In industries that rely on process pumps, water in the form of free water, steam, or moisture in the air is a common pollutant. Water not only causes rust and corrosion, but also reduces the strength of the oil film, resulting in poor overall lubrication conditions, which reduces the life expectancy of the bearing by as much as 50%. Therefore, maintaining low humidity and humidity in the bearing housing is critical to the overall life of the pump.
Most pumps use rolling bearings. In the element bearing, due to elastic hydrodynamic lubrication (EHL), the bearing raceway is separated from the rotating element. Under EHL conditions, the oil trapped between the moving element and the raceway will experience extremely high local pressures, usually in the range of hundreds of thousands of pounds per square inch (psi). Under these pressures, the viscosity of the oil will increase rapidly due to its pressure-viscosity coefficient. This increase in viscosity causes elastic deformation of the mating bearing surface. As a result, this distributes the load over a wider surface area. If there is any water in the load zone, it will affect the elastohydrodynamic oil film, and in some cases will cause flashing of water.
Although the traditional view holds that polar fluids such as water will not mix with non-polar fluids such as oil, in pump oils and other lubricants, oil and water can be mixed under certain conditions. When water and oil are mixed, water can exist in three different phases: dissolved, emulsified, and free. Dissolved water refers to water that is dispersed throughout the oil at the molecular level. All oils will retain a certain amount of water in the dissolved phase. Even if there is water in the oil, the oil with dissolved water will appear clear and bright. The amount of dissolved water depends on the type of oil, temperature and fluid conditions.
Most pump oils will remain between 100 to 150 parts per million (ppm) (0.01% to 0.015% volume/volume [v/v]) at operating temperature. Once this concentration is exceeded, the oil will be saturated with water, and the extra water will exist as free water at the bottom of the bearing seat or emulsify, causing the oil to appear milky white or turbid. Although all forms of moisture are harmful to bearings, free water and emulsified water are the most destructive.
Figure 1 shows the results of an empirical study on the bearing life as a function of the water-in-oil concentration 1, which is applicable to the ISO VG 68 Anti-rust and Oxidation (R&O) pump fluid based on standard minerals. According to this curve, a water-in-oil concentration of 100 ppm is arbitrarily set as the baseline for 100% bearing life. It can be seen that once the water concentration exceeds the oil saturation point, the bearing life will be significantly reduced. In fact, as long as 300 ppm, which is equivalent to 3 milliliters (ml) in a 10 liter (L) oil pan, the life expectancy of the bearing will be reduced by half.
The impact of water and other contaminants on pump life is well known, and it is the main reason why bearing isolators have become the standard configuration of many American National Standards Institute (ANSI) and American Petroleum Institute (API) pumps. Modern bearing isolators designed to provide dynamic and static sealing have been proven to effectively extend the life of the pump. However, studies have shown that the use of bearing isolators cannot completely eliminate the impact of water on pump life.
The reason lies in the way in which water and humid air interact-Henry's law can explain this effect. Henry's law states that at a constant temperature, the amount of a given gas (water vapor) dissolved in each type and volume of liquid (oil) is proportional to the partial pressure of the gas and the liquid when it is in equilibrium. In other words, if the relative humidity of the air above the oil is 60%, the saturation of the oil will also reach 60%, which means that the concentration of water is 60 to 100 ppm. Figure 2 shows how air humidity affects the water concentration in the same ISO VG 68 R&O fluid. The inference in Figure 2 is clear: Exposing pump oil to higher ambient humidity will increase the water-in-oil concentration, effectively reducing the life expectancy of the bearing.
In order to study the impact of environmental humidity on the humidity level of the pump headspace, both pumps are equipped with desiccant respirators equipped with smart humidity sensors. Seal one respirator and take out the silica gel drying medium as a reference, while the second respirator is installed in the bearing filling port to allow the silica gel to dehumidify the headspace. Both pump bearing housings are equipped with leading brand bearing isolators. The test was conducted in central Florida during the summer, with an average environmental humidity of 74% and a relative humidity of approximately 93 degrees Fahrenheit (34 degrees Celsius).
The results of the research are shown in Figure 3. As shown in the figure, a pump without headspace protection shows high humidity in the headspace, with relative humidity ranging from 50% to 65%. Interestingly, the humidity level fluctuates within 24 hours, reflecting the influence of day/night temperature fluctuations on the humidity in the headspace of the pump. In contrast, although the conditions are the same as the reference pump, the humidity level of the pump with silicone protection is lower. Based on these data, the pump oil saturation graph shown in Figure 2 and the relative bearing life prediction shown in Figure 1, it is reasonable to conclude that a pump without headspace humidity protection may have up to 20% to 40 reduction in bearing life %.
When the pump is running intermittently, the impact of headspace humidity on the life of the pump may be the most obvious. Since the relative humidity depends on the temperature, pumps that are running in stop/start mode or shut down for a long time are usually fully saturated with water, which means that the bearings are left idle in oil contaminated with free or emulsified water. This is common in process industries such as petrochemical or oil refining, where working pumps and standby pumps are common. In these cases, priority should be given to controlling headspace humidity to prevent accidental failure during pump startup.
Controlling headspace humidity is a three-step process.
RE Cantley ASLE transaction volume. 20. 3. 244-248, 1977
Mark Barnes is the vice president of Des-Case's reliability services team. Barnes has 25 years of experience in lubrication management, oil analysis and pollution control, and has published more than 175 technical articles and white papers. Barnes holds a PhD in Physical Chemistry from the University of Southampton, UK, and is a Certified Maintenance and Reliability Specialist (CMRP). For more information, please visit www.descase.com.