Hydraulic systems are employed in mobile, industrial, and aviation applications to convey power to work equipment. They are extremely efficient, compact, and lightweight comparative to a mechanical equivalent. Hydraulic fluids convey force in the system, and as such are carefully selected by the system maker. Chemical stability, viscosity, high flash and fire points, and oxidation resistance are all valued, and consequently, mineral and synthetic hydrocarbon fluids are chosen for industrial and mobile systems, while functional chemicals such as phosphate esters are selected for aviation and dedicated industrial applications.
Monitoring hydraulics with oil analysis is the only way to establish if the hydraulic fluid is impacting the anticipated performance. Contamination is the main cause of failure, typically water contamination or dirt ingression. All hydraulic systems have filters and systematic monitoring is carried out not only to detect initial signs of contamination and resulting wear, but also to establish how successful the filtration is.
Tolerances in a hydraulic system are usually very tight. The clearances on servo valves are mostly on the order of 40-80 µm and the clearances on actuators can be as low as 10 µm, so particles larger than 4 µm can cause serious issues in hydraulic systems. There are a number of ISO codes, ASTM Methods, and SAE guidelines dedicated to measuring particles in hydraulic systems. Most of these codes or techniques bin particles by concentration and size.
Hydraulic system manufacturers will stipulate what ISO code one should use when assessing system cleanliness. For example, they might suggest that ISO code 20/18/15 be applied to the pump in the hydraulic system, while ISO code 18/16/13 could be used for the actuator and valve.
The other contaminant that is frequently found in hydraulic systems is water. This contaminant can lead to corrosion and oxidization of the hydraulic fluid. Usually, water contamination should be maintained below 2500 ppm.
Key Parameters to Monitor
The measure of cleanliness of a fluid is a crucial test for hydraulic systems. Servo valves have very tight tolerances, and are vulnerable to jamming with improperly filtered fluid. All OEMs stipulate cleanliness levels to ISO 4406, so repeated, routine particle counting is vital. When counts are elevated, understanding the cause is critical. Latest technologies, such as LaserNet Fines, not only count particles and report to ISO 4406 or SAE AS 4059, but also offer more detail in terms of where the particles are originating from. Particle imaging enables the maintenance team to directly see sand or dirt particles and also the level of ferrous debris that is adding to the count. This level of detail allows smarter work orders to be created, concentrating on eliminating the main cause of the high counts.
Water is the most prevalent liquid contaminant in power plants globally, and therefore it has to be constantly monitored. Too much water in a system damages the ability of a lubricant to separate opposing moving parts, causing severe wear to occur with subsequent high frictional heat. Water contamination should not surpass 0.25% for the majority of hydraulic systems. Many new technologies are available to sense water contamination in lube oils, and on-site results correlate very well with laboratory methods.
Kinematic viscosity can be defined as the resistance of a fluid to flow under gravity. Viscosity is the most critical physical property of a lubricant. Lubricants must have ideal flow features to ensure that a sufficient supply reaches lubricated parts at various operating temperatures. The viscosities of lubricants differ based on their classification or grade, in addition to the degree of oxidation and contamination in service. Oil viscosity is projected to rise over time and use, and loss of viscosity is said to be more severe than an increase. New technologies make it simple to measure kinematic viscosity—solvent free, low volume systems are present that integrate user-friendliness with data logging capability.
Total Acid Number
Total Acid Number (TAN) denotes a titration technique meant to specify the relative acidity in a lubricant. The acid number is used as a guide to track the oxidative degeneration of oil in service and is often mentioned on OEM equipment or lubricant supplier guides. Oil changes are repeatedly indicated when the TAN value touches a preset level for a specified lubricant and application. A sudden rise in TAN would indicate irregular operating conditions (for example, overheating) that require analysis.
Elemental spectroscopy is a method for identifying and quantifying metallic elements in a used oil resulting from contamination, wear, and additives. The oil sample is energized to make each element release or absorb a quantifiable amount of energy, which specifies the concentration of elements in the oil. The results echo the concentration of all dissolved metals (from additive packages) and particulates. This test is the mainstay for all on-site and off-site oil inspection tools, as it offers information on machine contamination and wear condition in a relatively fast and accurate way. Its chief limitation is that its particle detection efficiency is weak for particles 5 µm in size or larger.
Oxidation by Infrared Analysis
Oxidation can be defined as a measure of the degradation byproducts in the hydraulic fluid. If oxidation becomes extreme, the lubricant can corrode important surfaces, and also deposit silt or lacquer deposits at servo valves. The more the "oxidation number", the greater the level of oxidation. Conditions such as sticky valves, varnishing, sludge deposits, lacquering and filter plugging take place in systems with oxidation issues.
WDA (Wear Debris Analysis/Analytical Ferrography)
WDA defines either a patch or an analytical method that divides magnetic wear particles from the oil and places them on a glass slide called a ferrogram. Microscopic inspection of the slide or patch allows characterization of the wear mode and probable sources of wear in the machine. This method is called analytical ferrography. While it is an excellent indicator of abnormal ferrous and non-ferrous wear, it is typically only performed by a skilled analyst.
This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.
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