When Water & Oil Mix, Assets Suffer
EP Editorial Staff | July 29, 2020
Controlling the amount of water contamination in your lubricants will extend rotating-equipment life.
By Mark Barnes, PhD CMRP, Des-Case Corp.
From an early age, we learn that oil and water do not mix. While that is not exactly true in the lubrication world, it is true to say that any degree of water in a lubricant can cause irreparable harm to lubricated components and the oil. When water enters an oil sump or reservoir, it exists in one of three distinct phases: dissolved, free, or emulsified. While each pose different challenges for lubricants, all three phases can significantly reduce equipment life, cause excessive maintenance costs, and lead to unscheduled downtime.
As the name implies, dissolved water means that the water and oil form a solution. While this may seem counterintuitive, since water is chemically polar while oil is largely non-polar, the presence of polar components such as oil additives, degradation by-products, and/or other contaminants, can make it possible for oil and water to mix. The amount of water that can be dissolved in a new oil is dependent on the type of base oil, the amount and type of additives contained in the oil, and the temperature.
For mineral oils or hydrocarbon-based synthetic oils, the affinity of the base oil for water is very low. However, for some base oils, in particular API Group V base oils, significant quantities of water can dissolve in the oil due to the polar nature of these fluids.
Likewise, additives have an impact of the solubility of water in oil. Oils that have small additive quantities, such as turbine oils, which contain less than 5% additives, are relatively resistant to water solubility. Oils that contain large amounts of additives, such as gear oils, hydraulic fluids, and engine oil, can hold much higher concentrations of water in the dissolved phase.
Similarly, temperature affects how much water can dissolve in oil. At room temperature, a conventional anti-wear hydraulic fluid may be able to hold as much as 200 ppm (0.02% v/v) of water in solution. As the oil cools, the solubility of water in oil decreases. At 40 F, the oil may only hold 50 ppm of water in solution. The maximum amount of water an oil can hold in solution at a given temperature is referred to as the saturation point.
When water comes out of solution, it will co-exist with the oil in one of two phases—free or emulsified. Free water has completely separated from the oil and settled to the bottom of the tank or oil sump. Emulsified oil refers to a suspension of small water droplets (the dispersed phase) in the oil (the continuous phase). Whether oil and water exist in the free or emulsified phase depends on a property of the oil known as demulsibility.
Like solubility, demulsibility is dependent on the type of oil, the degree to which the oil has degraded, and the presence of certain contaminants. Oils with small amounts of additives will separate (demulsify) fairly quickly—usually in less than 5 to 10 min. Separation takes much longer when larger amounts of additives are present and/or the oil is severely degraded. Some oils, particularly those that contain detergents, are severely degraded. Those that have contaminants, such as soaps or other process fluids, may lose the ability to shed water completely, forming a stable emulsion.
When it comes to the ability of a lubricant to do its job, the presence of free or emulsified water is of great concern. This is particularly true for wet-sump applications such as small process pumps or splash-lubricated gearboxes, since the lubricated components operate directly in the oil sump. Free and emulsified water in wet-sump applications not only causes rust and corrosion to occur but can significantly affect film strength, while accelerating oil degradation, leading to the formation of sludge and varnish.
Figure 1 shows the impact that water can have on equipment life. In this seminal study, researchers were able to derive an empirical relationship between the amount of water in an oil (in this case an R&O ISO VG 68 fluid) and the life expectancy of a rolling-element bearing. This would mirror exactly what might happen in a small centrifugal pump. As the graph illustrates, bearings that operate with water above the saturation point (approximately 100 ppm in this case) will have a significantly reduced life expectancy, often as low as 50% of the anticipated bearing life.
Water can enter an oil in a number of different ways, but one of the most common is when equipment is operated in a high-humidity environment. Figure 2 shows the impact on two identical centrifugal pumps operating outside in relatively high humidity (74% RH) and at high temperature (93 F). Both pumps were filled with pre-filtered oil to remove as much moisture as possible and were equipped with bearing isolators to prevent contaminants from migrating through the shaft-seal interface. In addition, both pumps were completely sealed from the outside environment by plugging the breather/fill port.
By installing breathers with integrated humidity sensors, the impact of controlling headspace humidity using a silica gel desiccating medium is obvious. Without headspace humidity protection, the first pump showed relatively high internal humidity, which fluctuated between 55% and 65% due to day/night temperature fluctuations. Despite being nominally sealed, clearly air exchange is occurring, likely through the bearing isolator.
By contrast, the pump with headspace protection maintained a very low internal humidity due to the silica gel actively dehumidifying the headspace. Based on the graph shown in Fig. 1, we can expect the bearing life for the pump with dry headspace to be 20% to 40% longer than the pump with non-protection. This indicates that bearing isolators and a desiccant breather are required for maximum bearing life.
Similar effects can be seen in most, if not all, wet-sump applications. For example, measurements were made with two different sets of splash-lubricated gearboxes, one with active headspace humidity protection and the other with no protection. Both sets of gearboxes were operating outside in high-humidity environments. Water concentrations dropped from an average of 350 to 400 ppm without headspace protection to 70 to 80 ppm with active protection.
Much is made of solid-particle contamination, and rightly so—particles are the leading cause of lubrication-related failure. However, do not overlook the importance of controlling moisture. For wet-sump applications, such as pumps and gearboxes, moisture is just as likely to induce a failure. Make sure you take every step possible to protect your critical wet sumps. EP
Mark Barnes, CMRP, is Senior Vice President at Des-Case Corp., Goodlettsville, TN (descase.com). He has 21 years of experience in lubrication management, oil analysis, and contamination control and has published more than 150 articles and white papers.