by David Kolstad, Pentair
December 17, 2011

Lubricated machines require clean, dry oil to work properly. Recent studies have determined that as much as 80 percent of all failures and wear problems that hydraulic and lubricated equipment experience are due to oil contamination. The most damaging forms of contamination are particulates and moisture.

All oil lubricated machines with rolling elements and pressure surfaces rely on the oil to separate moving surfaces. Examples include gears, roller bearings, journal bearings, side bearings, piston pumps, vane pumps, servo valves and dynamic seals. Operating pressures and loads on those elements dictate the required oil film thickness, and the film thickness that can be provided by the oil depends on its viscosity and temperature.

Particulate contamination is capable of bridging the film thickness gap, leading to contact fatigue, additional particle generation and accelerated wear. Water contamination reduces component life and productivity by causing rust and corrosion, oil oxidation, additive depletion, varnish deposits, hydrogen embrittlement and changes in viscosity. In addition, these phenomena can result in additional internal particle generation.

The Challenge

Particulate contamination is present to varying degrees in new oil, can be generated internally or may occur due to ingress during a lubricated machine's operation. As industry becomes more aware of the damaging nature of particulates, they have become more focused on the need for filtration. High efficiency (beta rated) filtration is widely accepted, and OEMs now often incorporate full-flow filtration into new equipment. However, in many cases there is still room for improvement.

Moisture presents a greater challenge, with some machines more at risk than others. Since water solubility will vary based on oil type and temperature, even a small amount of excess moisture can lead to equipment failure. Water can be present in the free, emulsified and dissolved states, and it typically infiltrates lubricated machines through condensation, seal leaks or ambient conditions, as well as from cleaning chemicals and water from equipment cleanup. For optimum performance, all forms of water contamination must be removed. Several methods are used for this purpose, but some have limitations.

Available Solutions

Some removal methods are better suited for one type of moisture than another. For free water removal, water- absorbing cartridges, settling tanks, centrifuges and coalescing filter/separators are often used-but many are high in cost and require significant labor. Their effectiveness can depend on the viscosity of the oil and volume of water.

Water-absorbing cartridges use hygroscopic media capable of removing trace amounts of free water from oil. Water-absorbing cartridges can be installed for low initial capital cost, but they are typically rated in grams of water per element and are not appropriate for systems with ongoing moisture ingression. They may not effectively remove emulsified or dissolved water and must be replaced when saturated, making them labor and cost-intensive for applications involving higher volumes of water.

Settling tanks, as their name indicates, are used to settle free water to separate it from the oil. These systems do not remove emulsified or dissolved water, or gases. While relatively inexpensive, effectiveness is a function of tank design, residence time, turnover rate, oil viscosity and additive chemistry. Due to the relatively large size tank needed even for small systems, space limitations often make settling tanks impractical.

Two additional systems, centrifuges and coalescing filter/separators, mechanically separate free water using gravitational forces. These are best suited for use on lower viscosity fluids and can be used to remove significant volumes of gross water. However, new centrifuge installations are costly, and with a large number of moving parts, this equipment can be labor-intensive and require dedicated operators. Coalescing filter/separators are subject to fouling from particulate contaminants and certain additives, and this equipment may require heat plus discharge cooling.

To remove dissolved and emulsified moisture as well as free water, vacuum dehydrators (sometimes referred to as a vacuum purifiers) have often been used. Such systems are typically placed in slipstream (kidney loop) oil circulation with the reservoir and use a vacuum to lower the boiling point of water, allowing moisture and gases to vaporize out of the oil. Vacuum dehydrators have moderate-to-high capital, operating and maintenance costs due to their relatively complex designs. In addition to requiring trained operators, they often require a heater to increase oil temperature to promote water vaporization and a cooler on the return. Along with these challenges, as with some oil purifiers, vacuum dehydrators are relatively large and difficult to relocate to different areas of a plant.

Since conventional water and particulate removal alternatives involve significant trade-offs, a new, advanced form of fluid conditioning has proven beneficial. This conditioning system was developed to use the fundamental principles of mass transfer. The system continuously removes all free, emulsified and dissolved water, as well as particulate contaminants-without the complexity and operator knowledge required with conventional methods.

Mass transfer principles dictate that moisture will naturally diffuse from a region of higher concentration to one of lower concentration. With an innovative filter-dehydration system, free, emulsified and dissolved water will diffuse from the oil to the system's dehydration contactors.

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