Varnish Kills Quietly
EP Editorial Staff | January 21, 2020
Operating conditions that cause lubricant degradation can result in damaging deposits on machinery surfaces.
By Mark Barnes, PhD, CMRP, Des-Case Corp.
Particle contamination is the number one cause of lubrication-related failures in rotating and reciprocating equipment. In fact, most industry experts agree that as many as 80% of hydraulic-system failures and 50% of bearing failures are caused directly or indirectly by particle contamination. What is less understood is the role that varnish plays in hydraulic- and lubrication-system reliability.
Varnish is a ubiquitous term used to describe the formation of deposits within a lubricated system. While that term might imply a single process at work, there are 20 to 30 lubricant degradation pathways. Each pathway involves complex tribo-chemical interactions and reactions between the lubricant base oil and additives, and system components such as pumps, valves, piping, coolers, and filters.
Varnish is like bad cholesterol in the human body. Once too many deposits form in a manufacturing system, performance is impaired, resulting in reduced oil flow, sluggish operation, loss of system control, bearing deposits, and pump failure.
The three most common ways that varnish forms are oxidation, thermal failure, and static discharge. In each case the physical and chemical processes at play are very different. As such, the byproducts of lubricant degradation are also different. However, what is common is that each results in the formation of soluble and/or insoluble deposits inside the hydraulic or lubrication system as the lubricant degrades.
Oxidation is the most common degradation pathway. Oil oxidation involves oxygen reacting with organic and organometallic components of the base oil and/or additives. Initially, oxidation generates intermediate compounds such as aldehydes, ketones, and peroxides. These byproducts are very reactive, particularly in the presence of metal catalysts within the lube oil or hydraulic system. As these chain reactions occur, smaller molecules chemically combine to form larger molecules. Eventually the molecules fall out of solution and deposit on surfaces.
Like any process in which a substance is dissolved in a liquid, the solubility of oxidation byproducts and other varnish-forming degradation pathways is temperature dependent. As such, varnish formation tends to occur in cooler areas within the system or where the oil remains static for extended periods of time.
Like most chemical reactions, oxidation rates increase with increasing temperatures. In fact, it is generally recognized that once oil temperatures reach/exceed the 130 to 140 F (55 to 60 C) range, the oxidation rate increases by a factor of two for every 18 F (10 C) increase in temperature. Oil oxidation is also an autocatalytic reaction, meaning the byproducts formed upon initial oxidation act as a catalyst for further oxidation. As such, once oxidation starts to occur, it tends to increase fairly rapidly unless remedial action is taken immediately.
Thermal failure doesn’t require oxygen. It results from heating alone. For most oils, temperatures need to exceed 400 F (200 C) for thermal degradation to occur, resulting in “burnt on” deposits. In most lubrication and hydraulic systems, thermal stress occurs through direct contact with a hot metal surface or by adiabatic compression. Adiabatic compression occurs any time the oil becomes aerated and passes from a low-pressure zone to a high-pressure zone such as in a hydraulic pump.
Static discharge occurs because most oils are fairly non-conductive, which is why oil has been used for many years to prevent arcing in transformers. In lubrication systems, as the oil flows through pumps, pipes, valves, and filters, static electricity can build up. Because the oil has very low conductivity, this static charge is retained by the oil rather than discharged to machine surfaces. As the voltage increases, dielectric breakdown can occur causing arcing from the oil to a metal component within the system, much like a lightning strike. The result is a very high temperature along the arcing pathway that thermally stresses the oil and causes deposits to form. In some instances, special filter media can be deployed within the oil-filtration system to help dissipate static charge and prevent voltage buildup within the oil.
Effects of Varnish
Varnish affects not just the fluid and components but system function as well. For example, Figure 1 shows the force required to move a hydraulic valve after a defined amount of dwell time in “new” fluid versus a degraded (oxidized) hydraulic fluid.
As the data shows, it takes twice as much force to move a servo valve that’s been idle for 10 min. or more in degraded fluid than it does in new fluid. Depending on the application, this extra force can cause the solenoid to work harder, increasing the likelihood of an electrical failure. Sluggish response times, often referred to as valve stiction, can also have a negative impact on precision-control circuits in critical hydraulic applications.
The secret to controlling varnish is to first know you have a problem. While routine oil analysis on hydraulic systems should be done monthly, it should be supplemented two to four times a year with an MPC (membrane patch colorimetry) test. This test indicates the “varnish potential” of a fluid by measuring the fluid color and the weight of small (0.45 μm and greater) particles suspended in the oil. Based on this, an MPC number, on a scale of 0 to 100, is assigned. For critical hydraulics, the MPC number should always be less than 20, but lower is always better.
Once varnish has been identified, rapid removal is paramount. While there are several ways to do this, the most effective, fastest, and most broadly applicable method is to use specialized varnish-removal cartridges. The cartridges force the oil through special cellulose media made with long-strand fibers. As the oil flows through the fibers and capillaries within the media, chemisorption and physisorption occurs, stripping varnish and varnish precursors from the oil. This approach is preferred over varnish-control methodologies that use electrostatics because cellulose is not affected as much by water and can be used at high temperatures where precursors are more soluble.
Unlike particles and moisture, varnish is often a “hidden” contaminant that lurks insidiously in the background, waiting to create problems. While varnish doesn’t always occur in hydraulics and lube-oil systems, if lubricating oil or hydraulic fluid has been in service for too long or you are experiencing an unacceptable number of valve failures, even if the fluid tests clean, take a sample and perform an MPC test. You may be glad you did. EP
Varnish: The Silent Machine Killer
Learn more about how varnish forms, its impact on machinery, and how to deal with it by listening to our webinar on the subject. Go to efficientplantmag.com/2019varnishwebinar to learn more about this silent killer.
Mark Barnes, PhD, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN (descase.com). He has 21 years of experience in lubrication management, oil analysis, and contamination control and has published more than 150 technical articles and white papers.