How to select HYDAC filter elements with corresponding filtration accuracies for different NAS fluid cleanliness classes?

A lower NAS 1638 cleanliness class number indicates cleaner hydraulic oil, which accordingly requires filter elements with higher filtration accuracy. Meanwhile, mainstream HYDAC glass fiber filter elements need to meet specified beta values to ensure reliable filtration performance.

Oil cleanliness of NAS Class 4 and below represents ultra-high cleanliness, commonly seen in servo control systems and precision hydraulic equipment. It is recommended to adopt filter elements with a filtration rating of 1 to 3 μm and a beta value of no less than 200, delivering a particle removal efficiency of over 99.5% to protect high-precision valve assemblies.

NAS Class 5 to 6 is the mainstream standard for industrial precision hydraulic systems serving proportional valves, high-pressure piston pumps and other components. 5 μm glass fiber filter elements with β₅ ≥ 200 are the preferred choice, which is also the most common configuration for HYDAC pressure line applications.

NAS Class 7 to 8 refers to general industrial cleanliness for a wide range of working scenarios. 10 μm filter elements with a beta value above 75 are sufficient to meet operational demands, striking a balance between filtration performance and flow capacity.

Oil rated NAS Class 9 to 10 contains relatively large amounts of contaminants and is mostly used for ordinary gear pumps and low-pressure hydraulic circuits. Filter elements of 20 to 25 μm are applicable here. Oil of NAS Class 11 and above is heavily contaminated and mainly used for coarse filtration at pump suction ports. Sintered stainless steel filter elements with a rating of 50 to 100 μm are recommended. They feature low flow resistance and can be cleaned and reused.

Avoid selecting excessively fine filter elements blindly, as over-high accuracy will accelerate clogging and raise operating differential pressure. A staged filtration setup is advisable for the entire hydraulic system: use high-precision filters for pressure lines, moderately lower the accuracy for return lines, and install large-pore coarse filters at suction ports. This approach optimizes the balance among oil cleanliness, filter service life and equipment operating efficiency.