Hydraulic contamination

Hydraulic contamination

Fig. 1. Portable particle counter operates in three sampling modes: high pressure (from 50- to 5,000-psi sources), low pressure (sources less than 50 psi), and from bottles. Typical analysis consists of three counting runs; instrument then displays cleanliness codes and average particle counts for seven micron-size ranges, and can produce reports from its built-in printer.

Fig. 2. Water sensor mounts in fluid line or hydraulic reservoir. LED display instantly indicates percentage of saturation and temperature, also flashes at 90% saturation to warn operator.

Fig. 3. Oil purifier removes free and dissolved water as well as free and dissolved gases. Nozzles spray water-contaminated fluid into its vacuum chamber; air with low relative-humidity passes through chamber to pick up water vapor and gases, then is discharged to atmosphere.

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; anti-oxidants; 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

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, polyol-ester, or phosphate-ester 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 (below) 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.

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.