Insane Hydraulics

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Strangest Failure of a Pressure Filter Element

Today I want to discuss the breakdown that I investigated a couple of weeks ago - it puzzled me so greatly that I feel I must document it (along with my unsuccessful attempts to figure out what the hell happened). Remarkably, the catastrophic failure of a very expensive pump that originated the investigation wasn't caused by the strange failure that I discovered - even though it totally could, which makes this case even weirder... You'll see for yourself.

It started with a large closed-loop drive that "lost speed". Probably because of this:

And here's the "shrapnel" the mechanic found stuck in the poppet of one of the fast couplings:

The three fragments seemed to fit well into one another, which meant they were essentially pieces of a single, possibly round, "puzzle", and I couldn't, for the love of me, tell which part of the pump or the motor this thin metal piece originated from! The loop consisted of a 250cc series 90 pump and a 355cc A6VM motor - both of which I knew in and out, and I was 100% sure that neither of these two had anything inside that could give birth to this "specimen".

Pretty soon I realized that it had to come from one of the high-pressure filters, installed in the loop (my opinion on the use of high-pressure filters in closed-loop hydraulic systems can be read here). I ran to the used filters bin, to see if I could fish out the elements and luckily all four of them were still there (it was one of those systems that used four UFI FPB34 high-pressure filters with proper external return flow by-pass check valves, and for its credit - it has worked problem free for the last twelve years). And there it was - one of the four filters had a nice round chunk of the bottom cap missing (the second picture shows you how the end cap is supposed to look):

I've seen all sorts of bad things happen to filters, but I confess that a bottom cap of a filter element with a perfectly round hole blown in its center is a (weird) novelty to me. And I have no explanation for how something like that could happen!

The filter heads are equipped with bypass valves that are in good condition. The elements may be loaded with catastrophic failure particles, but they show no signs of collapsing - which for these particular elements should happen at 20 bar, and 20 bar is not enough (by a huge margin!) to blow a hole in a relatively thick metal sheet.

I then thought that, maybe, the element was inserted upside down and the filter head tube cut into the cap:

But a quick test showed that when the filter element is inserted "the other way around" - there's no way for the bowl to even come near the threads - so that theory was discarded.

Then this situation made me think of another sneaky thing that can happen to a pressure filter of the classic design (by classic design I mean - a hefty cast iron head with a thick-walled steel bowl under it and a cylindrical filter element inside). This is a perfect occasion to bring this one up, actually - such "accidents" usually happen when the filter element is exchanged by an inexperienced person (which is a common occurrence due to the universal consensus that a filter exchange requires no skill at all) - and the person does not realize that when he/she removed the old filter element, a part of it - namely the top filter cap with the hole - decided to "stick around". This tends to happen more to "counterfeit" filter elements due to the inferior quality of the glue compound that is supposed to hold the caps in place. I've seen this more than once! Here's an illustration of what I am talking about:

What happens next is - the new filter element goes in under the "surplus" cap - the bowl is re-screwed and the conditions for something bad happening to an "over-sandwiched" filter element are created! This is why, whenever I replace a pressure filter element, I always check inside the head for "leftover caps":

So - it got me thinking, what if the following sequence of events happened:

What an intricate theory, isn't it? I tested this one, as well (remember that we had four filter elements to play with!). I removed the top cap (and I confirm to you that it indeed has very sharp edges), then dropped it to the bottom of the bowl, and assembled the filter with another element. I was surprised how small the added resistance of the cap crushed against the filter element is - you can totally re-tighten the bowl without noticing that something's not right. But... no matter how I tried - I didn't manage to punch through the end cap - the "stamping die" would simply crush and bend, without making even a dent in the cap:

So much for my theory!

At this point - the only explanation I have is: that it must have been a faulty filter element, in which the stamping of the round dome in the end cap went a bit too far, and made the perimeter so thin that a pressure differential spike was enough to bust it out. That's the best I can come up with. And if you, by any chance, can suggest a different theory - I'm all ears!

But if you think that it was the loose cap that broke the loop you would be wrong! If you look at the sealing surfaces of the pump's rotary group - you'll find no large object damage anywhere:

Surely, there are signs of cavitation (the charge pressure membrane accumulator was busted, by the way), and there are clear signs of small particle wear, but no signs of a chunk of sheet metal getting sheared between a valve plate and a barrel port. Believe me, it looks very different from what we're seeing here.

So, in a twisted chain of events, a pressure filter, which is supposed to be a good thing, gave origin to a "killer blank", and then a fast coupling, which is supposed to be a bad thing for any transmission line, was actually the one who caught and held onto it! But why did the pump fail then?

I believe that the pump failed due to old age and lack of maintenance. Not to the rig itself though - because in that regard all the filter/oil changes were done by the book, but rather due to the absence of scheduled pump overhauls. The rig went through at least 1000-2000 hours per year for the last 10-plus years and the pump had never been removed. That's a big no-no for a series 90 (for any pump, actually). You can tell that the pump has many hours by the wear of the shaft splines or the grooves on the swash-plate bearing surface:

This is, by the way, a low-pressure/high-flow system (medium working pressure below 250 bar), so these grooves translate into more hours than they would for a series 90 pump that operates at 400+ bar.

So, today's takeaways would be: