luisventura,

In response to your question, your first step should be to complete a particle count on your system to see how contaminated your system is. I would think with nominal rated filters that, unless your cooling circuit turns over your reservoir capacity numerous times per day, your fluid is fairly contaminated. Compare your oil cleanliness level to the contamination level recommended by your hydraulic component manufacturer. To determine the cleanliness level that is desireable for your system you should choose the recommended contamination level for that component which is most sensitive to contamination (i.e. servo valve). You mentioned that you had PF and PV pumps. The minimum contamination level for these pumps, if they are operating in the 2000-3000 psi range, is an 18/16/14 as recommended by the manufacturer. You would then pick this contamination level as your target to achieve. Since you appear to know the difference between nominal and absolute filters you are probably aware that you need to increase the efficiency of your filters by going to the absolute filters which are 98.7% efficient. You most likely want to use at least a 5 micron or less filter to achieve the best cleanliness level for your components. You must check what the clean element pressure drop is to make sure it does not exceed 1/3 of the indicator setting, which is recommended by most filter manufacturers to maximize element life after you have cleaned up your oil with the smaller micron rated and higher efficient filters. I am involved in the hydraulics industry and take this approach with numerous customers. Hydraulic filtration has made giant leaps in the last 20 years. We have finally fiqured out that finer filtration and higher efficiency filters leads to longer component life. When you change to a smaller micron and higher efficient filters you will consume a lot of elements. The life of each replaced element getting longer as you are cleaning your hydrauic fluid.

With out a full circuit diagram, my suggestion is to put a separate filtration circuit on the tank that has 5 micron filter elements and turns the tank volume a few times per hour. Be sure the filtration suction is away from the main pump inlets and the filtration return is positioned to turn the oil in the tank.