Varnish: The cholesterol in your plant equipment

Consider it the cholesterol of industrial equipment, because what it can do to machinery is almost identical to what hard, plaque-like cholesterol buildup can do to the cardiovascular system. We’re not talking HDL here, folks. There’s nothing healthy about this stuff. Varnish is a nasty, hard, sticky substance that forms on the surface of machinery components.


Consider it the cholesterol of industrial equipment, because what it can do to machinery is almost identical to what hard, plaque-like cholesterol buildup can do to the cardiovascular system.

We’re not talking HDL here, folks. There’s nothing healthy about this stuff.

Varnish is a nasty, hard, sticky substance that forms on the surface of machinery components. Just as cholesterol builds up in arteries and slows the flow of blood, varnish slows and, if left unaddressed, eventually stops the flow of lubrication through a machine. That’s a scenario manufacturers can’t afford.

As manufacturers demand more of their machines and components during continuous operation, Lean manufacturing and other initiatives, wear and tear increases. Machines generate more heat, and additional heat is murder on lubricants. It’s the number one cause of lubricant deterioration and degradation and is the key factor in the development of varnish and its close associate, oxidation.

What is varnish?

“Varnish is a thin, insoluble film deposit that occurs on the interior of lubricant systems and on components,” said Glen Sharkowicz, industrial lubricant specialist for ExxonMobil. “It’s usually sticky and difficult to remove by simply wiping it off.”

Varnish is composed of lubricant degradation byproducts that range in color from light yellow to dark brown, he said. The byproducts are high-molecular-weight substances that are unstable in the lubricant, making them insoluble.

“The chemical compositions of these insoluble materials vary from case to case with differences in additive technologies, outside stresses and contaminants affecting what are insoluble,” said Paul Sly, business solutions specialist, Isoclean Program for Chevron Products Company. “As these components that are normally dissolved in the oil become insoluble, they create soft, sub-micron particles that are polar and seek out other polar surfaces, like metal, to attach to.”

Consider lubricant formulation, filtration and oil analysis to reduce the likelihood of varnish forming on components.
Photo courtesy of Shell Lubricants

These deposits are small and soft, making them difficult to filter out. Often sticky once deposited, they can attract other solid particles in the oil flow, increasing the film thickness, Sly added.

What causes varnish?

Several factors contribute to the onset of varnish, but the one common catalyst for its production is high temperature.

“The hotter it gets, the more the varnish will come up and it will get very hard and brittle,” said Anne Brantley, a product manager for Alemite LLC, Fort Mill, SC.

As the temperature rises, chemical changes can occur within the lubricant. Both thermal degradation and oxidation deprive the oil of effectiveness and can contribute to the development of varnish.

“The cause is generally the result of one of two things: either thermal degradation or oxidation,” Sharkowicz said. “Thermal degradation is basically the destruction of an oil molecule by high temperatures, without oxygen being present. Oxidation is the reaction of the hydrocarbon in the lubricant with oxygen.”

During oxidation, a lubricant’s molecules react with oxygen and create acids, explained Walt Silveira, U.S. technical manager for Shell Lubricants. The continued increase in temperature induces, “an additional chemical change in the higher molecular-weight molecules that create varnish. It’s kind of like oxidation on steroids,” Silveira said.

While heat is a leading cause of varnish conditions, it’s not the only cause. Sly indicated that recent studies show formulation to be a factor in how resistant a lubricant can be to varnish.


Varnish is a notorious problem in turbine and hydraulic system

Photo courtesy of Shell Lubricants

The mess varnish causes

“The causes of varnish are varied and many are just beginning to be understood,” Sly said. “A very short time ago varnish was suggested to correlate with higher grade oils %%MDASSML%% group II and synthetic %%MDASSML%% but we have since learned that this was a misperception.

“More recent studies indicate a specific oil’s varnish tendency is most closely related to good formulation,” he continued.

If undetected or unaddressed, the consequences of varnish are disastrous.

“As this thickness increases in critical areas, oil flow and machine tolerances can be affected,” Sly said.

“You get reduced clearances where you need lubrication. It can create higher operating temperatures,” Silveira said. “It can plate out on the filters.”

Often, the filters will become covered with a sludgy, brown film, Brantley said. “You also see separation sometimes. If there’s a lot of heat, oils will separate.”

“It’s kind of like a catalytic effect: once you start to get varnish in the equipment, it starts to actually cause the oxidation process to accelerate. It gets plated out inside the piece of equipment, and it actually will attract other materials. Wear particles that may be generated and come through the equipment may be attracted to it, which may diminish the clearances,” Silveira said.

Varnish deposits, as seen on this spool, are difficult to remove and typically require high-velocity oil flushing with a cleaning agent.

Photo courtesy of Shell Lubricants

“Now all of a sudden, you’re not going to be able to pump any fluid. It’s just going to become a hard cake,” said Brantley.

“Even a small amount of varnish on the interior of (a servo) valve or on the filters that precede the valve can cause the valves to not operate or be faulty,” Sharkowicz said.

Dealing with varnish

Once varnish begins to form, it can be difficult to remove because of its insolubility. Methods for its removal exist, but the best way around it is to avoid it all together.

“As existing varnish can act as a catalyst for future varnish, removing old deposits when possible is a good idea,” Sly said.

“Typically, the only way to remove varnish from a system is through flushing %%MDASSML%% high-velocity oil flushing,” Sharkowicz said. “You would actually take the system and have oil at a high enough velocity that it becomes turbulent, and circulate that through the system. That’s usually done in conjunction with some sort of a cleaning agent.”

To prevent varnish conditions, careful maintenance is key. Lubricant formulation, filtration and oil analysis are a few recommended considerations to minimize the chances of varnish developing in systems.

“(Varnish) can be avoided. One of the key things is getting the right product in the application with the right well-balanced formulation,” Sharkowicz said. Keeping the oil free of contaminants, particularly dust and water and monitoring it to ensure it runs within the proper temperature ranges, all help avoid varnish deposits, he said.

Conducting regular oil analysis helps end users identify additive depletion and lubricant contamination %%MDASSML%% two factors in the development of varnish.

Photo courtesy of A2 Technologies

“Sometimes specialized filtration can help avoid having varnish deposits,” Sharkowicz continued. “Things like balance-charged agglomeration or electrostatic filters. Both of those filter types are specifically designed to remove varnish precursors from the oil through electrostatic charge.

“Electrostatic filters will have two plates in the filtration unit that are charged oppositely. As the oil flows through it, the polar materials %%MDASSML%% the varnish precursors %%MDASSML%% will be attracted to those plates. Balanced charge agglomeration is similar, but in this case, what you’re doing is taking this stream of oil coming into the filter and separating it. One stream gets charged negatively and the other stream gets charged positively. Then you recombine those two streams of oil and those oppositely charged particles agglomerate, and then they can be filtered out.”

Depth media filters may be another option for filtering varnish deposits, Sly said. Some of these filters are specifically designed to filter varnish precursors and can remove them, along with other small, hard particulates, before they are deposited on machinery components, he said.

Electrostatic or balanced-charge agglomeration filters can prevent the build-up of varnish on components such as servo valves or this bushing.

Photo courtesy of ExxonMobil

“Examine your filters very closely for burned spots,” Sly added. “If dry oil is pushed hard through a very fine filter media, it can build up an electrostatic charge in the oil which can arc across to the filter pleats or end caps. Close examination of the filter media has sometimes shown burnt spots, which is clear evidence of this electrostatic discharge. Using larger-pore filter media or using parallel-flow through a normally duplex filter can sometimes help if this is happening.

“Do thorough oil analysis on large oil reservoirs to monitor additive depletion. Minimize temperature variation throughout the system where practical. Sometimes heat tracing of servo valve supply lines can keep oil temperature warm (and precursors in solution) in these critical areas,” Sly said.

Consequences of varnish

Consequences of varnish deposits, according to Paul Sly, business solutions specialist, Isoclean Program, for Chevron Products Company:

  • Reduced oil-film clearance

  • Bearings run hotter

  • Sticking servo valves

  • Lower efficiency in heat exchangers

  • Reduction of heat transfer

  • Plugging of last-chance filters

  • Oil foaming

  • Increased component wear

  • Attraction of abrasive contaminants

  • Catalytic deterioration of the lubricant.

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