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 semipurifies fluid before it is once again welcomed into the reservoir. However, what if a single return filteris not enough? What if your cleanliness codes arenot achieved, even if you’ve upgraded to a finerfilter 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?

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.

Image courtesy of Schroeder Industries

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.

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.

Image courtesy of Schroeder Industries

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.

WHEN SHOULD YOU FLUSH A NEW OR REBUILT HYDRAULIC SYSTEM?

Particulate contaminants circulating in fluid power systems cause surface degradation through general mechanical wear, such as abrasion, erosion, and surface fatigue. This wear causes increasing numbers of particles to be formed, the result being that wear increases if this chain reaction of wear is not properly contained (by reducing contamination). Gaps within components grow larger, leakage oil flows increase in size and operating efficiency (e.g. of pumps, cylinders) decreases. To avoid this, you should always remove particulate contamination in new systems, before startup, and later when the system is in use.

The overall contamination level of a hydraulic system being built in an assembly plant is generally high. The total contamination (number of particulates) consists of contamination that already exists in the new fluid, contamination that is already on/in supplied components and contamination introduced during the build process from the surroundings. The sum of all the contamination can be considered the Initial Contamination Level.

The investment in time and equipment to reduce the Initial Contamination Level is worthwhile because of a reduction of costly warranty claims and for quality control and tracing purposes of products. If the fluid was not maintained to recommended cleanliness standards during use, a warranty claim could be rejected.

Fluid in the reservoir should be flushed with a filter cart or a kidney loop system. It is recommended to flush the system and all the sub-functions by activating them to allow fluid to circulate and to flush particulates back in to the tank where they can be captured, either by the system filters (e.g. return filters) or by external, off-line filter systems (filter carts, kidney loop systems).

A general rule is to achieve a cleanliness level of the hydraulic fluid in the reservoir that is 1 to 2 ISO Codes below the recommended Target Fluid Cleanliness level for the system.