In today’s industrial world, going small can result in big savings. Hydraulic systems, for example, are getting smaller as part of a larger effort to improve fuel economy in industries across the board. The logic behind this is that a physically smaller hydraulic system weighs less, and that translates into greater fuel economy. However, such a size reduction is not accomplished without risks.
First, flow rates are increased relative to oil volumes as a result of less oil use. Issues related to oil residence, such as very short oil residence times because of the compact nature of the smaller systems, can lead to situations in which oil remains at elevated temperatures for extended periods of time. The reduced residence times also mean that the oil has insufficient time to cool to an ideal level. In some cases, temperatures as high as 266 F have been measured. This kind of heat can result in a high amount of oxidative and thermal stress on hydraulic oil.
Another undesirable effect from reduced residence times in the reservoir is that contaminants—such as foam, water and wear debris—are unable to settle. Instead, the contaminants remain suspended in the thermally stressed oil, causing it to break down and form varnish, a tacky substance.
Varnish can cause poor hydraulic valve response, which occurs because of sludge and varnish buildup. Varnish can also result in an increased need to replace blocked filters. When varnish forms, it creates a chain reaction within the hydraulic system, ultimately causing erratic operation, increased wear on valves and pumps, shorter oil life because of increased oxidation, less fuel efficiency and increased downtime.
What Is Varnish?
When polymerized oil oxidation products and decomposition byproducts are produced as a result of thermal breakdown, varnish forms. Varnish is identified by its sticky brown appearance. It typically forms on pump housings, reservoirs and filters, and valve spools and bodies (see Image 1).
Image 1. Varnish coats the inner walls of a hydraulic fluid reservoir. Varnish deposits first appear as a soft, tacky brown coating on metal surfaces. If left untreated, it changes into a solid glaze that is difficult to remove.
Varnish is primarily composed of organic residues. As the hydraulic oil ages, more varnish is formed. This varnish can come in and out of solution with changes in temperature.
As the problem with varnish became more apparent, particularly in large turbines, one of the attempted solutions to prevent varnish (installing very fine filters to keep the oil cleaner) had the opposite effect and increased its formation. Varnish can form because of a high-temperature static discharge across high-flow filters. This phenomenon—called microdieseling—is caused by an implosion of entrained air bubbles, causing tremendous spikes in localized temperatures, some as high as 1,800 F. Such high temperatures can crack the oil molecule, generating auto-oxidation and creating varnish.
Varnish is polar, similar to many of the additives contained in hydraulic oils. The polar bonds present in varnish are attracted to metal surfaces—such as servo valves and pump components. As it coats metal surfaces, it prevents anti-wear additives in the oil from attaching to the metal. This results in increased wear on the engine and its metal components. Worse yet, the sticky-soft nature of the varnish attracts wear-causing debris and accelerates wear.
Why Is Varnish Bad?
Generally speaking, most operators realize that varnish has formed when the hydraulic actuator begins to operate erratically. The cylinder will appear to stick, but it is actually the spool in the valve body that is not operating freely. If operators are unaware of the cause of this sticking, they may inadvertently cause more equipment damage by calling for more flow than is needed.
Varnish increases friction, which negates any power savings that were gained by the smaller systems. Such friction also generates wear. Since varnish tends to attract and hold onto wear metals, friction and wear are increased. This wear can occur in the valves, the cylinders or the pump.
Cleaning Existing Varnish
Until recently, operators and maintenance professionals only had two key prevention techniques to rely on to prevent/limit varnish formation:
- Use a higher-quality oil with improved base stocks and increased levels of additives
- Implement a high-end filtration system
The oil’s potential to form varnish can be tested and continually monitored throughout its life cycle. As mentioned earlier, the use of high-level filtration has been shown to increase varnish formation. The microdieseling that takes place during filtration not only causes varnish to form but also breaks down the oil and additives, leading to increased wear and erratic operation.
Fortunately, a number of additive systems have been developed in recent years that simultaneously clean existing varnish and prevent future vanish formation on the metal components of hydraulic systems.
For example, one oil manufacturing company’s specially formulated varnish cleaner was submitted to a test using a vane pump. The test was run for 1,000 hours to allow for the formation of varnish precursors in a typical mineral-based hydraulic fluid. A valve was taken from a field application and submitted for the study. It rated 5.5 on a scale of 1 to 10, in which 1 is dirtiest and 10 is cleanest.