Plant Engineering – December 2000
FLUID POWER
MAINTENANCE
Cover feature: Maintaining hydraulic fluids
Joseph L. Foszcz, Senior Editor, Plant Engineering Magazine
Sidebar 1:
Sources of generated contamination
Sidebar 2:
Water contamination causes
Key concepts
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Hydraulic fluid contamination can be held to an acceptable minimum.
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Contamination comes from internal and external sources
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Portable instruments quickly and accurately measure particle contamination.
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Maintenance includes prevention, filtration, dewatering, and additive replenishment.
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Clean hydraulic fluid is the best friend any hydraulic system can have. It transmits pressure, lubricates components, suspends contaminants, and keeps the system cool. Unfortunately, when this fluid gets too contaminated it can wreak havoc by causing components to wear, seals to leak, and valve passages to block.
Much present-day thinking about hydraulic systems suggests that filters be changed when their indicator says so, fluid can be added straight from a drum of “clean” oil, and if a system is running, leave well enough alone ( Fig. 1 ).
The truth is that maintaining hydraulic fluids is an important job that can reduce or eliminate many problems. Key components of hydraulic fluid maintenance are reducing, but never eliminating, sources of contamination and oil degradation and regular fluid monitoring.
Studies have shown that up to 55% of hydraulic components fail when the contamination level is too high. Monitoring hydraulic systems for contamination can provide an early warning of component or filter failure, helping to avoid unplanned or catastrophic downtime.
Sources of contamination
There are four major sources of hydraulic fluid contamination: built-in, ingressed, generated, and new oil.
Built-in
All hydraulic systems, new and rebuilt, contain a certain amount of contaminants left over from fabrication and assembly procedures. Good assembly practices and thorough flushing can reduce, but not completely eliminate them. These contaminants are usually in the form of rag fibers, burrs, chips, dirt, dust, sand, moisture, pipe scale and sealant, weld splatter, paint, and flushing solution.
The amount of contamination removed during system flushing depends on the effectiveness of the filters used and the temperature, viscosity, and turbulence or velocity of the flushing fluid. Unless high velocities are reached, much of the contamination will not be dislodged until the system is in operation. Depending on filter location, catastrophic component failure is a possibility. A running-in period is essential for any new or rebuilt hydraulic system.
Ingressed
Reservoir breathers allow air exchange into and out of the reservoir to compensate for changes in fluid level caused by cycling cylinders and fluid thermal expansion and contraction ( Fig. 2 ). The air contains moisture and dirt particles, which enter the oil from the surrounding ambient conditions. It cannot be assumed that reservoir or component access plates will always be replaced. Good contamination control requires that reservoirs be designed to remain sealed during operation. Access plates that need to be removed for maintenance should be easy to reinstall.
Whenever a system component is opened for maintenance, there is an opportunity for contamination to enter. All possible care should be taken to ensure that open ports are covered or plugged and component rework is done in a clean area.
Cylinder rod wiper seals are not 100% effective in removing a thin oil film and fine contamination from cylinder rods. Environmental dirt that sticks to the extended rod is drawn into the cylinder and enters the hydraulic fluid.
Generated
The most dangerous system contamination is generated by system components. Particles are work-hardened to a greater hardness then the surface they came from and are very aggressive in causing further abrasive wear.
In a system operating with clean fluid, all components, especially pumps, create a small amount of particles. If these particles are not quickly captured, elevated contamination levels cause the number of additional generated particles to increase at a highly accelerated rate.
New oil
Hydraulic fluids are blended under relatively clean conditions. But after traveling through many hoses and pipes to drums or tanks, the fluid is no longer clean. It picks up rubber and metal particles from lines and flakes of rust, metal, and scale from storage tanks.
Detection
Approximately 70%%MDASSML%%80% of hydraulic component wear can be traced to solid particulate contamination. The rest is caused mainly by additive depletion and water. Common effects are wearing of pumps, valves, and cylinder rods, silt-sticking of valves, erosion of metering orifices, and oxidation.
Visually inspecting an oil sample by holding it up to a light does not provide any real measure of contamination. The human eye, even with perfect vision, cannot distinguish single particles below 40 microns in size ( Fig. 3 ).
The most complete determination of hydraulic oil contamination is done in a lab. Samples must be taken in a certified superclean glass container. Just certified clean containers, which are biologically clean, are not necessarily free of solid particulate. ISO 3722 should be followed for certifying that bottle cleanliness is free of solids.
The sampling method should be selected on the basis of the results desired. If representative particle size distribution is required, a dynamic sample from a tubing or pipeline must be taken. A static sample, from the reservoir, is used for particle counts, viscosity measurements, and water and other chemical analysis.
Laboratory tests produce a detailed analysis of hydraulic fluids.
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Photomicrography provides particle size and composition.
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Drawdown particle isolation determines insoluble contaminants.
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Viscosity measures fluid resistance to flow in relation to time and temperature.
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Water content predicts fluid and component performance.
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Particle counting defines solid contaminants, a common measure of fluid cleanliness.
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Moisture should be below the saturation point to avoid additive depletion and system corrosion.
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Total Acid Number (TAN) measures the acidic constituents and is associated with the remaining useful life of the fluid.
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Ferrous density is associated with the amount of wear metals in the fluid.
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Because of improved technology, basic particle counting and water concentration can now be conveniently done in the field using portable instruments that offer laboratory-quality results. Particle counters combine laser particle counting with a user-friendly interface in a compact size ( Fig. 4 ). Moisture detectors continuously monitor the hydraulic fluid for the percentage of water saturation ( Fig. 5 ).
Results from a lab or field testing should be reviewed regularly to observe trends in fluid contamination. These data are useful in determining if filters are operating properly and whether components are beginning to wear at an excessive rate.
Prevention
There are a number of effective measures that can be taken to help keep a hydraulic system clean and operating troublefree for a long time ( Fig. 6 ).
Filtration
There are three key places where contamination control filters should be located:
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