Contamination of the hydraulic fluid of a hydraulic system. It can be caused by solid particles and/or dissolved foreign matter, by mixing with foreign liquids, or by air entering the hydraulic fluid.
The solid particles may be hard or soft. While the latter simply clog the filters hard particles of dirt (metallic debris) may damage the seals and the metallic surfaces. Therefore, hydraulic fluids must be filtered.
Contamination with foreign liquids occurs through the entry of water, mixing with other hydraulic fluids or lubricants.
The contamination may already have come from the installation in the system (installation contamination) or it may be the result of wear and ageing during operations (contamination in operation). Suitable measures (scrapers on the piston rod, tank air filter, tank pre-load, etc.) must be taken to prevent external contamination from entering into the hydraulic system.