Many of today's hydraulic systems contain insoluble contaminants, which are too small to be trapped by filters — and too small to be detected by particle counting. This varnish formation is a serious problem that degrades system operation. Conclusive proof now exists that the root cause of tar and varnish formation is the buildup of insoluble soft contaminant oxidation byproducts.

What causes the formation of these insoluble soft contaminants? The process of oxidation is caused by a combination of water, heat, and oxygen. The heat is caused by static discharge, a process discovered several years ago that damages a filter's center core. Now, with additional study, there is solid visual proof that static discharge occurs throughout the entire hydraulic system, causing the formation of tar and varnish and reduced system performance.

Static discharge is generated through friction within the oil, and can generate sparks with temperatures greater than 10,000° C — hotter than the surface of the sun for a nanosecond. The heat generated "cooks" the oil, speeding additive decomposition and the formation of oxidation byproducts that coat tacky layers of varnish on all internal metal, rubber, and synthetic material surfaces. Over time, hard contaminant particles stick to the tacky varnish to create a sandpaper-like surface. The end result is sticky valves and reduced system performance. Left unchecked, system failure will occur.

In addition, the high acid content in varnish can cause brittleness in system seals and O-rings. Because the tacky nature of varnish can attract larger particles, seals in moving parts can be cut or otherwise damaged to the point of significantly reducing seal life.

There is good news: varnish formation can be eliminated before it causes problems. And, if you are already dealing with varnish issues, you can reverse the process back to the point of having clean oil continuously.

The culprit is heat

Hydraulic systems have four heat sources:
friction — the combination of oil against the metal surfaces in the system, as well as metal parts rubbing together, can generate temperatures between 180° and 450° C.
adiabatic compression — similar to dieseling, can generate temperatures between 600° and 900°C.
dark stream spark discharge — with no flash of light, temperatures between 5000° and 10,000° C can be generated.
full spark discharge — with flash, will generate nano-second temperatures from 10,000° to 20,000° C.

Note that spark discharge also produces metallic ions and water. Oil molecules have hydrogen, and oil contains 9.5% air, 21% of which is oxygen.

Spark discharge can occur by friction of the oil, high pressure and temperature, or pressurization of filters in a return line. Friction alone is sufficient to cause spark discharge. If your hydraulic system has the combination of high pressure and temperature, spark discharge will definitely occur.

Spark discharge in a filter

You would logically expect to see varnish form on the upstream side of filter elements. But, for years it has been observed to form on the downstream side. The reason is spark discharge that takes place between the pleated edges and grounded center core.

Figure 1 shows an example of spark discharge damage on a traditional filter after three months of use. The inner surface deposits are shown on the center core, looking down from the top into the grounded filter. Sheared free radicals are absorbed on the inner surface and propagate.