2, 10, OR 30 MICRONS?
No discussion of primary fuel filtration would be complete without some mention of the great micron debate. The question is often asked: what is the proper micron rating for my primary filter element, 2, 10, or 30? Engine and filter manufacturers are in near universal agreement (although not always for the same reason) that the primary filter should be either 30 or 10 microns. Many cruisers and vessel operators, both commercial and recreational, are adamant about using only 2-micron elements in their primary filter bodies. Their argument goes something like this: Why change two filters when I can change just one? If all of the debris gets captured by the primary filter (which, incidentally, is usually much easier to change), why rely on the secondary filter for anything other than a backup?
While I know that at least half of the readers and one editor will disagree with what I’m about to say, here’s my position. (Some of what follows is paraphrased from an article by Gary Morgan on the boatdiesel.com website.) The distribution of contamination particles in a fuel system follows a protocol known in the particlecounting business as log normal distribution, which states that the number of particles is inversely proportional to the size of the particles. Thus, if a sample of contaminated fuel contains 1,000 30-micron particles, it may have 10,000 10-micron particles and 20,000 2- micron particles. The average 2-micron secondary filter is especially efficient at capturing millions of fine particles until called upon to also capture larger, “gravel”-sized particles as well. The reason for this is straightforward. In spite of their absolute-sounding ratings of 2, 10, and 30 microns, these filters do not possess an absolute ability to capture all particles of their respective ratings and larger. Some larger sized dirt inevitably slips through.
However, because of a phenomenon known as microcaking, as the filter begins to capture some contaminants, it becomes more efficient at capturing more of the smaller particles. The dirt actually becomes part of the filtering process. If both large and small particles are included in the mix, the micro-caking process is hindered; the larger “gravel” leaves holes in the caked surface layer, allowing some debris to pass through the filter. Picture a bucket with a few holes in the bottom. If you pour fine sand into the bucket, some of the sand will filter through the holes, but eventually the sand itself will capture more new sand than is filtering through the holes. But if you pour a mixture of fine sand and gravel into the same bucket, the gravel will help keep pathways open for the sand to continue to filter through to the holes in the bottom.
Additionally, using a 2-micron element as the primary filter means, in theory, that all or most of the contamination will be captured in this element and that virtually nothing will be captured by the secondary, on engine filter. Thus, you’ve effectively halved your filter media surface area; you now have only one filter that must contend with all of the contamination. If you happen to take on a batch of particularly dirty fuel, this primary filter may quickly become overwhelmed. If, on the other hand, your primary filter has a 30- or 10- micron rating, then the contamination can be divided among the two filters, offering greater overall media surface area.
The finer filter, because of the lack of large particles, will operate more efficiently to boot. While individual results may suggest that 2-micron primary filtration works, this may only be the case when dealing with relatively clean fuel. Once dirty fuel is encountered, segregating the contamination into two locations is simply more efficient and desirable, and most engine and filter manufacturers agree with or require this approach.
(Editor’s note: The editor Steve is referring to is, of course, me. I would like to comment that I follow a 2 micron strategy because I have a fuel polishing system, which routinely filters the fuel in all tanks down to 2 microns through a large Racor 1000 filter. If I did not have this system, I would add an additional Racor filter, with a 10-micron element, between the fuel tank and the switchable, 2-micron Racor filters. That would provide multi stage filtration while keeping the on-engine filter off my dance card. The small on-engine filter is impossible to monitor, and I will never accept having to change it when it shuts the engine down in a seaway. Sorry, but I think creating this single point of failure is a rather reckless form of Russian roulette.
THE SECONDARY FILTER
The secondary fuel filter, the one that’s nearly always mounted on the engine, is designed to capture the finest of particles in the most efficient manner. Many secondary filters also possess the ability to separate and capture water, and some include a water sensor and alarm as well. The micron rating of this filter, while not user selectable, is usually between 2 and 7 microns. Because the secondary filter is under pressure, it cannot be equipped with a vacuum gauge, so the proper replacement interval may be unclear. The micro-caking phenomenon would lead one to believe that a filter that isn’t clogged should never be replaced. However, most engine manufacturers recommend annual or seasonal replacement, and this makes good sense. (Over time, the fuel-saturated paper of the pleated element may soften and tear.) Because these filters are opaque, it’s worth pouring the contents into a container for inspection when they are removed for replacement. If free water is evident, closely inspect your primary water-separating filter for internal damage or malfunctions.
A SECONDARY ALTERNATIVE
If you simply loath the thought of replacing your secondary fuel filter because it is maddeningly difficult to access or you have a primal fear of allowing air to enter your fuel system, there is hope. You can simply install two Racor or equivalent filters, in series, one after the other. The first one is your primary filter; equip it with a 30- or 10-micron filter element. The second one becomes your secondary filter; equip it with a 2-micron filter. You’ve created an easily serviced, multistage filtration system. The on-engine (now tertiary) filter should never require replacement; however, replacing it annually at your convenience while dockside makes good sense and will keep you on the right side of your engine manufacturer’s guidelines.
