Maintaining hydraulic fluids

Key concepts Hydraulic fluid contamination can be held to an acceptable minimum. Contamination comes from internal and external sources.



Hydraulic fluid contamination can be held to an acceptable minimum.

Contamination comes from internal and external sources.

Portable instruments quickly and accurately measure particle contamination.

Maintenance includes prevention, filtration, dewatering, and additive replenishment.

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.


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.


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.


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.


Approximately 70-80% 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.

  • Photomicrography provides particle size and composition.


      • Drawdown particle isolation determines insoluble contaminants.


          • Viscosity measures fluid resistance to flow in relation to time and temperature.


              • Water content predicts fluid and component performance.


                  • Particle counting defines solid contaminants, a common measure of fluid cleanliness.


                      • Moisture should be below the saturation point to avoid additive depletion and system corrosion.


                          • Total Acid Number (TAN) measures the acidic constituents and is associated with the remaining useful life of the fluid.


                              • Ferrous density is associated with the amount of wear metals in the fluid.

                                • 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.


                                  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).


                                  There are three key places where contamination control filters should be located:

                                  • Directly downstream from the pump


                                      • In return lines


                                          • In a continuously recirculating filter loop.

                                            • Filters should be fitted with differential pressure switches-electric or visual-to indicate when a filter is becoming clogged. The switch signals the need for element change before the bypass relief valve opens and passes contaminants. Changing elements prevents system component damage and premature replacement of partially used filter elements.

                                              Recirculating loop filtration is cost-effective. Filter efficiency and dirt-holding capacity are at a maximum when fluid flow is steady. The benefit is that loop filters have a lower cost per ounce of captured dirt than inline filters.

                                              All tank breathers should capture dirt particles and adsorb water when air comes in and remove oil vapors when air is expelled (Fig. 7).

                                              Other acceptable options are bladders or flexible rubber barriers. They prevent the exchanged air from coming in contact with the surface of the hydraulic fluid while allowing relief protection against over-pressurizing the reservoir.

                                              If it is not possible to prevent dirt from falling on cylinder rods, at least install rod boots or bellows with vents to exclude most of the dirt.

                                              Contamination should be removed from new fluids before adding them to a system. Use a portable transfer cart pump fitted with high-efficiency particulate and water removal filters to replenish reservoirs (Fig. 8).


                                              When moisture content in the oil gets too high, it can be removed with a portable dewatering unit (Fig. 9). System oil is drawn into the unit and heated. The hot oil is exposed to a vacuum, which vaporizes and distills the water. Clean filtered and dry oil is returned to the system. Operation is automatic and unattended.

                                              Other types of water removal-coalescence, centrifugation, and absorption-only remove free water. Vacuum dehydration removes free and dissolved water without generally altering the physical or chemical properties of the hydraulic fluid. In some cases, antiwear additives may be volatilized.

                                              Additive replenishment

                                              An emerging technology has the capability to replenish depleted antiwear and antioxidant oil additives. Fourier transformed infrared spectroscopy analyzes oil samples in real time. Portable units can be installed online for continuous monitoring or off-line for semiautomatic operation. This technology optimizes oil longevity and reduces the volume of waste oil for disposal by adding preset quantities of the appropriate additive. Care must be taken to ensure there is no unwanted additive reaction.

                                              Plant Engineering magazine extends its appreciation to Analysts, Inc., Fluid Technologies, Inc., and STLE for their assistance in the preparation of this article. The cover picture was supplied by Eaton Hydraulics.

                                              -Joseph L. Foszcz, Senior Editor,


                                              &HEADLINE>More info&/HEADLINE>

                                              Two previous articles discussed hydraulic oil filtration: "Optimum filtration for fluid power systems" (PE, December 1999, web site, File 2070) and "Keeping hydraulic fluid clean" (PE, March 1997, p 86, File 2070).

                                              See the " Power transmission " channel on our web site at for the text of the articles mentioned above and others related to this topic.

                                              The internet holds a wealth of information about hydraulic fluid filtration, monitoring, and testing. For starters, check out the following web sites.

                                              Additive replenishment


                                              General information















                                              &HEADLINE>Sources of generated contamination&/HEADLINE>

                                              Abrasion- Hard particles bridge two moving surfaces, scraping one or both.

                                              Adhesion- Loss of oil film allows metal-to-metal contact between moving surfaces.

                                              Aeration- Air bubbles in the fluid implode, breaking away surface material.

                                              Cavitation- Restricted inlet flow to a pump causes fluid voids that implode, creating shocks that break away critical surface material.

                                              Corrosion- Water or chemicals in the fluid cause rust or a chemical reaction that degrades a surface.

                                              Erosion- Fine particles in a high-speed stream of fluid eat away a metering edge or critical surface.

                                              Fatigue- Particles bridging a clearance cause a surface stress riser or microcrack that expands into a spall due to repeated stressing of the damaged area.

                                              &HEADLINE>Water contamination causes&/HEADLINE>

                                              • Fluid breakdown, such as additiveprecipitation and oil oxidation

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