Insane Hydraulics

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Three Common Filter-Related Misconceptions

A couple of days ago I had an interesting discussion with a contractor who had to perform an oil transfer operation for an important equipment commission and they were looking for filter elements for their cart. Their client requested that the oil was to be filtered through filters with at least a 10-micron rating. We had 10-micron cellulose elements and 25-micron glass fiber elements in stock and I advised the man to go for the glass fiber because it would provide better filtration, however, the proposition was rejected and they insisted on the 10-micron elements because this was the "specific client's request".

This was a bad call, in my opinion, but I've seen similar situations many times, and so this post will be about the three common filter-related misconceptions that I run into all the time (even though the internet is overflowing with high-quality articles about all the nuances of the filtration of hydraulic oil).

The first filtering mistake would be the religious belief in the micron rating of a filter. If it says it's a 10-micron, then it's a ten-micron filter, damn it! My friends, this is simply not the case. Micron rating is just a number that a manufacturer put on his product, a ballpark target particle size if you will, but it definitely does not mean that it will capture all the particles above the target size and let through all the particles below.

There are a lot of details about the mechanisms that retain particles in the filtering media, but what a technician or an end user needs to know to make a correct choice is the beta ratio and the peculiarities of the chosen filtering media. A beta ratio of a filter is the ratio of the number of particles of a certain size upstream of a filter to their number downstream of the said filter. A beta of 200 for 10 microns means that for every 200 10-micron particles roughly one fill "slip through". But if we were to plot out how the beta is related to the particle size, we would see something similar to this (image taken from a UFI/Sofima catalog):

This is just an example, but you can see, a filter with the type "FD" filtering media (UFI's fiberglass 21 µm(c) β>1.000) would have a nice beta of 1000 for 21-mic and a beta of 10+ for 10-micron particles, which is actually five times better than a 10-micron cellulose element with its beta of only 2! All this without mentioning the much superior dirt-holding capacity of fiberglass filters.

In short - a micron rating is just a number. A good filter with a higher micron rating filters better than a cheap paper filter with tighter microns but a measly single-digit beta ratio.

Note that this does not mean that you can't create a cellulose fiber filter with enormous beta - special depth filters for extreme filtering do this all the time, but they also cost an arm and a leg, and usually operate with extremely low flows inside specialized filtering carts to achieve their ridiculous ratings.

The second mistake or misconception that I see quite often is the religious belief in water-absorbing filters. By "religious belief" I mean gross over-estimation of their water-removing capacity. I am referring to filters that use these special polymers that absorb water from oil.

Don't get me wrong, these filters are great, but I often see that people intend to use them in cases like a blown-up water cooler that contaminated an oil tank with a couple of buckets of water, and then wonder why they don't work.

Usually, such filters are capable of removing (retaining) tens to hundreds of grams of water before becoming useless, and they can never be used in severe water contamination cases. Their purpose is prevention only - i.e. capturing these few drops of water that accumulated at the bottom of the oil drum.

This does not mean that you can't filter out free water from oil. You can, actually. By pure mechanics. You can either use a centrifuge filter (extremely rare these days, for some reason), or a mechanical centrifuge (never seen one in action), and the best choice would probably be a coalescent filter - something that, once again, you don't see too often.

In our area, the best choice would be the likes of CJC™ filtering solutions (no affiliations here), which put a large stainless wire mesh coalescent filter underneath the particle filter and flush it with a tiny flow. I've seen quite a few of these kidney loops in action at our mine and they work surprisingly well. At least they manage to turn "white oil" into an "oil-color" oil in a couple of days. Quite expensive to my taste, but still cost much less than vacuum dehydration systems. I am planning on running some "coalescing" tests of my own, to see if I can come up with a cheaper free-water removing solution. I'll make sure to report on them when "I get there".

Now to the third filtering misconception - tank breathers. I see that people underestimate the value of desiccant breathers. It usually goes like this: "How much did you say this costs? Well, I am not paying that much for a simple breather!" An boom - there goes your desiccant breather. Either this or it never ever gets serviced or replaced!

These are the type of situations where common sense should kick in. In certain areas (e.g. an air-conditioned room) an expensive desiccant breather may not be required at all, but in other situations, the presence of a moisture absorbent breather on the tank can help remove water from the oil, and I am talking about relatively large amounts of it!

One of the cheap ways to dry a batch of water-contaminated oil would be by continuously ventilating the tank head space with air that's fed through a desiccant breather. If there's compressed air around (and there's always compressed air around) - you can make a simple venturi vacuum pump to pull the air into the headspace through such a filter - and the water will eventually "fly out of the window". Zero moving parts! Of course, the oil will have to be hot, and the air around the rig should be "normal" (no water showers or high atmospheric steam content, obviously). It can take several days to remove water contamination caused by a blown water cooler, but it will definitely be beating a dehydration unit price-wise! Once again - I have a couple of experiments lined up and will be sure to report on them here, so stay tuned.

In short - desiccant breathers are great for oil tanks because they can contribute to keeping the oil dry more than a lot of techs would think!

So - there you have it. Three filter-related things a lot of hydraulic folks I come across get "almost" right.


Go find that filtering brochure of the brand that your company represents, and give it a thorough read, Then do the same with your competitor's brand. You'll find everything I just said and a ton more!