Moisture, upon contaminating hydraulic and lubricating oils, has a degrading effect to both the lubricant and the machine itself. While some additives adsorb to the water and are removed when the water separates from the oil, others are destroyed by water-induced chemical reactions. Water also promotes oxidation of the oil's base stock, causes rust and corrosion of machine surfaces, and reduces critical, load-bearing film strength. Essentially, water represents a real risk to equipment and should be aggressively controlled.
The Varying States of Water
Water coexists with oil in either a dissolved or a free state. When single water molecules are distributed throughout the oil due to the water's chemical attraction to the fluid, it is in a dissolved state. Numerous factors such as viscosity, base stock type and condition, impurities, and additive package determine the volume of water that will be dissolved by the oil.
Additionally, the dissolved volume is a function of the oil's temperature, thus the humidity is reported as relative humidity (depending on the temperature). If the oil has dissolved all of the water possible at a given temperature, it is saturated. Dissolved water is difficult to control but causes only minimal harm to the machine and oil.
When the saturated oil experiences a temperature decrease, it reaches a point where water will not condense into a free form. This is called the dew point temperature. Free water is the other state in which water coexists with the oil. Water is in a free state when undissolved globules of water are physically suspended in the oil. Large globules tend to separate to the bottom of the reservoir or sump. However, in mechanical equipment, the shearing forces of gears, pumps, bearings, etc. tend to crush the water into such small globules that a stable emulsion exists.
An emulsion is the stable state of physical coexistence of chemically insoluble substances, like oil and water. Additives and impurities that lower the oil's surface tension can serve as agents to strengthen the emulsion. Free and emulsified water pose the greatest risk to the machine and the lubricant, and should be placed under strict control.
A Simple Solution
There are numerous methods of measuring the presence of water in oil, but most are complicated, expensive or difficult to use in the field because they employ wet chemistry. An easy way to detect the presence of free and emulsified water, the most dangerous forms of water in oil, is with the hot plate crackle test. This simple method alerts the user to the presence of free water.
For years, oil analysis laboratories have screened samples with the crackle test, performing further analysis only when the crackle test results are positive.
Advancing the Basics
The crackle test has been used as a reliable indicator of emulsified water. However, with practice and keen eyes and ears, the procedure can be advanced considerably and made more quantitative.
1. J.C. Fitch. Oil Analysis for Maintenance Professionals (Coursebook), Noria Corp., 1998.
2. Komatsu Oil and Wear Analysis (KOWA), 5th Edition, Procedure Manual.