Deliquescent Breathers:

Bill Needelman, Donaldson Company, Inc. David Webb, Donaldson Company, Inc.
Tags: water in oil, contamination control

Water covers 70 percent of the Earth's surface, with the majority located in oceans, lakes and rivers. Water is necessary for life. Before the Industrial Revolution, water was the primary production force in manufacturing, transportation and agriculture industries, from water wheels to beasts of burden. Of course, this is no longer the case as industry currently relies on high-performance machinery. Most of these machines use oil for lubrication, heat removal and power transmission. Modern essential oil-wetted systems include hydraulics, steam and gas turbines, engines, motors, gearboxes and electrical transformers.

As essential as water is for biological life, it can be devastating for machine life. Along with particles from dirt and wear, water is one of the two most harmful contaminants. Water problems range from corrosion to oil degradation and from plugging gels to flourishing microbial colonies. Minimizing water contamination maximizes performance, fuel efficiency, productivity and machine life.

Water Problems
Below is a list of problems caused and/or aggravated by water.

Corrosion: It is a significant problem with free water in oil. It also produces abrasive oxides, such as iron rust that abrade surfaces, block clearances and break off to damage moving parts.

Loss of Film Strength: When water contaminates the film it displaces the oil. Water cannot keep the surfaces apart, resulting in high friction, adhesive wear and even seizure.

Oil Oxidation: Water accelerates oil oxidation. Negative consequences include excessive viscosity, acidity and insoluble resins.

Additive Depletion: When additives migrate into free water, the concentration of some additives falls below effective levels.

Hydrolysis: In the presence of water, ester-based additives and synthetic fluids (such as phosphate esters and polyol esters) decompose into alcohols and acids.

Reduced Fatigue Life: Dissolved water enters microcracks in rolling contacts, dissociates into hydrogen gas and weakens steel by hydrogen embrittlement.

Microbial Growth: Negative consequences include rancid foul odors, human health problems, biomass slimes, foaming and acidic oil.

Gels: Some additives interact with water to form gels. These gels foul flow passages, reduce heat rejection and plug filters.

Transformers: Even minute amounts of water contamination will reduce the life and efficiency of a transformer.

Contamination Control
For minimum protection, it is recommended to keep water below the saturation level, generally 200 to 500 ppm for many oils and 10 ppm for transformer oils. For optimum protection it is recommended to maintain water levels at or below 30 percent saturation, generally 75 to 150 ppm for most machines and 3 ppm for transformers. Maintaining water levels at or below 30 percent saturation alleviates the problems related to water as well as provides a safety margin against accidental spikes of contamination.

Donaldson has developed a new method for preventing the ingression of humid air. It is based on thin film technology and the fact that warm air leaving a reservoir (exhalation) has lower relative humidity than cool air entering a reservoir (inhalation).

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Figure 1. Inhalation

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Figure 2. Exhalation

The T.R.A.P.™ (Thermally Reactive Advanced Protection) is manufactured by coating the walls of a porous network with a thin film of water-absorbing chemicals called a deliquescent salt. The resulting high surface area of absorbent provides rapid removal of water vapor from air while maintaining size and weight. Unlike desiccant breathers, the open porous structure presents minimal air flow restriction, so fluid flow into and out of the reservoir is not impeded. In addition, the proprietary absorbents are not sensitive to oil mists entrained in the air leaving the reservoir.

The breather unit includes a pleated 3 µm filter to protect against the ingression of hard abrasive contaminant particles that contribute to the wear of mechanical components. It is manufactured from materials that can be safely disposed of or recycled.

As illustrated in Figure 1, during "inhalation" cold humid air entering the system is drawn over the large absorbent surface area inside the breather. The high humidity drives water into the absorbent and the majority of water vapor is removed. This dry air maintains the water concentration below 30 percent saturation. Once inside the system, the air contacts the warm fluid and metal surfaces which increases air temperature and further reduces the air's relative humidity.

During exhalation (Figure 2), warm, dry air passes over the same absorbent. The low humidity air pulls water out of the thin films of absorbent. The rehumidified air exits the unit and is emitted into the surroundings.

The difference between the desiccant breathers and deliquescent breathers is the deliquescent's ability to release moisture back in to the air at standard temperatures. For a desiccant breather to release moisture the temperature need to be in excess of 360°F. The ability of the deliquescent to release moisture at standard temperatures gives the unit the ability to last longer.

The deliquescent breather is either absorbing or releasing moisture, depending upon the humidity of the air. This difference in relative humidity - high during inhalation and low during exhalation - is the force that drives the process. It is also aided by the temperature change to increase the difference between the low and high humidity air. The heating of the air in the headspace creates an air that is dryer than the air that went through the breather. The result is dry oil and regeneration of the absorbent during each cycle.

Water contamination causes major problems in oil-wetted machinery. Upon invading the system, its presence may be hidden from the outside eye. Preventing water ingression in an oil reservoir is the best cure for contamination. Desiccant breathers are successful at reducing the ingression of moisture, but they are limited by their low water-holding capacity and the need for frequent replacement. The self-regenerating breather, the T.R.A.P.™, effectively maintains the water concentration well below the saturation level of the oil. It is small, lightweight, and doesn't harm the oil.

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