Question:
Joel White of Weyerhaeuser was curious about the effect of flow rate, contaminant level, and viscosity on the performance of a filter, comparing the multi-pass Beta test (ISO 4572) performance to the real world. Leonard Bensch, Ph.D. of Pall Corporation and U.S. Delegate to ISO on contamination control, offered the following enlightening response. We at Practicing Oil Analysis felt the information would be helpful to anyone interested in selecting filters and controlling contamination.

Answer:
1. Flow Rate Influence. Relative to flow-rate, the efficiency, or Beta ratio, of a filter may change slightly but this is generally not that significant unless you are talking about wide extremes of flow. We can test an element rated at 30 GPM at flows ranging from 10 to 50 GPM without much change in Beta. We will, however, see a change in the initial (clean element) differential pressure and the dirt capacity of the filter. Pressure drop increases roughly proportional to the flow and the dirt capacity will decrease with increasing flow, but not necessarily linearly.

2. Influence of Particle Ingression Rate. If dirt ingression is within reasonable limits there will be little effect on the Beta ratio or dirt capacity. When we first developed the multi-pass test at Oklahoma State University, tests were run on a wide range of filters from 3 mg/L up to over 100 mg/L. In the range of 3 to 25 mg/L, we did not see significant differences. 10 mg/L was chosen for the original test procedure, just for standardization. The ISO 4572 Multi-pass test is now undergoing revision to include, among other things, on-line particle counting, a new automatic particle counter calibration procedure, and a new test dust to replace the obsolete AC Fine Test Dust. This revised standard will also include three separate test conditions for dirt ingression -- 3, 10, and 15 mg/L. Test results were presented to the committees which showed very little change in Beta or dirt capacity with these three conditions. Laboratories are allowed to choose any of the three conditions to help in establishing a convenient test time. However, the standard procedure will state that for comparison of filter performance, the comparison should be done at the same test condition.

This new Multi-pass test will be designated ISO 16889 and is in the final stages of acceptance at ISO, which is anticipated this year.

3. Viscosity Influence. With respect to viscosity effects on beta, we have found that the efficiency is generally not highly affected unless the viscosity ranges are extreme (I'm beginning to sound like a broken record). I did a study at Oklahoma State University many years ago on this effect and found that dirt holding capacity was affected, generally decreasing, as viscosity was increased. Such studies were run on the same oil while viscosity was adjusted by changing temperature. We have since run tests at Pall Corporation where the viscosity was changed by changing the test oil from the standard lab method using Mil-H-5606 aircraft hydraulic oil (15 cSt @ 40° C) to some other higher viscosity fluids. In such case, we have found a difference in filter performance, which we attribute to both viscosity effects as well as the influence of other fluid properties, including additives, conductivity, etc. We are currently extending this work to include operating conditions from typical field applications to help us be better able to predict how a filter will perform in a specific field application with operating conditions and oils which are different from the "mild" laboratory multi-pass test. We know that field performance of filters cannot be directly correlated to laboratory multi-pass test results as actual field systems often have different oils, variable flow, pressure surges, temperature cycling, vibration, and a much lower ingression rate than the lab test. The research we are currently conducting will give us better insight into correlating lab test results to actual field performance. We hope to have something published on this within this year.

Dr. Bensch's comments to Joel White's questions are very timely. Increased demand for reliability has led to aggressive efforts to control contamination. This thrust, of course, has created many questions about filtration system design, element selection and performance assessment.