- Buyer's Guide
Most of us are well aware of the enormous damage water can exact on a machine and its lubricants. However, the magnitude of this potential destruction seems to depend directly on five enabling factors. These factors are listed below and are further diagramed in Figure 1:
1. Sensitivity of the machine and lubricant to water. Some machine types and components have unique sensitivity to water-induced damage, such as corrosion, cavitation, hydrogen embrittlement, silt lock, elastohydrodynamics (EHD), etc. Likewise, certain lubricants (both base oils and additives) are more prone than others to degradation in the presence of water. The degradation or depletion modes include hydrolysis, oxidation, water washing and others.
2. How long water stays in the oil. The longer water exists in a suspended state in the oil, the higher the risk it poses to the machine and the lubricant. The longevity of water in the oil is influenced by factors such as turbulence, emulsification, the rate of water ingression and the rate of water removal, including evaporation, dehydration and settling.
3. Amount of oil/water interfacial surface area. If water and oil are allowed to become emulsified, high surface area between the oil and water results. This occurs when larger droplets of water are crushed into numerous microglobules of water, which can increase the interfacial surface area by more than one million times. When surface area multiplies so do chemical reaction sites between the water, additives, base oils and other contaminants.
4. Mobility of the water within the machine. Water is less likely to crush into microglobules unless it is mobile and able to be carried into turbulent and mechanically dynamic zones within the machine. Once emulsions are allowed to form and stabilize, the risk water poses to the lubricant and machine is significantly magnified. One could say that the movement of the oil gives “legs” to the water, spreading its destructive potential. The water is then able to go wherever the oil goes, including reliability-sensitive zones within the machine.
5. Heat. Heat provides the activation energy necessary to initiate chemical reactions (corrosion, oxidation, hydrolysis, etc.) and hinder effective lubrication (film strength) in highly loaded frictional machine surfaces.
Figure 1 illustrates the principal conditions that enable the five factors to potentially inflict damage from water contamination. For instance, how long water stays in the oil is influenced ultimately by more than 16 other conditions. Not all of these conditions are likely to occur at any one time, but then again, only a few are actually required. Perhaps the best thing to do, using the diagram, is to perform an inventory of the applicable influencing conditions to rate your machine’s tendency for water-induced failure.
Controlling Water’s Grip
It is frequently said that water is the most destructive when it is emulsified in the oil - perhaps its most prevalent state of coexistence. As previously stated, an emulsion not only engages the oil and its additives via high interfacial surface area but also aids its mobility in the flowing fluid to load-bearing and metallurgically sensitive surfaces. If the water wasn’t emulsified (trapped in the oil) it couldn’t be so easily transported.
Let’s take a closer look at how this happens. To have stable oil/water emulsions you need not only a mixture of oil and water but also emulsifying agents, also known as water handles. This is illustrated in Figure 2. In fact, there is convincing evidence that these water handles are perhaps the core enabler of most water-related problems. Without these handles, water - being heavier than the oil - has nothing to prevent it from settling out of the way.
As a practical matter, whatever is in the oil that is “water-loving” will effectively serve the function of being a water handle. Most water handles are polar soluble suspensions within the oil. For certain oils, these polar handles are primarily polar additives such as detergents and dispersants. In such cases, they can’t be avoided - they are a part of the oil’s formulation. In other cases, they are both uninvited and unwanted guests. These include oxides, surfactants and many common solid and dissolved suspended contaminants.
Seeing the Water Hazard
Eventually water will enter your lubricants. The damage it inflicts will largely depend on how long it stays and the surface area it attains in contact with the oil (enabled by water handles).
A good start at controlling the risk is to monitor both root causes and symptoms of the risk. This includes monitoring not only the amount of water in the oil, but also its state of coexistence (dissolved, emulsified or free). Next, assess the oil’s population of water handles. Perhaps the best way this can be done is to use the demulsibility test (ASTM D1401). Start with new oil to get a baseline, then assess the condition of in-service lubricants. Some oil analysis labs have began using D1401 for routine oil analysis of turbine oils, hydraulic fluids and paper machine oils.
Any significant failure of demulsibility should be investigated. Don’t simply change the oil and go on down the road. Knowing where the handles came from will give you a better idea of how control their recurrence.
Finally, go back to the basics and take stock of where water comes from, then develop a workable plan to control its ingression. Plainly stated, proactive maintenance is always the best machine reliability strategy whether you are trying to control dirt, heat, misalignment or water contamination.