FLUID POWER HANDBOOK
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HYDRAULIC
FILTRATION SYSTEMS Filtration is always on a hydraulic maintenance technician’s mind. But for some machinery or in some plant setups, inline hydraulic filters are not enough to keep a hydraulic system functioning properly. 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
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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 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 they do 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-μm offline filters with high beta ratios.
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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. This design allows for live selection of either filter so the other can be replaced. Most filter manufacturers offer a filter system dedicated to the offline filtration market that is highly efficient and offers 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. Permanently mounted offline systems are now more commonly used as well. They are often mounted to a panel, either near the reservoir or directly attached to it. Eliminating intermittent filtration of the portable type ensures that fluid is clean from storage to service. Some of these filter 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, note that some hydraulic distributors will rent these machines out for a reasonable cost.
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7/15/19 1:31 PM
