There are four primary sources for solid contamination to enter a lubricant or hydraulic fluid. They are: contaminated new oil, built-in contamination, ingressed contamination and internally-generated contamination.

Although hydraulic and lubrication fluids are refined and blended under relatively clean conditions, the fluid travels through many hoses, pipes & tanks before it is stored in drums or in a bulk tank at the user’s facility. At this point, the fluid is no longer clean as the fluid lines it has traveled through have contributed metal and rubber particles, and the tanks & drums have added flakes of metal or scale. Storage tanks are a real problem because water condenses in them causing rust particles. Contamination from the atmosphere can also find its way into the tank unless satisfactory air breathers are fitted. If the fluid is stored under reasonable conditions, the principal contaminants on delivery to the machine will be metal, silica and fibers. Supercleaned new oil could be of cleanliness code18/16/14 but new oils are often 22/21/18 or dirtier. Fluids should be filtered into the system before the contamination enters and damages the components in the system. For critical hydraulic fluids, using a portable transfer cart fitted with a high efficiency filter should be considered.
New machinery always contains a certain amount of built-in contamination. Care in system assembly and in new component flushing reduces this but never eliminates it. Typical built-in contaminants are burrs, chips, flash, dirt, dust, fiber, sand, moisture, pipe sealant, weld splatter, paint and flushing solution. The amount of contamination removed during the system flush depends not only on the effectiveness of the filters used, but also the temperature, viscosity, velocity and “turbulence” of the flushing fluid. Unless high velocities and turbulence are attained, much of the contamination will not be dislodged until the system is in operation, with catastrophic component failure a possible result. Irrespective of the standard of flushing executed by the machine builder, an off-load period of “running-in” should be regarded as essential for any new or rebuilt hydraulic or lubrication system.
Contamination from the immediate surroundings can enter into the fluid power or lubrication system. The key is to severely limit the access environmental contamination has to enter the hydraulic or lubrication system. Sources of Ingressed Contamination
(a) Reservoir vents: allow air exchange into and out of the reservoir to compensate for changes in fluid level caused primarily by cycling cylinders and thermal expansion and contraction of the fluid.
(b) Maintenance: Whenever a system is opened for maintenance, there is an opportunity for environmental contamination to enter the system. All possible care should be taken to ensure that open ports are kept covered or plugged, and component disassembly and rework is done in an area that is protected from excessive airborne dirt and contamination. Lint free rags and oil absorbent materials in “socks” (rather than in loose form) should be used for component wiping and area clean up. Tank cover sealing should be restored on completion of work.
( c) Cylinder seals: Rod seals are rarely 100% effective in removing the thin oil film and the fine contamination from the cylinder rod. Environmental dirt that sticks to an extended rod is drawn back into the cylinder and washed off into the system fluid. Every effort should be made during machine design to avoid dirt or other contaminants from landing directly on extended cylinder rods. When this is unavoidable, the filters should be positioned and sized to capture this abundance of dirt.
The most dangerous contamination to a system is the contamination generated by the system itself. These particles are “work hardened” to a greater hardness than the surface from which they came, and are very aggressive in causing further wear in the system. In a system running on properly cleaned fluid very few particles are generated, although all components (especially pumps) create a small amount of particles during routine operation. In a system where these particles are not quickly captured the elevated contamination levels will cause the number of additional generated particles to increase at a highly accelerated rate! The best way to prevent contamination generation within a system is to start with a clean (fully flushed) system and keep the system fluid clean.