Lube-Tech
colder surfaces, such as cold elbows in the system, tank walls or any cooling devices. If a cooler is subject to sludge build up, the heat exchange capability will be reduced; this will raise the system temperature and further contribute to the oil ageing process.
If these symptoms are occurring within a system, what can be done to confirm that varnish is the cause?
Laboratory testing is the definitive solution to determining whether oxidation products are at fault, and there are a variety of tests to consider.
Figure 4: MPC test equipment
Spectroscopy; through infrared absorbance, different compounds can be identified thanks to their unique spectroscopic signatures. Oxidation products are one such set of compounds that can be observed through FTIR; identifying spectrum peaks around the 1750 cm-1 region typically translates to the presence of these materials.
The MPC (Membrane Patch Colourimetry) Test – traditionally developed for turbine lube oil applications, this procedure extracts any insoluble contaminants onto a filter membrane and assigns an index value based on colour; essentially, the darker the membrane, the greater the risk of varnish deposition in the system. The number scale is typically interpreted as follows: • 0-30 Good – No apparent varnish problem at the moment.
• 31-50 Action – Increased value due to soft contaminants caused by oil degradation. Varnish deposition on bearings, valves and gears as well as reduced filter lifetimes are to be expected.
• >50 Critical – MPC value has drastically increased due to soft contaminants caused by advanced additive depletion and oil oxidation. Varnish has deposited on bearings, valves, tank and gears. Very short filter lifetimes are to be expected.
Many different tests are available and choosing a wider range of analyses will provide the most information to allow for a greater informed decision. Once tests have confirmed the presence of varnish, what can then be done to mitigate this damaging material? A variety of options are available, dependant on factors such as oil volume, budget and accessibility. Typically on large volumes, the varnish can be removed from the oil via conditioning units. These units are able to efficiently remove these oxidation products by cooling the oil and filtering out the precipitated varnish. Where the oil volume is low, it may not be cost effective to pursue this; in these instances, an oil change and complete system flush is recommended.
As the title of this article suggests we are looking at using traditional optical particle counters as a measurement technique for varnish by manipulating the test temperature, either online or in laboratory settings, to determine the particle distribution in the
LUBE MAGAZINE NO.151 JUNE 2019 25
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.122 page 4
The RULER (Remaining Useful Life Evaluation Routine) – a technique using linear sweep voltammetry that measures the phenolic and aromatic amine antioxidants; by comparing used and new samples, a percentage of useful remaining antioxidant content can be obtained. These antioxidants act as a scapegoat for the base oil, reacting with oxygen before any harm comes to the oil itself; therefore it stands to reason that, if the antioxidants have depleted, the rate of base oil oxidation will increase and more varnish will build up within the system. A final analysis of note here is Fourier Transform Infrared Spectroscopy (FTIR).
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