Editor’s note: This article was updated with links and additional text on February 6, 2025.
By Josh Cosford, Contributing Editor
Sometimes, inline hydraulic filters are not enough to keep a hydraulic system happy. Most hydraulic systems are installed with at least a return filter, which semi-purifies fluid before it is once again welcomed into the reservoir. However, what if a single return filter is not enough? What if your cleanliness codes are not achieved, even if you’ve upgraded to a finer filter media? A pressure filter is an option, which will keep the components downstream of the element one step cleaner. But what if using a pressure filter is impossible, due to plumbing difficulties or pressure drop considerations?
There is more than one way to skin the proverbial cat, and a solid option to increase filtration effectiveness is with an offline filter system, often called kidney loop filters. Offline filtration uses a dedicated lower pressure pump (still often a hydraulic pump), which draws fluid from the reservoir and then flows that fluid through a dedicated filter assembly—usually of a very high quality medium—and then right back into the tank.
Sometimes hydraulic power units have dedicated offline filter systems, whose only jobs are to circulate fluid from their reservoirs and filter it as it does so. Because a kidney loop filter neither affects nor is affected by the main hydraulic system, it is a consistent and stable way to keep the oil clean. The pressure drop of often low-micron filter media will never be additive to system pressure drop, especially those related to flow surges in the tank lines of machines with rapid cycle times of cylinders. It is not uncommon to see 5- or even 3-micron offline filters with very high beta ratios.
Offline filtration also enables changing of filter elements while the machine is running, as shutting down the kidney loop has no association with machine operation. Some filtration systems employ duplex filters, which are two filter assemblies installed in parallel, separated by a three-way ball-valve, allowing live selection of either filter so the other can be replaced. Most filter manufacturers offer a filter system dedicated to the offline filtration market, which are highly efficient in addition to offering high dirt holding capacity.
Offline filtration is typically the highest quality in a manufacturer’s product line, which is reflected in the cost of these products. To help justify the purchase of such a system, they are often sold as portable units; small units that can be carried by a handle or large units requiring a wheeled cart to manage their bulk. These units can be wheeled from machine to machine, where a suction tube is placed into a port of the reservoir and then passes through its own filters before being injected back into the tank. Depending on the size of the tank, the filter system’s flow rate and filter quality, one might leave the filter system running on the machine for hours or perhaps days.
Some of these offline filtration systems are installed with auxiliary electronics, such as particle counters. A particle counter will give you a live reading of the ISO Code of the oil passing through the unit, so you can leave the unit running until the desired code is achieved. If this type of system seems out of your reach as a small end-user, the good news is most hydraulic distributors rent these machines at a fair rate, so even the little guy now has no excuse for dirty oil.
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