It is a common saying in the lubrication industry that you should not mingle water and oil. That begs the question, though: what does it mean? It’s true that contaminated water presents problems, but what standards exist for gauging water quality? Can we keep it under control? Is there a foolproof method to get rid of it? Hydraulic and other lubrication systems are discussed, as are the consequences water pollution may have and the methods used to assess, regulate, and remove water.
Hydrological Regions
In either a dissolved, emulsified, or free form, water is always present in hydraulic fluids and other lubricants. The saturation level is the concentration of dissolved water at which the fluid becomes completely saturated. Free water occurs when there is more water in a fluid than the fluid can contain, and it may exist as a distinct bulk water phase or an emulsion.
When a fluid is too saturated, it takes on a murky appearance. The amount of water a fluid can store at saturation varies significantly depending on the kind of fluid base stock, the additive package, the temperature, and the pressure. For instance, at 70 degrees Fahrenheit, 100 parts per million (ppm) of water is roughly the maximum amount that highly refined mineral oils with minimal additions can contain before being saturated.
On the other hand, hydraulic fluids based on esters, such as those used in rolling mill applications, may have saturation values of more than 3,000 ppm at 70°F and greater yet at higher temperatures.
If you look at the saturation curve for a typical turbine lubrication oil, you can see how the saturation level changes as the temperature rises and falls. When a system is operating at 100°F and the fluid only includes dissolved water (100 ppm), lowering the temperature to 70°F, like during a shutdown, would result in the presence of free water in the system since the saturation level under ambient circumstances is less than 100 ppm.
Conversely, if the saturation threshold is achieved or surpassed (200 ppm at 100 °F), entrance of water at working temperature might also lead to the existence of free water.
Water Supply Options
Several different things may be the starting point for water. Seepage via reservoir lids, access panels, breathers, or old seals; condensation from air in reservoirs and other system regions; and rain leaking into exterior reservoirs are all examples of environmental infiltration. Leaky heat exchangers or coolers, as well as direct ingression of process water (such as cooling water, washdown water, or steam), may also introduce water into the fluid system.
While careful system design and maintenance may help reduce water infiltration, completely eliminating all potential entry points is challenging (and expensive).
A Look at Water’s Repercussions
Water in hydraulic fluids and lubricants may cause a variety of problems for the system. The availability of free bulk water is directly related to surface corrosion, the most readily apparent consequence. Even if all the water in the fluid is dissolved, it may still be conducive to accelerated metal surface fatigue, such as in bearings. The impact of dissolved water on the fatigue life of tapered roller bearings was researched by Cantley in 19771.
With the use of an SAE 20 fluid incorporating rust- and oxidation-inhibiting additives, Cantley derived an equation that correlates the relative bearing life to the water content of the lubricant used in the testing. Using a test temperature of 150 °F, he demonstrated that by reducing the amount of dissolved water in the bearing lubricant from 400 ppm to 25 ppm, the bearing life could be increased by a factor of five. Water content has a substantial link with relative bearing life, as shown in Figure 2 (an adaption of Cantley’s results).
Reduced lubricating qualities (lubricant layer thickness, load-carrying capacity, etc.) induced by the presence of water may lead to higher component wear2, and ice crystals generated at low temperatures can cause components to jam.
Water has a physical and chemical impact on hydraulic and lubricating fluids, as well as the components of the system. Many aspects of matter’s physical makeup are modified by water.
power transfer properties (compressibility), including viscosity and load-carrying capacity, are particularly important in hydraulic systems.
There are a number of chemical characteristics that may be noticeably altered by the addition of even a little quantity of water, including:
Immunity to heat and oxygen. Hotter temperatures and the presence of water speed up the formation of oxygenated compounds from oxygen reacting with a fluid basestock. Metal particles from wear may be a catalyst. 3 Higher viscosity and deposits like polymeric polymers or sludges are the end results of oxidation.
Hydrolysis, the breakdown of ester-based fluids by heat and water, produces acids and alcohols, increasing their corrosiveness.
The Features of Deposition (soot, coking)
Negative effects on fluid performance due to premature additive depletion and additive precipitation
Two popular ways of expressing the amount of water present in hydraulic and lubricating fluids are: The ppm value may be used to indicate the water content as a percentage of the total mass or volume. This technique is often used for defining water content requirements. The saturation level of fluid water at a certain temperature is indicated by the relative content, which is represented as a percentage of saturation. Clearer forewarning of the emergence of free water is provided.
The amount of water present in hydraulic fluids and lubricants may be determined in a number of ways. Determine if a rapid evaluation or an accurate measurement is more important to you before making a final decision. The following methods are examples of those most often used:
Only the more precise quantitative approaches, such as Karl Fischer titration and the use of capacitive water sensors, are of any value in keeping tabs on the water content.
The standard method of water content monitoring includes collecting a representative sample for laboratory examination (often using Karl Fischer titration). Although this technique yields precise data, there is a significant delay between the sample and the analysis.