Insane Hydraulics

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Use of Pressure Filters in Closed Loop Hydraulic Systems

Today I want to talk about the use of pressure filters in closed-loop systems, and share my general opinion about pressure line filtration.

In my "early hydraulic days" I would recommend pressure filters to everybody! It was literally like - want your hydraulic system to last longer? Slap a pressure filter in it! Even longer still? Slap two! Want to triple your productivity? Slap three pressure filters in a row! Improve your satellite dish reception? That's right, install a pressure filter! You get the picture, right?

Of course, I am exaggerating for the purpose of drama, but I really liked promoting pressure filtration, and, truth be told - always had good results with it.

So, what changed? Well - not much. I still love pressure side filtration and give due preference to it, but there’s one particular "place", namely - the bi-directional closed loop - where I stopped promoting the application of pressure filters like I used to in the past. I would say that now I defend the need for a pressure filter in such a system only if one particular criterion is present (more on this later).

Three reasons for that:

Hoops you have to jump through to pressure-filter a bi-directional line.

This one is probably the hardest. A closed-loop transmission, when bidirectional (which it is in most cases) is tricky to filter, because you can’t just install an inline filter, and hope that the particles caught in the media don’t flush out when the flow reverses. Of course, they will! At least some of them. So you have to either engineer a reverse-flow bypass solution (two check valves, four check valves, etc…) or use a filter that has a built-in reverse flow bypass. This sounds like a straightforward task - but it is not, actually, because you are introducing a breakable component into the loop, and even if the filter keeps the oil super clean, a broken check valve passing through the pump will render all of the filtration benefits useless in a snap!

But wait, check valves don't break, do they? And if a pressure filter manufacturer puts a reverse-flow capable pressure filter on the market surely they did their homework before launching the product, didn't they? Well, I thought so too. That is - I did before I had a very, very disappointing experience with a respectable line of reverse-flow capable pressure filters. This means that you should be very careful when choosing such a filter for your closed loop (if you have to use a pressure filter in a closed loop). I don't want to bash or support any particular brand here, but I do advise you to give the built-in by-pass solution of your current filter supplier a good check-out before you use it. Just in case.


I overhaul hydraulic pumps and motors, and even with our small operation receive components from everywhere. I always try to obtain as much information as possible about the hydraulic system that "donored" the unit I am working on, and many of the systems I actually know first hand, so I'm in the perfect position to form an opinion about how the way a closed loop is built and maintained is reflected in component wear. After you've overhauled your first thousand units - you start seeing patterns, you know...

Here's my take. I have worked on similarly built systems that use four pressure filters in the loop (with external check valve by-passes) and systems that didn’t use pressure filters at all. I can state that in closed-loop systems that have no pressure filtration but are equipped with a proper charge pressure filtration, a correctly sized and set loop flushing system, an adequately designed tank, and are commissioned, operated, and serviced by professional crews, I saw absolutely no difference in wear patterns in pumps and motors.

I found this to be true even for systems equipped with fast couplings (a.k.a. universal closed-loop killers) that work in polluted environments and run their pumps at close to max. rated pressures.

Filter quality.

Filtration technology simply got better over the years. Modern fiberglass filters are nothing like their cellulose ancestors. We all can sleep better these days, because it is very easy to build a closed-loop system that maintains oil pristine using only (properly sized) charge pressure filtration (although in this case, I do always give preference to the pressure-side filters).

But what is the only criterion to use a pressure filter in a closed loop I mentioned earlier then?

In my opinion, there can be only one justification for the use of a pressure filter in a bi-directional closed-loop transmission: a real and "unfixable" risk of line contamination due to external factors (most likely service or operation related). Think about it - a line of a closed loop gets contaminated either due to unprofessional service (hose exchange), or due to careless operation (negligent attitude towards fast couplings). These are human factors, and, in theory, they should be easy to fix, but in real life... Let's just say that in certain situations a pressure filter in a closed loop can, indeed, lend the transmission an extra couple of years of service.

Here are my personal guidelines for applying pressure filters in hydraulic systems:

If, however, you must use pressure filtration in a closed loop because there’s a real risk of in-loop contamination due to the factors described above - be very, very careful to study the by-pass system of a pressure filter of your choice, and if it looks flimsy - do not use it. Go for a different brand, or design your own custom by-pass solution. Just remember one thing - any by-pass has maintenance and wear, and must be mounted and installed in a way that allows for easy inspection and replacement. And do verify these valves regularly.