Detrimental Effects of Water ContaminationHydraulic and lubricant fluids are carefully formulated for specific areas of application and are comprised of a base stock and an additive package4. The additive package consists of chemical compounds designed to protect the base stock, as well as system components, and to ensure proper performance of the system. Typical additives in hydraulic/lube fluids include rust & oxidation (R&O) inhibitors, anti-corrosion, anti-wear, anti-foaming and extreme pressure (EP) agents, and viscosity index improvers. Water contamination can arise from a number of sources including leaks from inadequate sealing surfaces, condensation of humid air or from open reservoirs. In addition a reduction in temperature of the fluid can lead to the generation of free water from dissolved water. Each fluid type has a typical water saturation profile based on the base stock and additive package. Water contamination affects both the base stock and additives adversely as discussed below. Water is a poor lubricant, and significant concentrations of water in hydraulic and lubricating fluids can result in decreased viscosity, load carrying ability, and dynamic film thickness. This can lead to greater surface-to-surface contact at sliding and rolling dynamic clearances, and hence, increased component wear. Water, in combination with a metal catalyst, can lead to the formation of oxygenated compounds, notably acidic compounds in the case of hydrocarbon, poly-ol ester, or phosphate ester base stocks, and, eventually, high molecular weight polymeric compounds. The polymeric compounds often are insoluble and settle out of the fluid as gums, resins or sludges. Table 1 summarizes data from tests carried out to determine the effect of metal catalysts and water on oil oxidation a turbine grade oil as per ASTM D943.
Table 1. Effect of metal catalysts and water on oil oxidation.5The neutralization number, tabulated in the last column, is a measure of the extent of oxidation and the results show that the extent of oxidation is greatly increased: roughly 48-fold for iron/water and 65-fold for copper/water within 400 hrs. and 100 hrs, respectively, compared to the baseline test without water and metal catalyst (Table 1). Fluid base stocks that are comprised of ester compounds, such as poly-ol esters and phosphate esters, can undergo hydrolysis in the presence of water under operating conditions in the system. This results in the formation of acidic compounds that can react with materials of the system components leading to corrosion and insoluble corrosion products. Depletion of additives can occur either by their physical removal from the fluid or by chemical reactions in which they are converted to non-functional products. The solubility of many additives is critically dependent on fluid composition. The presence of water can lead to the precipitation of these additives from the fluid, as shown in Figure 1 below. In addition to being rendered non-functional, the precipitated additives contribute to the particulate contamination level in the fluid.
Figure 1. Ca/S additive precipitated from a paper machine oil.
Monitoring Water in FluidsA variety of on-line monitors are available for monitoring water concentration in fluids. Some monitors can be interfaced with fluid purification devices so that fluid purification is automated (i.e. without operator intervention) based on preset threshold concentration limits for water content. Typical on-line water monitors are adaptations of relative humidity measurement devices. They measure the percent saturation of water in the fluid, i.e., free water is 100 % saturation. Figure 2 depicts a portable water monitor that can be incorporated in the fluid system for on-line sampling in a continuous mode. The normal output of the water monitor is dissolved water concentration as percent saturation along with the corresponding temperature.
Figure 2. Portable water sensor.The percent saturation value can be converted to an absolute water content by performing a calibration for the fluid in question. A standard procedure for this type of calibration has been developed, allowing easy inter conversion between % saturation and absolute water concentration in various fluids.
Removal of Water ContaminationExternal purification devices (fluid purifiers), in addition to removing particulate contamination and water, also remove volatile solvents and dissolved gases. Two common types of external purification devices are vacuum flash distillation and vacuum dehydration systems. In the former device, pre-heated fluid is introduced into a high vacuum chamber. The combination of high heat and vacuum causes free and dissolved water, gases and solvents to be distilled off resulting in dehydrated fluid. One of the drawbacks of the flash distillation method is that it exposes the fluid to elevated temperatures which may lead to thermo-oxidative degradation of the fluid base-stock and/or additives. In vacuum dehydration systems, as shown in Figure 3 below, the fluid is exposed to a low humidity atmosphere in a partial vacuum chamber resulting in the transfer of free and dissolved water, solvents and gases from the fluid to the atmosphere in the vacuum chamber. To facilitate this transfer, ambient air is introduced into the vacuum chamber. As it expands in the low humidity chamber, the air becomes drier and moisture and other volatile gases are transferred from the fluid to the air stream. This mixture of gases is then exhausted through a de-mister filter to remove any residual fluid in the air stream. Vacuum dehydration systems operate at lower temperatures compared to flash distillation units with the advantage that they do not degrade the fluid.
Figure 3. Schematic of vacuum dehydrator.In the final stage of the purifier, the de-aerated and de-hydrated fluid exits the vacuum chamber through a particulate contamination control filter. These fine filters have high efficiency particle removal characteristics in the smaller size ranges while exhibiting a high dirt retention capacity.
Field Experience with Oil MonitoringIt has been demonstrated that predictive oil monitoring is more cost effective than a preventive or reactive oil monitoring6. Incorporating both water detection and water removal technologies into an effective predictive condition monitoring program was implemented at a US paper mill. This program monitored the water content at various locations within the paper mill. The frequency of the monitoring depended on the level of water content in that particular fluid system, with more frequent monitoring occurring when the water content was above 1000 ppm (0.1%). The data from the fluid analyses was stored in a centralized database which was color coded to ensure easy identification of locations/systems which were potentially problematic. If the water content reached a threshold level of 500 ppm (0.05%), which is typically more that the saturation level of most hydrocarbon based mineral oils, then a water reduction program using vacuum dehydrator technology was installed to lower the water content. In this article, the role of both particulate and water contamination in contributing to the degradation of fluid properties and component performance in hydraulic and lubrication systems was discussed. Consequently, there are significant costs associated with fluid disposal and replacement, labor costs due to increased maintenance as well as costs incurred from repair and/or replacement of system components. An effective oil monitoring program can mitigate these costs by taking a pro-active approach to detecting the ‘health’ of the fluid on a continuous basis. More important, a predictive oil monitoring program can utilize the information that is obtained by monitoring and applying corrective action on a real-time basis. Data from this type of approach to ensure effective water removal at a paper mill was presented.
- Beercheck, R. C. (1978 July). Machine Design, “How Dirt and Water Slash Bearing Life”.
- 2. Cantley, R. E. (1976). 31st Annual ASLE Meeting, Philadelphia, Pennsylvania, “The Effect of Water in Lubricating Oil on Bearing Fatigue Life”.
- Swain, J. C. and Adams, C. E. (1970). Proceedings of the National Conference on Fluid Power, Vol. 24: 214, “Some Effects of Dirt and Water Contamination on Vane Pump Life”.
- Editor: Satriana, N.J. (1982). No. 207, Chemical Technology Review, “Synthetic Oils and Lubricant Additives – Advances since 1979″.
- Weinschelbaum, M. (1969). Proceedings of the National Conference on Fluid Power, “A Study of the Invisible but Measurable Particulate Contaminant in Hydraulic Systems”.
- Electric Power Research Institute (2006), “The Benefits of a Pro-Active Approach using Preventive and Predictive Maintenance Tools & Strategies – Actual Examples and Case Studies”.