerts at user-defined thresholds enable maintenance actions before critical additive depletion. Compared to voltammetric techniques, FTIR offers direct weight-percent results, no solvent extraction, improved operator independence, and simultaneous detection of water and nitration.

Benefits and Practical Applications

• Onsite, rapid analysis requiring only a drop of neat oil
• No sample preparation, reagents, or electrode maintenance
• Independent monitoring of phenolic and aminic additives and oxidation levels
• Early warning of additive depletion to optimize bleed-and-feed, top-off and re-additization strategies
• Detection of contaminants such as hydraulic fluids or gear oils

Future Trends and Opportunities

Further developments may integrate FTIR monitoring with predictive maintenance platforms and remote diagnostics. Expanded chemometric models could enable simultaneous quantification of additional additive classes and degradation products. Miniaturized FTIR sensors and real-time data analytics may support continuous online monitoring and advanced asset health management.

Conclusion

FTIR spectroscopy using the Agilent 5500t provides an efficient, reliable, and user-friendly approach for continuous monitoring of antioxidant depletion and oxidation in turbine oils. Its rapid, onsite capability enhances lubrication management, prevents unexpected downtime, and extends oil service life.

References

Higgins F. Onsite additive depletion monitoring in turbine oils by FTIR spectroscopy: Fast, easy antioxidant measurement. Agilent Technologies Application Note 5990-7801EN, 2011.