Water can be present in lubricating oil in three different forms, dissolved water, emulsified water and free water. Dissolved water refers to water that has been chemically absorbed into the oil. Emulsified water is water that maintains its chemical integrity, but is held in suspension in the oil by additives and contaminants. Free water describes water that is present in the oil, but not held in suspension.

Effects on Lubrication
In general, water is a poor lubricant. Proper lubrication is dependant on the formation of an oil film to separate opposing friction surfaces. When pressure is applied to an oil film, the oil’s viscosity will increase proportionally to maintain protection. Water does not exhibit this tendency and will cause boundary lubrication to occur where full fluid film lubrication would otherwise be present.

Alarm Limits
Water is considered to be among the most detrimental of possible contaminants. Studies have shown that water present at any level can reduce bearing life from 10 to 100 times. While the overall tolerance for water varies from machine to machine, a good general alarm limit for water contamination is 0.05% (500 ppm). Systems may be more tolerant to water, while others may be more sensitive. Some manufacturers have set limits as low as 30 ppm. Best practice would tend to indicate that it is beneficial to keep water contamination at the lowest level possible.

Controlling Water Contamination
Controlling water contamination requires two individual considerations. First, we must address the issues of keeping water out of the system. Water can contaminate a system by condensation when moist air enters the component and a change in temperature causes the water to condense and drop into the oil. Water can also enter a system through non-drying breathers, faulty hatches and seals, and internally from leaking heat exchangers.

Second, steps must be taken to remove water once it has entered a system. This can be accomplished with physical filtration, vacuum dehydration or centrifuge. For systems prone to contamination, these types of purification can be installed in the system to remove water continuously or on demand.

Systems that are prone to water contamination should use an oil that rapidly separates from water. An oil’s ability to separate from water is known as its demulsibility. High detergent oils tend to be poor demulsifiers, while turbine oils and many industrial lubricants have better demulsibility. Circulating systems prone to water influx, such as those found in rolling mills and power generation plants, can simply drain the water from the reservoir to control contamination, provided the oil exhibits good demulsibility. In these situations, the oil’s demulsibility should be monitored to ensure that water contamination does not reach the lubricated components