5.1 Precision equipment and high pressure hydraulic machinery require filtered lubricants and fluids to prevent damage from the circulation of hard particulate contaminants. Three types of particulate contaminants are present in lubricants and hydraulic fluids: built in contaminants from the machinery assembly process, generated contaminants from equipment wear, and contaminants that enter from external sources. Water can also enter machinery lubrication and hydraulic systems through fill ports, defective seals, corroded heat exchangers, and reservoir breathers in the form of rain water, cleaning solutions, process water, metalworking fluids, coolants, and humid air.

5.2 The ability of lubricants and hydraulic fluids to retain their filterability in the presence of moisture is critical for efficient and reliable machine performance. Normally, the pressure differential across a filter will increase gradually as it accumulates dirt, sludge, and wear debris. In order to prevent the filter from collapsing, bypass valves in the filter assembly open when the differential pressure gets too high. If a filter becomes blocked by additives that precipitate due to the presence of contaminating water, the bypass valve will open. This can lead to a machine shutdown or circulation of damaging particles throughout the machine.