Spark discharge inside a reservoir is shown in Figure 2. The top access plate has been removed to show a 6-in. return line. The nanosecond long spark being generated without the use of a filter is 20,000° C. A video of this spark discharge test can be found at

To eliminate the potential of oilto-air friction from a grounded surface, make sure that return oil pipes actually come down into the oil. Otherwise, a situation exists much like that of a big capacitor, where a charge builds in the air to the point of discharging and burning the oil.

Oxidation byproducts
Wherever oil touches any part of the hydraulic system, it deposits varnish. If varnish is in your reservoir, you will find it throughout the system — anywhere and everywhere the oil comes in contact with a surface. In addition to metal surfaces, varnish formation will affect seals and Orings. It does not make a difference what type of hydraulic fluid is used. Even chemical blends, such as phosphate esters, will generate varnish.

The electrostatic solution

Traditional oil cleaning has been through strainers, centrifuges, and mechanical filtration. These methods are effective in removing hard contaminants. But the only way to remove the insoluble sub-micron size soft oxidation products is through continuous electrostatic oil cleaning that is independent of contaminant size.

Because oil always seeks equilibrium, electrostatic cleaners use the oil in the system to clean the system. This balancing process draws the oxidation by-products from the system surfaces back into the oil where they are removed through a process of re-absorption.

The electrostatic contamination collection process is shown in Figure 3. Varnish, by its nature, is polar — both positive and negative. A combination of a high voltage dc plate along with a ground plate is used in the electrostatic collector. The positive polar varnish is pulled to the negative side. Negative charged particles are pulled to ground and repulsed. The sub-micron size oxidation byproducts are trapped in a cellulose collector on a continuous oil-cleaning basis.

Electrostatic oil cleaners can be an essential part of good oil maintenance. However, they should not be solely relied on because they only remove the soft contaminants that cause varnish.

Proven benefits

Continuous electrostatic oil cleaning to remove the sub-micron size soft contaminants yields several proven benefits:
avoids unplanned system outages
eliminates the need for costly system flushes
significantly enhances mechanical filter, seal, and O-ringlife
reduces oil consumption and nearly eliminates oil disposal costs, and
is virtually maintenance free.

The use of continuous electrostatic oil cleaners will change the percentages in your favor. Instead of having to deal with the 85% of failures traced to static discharge and oxidation related problems, you will only have to be concerned with the remaining 15%.

Lewis Lucas application engineer at UAS Kleentek, Cincinnati. For more information, email or visit