One of the most
important contaminant intake point in hydraulic plants, is the filling plug.
Very often this plug is open or not correctly fitted and at the best and where
present, it’s installed a plug with inner filter.
The inner filter is usually a 40um nominal sintetic filter media. Pondering that particles to be trapped by filter cartridges present on system board, are surely smaller than 40um, it’s not rare that hydraulic filters blocking could be due to particles entering from this filtered breathing plug, instead from the inner at the system normal production. In some cases, it’s possible to find what visible in picture, that is a 3um absolute ratio breathing filter (A) and a 40um nominal filtered filler plug (B) working together on the same reservoir.
Since a 3um absolute media makes more resistance to the air flow that a 40um nominal filter media, most of the suspended dust present in air, will flow inside the reservoir from the plug, polluting the contained oil. The correct operation is to install a 3um absolute rating breathing filter media, but applying a sealed filling plug.