New Test For Measuring Dynamic Efficiency Of Hydraulic Filters

Diesel Progress North American Edition, April, 2001 by Aaron Hoeg, Rob Murad

When developing or verifying the performance characteristics and limitations of any technology, one of the key factors is testing. Ideally, test procedures should be structured to provide comprehensive information on how a component or system will perform in literally any operating condition it might meet in the real world. Yet often, for any number of reasons, this kind of testing protocol is difficult to achieve.

One of the most critical aspects in modern mobile hydraulic technology is filtration. With machine hydraulic systems facing ever-higher demands for performance and efficiency component tolerances have been reduced to unprecedented levels. The presence of entrained particles, sediment or even air, has gone from being a nuisance to being unacceptable.

Until recently, steady state testing has been the only form of multipass testing to evaluate filter performance. The latest ISO standard, ISO16889, was adopted in December of 1999 and is generally considered an improvement over the previous testing protocol. Yet even this standard has its shortcomings in measuring the efficiency of filters in dynamic -- that is to say -- real world operating conditions.

Three years ago, in an effort to provide a more representative test procedure, Larson Testing Lab, working with Hy-Pro Corp., began the development of Dynamic Filter Efficiency (DEE) testing. The development was initially inspired by a servovalve company experiencing contamination related failures. Since then, DFE has sparked the interest of a mobile equipment manufacturer and several government agencies. DFE testing has since been proposed to the NEPA and SAE committees as a new standard for multipass testing.

To best understand the benefits of DFE testing, it's important to review multipass testing as it exists today. The multipass test consists of a closed hydraulic circuit that continually circulates fluid through a filter. A known quantity of contaminant is gradually introduced into the system as the fluid makes repeated passes through the filter. The level of contamination is measured upstream and downstream of the filter to determine how efficiently that filter removes the particulate, and approximately how much particulate the filter can retain.

The ISO 16889 standard specifies that the filter elements will be tested at one flow rate throughout the life of the test. The drawback is that very few filters are fortunate enough to operate in such a friendly environment in the real world. DFE testing seeks to bridge the gap between ideal lab simulation and real world operating conditions by cycling the flow rate up and down between two flow values throughout the test. Thus, DFE testing combines the concepts of flow fatigue and multipass testing to truly show the vital signs of a filter in a real life system.

Along with the multiple flow values, DFE testing also includes vibration analysis, which is also not addressed by current multipass test methods. Virtually every hydraulic system is subject to vibration, either generated by the pump in the form of pressure pulsations (pump ripple) or actuation and movement of equipment. This vibration can have an adverse effect on filter performance especially if one or more of the frequencies in the system coincide with the harmonics of the filter element. When this occurs, the excited element can resonate and release most of the contaminants previously captured.

The DFE test procedure monitors the vibration characteristics of the filter throughout the life of each test to assure that an element does not operate at a harmonic frequency across the performance envelope.

A series of tests on similar elements from different manufacturers was performed to compare performance under ISO 16889 and DFE procedures. Not surprisingly, filters developed and tested under current ISO standards did not perform as well when subjected to DFE testing (see Table 1). Depending on the manufacturer, there were different phenomena that occurred including unloading, media breakdown and reduced capacity Here's a closer look at some of those occurrences.

Unloading: When the flow was cycled up and down a range from 15 to 30 gpm there was a tendency for the elements to unload particles during transitional periods, releasing clouds of contamination downstream of the filter. Shortly after a flow change the fluid cleanliness would stabilize, but there were noticeable decreases in efficiency. The unloading also occurred when reducing flow from 30 to 15 gpm, but was most dramatic when flow was increasing (Figure 2). This contamination can move the cleanliness level in and out of the acceptable ranges required by manufacturers and it typically consisted of very high concentrations of silt with larger particles.

Media Breakdown: Some of the test elements showed integrity problems when simultaneously challenged with contamination and cyclical flow rates. As the test went on not only was there a decrease in efficiency at flow change sequences, some elements displayed a continual decay in overall filter efficiency (Figure 3). The element illustrated in Figure 3 was true to its rating of B10(c) = 200 (99.5 percent removal efficiency) at a clean pressure drop of 2 psid. At 34 psid, after several flow changes, the same element had a filtration ratio of B10(c) = 7 (85.8 percent removal efficiency). The result was a filter that started out providing the rated cleanliness level, then fell significantly.