SYMPOSIA PAPER Published: 01 December 2022
STP163420200079

Lubricant Condition Monitoring by Fluorescence Spectroscopy

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Varnish is produced by oxidative lubricant breakdown, and its detrimental impact on the performance and reliability of critical industrial systems is well established. Indeed, varnish leads to numerous problems, including filter plugging, restricted flow, poor heat transfer, valve sticking, and of particular impact in gas turbine applications, costly unit trips. For this reason, industrial lubricants contain antioxidant additives to protect their base oils from oxidation and the concomitant formation of harmful varnish deposits. Depleted antioxidant additive levels are among the turbine oil-replacement criteria outlined by ASTM D4378. Antioxidant levels are currently monitored directly and indirectly by ASTM D6971 and ASTM D2272, respectively. However, these techniques require expensive laboratory equipment, sample preparation, consumables, and operation by trained personnel. Moreover, linear sweep voltammetry is a relative measurement that requires antioxidants to be efficiently extracted from lubricant samples; in cases in which these additives are not fully extracted into the measurement solution, dubious results may be obtained. This paper discusses the development and application of an alternative technique using fluorescence spectroscopy that allows for more efficient, direct, and repeatable monitoring of antioxidant additive levels. Initially, fluorescence excitation-emission matrix spectroscopy was used to identify spectral regions of interest corresponding to remaining and spent antioxidants. The spectra obtained in these regions were shown to correlate with the breakdown of most commercially available turbine lubricants under lab conditions. Several hundred in-service turbine oil samples were also measured and used to develop “condition indicators” that can alert end users to impending oil-related problems. Critically, fluorescence spectroscopy is well suited to real-time, on-line monitoring of antioxidant levels and oil condition. The use of an on-line fluorescence-based sensor would allow end users to make better and more timely maintenance decisions than is possible using existing lab-based techniques.

Author Information

Sosnovski, Oleg
Gastops Ltd., Ottawa, CA
Vieira, Marianna
Gastops Ltd., Ottawa, CA
Suresh, Pooja
Gastops Ltd., Ottawa, CA
Hobbs, Matthew, G.
EPT, Calgary, CA
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Pages: 122–143
DOI: 10.1520/STP163420200079
ISBN-EB: 978-0-8031-7715-4
ISBN-13: 978-0-8031-7714-7