Onsite quantitative FTIR analysis of water in turbine oil

Importance of the Topic

The presence of water in turbine oil critically affects equipment performance and lifespan. Excess water alters viscosity, density, and surface tension of the lubricant, accelerating additive depletion and chemical degradation processes such as oxidation, nitration, and varnish formation. Rapid, on-site detection of water content helps prevent premature turbine failure and supports reliable maintenance decisions.

Study Aims and Overview

This application brief evaluates the Agilent 5500t FTIR spectrometer for real-time monitoring of water in turbine oils. The study compares two FTIR-based quantitative methods—Beer’s Law absorbance and partial least squares (PLS) regression—against the conventional Karl Fischer (KF) coulometric titration. Objectives include demonstrating accuracy, reproducibility, and on-site suitability of FTIR analysis across a wide water concentration range (≈30–1500 ppm).

Methodology and Instrumentation

Sample Preparation and Standards:

• High-water standard prepared by hydrating used turbine oil and aging at 70 °C.
• Serial dilutions created with in-service and degraded oils to cover 22–3720 ppm water.
• Standards measured by KF titration (Metrohm 756 Coulometer) and FTIR in triplicate.

Instrumentation:

• Agilent 5500t FTIR spectrometer equipped for on-site analysis.
• Metrohm 756 Karl Fischer Coulometer for reference measurements.

Main Results and Discussion

Beer’s Law Model:

• Single-band O–H stretching absorbance calibration produced linear fit (R² = 0.996) when including high-ppm standards.
• Detection limit ~30 ppm; standard error of validation ≈40 ppm in the 20–270 ppm range.

PLS Chemometric Model:

• Multi-region spectral analysis delivered robust predictions across 30–1500 ppm (R² = 0.983).
• Standard deviations between PLS predictions and KF values were generally <30 ppm for the 0–700 ppm range.

Validation on 15 Unknowns:

• Both FTIR methods classified water concentrations into industry-relevant bands (<100, 100–200, 200–500, >500 ppm).
• PLS provided slightly better accuracy for moderate to high water levels (100–1500 ppm), while Beer’s Law excelled at <100 ppm.

Benefits and Practical Applications

• Analysis time reduced to ≈2 minutes per sample without overnight equilibration.
• No hazardous reagents or extensive sample handling required.
• On-site measurements maintain true water content by eliminating evaporation and demulsifier effects.
• Enables real-time lubrication health monitoring and timely maintenance interventions.

Future Trends and Applications

• Integration of proprietary water-stabilizer techniques (Agilent AN 5991-0672EN) to enhance accuracy across diverse oil types.
• Extension to other lubricant analyses such as additive depletion, oxidation, and nitration.
• Deployment within automated condition-monitoring systems for continuous turbine maintenance.

Conclusion

The Agilent 5500t FTIR spectrometer offers a fast, accurate, and reproducible alternative to Karl Fischer titration for water measurement in turbine oils. Its on-site capability, reagent-free operation, and robust chemometric calibration support proactive lubrication management and reduce downtime risk.

References

• Higgins F. Onsite quantitative FTIR analysis of water in turbine oil. Agilent Application Note 5990-7806EN. June 2013.
• Agilent Application Note 5991-0672EN: Onsite FTIR quantitative analysis of water in mineral-based oils using a novel water stabilization technique.