Oil Filter Performance Ratings

You may have heard someone say, “a filter is a filter.”  Although catchy, this phrase is very misleading.  It is said assuming any filter will get the job done.  Not all filters and lubricants are created equally.  There is no one size fits all in this world of lubrication.  Lubricants and filters must be selected based on the application.  So why do we view oil as a consumable instead of an asset? Why do we see changing oil filters as a task that we complete off a schedule or perform filtration only when the oil lab says the oil is dirty?  An oil filter to our oil is like a liver to our blood.  With that understanding, we can appreciate the importance of proper filter selection and the decision to use a filter cart vs a permanently mounted filtration system.  The first thing that may come to mind when choosing a filter is the micron size.  Here comes the fine print.  You may see a few specs on most filters such as nominal rating, absolute rating, and beta ratio.  If you do not see either, keep shopping.  Absolute rating is generally defined as the smallest pore size in a filter’s media or the largest particle size that can pass through the media.  A nominal rating is an arbitrary number used to define a filter’s ability to remove particles of a given size as a percentage of weight.  Although both Nominal and Absolute define a particle micron rating, none are specified using a universally accepted testing method to validate the performance of the filter.  Pulling your hair out yet?  Do not worry, take consolation in the Beta Ratio.  The Beta Ratio is a value determined through the Multi-pass Method in ISO 16889:1999.  During this test, particles of a known size and quantity are added upstream of the filter and are measured with particle counters both upstream and downstream.  The upstream count is divided by the downstream count and the result is defined as the beta ratio.  Essentially the beta ratio can be understood as the filter’s efficiency to remove a specific particle size.  Beta ratios are defined as 2, 10, 20, 75, 200, 1000, and 2000.  A 3-micron filter with a beta ratio of 2 means that the filter is 50% efficient at removing particles 3 microns and larger.  A 3-micron filter with a beta ratio of 2000 means the filter is 99.95% efficient at removing particles 3 microns and larger.

Oil Filter Performance and Selection

Ok, now that we have discussed the details of filter performance ratings, let’s discuss the other important factors when choosing a filter.  Initially, after learning a little bit about filtration, you may ask the supplier to give you the smallest micron rating with the highest beta ratio available simply because it will clean the oil better.  Whoa, there is more to consider.  Things such as the machine and bearing type will help determine the target oil cleanliness level.  After all, we need to make a reasonable cost/benefit decision here. What about the oil properties?  Can I go too small and filter out additives?  The short answer is yes. Additives such as viscosity improvers and antifoam agents can be filtered out, especially at the 3-micron range.  It is wise to consult with your oil manufacturer to help determine the ideal filter size and oil temperature range for filtration.  What about the application?  This is important to consider to know how clean of an ISO target you need to meet.  Are you looking to upgrade the existing filter on the factory oil circulation system?  If so, you need to be aware of certain properties of the filter housing and media such as burst psi, operating psi, psi drop across the media and housing, differential psi, and oil compatibility.  It is safe to assume that a smaller micron filter with a high beta ratio will lead to a higher psi drop across the filter because it will require more psi to flow through the media.  Other variables that increase psi drop are viscosity and flow rate.  Why the obsession with psi and psi drop?  For pressure, it boils down to the psi rating for the housing and the pump.  Let’s say that the existing pump ran at 80 psi with the current very porous and ineffective filter, and the pump has a bypass psi of 100.  If you decide to install a 7 micron Beta 1000 filter you may create enough restriction inflow that the pump will hit 100 psi and go into bypass.  In this case, the pump is not pumping oil through the filter, but bypassing it internally or externally, thus providing no filtration, no flow, and creating heat.  What about psi drop and differential psi?  Differential psi is the difference between upstream and downstream psi.  It is important because it is an indicator of the filter service life and saturation or, “how dirty it is.”  If you currently change your filter based on calendar days or operating hours you may not be aware of the importance of differential psi across a filter.  As a filter gets saturated with contaminants the downstream psi decreases as the upstream psi increases.  Eventually, if equipped with a bypass, the filter will go into bypass to prevent it from collapsing or rupturing releasing all the contaminants back into the system.  Most inline filters are equipped with a differential psi gauge or a pop-up indicator that will let you know when it is time to change the filter.  Some filters have a bypass at 15 psi and others at 65 psi.  Therefore, readings in those ranges mean it’s time to change the filter. What if you install that new filter for the first time and instantly the differential psi is indicating a dirty filter?  This scenario is very much a possibility if you overlooked details that play a role in psi drop such as viscosity and flow rate.  For example, if you select a filter housing size to accommodate a gearbox recirculation system with a flow rate of 5 GPM that circulates an oil with a viscosity of 220 Centistokes, your filter will work just fine if the flow rate stays at 5GPM and the viscosity stays at 220 Centistokes.  The problem in this scenario is that although the pump may provide a consistent flow rate of 5 GPM the oil viscosity varies with temperature.  In this case, the gear oil increases in viscosity as the temperature drops, therefore increasing the psi drop across the filter.  What can you do?  You can increase the filter housing size or, although not ideal, you can mitigate the variable of oil temperature with an oil heating element.

Oil Filtration Systems

Now that we have talked about filter performance and selection, should you have a permanently mounted oil filtration system or filter with a filter cart periodically based on the oil analysis report?  Well, folks, this is a loaded question. Ideally, a dedicated filtration system is the best choice as it ensures clean oil while in operation.  The problem here may be the cost of running power to support the recirculation system.  However, if the asset is very high in the criticality ranking it may be easy to justify.  The case may be the opposite for an asset that has very low criticality, although best practice may be to have a dedicated filtration system, the benefit may not outweigh the cost.  In this case, periodically using a filter cart may suffice.  Other things to consider are factors such as safety and environmental impact.  If the machine is 10 floors up in the air and you must haul a 65 lb. filter cart up those stairs, the safety impact of doing that task may merit the investment of installing a dedicated system or purchasing a filter cart to leave on that 10^th floor.  What if you are in the unfortunate position of not being able to do either?  In this case, filter the oil to the specified ISO target before topping off the gearbox and mitigate the risk of contamination by installing high-quality breathers, sample ports, and level indicators.  In summary, filtration and lubrication is a very technical responsibility, that should be treated with the utmost importance.  Unfortunately, in some cases, the role of lubrication is viewed as a secondary task to maintenance or viewed as an entry-level responsibility.  I hope that in reading this blog we can give lubricants the respect they deserve, after all, they are the lifeline of our machines!  Please contact us to learn more about lubrication best practices and see what we can do to help you Keep It Running.

Watch our 5-Step Acoustic Lubrication Procedure Motion Graphic which outlines an effective way to grease your bearings right and bring you one step closer to best practices for your lubrication program.

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