Hydraulic filters are an example of a product that began as a very simple solution but progressed over the years to be quite complex. Jeremy Crookshanks Product Manager, Hydraulic & Lube Filtration for Schroeder Industries said that while they still perform the same basic task of protecting sensitive hydraulic components from contaminates, due to industry requirements, the materials and methods to do so are now incredibly advanced.

He explained that—as illustrated below—the components of a standard hydraulic filter have not changed much in the last 50 years. “However, the design and innovation that go into the components we use today seem like light years ahead of what was done in the 1960’s, “ he added. “Understanding the design of an element, all basic components of a standard top ported filter and the 4 key characteristics to examine when selecting a filter for your system is a priority.”

Basic hydraulic filter components
• Filter head: The ported casting that fluid flow both enters and exits the filter from. Additionally, it can also serve as a mounting pad when mounting to a machine.
• Filter bowl: The vessel that threads into the filter head and contains and protects the element. This allows dirty fluid to flow evenly around the outside of the element and the clean fluid to exit through the center of the element.
• Element: The cartridge that holds the filter media. The element is responsible for trapping contaminates entrained in the system fluid.
• Bypass valve: Valve designed to protect the system when the filter becomes overloaded with contaminates.

Filter element design
“It’s the advanced filter element design that makes hydraulic filters so complex today,” Crookshanks said. “Even our most basic filters are an intricate combination of microglass media, wire mesh, and bonded scrim, pleated together at very tight tolerances. The Schroeder’s Excellement Z-Media illustrated below is a six-layer media pack specifically designed for better efficiencies, excellent stability, lower pressure drops, and higher dirt holding capacities.”

Crookshanks noted that when evaluating the performance of hydraulic filter elements, the most important parameters to consider are:
(a) Efficiency, or filtration ratio, expressed by “Beta” (ß) relates to how well an element removes contamination from fluid. Higher efficiency translates to cleaner oil, better protection of system components, less down time for repair, and lower maintenance costs.
(b) Beta stability is defined as an element’s ability to maintain its expected efficiency as differential pressure across the element increases. Differential pressure will increase as contamination is trapped, or with an increase in fluid viscosity (cold start). Beta stability is important because it relates to how well an element will perform in service over time. When the element is loaded with contamination, or when it is subjected to cold starts, will it perform as well as it did when new?
(c) Dirt holding capacity (DHC) is the amount of contamination that an element can trap before it reaches a predetermined “terminal” differential pressure. Dirt holding capacity is related to element life. Since elements with higher DHC need changed less frequently, DHC has a direct impact on the overall cost of operation. When selecting filter elements, it is beneficial to compare DHC of elements with similar particle removal efficiency.
(d) Pressure drop vs. flow is simply a measure of resistance to fluid flow in a system. It is important to consider the initial pressure drop (Δp) across the filter element (and housing). Ideally, a filter element should be sized so that the initial pressure drop across the clean element (plus the filter housing drop) is less than half the bypass valve setting in the filter housing.

When selecting a filter element for your system, be sure to consider all four of these performance criteria. If an element is strong in three areas, but weak in another, it may not be the right choice. At every level of filtration, Schroeder’s Excellement Z-Media elements offer the best combination of high efficiency, high beta stability, high dirt holding capacity, and low pressure drop.

Best system locations for hydraulic filters
There are many variables to consider when choosing the location for a hydraulic filter: system cleanliness requirements, system pressure, where in the system are the components that are trying to be protected, pump flow, total return flow, just to name a few. The following are typical locations with schematic’s to consider when designing a hydraulic circuit:

Pressure filtration: Pressure filters usually produce the lowest system contamination levels to assure clean fluid for sensitive high-pressure components and provide protection of downstream components in the event of catastrophic failures. Systems with high intermittent return line flows may need only be sized to match the output of the pump, where the return line may require a much larger filter for the higher intermittent flows.

Return line filtration: Return line filters are often considered when initial cost is a major concern. A special concern in applying return line filters is sizing for flow. Large rod cylinders and other components can cause return line flows to be much greater than pump output. Return lines can have substantial pressure surges, which need to be taken into consideration when selecting filters and their locations.

Re-circulating filtration: While usually not recommended as a system’s primary filtration (due to the high cost of obtaining adequate flow rates) re-circulating, or off-line, filtration is often used to supplement on-line filters when adequate turnover cannot be obtained with the latter. It is also often an ideal location in which to use a water removal filter. Off-line re-circulating filters normally do not provide adequate turnover flow rates to handle the high contamination loading occasioned by component failures and/or inefficient maintenance practices.

Suction filtration: Micronic suction filters are not recommended for open-loop circuits. The cavitation these filters can cause significantly outweighs any advantage obtained by attempting to clean the fluid in this part of the system. SKB magnetic suction separators are recommended, as they will protect the pump from large and ferrous particles, without the risks of cavitation.

Air Breather filtration: Efficient filter breathers are required for effective contamination control on non-pressurized reservoirs and should complement the liquid filtration component.

Multiple filtration: For systems incorporating large total fluid volumes, it may be necessary to employ filters in more than one location. Multiple pressure filters, pressure and return line filters, and recirculating filters are examples of multiple filtration applications.

In closing, Crookshanks advises that whether setting up a new system or updating older systems, always do a complete checklist for what is needed. If there is any question about what may be the best solution, simply consult with an accredited hydraulic system expert or your equipment system supplier.

By Joyce Laird