Return line filters usually have low-pressure housings, which are less expensive than their high-pressure counterparts. Return-line filters collect contaminants from the oil as it returns to the reservoir. Much like the kidney loop, return-line filters are versatile. They can be positioned almost anywhere within the return line circuit, either mounted inline or built into the reservoir.

Disadvantages are few, but worth noting. Return-line filters can be sensitive to flow surges that can degrade their performance. These surges can be caused by multiple valves shifting simultaneously or from flow intensification from cylinders. This means it is important to size return-line filters for maximum anticipated flow.

Return-line filters also do not clean fluid entering the reservoir through drain lines. Although return filters are widely used, few standards exist to regulate consistency from one manufacturer to another. This means that replacement cartridges from sources other than the original may not be completely interchangeable.

Return-line and kidney-loop filters offer great value and are easy to maintain. They are available in a wide variety of media choices, making them suitable for most industrial applications.

*Cost: $30 to $1000     Benefit: high

High-pressure filters, sometimes called last-chance filters, clean the fluid delivered by the pump. This protects expensive downstream components, such as servovalves and actuators. High-pressure filters offer protection from catastrophic pump failure and are a worthwhile investment for high-performance systems. They find wide application in aircraft, paper and steel mills, and injection molding and die-casting machines.

One disadvantage of high pressure filters is, ironically, the high pressure. Housings for these filters must withstand pressures of 6000 psi or higher, so they can be heavy and difficult to handle — as anyone who has ever tried to change a slippery, 200-lb cast-iron filter can attest. Unless a duplex configuration is used and strict safety practices followed, the entire hydraulic system must be shut down to service a high-pressure filter.

*Cost: $200 to $2500    Benefit: high

Evaluate costs and benefits

Deciding which of these filtration techniques are essential, and which you can do without, requires careful study of the application at hand. Historical data and experience are some of the best tools to aid in making this decision. For new applications, as much data as time allows should be collected and studied. This should include the amount and nature of contamination anticipated as well as the operating conditions of the hydraulic system.

Curt Rausch is Industrial Hydraulics Product Manager, Donaldson Co., Inc., Minneapolis. Click here to visit Donaldson's Industrial Hydraulics Filtration website.

*Prices mentioned above may vary due to the economic changes since this article was originally published.