DFE - Rated Filter Element
All hydraulic and lube systems have a critical contamination tolerance level that is often deifned by, but not limited to, the most sensitive system component such as servo valves or high speed journal bearings. Component manufacturers provide fluid cleanliness levels, per ISO4406 or ISO4406:1999, required for optimum performance and predictable life. An operating system is at risk whenever the critical contamination level is exceeded. Contamination levels determine the individual component's wear rate and ability to perform as intended.
System design, fitter performance and maintenance practices largely determine the contamination level in a system. Filters are expected to maintain contamination below critical tolerance levels. Filter performance in a dynamic operating system is variable based upon flow rate and flow density, changes in flow rate (duty cycle), viscosity, fluid and structure borne vibration (Hz), contamination levels, ingression rate and several other conditions. All filters are subjected to some form of system dynamics. Hydraulic filters encounter frequent and rapid changes in flow rate accompanied by frequency changes. Lube filters typically experience dynamic conditions during start up and shut down. Two key characteristics of filter performance are capture efficiency and retention efficiency. Capture efficiencf/ can be thought of simply as how effectively a filter captures particles while retention efficiency is a measure of how effectively that filter retains the particles it has captured. A filter is not a black hole, and its performance must not be based solely on how efficiently it captures particles. If not properly designed and applied, a filter can become one of the most damaging sources of contamination in a system.
The Dynamic Filter Efficiency Test (DFE) is the evolution of hydraulic and lube filter performance testing. The DFE test goes further than current industry standards to bridge the gap between lab and real world by inducing dynamic duty cycles and measuring real-time performance before, during and after the cycles. DFE testing quantifies both capture and retention efficiency in real time so that we may predict the worst case fluid cleanliness along with average fluid cleanliness. The DFE test method was pioneered in 1998 during a joint effort between Scientific Services Inc (SSI) and Hy-Pro Filtration.
DFE multi-pass enhances the industry standard by inducing dynamic conditions (duty cycle) and measuring the affects of the duty cycle in real time instead of looking at normalized numbers over a time weighted average. DFE also addresses the inherent problem of IS016889 where fluid is added and removed throughout the test, thus creating a small mathematical error that must be corrected in final calculations. In addition to the capture efficiency, DFE also quantifies retention efficiency in real time. A filter that does not properly retain previously captured contaminant can be identified. The phenomenon of releasing captured contaminant is called unloading, and can result in temporary contamination levels that are well above the critical contamination tolerance level of a system.
The DFE test circuit also utilizes upstream and downstream particle counters, test filter and injection point before the upstream particle counter much like IS016889. That is where the similarity to IS016889 ends. The DFE flow rate is not constant like IS016889, but rather hydrostatically controlled so flow changes can be made quickly while maintaining full system flow through the test filter. Particle counter sensor flows remain constant during all particle counts and no intermediate reservoirs are used to collect the particle counter flow before it is counted. This ensures that the fluid counted is representative of the system contamination level. Counts are taken before, during, and after each flow change. The total number of particle counts is determined by the duty cycle of the specific test. The efficiency results are reported in Filtration Ratio (Beta), efficiency percentage and actual particle levels per milliliter.
The raw data is digitally tagged so filter efficiency may be reported for various combinations of flow conditions as a time weighted average and specific ranges related to differential pressure across the filter element. Some typical combinations include all maximum flow counts, all low flow counts and all flow change counts (low to high or high to low). Rapid particle counting with proper timing is how DFE allows Hy-Pro to analyze and understand both capture efficiency and retention efficiency characteristics of each filter tested while contaminant is being introduced upstream of the filter or when there is no contaminant being injected.