Reservoirs aren’t the only source of dirt and moisture contamination in hydraulic systems. These elements can also enter through actuators, especially piston rod seals on cylinders. Moreover, contaminants may be self-generated by pumps, motors, and degradation products from the fluid itself. So, even if you completely eliminate dirt and moisture from entering the hydraulic system through the reservoir, you still need to add filters within the system.
One of the most effective filtration methods is offline filtration, often referred to as a kidney loop. A kidney loop uses a circulating pump to draw hydraulic fluid from the reservoir, route it through a filter, and pump the clean fluid back into the reservoir. Heat exchangers often are incorporated into the kidney loop to control fluid temperature within prescribed range.
The problem is, installing a kidney loop or adding an auxiliary circuit to an existing system requires making sure any lines you work on are not pressurized after shutting off power. Only at this point should you open the hydraulic line you'll be tapping into, drain the fluid from the line, install a tee or other fitting, and then connect everything back up. You then have to repeat this process for the return line. However, you're still not done, because you may have to bleed air from the line, and you certainly should check for leaks.
