States of Coexistence
Moisture is the second-most-destructive contaminant found in machinery, next to particle contamination. Moisture can exist in oil in the following three states or phases:
Contrary to what I was taught, oil and water do mix. The volume of water that will dissolve into the oil depends upon the oil’s base stock, condition, additive package, contaminant load and temperature. Typically, new, high-grade oils with minimal additive loads will hold little dissolved water. Conversely, oxidized, lower grade oil that is heavily additized can hold as much as 2,000 ppm water in the dissolved state. In this state, water is not visible in the oil.
Once the amount of water has exceeded the maximum level for it to remain dissolved, the oil becomes saturated. At this point, the water is suspended in the oil in microscopic droplets known as an emulsion. Emulsified water is often referred to as having a hazy appearance.
Adding more water to an emulsified oil/water mixture will lead to a separation of the two phases, producing a layer of free water. This water separates from the oil due to inherent insolubility and the specific gravity difference between the two fluids. In most cases, free water is found at the bottom of tanks and sumps.
How Moisture Affects Components
In a lubricating system, the two most harmful phases are free and emulsified water. According to SKF, as little as 1/10th of a percent water in oil can reduce the life expectancy of a journal bearing by as much as 75 percent. For rolling element bearings, the situation is even worse. The main cause of this shortened life cycle is the weakening of the oil film strength. The weakened film leaves the component more susceptible to abrasive, adhesive and fatigue wear. Not only will water destroy the oil film strength, but both free and emulsified water under the extreme temperatures and pressures generated in the load zone of a rolling element bearing can result in instantaneous flash-vaporization, causing erosive wear to occur.
How Moisture Affects the Lubricant
Not only does water have a direct harmful affect on machine components, but it also plays a direct role in the oxidation (aging) of lubricating oils. The presence of water in a lubricating oil can cause the progress of oxidation to increase tenfold, resulting in premature aging of the oil, particularly in the presence of catalytic metals such as copper, lead and tin. Where free water accumulates in a system, microorganisms can grow. These microbes feed on the oil and decompose to form acids, which promote further oxidation of the oil.
There are five basic test methods to determine the moisture content of a lubricating oil. These methods range from a simple apparatus to a more complex chemical test or slightly more expensive percent saturation probe test.
The most basic is the crackle test. In this test, a hot plate is held at 320°F (130°C) and a small drop of oil placed in the center. Any moisture present in the oil is reflected in the number of bubbles observed as the water vaporizes. Depending on the lubricant, relatively few small bubbles indicate approximately 500 to 1,000 ppm (0.05 to 0.1 percent) water. Significantly more bubbles of a larger size may indicate around 1,000 to 2,000 ppm water, while an audible crackling sound indicates moisture levels in excess of 2,000 ppm. The crackle test is sensitive only to free and emulsified water.
Another simple on-site test is the use of a pressure cell, where the sample is prepared with a chemical reagent (calcium hydride) and placed in a container and shaken vigorously. A change of pressure within the cell is monitored to determine if free water is present. The cost of this type of product is relatively low, although the operational costs must be considered with regard to the reagents, as well as the health and safety issues of these reagents.
Relative Humidity Sensor
A third type of on-site screening test for water is a relative humidity sensor. The sensor uses a thin film capacitance grid that can determine the amount of moisture permeating through the film. The advantage of this method is its relatively low running costs and that it can be permanently mounted on critical plant equipment to provide real-time monitoring.
Fourier Transform Infrared Spectroscopy
Aside from the on-site screening methods, another commonly used method to screen for water is Fourier Transform Infrared Spectroscopy (FTIR). This test is sensitive to free, emulsified and dissolved water; however, it is limited in precision to a lower detection limit of approximately 1,000 ppm. This is adequate for some applications but insufficient for typical industrial applications. Commercial laboratories that use this method often report that less than 0.1 percent volume of water is present in the sample.
The most precise method for determining the amount of free, emulsified and dissolved water in a lubricating oil is the Karl Fischer moisture test. When used correctly, the Karl Fischer test is capable of quantifying water levels as low as 10 ppm, or 0.001 percent, and should be the method of choice when more exact water concentrations need to be known.
Once detected, the root cause of the ingress of the moisture should be investigated. If detected early enough and the oil’s physical and chemical properties are not compromised, the water can be removed and the oil kept in service. Following are a few methods for this removal:
Free water can be removed from oil by providing a good settling location. This is typically provided in the form of a bottom sediment and water bowl (BSWB). Settling, however, will not remove dissolved or emulsified water.
The settling process can be accelerated when the forces of gravity are magnified using a centrifuge. While more effective than regular gravity separation, centrifugal separation fails at removing dissolved and emulsified water.
In a vacuum dehydrator, oil is put under a vacuum and the temperature is elevated. This effectively vaporizes the water from the oil at a temperature that does not severely harm the lubricant.
These filters look like typical, spin-on or cartridge-style filters, but they contain a filter media impregnated with super absorbent polymer. The polymer absorbs free and emulsified water and forms a gel.
Water is a major cause of lubricant failure, component failure and poor machine reliability. Like all contaminants, it is important not only to recognize its presence, but also to take steps to control or eliminate the source of water ingression. If possible, moisture levels should be kept at an absolute minimum. Whether you choose to install desiccant-style breathers, improve seals, or to use a centrifugal filter or a large vacuum dehydration unit, reducing the level of water in all types of equipment can dramatically extend the life of the lubricant and the machine.