Lube-Tech
Introduction Varnish formation is a by-product of oil that has oxidised and degraded over time. The source of varnish formation can also come from electro- static discharge, spark discharge, micro-dieseling, hydrolysis, thermal degradation, fluid incompatibility and catalysed wear particles just to name a few. Initially varnish will form as micro-agglomerations that remain soluble in the oil, over time varnish continues to form saturating the oil. As a result, the suspended varnish will begin to fall out of solution and plate out and adhere to the surfaces of the system. Varnish can take on various formations varying from gummy like agglomerations, hardened abrasive particles to baked on layers on the surfaces of the system. These formations will increase wear in a system, restrict moving mechanical parts, plug filters and even create hot spots leading to a loss of heat transfer and increase in operational temperatures.
For more than 30 years, varnish has been plaguing turbines, hydraulic and compressors. Causing issues with valves and spools sticking and fail to start modes, varnish can also act as an insulator of heat, creating hot spots and micro-dieseling issues. Several useful tests have been developed to help detect varnish and they have been a staple for monitoring varnish formation and aide in the mitigation of varnish. While very useful, they are not able to detect varnish type
26 LUBE MAGAZINE NO.169 JUNE 2022
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.140 page 1
Varnish Analysis: Introducing New Testing Techniques
Elaine A. Hepley CLS, OMA, Data Analysis Manager, POLARIS Laboratories®
formation, soluble varnish or the presence of wear because of varnish. Below is a list of common tests performed to monitor varnish formation.
Test Methods
MPC ASTM D7843 – Detects the potential for varnish formation that is insoluble by measuring the concentration of colour and includes the L*a*b* values. Provides overall severity.
S-MPC (internal method) – Detects the potential for varnish formation by measuring colour concentration that is both soluble and insoluble in the oil and includes L*a*b* values. Provides overall severity.
RPVOT ASTM D7220 – Detects the fluid oxidative stability, remaining useful life. A value of <25% can mean the oil has undergone significant oxidisation, and the potential for varnish can increase. Reported in minutes.
LSV (Ruler) ASTM D6971 – Measures the depletion of antioxidants (amines and/or phenols) through applied current. As the antioxidants deplete, the potential for varnish will increase. Reported as % of new.
FTIR – ASTM E2412 – Detects the oxidation value of the fluid as well as phenolic degradation and chemical change.
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