This schematic show the setup for the ISO 16889 multi-pass filtration test.
The main issue with the current standard is that we’re designing equipment for filters that can hold a certain amount of contaminant. In reality, though, filters cannot hold as much contaminant as they did in lab tests because the varying flow rates in actual field usage means that contaminant caught by the filter can be released back into the fluid stream. One or more factors are the cause: media pleats, backing material, or any other structure or support in the filter can underperform under stress. Theoretically, the new standard will make filter specifications more accurate and, in turn, equipment will operate more efficiently.”
ISO/NWIP 23369 in Practice
The ISO committee includes members of major oil filtration manufacturers, a major reason the need for additional testing is recognized. No manufacturer wants to be accused of making filters that seem less efficient than advertised, which can happen when current approved filters operate under stress.
ISO developed this cyclic flow multi-pass test procedure for hydraulic filters to supplement the basic steady-state flow test of ISO 16889 for filter elements that will be used in cyclic flow environments. Using an industry survey and a round-robin laboratory testing procedure, ISO 16889 guidelines recommend a more stringent flow-rate cycle of 0.1 Hz, although it also notes that if much higher cycle rates are expected in actual service, “the test should be conducted at that frequency to produce more meaningful results.” However, only values resulting from testing at 0.1 Hz will be recognized.
Multi-pass tests using a cyclic flow rate require operators to choose a cyclic ratio of current change normally between 2:1 or 4:1. These ratios will stay consistent throughout testing and offer a one-step-closer approach to real-world filter performance results by showing the shedding of particles (slough) from filters being tested during current changes in the test fluid. The new standard suggests that flow rates (measured in lpm) change every 5 sec. at a 4:1 ratio.
Just as importantly, the test requires fine dust (smaller than 1 micron) vs. the medium dust (5 µm or larger) required by ISO 16889. The 5-sec. changes and varying rates means it’s possible that twice as much data can be recorded, although the data is averaged instead of exact.
What Does a New Standard Mean for You?
Everyone who uses filtration expects it to protect costly equipment, and minimize downtime and unexpected costs. Our international standards are in place to guarantee a certain level of quality and performance. Because of that, our standards need to be constantly evaluated to stay in line with current technology and equipment use.
Other than the obvious benefit of more efficient and reliable filtration, the primary benefit of this new standard for end users and equipment manufacturers is fewer surprises. Whether for equipment design or actual application, the purchase of a filter comes with a certain trust that comes from the certification on the label. A filter has to reliably capture and retain contaminants. If it can’t capture contaminants of a specific size at a specific efficiency, it cannot be relied upon. But retention capacity is as important as efficiency, if not more. If the filter is cleaning fluid efficiently but has no room to retain the contaminants, ironically, it can become a damaging source of contamination.
Which brings us back to cyclic flow. In current lab testing, the flow rate is constant, not taking into account that equipment at work can send fluid through a filter at varying rates. If not equipped to handle these varying rates, the filter becomes less efficient, and its capacity can be reduced. If a user expects 100-g contamination capacity, the filter must hold 100 g.
Some filters currently on the market meet the current standard in every possible way. The standard just isn’t good enough, so unexpected failures are happening. It’s often not the filter’s fault; it’s just being held to a standard that makes it seem more effective than it is. If enacted, ISO/NWIP 23369 will go a long way toward assuring users that they’ll receive a level of performance they expect from their filters, all other things being equal.
John Aune is hydraulics product manager at Donaldson Co., Minneapolis. For more information, visit www.donaldson.com.