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Today's emphasis on pollution prevention and waste stream minimization has made the benefits of contamination control programs for hydraulic fluids even more compelling.
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Concern for the environment has become a fact of life for most companies. For the companies that use hydraulic equipment, that concern is reinforced by stringent federal and local regulations requiring pollution prevention and governing disposal of used industrial fluids. Higher costs (and greater potential liabilities) for fluid disposal are a direct result of these regulations. This adds another economic item to the list of benefits derived from controlling contamination to extend the service life of hydraulic fluids. The list now includes:
• lower cost for fluid needed for replacement and replenishment
•more consistent hydraulic system performance
•less component wear, and
•reduced fluid disposal costs.
Contamination's detrimental effects
It is an established fact that particulate contamination and water in hydraulic fluids can have serious adverse effects on the fluids' physical and chemical properties. The loss of crucial fluid properties, which are central to useful service life, can result in inefficient system performance and accelerated mechanical and chemical wear processes.
Hydraulic fluids are carefully formulated for specific areas of application. They usually are comprised of a base stock and an additive package. The additive package consists of chemical compounds designed to protect the base stock — as well as the components in the hydraulic system — and to ensure proper performance of the system. Typical additives include dispersants and detergents; antioxidants; anti-corrosion, anti-wear, anti-foaming and extreme pressure (EP) agents; and viscosity-index improvers. Particulate contamination and water adversely affect both the base stock and the additives.
Water is a poor lubricant, and significant concentrations of water in hydraulic fluids can decrease their viscosity and load-carrying ability, as well as hydrodynamic-film thickness. This can lead to greater surface-to-surface contact at sliding and rolling dynamic clearances, and hence, increased component wear. The presence of free water in systems that could be exposed to temperatures below the freezing point of water can lead to icing, which will degrade system performance and can produce malfunctions.
The presence of both water and particulate contamination can lead to the formation of insoluble precipitates, and viscous sludges and gels. These materials induce excessive stress on system components — especially pumps — and can clog orifices, nozzles, and jets.
Degradation of fluid base stock
Dealing with acidic components
Phosphate-ester fluids are particularly susceptible to hydrolysis during service, resulting in an accumulation of acidic hydrolysis products. A new trend in fluid purifiers is the incorporation of ion-exchange resin cartridges to remove acidic components from phosphate-ester fluids. If acidic components are a problem, these purifiers provide a double benefit: they remove water from the fluid to reduce the possibility of hydrolysis, and they adsorb any acidic hydrolysis products that already exist in the fluid to minimize acid buildup.
Oxidation of the fluid base stock is a primary chemical-degradation process in many hydraulic fluids. The oxidation process proceeds through a series of chemical chain reactions and is self-propagating — with the intermediate, reactive chemical species regenerating themselves during the process. The result is the formation of oxygenated compounds (notably acidic compounds in the case of hydrocarbon, polyolester, or phosphateester base stocks) and, eventually, high-molecular-weight polymeric compounds. All of these compounds often are insoluble; they settle out of the fluid as gums, resins, or sludges.
Oxidation is significantly accelerated in the presence of metals and water. Metals act as catalysts, and fine metallic wear debris, commonly found in hydraulic systems, are especially active, due to their large effective surface area.
Table 1 summarizes data from tests that were carried out to quantify the effect of metal catalysts and water on oil oxidation. The tests were conducted on turbine-grade oil in a pure oxygen atmosphere, according to the ASTM/D-943 oxidation test procedure. The neutralization number, tabulated in the last column, is a measure of the extent of oxidation. 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 and 100 hours, respectively — compared to the baseline test with no water and metal catalysts. Even with only a single contaminant present (either water or a metallic catalyst), the neutralization numbers increase.
Table 1. Effect of metal catalysts and water on oil oxidation
Hydrolysis — the alteration or decomposition of a chemical substance due to the presence of water — is another potential problem in hydraulic fluids. Fluid base stocks that are comprised of ester compounds, such as polyol esters and phosphate esters, can undergo hydrolysis under typical hydraulic system operating conditions. The acidic compounds that may form during hydrolysis can react with materials of the hydraulic-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 which convert them 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. In addition to being rendered non-functional, the precipitated additives contribute to the particulate contamination level in the fluid.
Additives that protect the base stock can be depleted rapidly due to the enhanced degradation of the base stock in the presence of both particulate contamination and water. A notable example is the depletion of antioxidants. A summary of the detrimental effects of particulate contamination and water is presented in Table 2.
Table 2: Effects of particulate contamination and water on hydraulic fluids
|Fluid breakdown||Cause||Effect on system|
|Physical properties||a. Agglomeration and precipitation of particulate contamination |
b. Oxidation/hydrolysis products - gums and sludges
c. Reactions involving additives - sludges and solids
d. Free water
|a. Oxidation |
|Additive depletion||a. Precipitation of additives |
b. Adsorption by particulates
c. Reactions involving additives
d. Abnormal degradation of base-stock