A kidney loop filtration system is just an outside loop on an oil storage tank. The advantages to these systems are they simple to install and they are out of the critical path, so their operation does not affect the availability of the lubrication system.
The actual system can consist of a pump and filter for a simple system. More advanced systems, like the ones we make, have an active filtration system to remove particles down to submicron levels. There are also small systems that consist of just a filter that is placed in the return line from a pressure regulator in a hydraulic system.
The system you need can be determined by the tank size and the level of cleanliness you wish to achieve
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